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Interesting paper here about the perils of shallow cycling LiFePO4 batteries and how to recover the lost capacity. Maybe it was a mistake to get 600Ah, which I'm going to struggle to deep cycle. But at least the problem is recoverable.

Edited by nicknorman
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7 minutes ago, nicknorman said:

Maybe it was a mistake to get 600Ah, which I'm going to struggle to deep cycle.

 

Why?  Don't charge them for a while (several days?) and they'll go well down, then reconnect your charging / throw the switch on the regulator controller and cruise for a few hours.

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7 minutes ago, nicknorman said:

Interesting paper here about the perils of shallow cycling LiFePO4 batteries and how to recover the lost capacity. Maybe it was a mistake to get 600Ah, which I'm going to struggle to deep cycle. But at least the problem is recoverable.

I have just skim read the link and this looks like the 'memory' effect that has been discussed on the Crusier forum - and to which I have referred to in a number of posts when I set out with Li's. On that forum, I seem to remember the general consensus was that only charging to 70% ish caused the issue and the way round it was to go up to 100% now and again. This is why I take mine up to 100%, 3 or 4 times a year - to reset the BMV but also to get rid of the memory effect. I know this was discussed in a lot of my earlier posts in 2018.

I will do a 'deeper' read tomorrow.

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3 minutes ago, TheBiscuits said:

 

Why?  Don't charge them for a while (several days?) and they'll go well down, then reconnect your charging / throw the switch on the regulator controller and cruise for a few hours.

Why? Because we cruise most days. But you are right, i can just set the alternator regulator switch to 50% SoC target. And use the toaster. And the electric kettle. And the tumble drier if i'm deperate. That should give 'em something to think about!

4 minutes ago, Dr Bob said:

I have just skim read the link and this looks like the 'memory' effect that has been discussed on the Crusier forum - and to which I have referred to in a number of posts when I set out with Li's. On that forum, I seem to remember the general consensus was that only charging to 70% ish caused the issue and the way round it was to go up to 100% now and again. This is why I take mine up to 100%, 3 or 4 times a year - to reset the BMV but also to get rid of the memory effect. I know this was discussed in a lot of my earlier posts in 2018.

I will do a 'deeper' read tomorrow.

What is interesting is that they claim the effect can be reversed by leaving for a while at either 100% SoC, or at 0% SoC. Something to do with Li ion gradients being at a minimum mid charge, and at a maximum at either 100% or 0%.

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My drive batteries were heavily used ex bus batteries, due to circumstances they arrived in some batteries flat and others with 3 -15 volts remember these are 36 volt batteries out of 30 3 would not recover. I fully charged them using solar connected to the puter they all balanced up and have been taken right down when I did 10 hours cruising with little or no sun, they are now at 60% for the winter. The 3 duf batteries were replaced and the duff ones stripped to find a couple of knackered cells in each. After reading the article I will fully charge the batteries a couple of times a year and discharge them as well, good find Nick 

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Still discharging (switched it off overnight) and now down to 8% Soc (185Ah drained) but voltage is still up at 12.7v (no load) so it looks like I'm going to get my 200Ah. I have set the "emergency disconnect" relay to operate when the lowest cell reaches 2.9v with warning beeps below 3v, so that will be a good test of it. Cell imbalance is now up to 10mV so obviously they aren't going to perfectly bottom balanced but still, not bad.

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Its nearly been a month since the new battery monitor was switched on. Still some work to do to move the current measurements to the new monitor so that the prototype system can be switched off.

The graphs are relatively self explanatory. and are beginning to show the effect of temperature on performance.

The near vertical steps on the blue Charge trace show when the alternator was used. This charges initially at about 100A, before dropping back to about 80A as it warms up. The charge current is engine RPM dependant, and quite possibly not consistent between charges.

The more gradual charging is caused by solar, which can provide a maximum current of 30A but I doubt that has been realised in these graphs. Maximum value is probably around 20A, on a sunny day.

 

The temperature graph shows a number of effects. The obvious one is the internal battery heating caused by charging, which appears as the steep temperature rises on the graph. As the engine room remains warm for several hours after running, the batteries continue to warm up after charging, albeit at a much slower rate.

There are a couple of obvious instances on the temperature graph where the battery heaters were switched on.

The first being at about midnight on 11th Nov, running till about 9am. The second being midnight of 29th Nov, still running now.

A noteworthy point regarding the heaters is that my previous comment about their efficacy was wrong by quite a long way. I think i previously stated a temperature gain of about 20C above ambient. These graphs show it to be at best 10C. This is obviously just the equilibrium point where the enclosure is losing (to ambient) the same power as we are providing in heat. It should be possible, given a clean enough heating curve, to estimate the thermal resistance of the enclosure, and the heat capacity of the batteries. All rather academic, although it shows the insulation of the enclosures could be improved.

