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Cheap LiFePO4 BMS?


jetzi

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7 hours ago, Tom and Bex said:

 

But surely comparing cell voltages at end of charge is all you need? As long as all cells reach the knee as closely as possible at your charging currents what more is needed? We probably charge at higher currents than others here, and haven't noticed any difference in which cell reaches the knee first whether charging at 90A via alternator, or 10-15A via solar (in summer!).

 

My point was that during charge the cell voltage is a combination of the SoC and what might be simplified as the cells IR voltage drop (current x internal resistance) - although in reality that simplification no doubt is a combination of both the ohmic resistance and something to do with reaction rates / ion migration rates. At faster charge rates, the latter effect becomes more significant and if cell "resistances" are not well matched a cell voltage difference might make it seem that there is a SoC imbalance when in fact