 

The most interesting effect is how the charge trip-off point is effected by temperature, and these graphs are helping to unravel some of the mysteries. Firstly a quick note about the charging scheme; the alternator charges at full current until a voltage threshold is crossed, and then trips out. Hence constant current charging only. I hope to improve this now that we have a better monitoring solution. The first observation is that the internal cell resistance is temperature dependant (negative coefficient) - no surprise.  Cell voltage probably is too, but that is less easy to correlate from the graphs.

At low temperatures, the internal resistance is higher, hence during charging the trip-out voltage threshold is crossed earlier.

However the batteries warm up as they are charged. So if charging from a lower state of charge, there is more time to warm up, hence a higher temperature is achieved, lower internal resistance, and a higher state of charge is achieved.

One final effect is that when charging for long periods of time, the engine room temperature rises further, and causes the alternator to become less efficient. The output might drop to 75A. Due to the effect of internal resistance, this causes more charge to be absorbed before tripping out.

Clearly several inter-related temperature effects which are in a bit of a positive feedback loop.

 

I've tried on several occasions to "model" the internal resistance of the battery; probably a bit of an over-technical term. This is largely academic, but has some value. If the internal resistance can be estimated, it would be possible to compensate for the effect of load/charge current on the measured voltage. This would also lead to better state of charge estimation, especially when it is not possible to take a clean voltage measurement with minimal load. Now granted, on a boat it is not really that necessary to precisely know the state of charge. A Volt meter is a bare minimum, but perfectly sufficient means of estimating SOC; this is all we had for the first couple of years. On my "quest" to understand the behaviour deeper, I discovered fairly early on that the resistance is non-ohmic. As the current is increased, the resistance reduces.

 

The graphs also show another well-known effect, that the voltage is dependant, not just on the instantaneous current, but on the current in a window several hours prior. I've demonstrated that this can easily be modelled us an exponential decay function, and it is relatively trivial using a "goal-seek" to estimate the time constant and magnitude.This modelling has always been thrown out by the fact that the voltage/current relationship is non-linear. As always, in the data there are too many uncontrolled variables. Some experimentation under more controlled conditions might be needed.

Screenshot from 2020-11-30 21-55-47.png

Screenshot from 2020-11-30 21-56-23.png

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45 minutes ago, Craig Shelley said:

Its nearly been a month since the new battery monitor was switched on. Still some work to do to move the current measurements to the new monitor so that the prototype system can be switched off.

The graphs are relatively self explanatory. and are beginning to show the effect of temperature on performance.

The near vertical steps on the blue Charge trace show when the alternator was used. This charges initially at about 100A, before dropping back to about 80A as it warms up. The charge current is engine RPM dependant, and quite possibly not consistent between charges.

The more gradual charging is caused by solar, which can provide a maximum current of 30A but I doubt that has been realised in these graphs. Maximum value is probably around 20A, on a sunny day.

 

The temperature graph shows a number of effects. The obvious one is the internal battery heating caused by charging, which appears as the steep temperature rises on the graph. As the engine room remains warm for several hours after running, the batteries continue to warm up after charging, albeit at a much slower rate.

There are a couple of obvious instances on the temperature graph where the battery heaters were switched on.

The first being at about midnight on 11th Nov, running till about 9am. The second being midnight of 29th Nov, still running now.

A noteworthy point regarding the heaters is that my previous comment about their efficacy was wrong by quite a long way. I think i previously stated a temperature gain of about 20C above ambient. These graphs show it to be at best 10C. This is obviously just the equilibrium point where the enclosure is losing (to ambient) the same power as we are providing in heat. It should be possible, given a clean enough heating curve, to estimate the thermal resistance of the enclosure, and the heat capacity of the batteries. All rather academic, although it shows the insulation of the enclosures could be improved.

 

The most interesting effect is how the charge trip-off point is effected by temperature, and these graphs are helping to unravel some of the mysteries. Firstly a quick note about the charging scheme; the alternator charges at full current until a voltage threshold is crossed, and then trips out. Hence constant current charging only. I hope to improve this now that we have a better monitoring solution. The first observation is that the internal cell resistance is temperature dependant (negative coefficient) - no surprise.  Cell voltage probably is too, but that is less easy to correlate from the graphs.

At low temperatures, the internal resistance is higher, hence during charging the trip-out voltage threshold is crossed earlier.

However the batteries warm up as they are charged. So if charging from a lower state of charge, there is more time to warm up, hence a higher temperature is achieved, lower internal resistance, and a higher state of charge is achieved.

One final effect is that when charging for long periods of time, the engine room temperature rises further, and causes the alternator to become less efficient. The output might drop to 75A. Due to the effect of internal resistance, this causes more charge to be absorbed before tripping out.

Clearly several inter-related temperature effects which are in a bit of a positive feedback loop.

 

I've tried on several occasions to "model" the internal resistance of the battery; probably a bit of an over-technical term. This is largely academic, but has some value. If the internal resistance can be estimated, it would be possible to compensate for the effect of load/charge current on the measured voltage. This would also lead to better state of charge estimation, especially when it is not possible to take a clean voltage measurement with minimal load. Now granted, on a boat it is not really that necessary to precisely know the state of charge. A Volt meter is a bare minimum, but perfectly sufficient means of estimating SOC; this is all we had for the first couple of years. On my "quest" to understand the behaviour deeper, I discovered fairly early on that the resistance is non-ohmic. As the current is increased, the resistance reduces.

 

The graphs also show another well-known effect, that the voltage is dependant, not just on the instantaneous current, but on the current in a window several hours prior. I've demonstrated that this can easily be modelled us an exponential decay function, and it is relatively trivial using a "goal-seek" to estimate the time constant and magnitude.This modelling has always been thrown out by the fact that the voltage/current relationship is non-linear. As always, in the data there are too many uncontrolled variables. Some experimentation under more controlled conditions might be needed.

Screenshot from 2020-11-30 21-55-47.png

Screenshot from 2020-11-30 21-56-23.png

Very nice, I have discovered over 3 years or so that if the sun shines my solar charges the batteries, they live inside next to central heating pipes so are cosy and if the sun doesn't shine my whispergen starts up and charges them instead! I decided at the beginning that my lithium batteries had to be fit and forget they are and I have 

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12 hours ago, Craig Shelley said:

Its nearly been a month since the new battery monitor was switched on. Still some work to do to move the current measurements to the new monitor so that the prototype system can be switched off.

The graphs are relatively self explanatory. and are beginning to show the effect of temperature on performance.

The near vertical steps on the blue Charge trace show when the alternator was used. This charges initially at about 100A, before dropping back to about 80A as it warms up. The charge current is engine RPM dependant, and quite possibly not consistent between charges.

The more gradual charging is caused by solar, which can provide a maximum current of 30A but I doubt that has been realised in these graphs. Maximum value is probably around 20A, on a sunny day.

 

The temperature graph shows a number of effects. The obvious one is the internal battery heating caused by charging, which appears as the steep temperature rises on the graph. As the engine room remains warm for several hours after running, the batteries continue to warm up after charging, albeit at a much slower rate.

There are a couple of obvious instances on the temperature graph where the battery heaters were switched on.

The first being at about midnight on 11th Nov, running till about 9am. The second being midnight of 29th Nov, still running now.

A noteworthy point regarding the heaters is that my previous comment about their efficacy was wrong by quite a long way. I think i previously stated a temperature gain of about 20C above ambient. These graphs show it to be at best 10C. This is obviously just the equilibrium point where the enclosure is losing (to ambient) the same power as we are providing in heat. It should be possible, given a clean enough heating curve, to estimate the thermal resistance of the enclosure, and the heat capacity of the batteries. All rather academic, although it shows the insulation of the enclosures could be improved.

 

 

 

 

Craig, its all good information but all this stuff on internal resistance seems over the top except for the very few peeps who understand electrickerty to the nth degree. I worry that this may put off the average 'Joe' who may come across this thread and think he needs to understand it. I see internal resistance of batteries very much like the heat capacity of the steel used to make a kettle to put on the hob. You fill the kettle full of water, put it on the stove and turn on the gas. It heats up. When its boiling you pour you cup of tea. You get used to the inputs of if its cold – you can touch it – if its getting over 80°C – it starts to make a noise – when it boils – it sings. No need to understand the heat transmission data of the steel! Lifepo4s are similar. You charge and discharge looking at voltage. The effect of the internal resistance is not important in the grand scheme of things. Maybe if you want to estimate when you get to 79% charged at 14.6°C, but if you just want to know if you are 60-70% full then just looking at the volt meter and a glance at how much power is going out is good enough. If I'm down to 12.8V on discharge, I need to charge (as long as Mrs Bob isnt running the Nesspresso machine). If I'm up to 13.8V on charge, I stop charging. Simple. Not unlike managing a kettle.

The graphs are interesting but if I was after more information, I am not sure what they tell me. How much capacity have you got? Quite important! What type of batteries?

You only seem to be charging to 13.55V normally via the alternator with one charge going to 13.7V and then a load of 'lesser' charge voltage peaks which I assume are the solar. I dont understand why you are tripping your system at 13.55V which seems very low – although there is a peak at 13.7V – and a few 'alternator' peaks at less than 13.55V. Similarly you never drop the voltage below 13.1V.

On my system I charge to 13.7V or occasionally 13.8V if the 2 BtoB's are having an argument which gives me 70-85% SoC dependent on how much current is going in on charge. If you are charging to 13.55V with 80A, on my system that would suggest only 70% charged! I know each system is different and my Lifepo4s are at the end of 5M of 50mm cabling which causes a voltage drop – but mine go down to 12.8V before starting to see the low voltage knee. Are you cycling between 40% and 70% charged? I am sure everyones voltages will be different.

The temperature information is interesting but I knew that with my Tesla. In summer above 25°C, I get 10% more range than at 15°C despite the air con running flat out and to charge most efficiently the batteries need to be heated up – they call it preconditioning. It seems like you are showing the batteries are at a higher SoC for a given cut off voltage at higher temps. My batteries vary between about 14°C in the winter (in a cupboard inside at the back of the boat) and 28°C in the summer and to be honest, that spread is fine. Ok I could set my voltage cut off at 0.1V higher in the winter to get more charge in but is it worth the hassle? I've never had my batteries down at 5°C and charged them but I wouldn't worry about that. Heating obviously helps but not that much.

I would however be interested to see some 'short duration' graphs ie voltage vs current flow during charging over the time you were running the engine and the few hours either side. Interested to see if the voltage profiles are similar to mine. That interests me as 3 or 4 times a year I manually take the batteries up to 14.0V to reset the BMV and to get rid of memory effects (or restricted capacity as Nick identified) and I dont have much info on the current performance in the constant voltage phase at a range of high currents. That's just the 'nerd' in me though. You can run a set of Lifepo4s without looking at graphs etc.

I do very much appreciate your data though.

 

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49 minutes ago, Dr Bob said:

Craig, its all good information but all this stuff on internal resistance seems over the top except for the very few peeps who understand electrickerty to the nth degree. I worry that this may put off the average 'Joe' who may come across this thread and think he needs to understand it. I see internal resistance of batteries very much like the heat capacity of the steel used to make a kettle to put on the hob. You fill the kettle full of water, put it on the stove and turn on the gas. It heats up. When its boiling you pour you cup of tea. You get used to the inputs of if its cold – you can touch it – if its getting over 80°C – it starts to make a noise – when it boils – it sings. No need to understand the heat transmission data of the steel! Lifepo4s are similar. You charge and discharge looking at voltage. The effect of the internal resistance is not important in the grand scheme of things. Maybe if you want to estimate when you get to 79% charged at 14.6°C, but if you just want to know if you are 60-70% full then just looking at the volt meter and a glance at how much power is going out is good enough. If I'm down to 12.8V on discharge, I need to charge (as long as Mrs Bob isnt running the Nesspresso machine). If I'm up to 13.8V on charge, I stop charging. Simple. Not unlike managing a kettle.

The graphs are interesting but if I was after more information, I am not sure what they tell me. How much capacity have you got? Quite important! What type of batteries?

You only seem to be charging to 13.55V normally via the alternator with one charge going to 13.7V and then a load of 'lesser' charge voltage peaks which I assume are the solar. I dont understand why you are tripping your system at 13.55V which seems very low – although there is a peak at 13.7V – and a few 'alternator' peaks at less than 13.55V. Similarly you never drop the voltage below 13.1V.

On my system I charge to 13.7V or occasionally 13.8V if the 2 BtoB's are having an argument which gives me 70-85% SoC dependent on how much current is going in on charge. If you are charging to 13.55V with 80A, on my system that would suggest only 70% charged! I know each system is different and my Lifepo4s are at the end of 5M of 50mm cabling which causes a voltage drop – but mine go down to 12.8V before starting to see the low voltage knee. Are you cycling between 40% and 70% charged? I am sure everyones voltages will be different.

The temperature information is interesting but I knew that with my Tesla. In summer above 25°C, I get 10% more range than at 15°C despite the air con running flat out and to charge most efficiently the batteries need to be heated up – they call it preconditioning. It seems like you are showing the batteries are at a higher SoC for a given cut off voltage at higher temps. My batteries vary between about 14°C in the winter (in a cupboard inside at the back of the boat) and 28°C in the summer and to be honest, that spread is fine. Ok I could set my voltage cut off at 0.1V higher in the winter to get more charge in but is it worth the hassle? I've never had my batteries down at 5°C and charged them but I wouldn't worry about that. Heating obviously helps but not that much.

I would however be interested to see some 'short duration' graphs ie voltage vs current flow during charging over the time you were running the engine and the few hours either side. Interested to see if the voltage profiles are similar to mine. That interests me as 3 or 4 times a year I manually take the batteries up to 14.0V to reset the BMV and to get rid of memory effects (or restricted capacity as Nick identified) and I dont have much info on the current performance in the constant voltage phase at a range of high currents. That's just the 'nerd' in me though. You can run a set of Lifepo4s without looking at graphs etc.

I do very much appreciate your data though.

 

Exactly this Bob I wasn't being sarcastic with my comments but I wanted to show how simple they are to manage.  I suppose 1% of boaters will find it interesting but a lot will be scared witless by this information me as I have said sun shines batteries charge

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Well I found it interesting! One thing I’ve seen repeatedly is the need to clean the surface corrosion / patina from the copper links and terminal tops using fine sandpaper before connecting. Failure to do this can cause slight resistance which translates to heat under heavy charge or discharge currents. I therefore wonder if some of Craig’s temperature rise is due to ohmic loss at the terminals, as opposed to internally generated heat. Thermal expansion at the terminal connections might also affect the connection pressure and hence resistance, messing up any attempts to determine internal resistance.

 

For me I’ve finished cycling the second set of 200Ah batteries. I think I made a mistake in not keeping the 3 batches (sent separately from China) separated as delivered. The first set I cycled was first delivery, but I’ve muddled up the second and third deliveries so my second batch I think is a mongrel. Considerably more split in voltages (though still not much) and more effort to top balance. What I don’t understand is that after fully charging, during discharge one cell was consistently high and was the last to fall off its “knee” at the bottom (discharge stopped when lowest cell got to 2.90v whilst the highest cell was just over 3v, but falling quite quickly). But now I have recharged to 30% SoC the rested cell voltages are all within 1mV. Weird!

 

Annoyingly I left the battery charger at my caravan (85 miles away) so the last batch is going to have to be charged using my 5A power supply. That is going to take a long time!

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1 hour ago, nicknorman said:

Well I found it interesting! One thing I’ve seen repeatedly is the need to clean the surface corrosion / patina from the copper links and terminal tops using fine sandpaper before connecting. Failure to do this can cause slight resistance which translates to heat under heavy charge or discharge currents. I therefore wonder if some of Craig’s temperature rise is due to ohmic loss at the terminals, as opposed to internally generated heat. Thermal expansion at the terminal connections might also affect the connection pressure and hence resistance, messing up any attempts to determine internal resistance.

 

For me I’ve finished cycling the second set of 200Ah batteries. I think I made a mistake in not keeping the 3 batches (sent separately from China) separated as delivered. The first set I cycled was first delivery, but I’ve muddled up the second and third deliveries so my second batch I think is a mongrel. Considerably more split in voltages (though still not much) and more effort to top balance. What I don’t understand is that after fully charging, during discharge one cell was consistently high and was the last to fall off its “knee” at the bottom (discharge stopped when lowest cell got to 2.90v whilst the highest cell was just over 3v, but falling quite quickly). But now I have recharged to 30% SoC the rested cell voltages are all within 1mV. Weird!

 

Annoyingly I left the battery charger at my caravan (85 miles away) so the last batch is going to have to be charged using my 5A power supply. That is going to take a long time!

My car packs have tinned aluminium links less prone to corrosion.  All the valance batteries are brass I think I polished them to a bright shine and and used a terminal spray on them 

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1 hour ago, nicknorman said:

What I don’t understand is that after fully charging, during discharge one cell was consistently high and was the last to fall off its “knee” at the bottom (discharge stopped when lowest cell got to 2.90v whilst the highest cell was just over 3v, but falling quite quickly). But now I have recharged to 30% SoC the rested cell voltages are all within 1mV. Weird!

I have one like that too. (Well, three in parallel). Tends to have slightly lower voltage during charge too. I think it has significantly greater capacity than the rest. If I was starting again, I'd carefully measure the capacity of all the original cells and arrange them in sets with as close to the same capacity as possible, That's not possible whilst we're living aboard unless we have a long sojourn in a marina.

 

MP.

 

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7 minutes ago, MoominPapa said:

I have one like that too. (Well, three in parallel). Tends to have slightly lower voltage during charge too. I think it has significantly greater capacity than the rest. If I was starting again, I'd carefully measure the capacity of all the original cells and arrange them in sets with as close to the same capacity as possible, That's not possible whilst we're living aboard unless we have a long sojourn in a marina.

 

MP.

 

Yes, I've got one as well. First into the knee at the top and 2nd into the knee at the bottom. Just work with the voltages in between!

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10 hours ago, Dr Bob said:

I would however be interested to see some 'short duration' graphs ie voltage vs current flow during charging over the time you were running the engine and the few hours either side. Interested to see if the voltage profiles are similar to mine.

Attached is a "zoomed in" view of the charge profile from yesterday. I agree that the voltage we are tripping the alternator off at is a little low. It would be nice if the alternator controller could benefit from some of the monitoring improvements which are currently going in. I might add a comms link back to the Pi to enable a proper CV/CI charging profile.

If it's useful, I can export the data from google sheets, or provide a link to the live sheet.

From experience, increasing the alternator cut-off voltage further doesn't tend to lead to a great deal more charging time because the voltage at this point in the charge profile begins to ramp quite steeply.

The data on the sheet and graphs is decimated down to a 10 minute sample interval, but is internally sampled at a much higher rate to enable accurate AHr counting.

The cycling of the fridge can clearly be seen on the graph, and it is possible that the fridge compressor cutting off might have triggered the alternator to then trip off too.

 

9 hours ago, nicknorman said:

One thing I’ve seen repeatedly is the need to clean the surface corrosion / patina from the copper links and terminal tops using fine sandpaper before connecting. Failure to do this can cause slight resistance which translates to heat under heavy charge or discharge currents.

I did wonder if that might be the case. When I originally assembled the pack, I polished the mating faces of the terminals, perhaps too obsessively, using several grades of sand paper. The sand paper was wrapped around a wooden block to ensure flatness of the surfaces. They looked like mirrors when they were done! In late 2018 the pack was split up and installed into enclosures. I remember noticing that there was no longer a bright mirror finish to the copper, although we didn't have time to faff about re-polishing. They looked good enough. The temperature probes are tightly attached to the centre links of the packs, my rationale being that the copper terminal must surely extend deep inside the cell, and hence would give a closer indication of the internal temperature than sticking it onto the plastic casing. In the temperature graph, the battery heaters were running throughout the entire plot, although their effect is barely noticeable at that scale.

Screenshot from 2020-12-01 22-10-45.png

Screenshot from 2020-12-01 23-09-08.png

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5 minutes ago, nicknorman said:

So my question is, should the bank be assembled with some sort of goo on the connections (after having cleaned them to bright) such as vaseline or proprietary terminal paste? At the very least, to keep corrosion at bay.

I use a terminal spray on them Nick which seems to work. Maybe copperslip would work as that's a grease with copper in it?

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1 hour ago, peterboat said:

I use a terminal spray on them Nick which seems to work. Maybe copperslip would work as that's a grease with copper in it?

I just bolted mine up 2 1/2 years ago with no cleaning or other faffing. The ecofan makes the odd noise now and again but other than that no problems.

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3 hours ago, Dr Bob said:

I just bolted mine up 2 1/2 years ago with no cleaning or other faffing. The ecofan makes the odd noise now and again but other than that no problems.

But obviously it depends on how much current one intends to stuff in and out. I can charge at 275A and discharge at 200A or so, so it will be important to minimise connection resistance. It would not be hard to be in a situation whereby the connection resistance is more than the internal resistance of the battery. And resistance, at a couple of hundred amps, equals a lot of heat.

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On 02/12/2020 at 11:57, nicknorman said:

So my question is, should the bank be assembled with some sort of goo on the connections (after having cleaned them to bright) such as vaseline or proprietary terminal paste? At the very least, to keep corrosion at bay.

 

When I was working, power semiconductors were always bolted to the heat sink and interconnects using thermal grease and 

 

Typical currents were 100 - 1000 amps and connections were always bolted down using a calibrated torque wrench.

 

Approximate current balance on parallel devices was measured by measing the millivolt drop across connections.

Edited by cuthound
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I've just ordered a new pcb for my battery monitor. My first foray into a 4 layer circuit board (circuit tracks top and bottom as usual but also 0v and 3.3v power planes sandwiched in the middle) but it means I have been able to shrink it to fit on a 72 x 72mm board which will fit inside commonly available enclosures. Amazingly, 5 4 layer boads has cost me £9. Roughly £5 for the boards (ie £1 each) and £4 for snail mail postage from China. So it will be a while, but no rush.

 

I have got the idle power consumption down to 3 mA which is not quite as good as I'd hoped, but acceptable I think bearing in mind the CANBUS interface is live. It's about 15 mA with the display on, but that times out after a while.

 

Which brings me to a question for BMV712S users. Having disconnected everything from the load side of the shunt (this is all still in my house, not connected to the boat), with 200Ah of batteries at around 40% SoC, the SoC indication on the BMV has crept up slightly, roughly by 0.1% per day. I have tried the "zero current calibration" - item 09 on the setup. Which doesn't seem to help. I select setup item 09, I press "select" again, and it comes back with "ERROR" so I am not convinced it is doing anything. I also notice that if I set the "current threshold" - item 07 - to zero, the current display shows fluctuations between 0 and 0.02A. Maybe not a lot but 20mA is 1/2Ah per day which is also not a lot if one is routinely synchronising the BMV by charging to 100% as one would do with LA  batteries. But of course that is not the plan. 0.25Ah per day for 6 months is 87Ah which definitely is significant. 6 months is the battery manufacturer's recommended interval for a full charge/discharge cycle for the batteries.

 

I also notice that this "phantom" current indication varies according to whether the BMV backlight is on or not. Presumably the small current taken by the BMV backlight is dropping a tiny voltage in the rather long cable supplied with the BMV. You would think they would use separate wires for the shunt vs the device's power supply, but maybe not!

This is a popular device, has anyone else had similar issues?

 

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36 minutes ago, nicknorman said:

I

 

Which brings me to a question for BMV712S users. Having disconnected everything from the load side of the shunt (this is all still in my house, not connected to the boat), with 200Ah of batteries at around 40% SoC, the SoC indication on the BMV has crept up slightly, roughly by 0.1% per day. I have tried the "zero current calibration" - item 09 on the setup. Which doesn't seem to help. I select setup item 09, I press "select" again, and it comes back with "ERROR" so I am not convinced it is doing anything. I also notice that if I set the "current threshold" - item 07 - to zero, the current display shows fluctuations between 0 and 0.02A. Maybe not a lot but 20mA is 1/2Ah per day which is also not a lot if one is routinely synchronising the BMV by charging to 100% as one would do with LA  batteries. But of course that is not the plan. 0.25Ah per day for 6 months is 87Ah which definitely is significant. 6 months is the battery manufacturer's recommended interval for a full charge/discharge cycle for the batteries.

 

 

 

I assume you mean the SoC has reduced?

Over the past month (hooked up to shore power in a marina) our Li's have been isolated at around 40% SoC. Most of the time I have had both the BEP auto switch live, the BMV live and the cell monitor live. The battery voltage has dropped around 0.10V over 21 days so that is 0.01V every few days. A week ago I turned the BEP switch off and disconnected the cell monitor and the system has stayed at 12.95V for 7 days. The current drain from the BEP switch and the cell monitor dont show up on the BMV. The cell monitor is wired directly onto the cells and not via the shunt but I am sure the BEP is wired via the shunt. Once I isolate the bank, the SoC or Ahrs used remain constant.

On my system, a drop of 0.1V is equiv to around 50Ahrs of capacity and I find that after a couple of months out and about with not going over 80% SoC, when I charge to full to syncronise the BMV I usually have to put in 50Ahrs ish past when the Ahrs out gets back to zero. I guess this is the 0.01V loss from the system every few days to 'power the system'. It could however be a BMV issue. I've given up worrying about it as 50Ahrs is half an hour of engine running!

BMVs are a thing of mystery.

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1 hour ago, nicknorman said:

I've just ordered a new pcb for my battery monitor. My first foray into a 4 layer circuit board (circuit tracks top and bottom as usual but also 0v and 3.3v power planes sandwiched in the middle) but it means I have been able to shrink it to fit on a 72 x 72mm board which will fit inside commonly available enclosures. Amazingly, 5 4 layer boads has cost me £9. Roughly £5 for the boards (ie £1 each) and £4 for snail mail postage from China. So it will be a while, but no rush.

 

I have got the idle power consumption down to 3 mA which is not quite as good as I'd hoped, but acceptable I think bearing in mind the CANBUS interface is live. It's about 15 mA with the display on, but that times out after a while.

 

Which brings me to a question for BMV712S users. Having disconnected everything from the load side of the shunt (this is all still in my house, not connected to the boat), with 200Ah of batteries at around 40% SoC, the SoC indication on the BMV has crept up slightly, roughly by 0.1% per day. I have tried the "zero current calibration" - item 09 on the setup. Which doesn't seem to help. I select setup item 09, I press "select" again, and it comes back with "ERROR" so I am not convinced it is doing anything. I also notice that if I set the "current threshold" - item 07 - to zero, the current display shows fluctuations between 0 and 0.02A. Maybe not a lot but 20mA is 1/2Ah per day which is also not a lot if one is routinely synchronising the BMV by charging to 100% as one would do with LA  batteries. But of course that is not the plan. 0.25Ah per day for 6 months is 87Ah which definitely is significant. 6 months is the battery manufacturer's recommended interval for a full charge/discharge cycle for the batteries.

 

I also notice that this "phantom" current indication varies according to whether the BMV backlight is on or not. Presumably the small current taken by the BMV backlight is dropping a tiny voltage in the rather long cable supplied with the BMV. You would think they would use separate wires for the shunt vs the device's power supply, but maybe not!

This is a popular device, has anyone else had similar issues?

 

I suspect that this effect is so small compared to the inaccuracies inherent in Ah counting lead-acid batteries (which is what it was designed for) that very few people have noticed it.  That error is the about the same as the best I can do with a Hall-effect current sensor driving the AtoD converter in an Arduinio, but I do have to adjust the zero offset fairly often to get it that good. I think temperature change is the main source of long-term drift. Such small systematic errors don't bother us as we live aboard and the batteries are cycling regularly, plus the Kalman filter incorporating voltage into the SoC estimate corrects any long-term drift.

 

Very good prices on PCBs. If we ever go back  to spending a long period on shore power, I'd be tempted to re-implement my system more professionally. Whilst we're using it daily, I can't afford to fiddle with it too much.

 

MP.

 

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22 minutes ago, Dr Bob said:

I assume you mean the SoC has reduced?

Over the past month (hooked up to shore power in a marina) our Li's have been isolated at around 40% SoC. Most of the time I have had both the BEP auto switch live, the BMV live and the cell monitor live. The battery voltage has dropped around 0.10V over 21 days so that is 0.01V every few days. A week ago I turned the BEP switch off and disconnected the cell monitor and the system has stayed at 12.95V for 7 days. The current drain from the BEP switch and the cell monitor dont show up on the BMV. The cell monitor is wired directly onto the cells and not via the shunt but I am sure the BEP is wired via the shunt. Once I isolate the bank, the SoC or Ahrs used remain constant.

On my system, a drop of 0.1V is equiv to around 50Ahrs of capacity and I find that after a couple of months out and about with not going over 80% SoC, when I charge to full to syncronise the BMV I usually have to put in 50Ahrs ish past when the Ahrs out gets back to zero. I guess this is the 0.01V loss from the system every few days to 'power the system'. It could however be a BMV issue. I've given up worrying about it as 50Ahrs is half an hour of engine running!

BMVs are a thing of mystery.

No, I meant the SoC has increased! The "phantom" current is positive.

 

My battery monitor is still connected but it is direct to the cells in order to avoid the voltage drop across the shunt. I did consider redesigning the board with separated 0v wires - one to the shunt to power the thing, and the other direct to the battery -ve to measure the cell voltage, but in the end I decided it was too difficult, and to live with the ~3mA error. Which, based on my experience so far with the BMV accuracy, was the right decision!

 

I think the difference between our approaches is that I intend to charge to a specified SoC rather than to a specified voltage, but for that to work the system has to have a reasonably accurate measurement of SoC.

Of course my intention is to use the SoC from the Mastershunt, maybe that will be more accurate (it should be, as the current is digitised within the shunt, not at the other end of some rather long wires).

And of course I could use a kalman filter as per Mr Moomin to keep an internal track of the SoC, but that is going to be a bit confusing if the alternator is going to float at some random (in terms of what the mastershunt or BMV is showing. Ah, Mr Moomin has just pinged up on the computer, I bet that is what he is going to say!...

4 minutes ago, MoominPapa said:

I suspect that this effect is so small compared to the inaccuracies inherent in Ah counting lead-acid batteries (which is what it was designed for) that very few people have noticed it.  That error is the about the same as the best I can do with a Hall-effect current sensor driving the AtoD converter in an Arduinio, but I do have to adjust the zero offset fairly often to get it that good. I think temperature change is the main source of long-term drift. Such small systematic errors don't bother us as we live aboard and the batteries are cycling regularly, plus the Kalman filter incorporating voltage into the SoC estimate corrects any long-term drift.

 

Very good prices on PCBs. If we ever go back  to spending a long period on shore power, I'd be tempted to re-implement my system more professionally. Whilst we're using it daily, I can't afford to fiddle with it too much.

 

MP.

 

Right on cue! Yes there will no doubt be some temperature drift. Actually since I wrote earlier, I had the idea to permanently switch off the backlight, then reset the zero calibration, then turn the backlight back on (it goes off after a minute of inactivity) so that the zero calibration was done in the state the system will be for 99% of the time - backlight off.

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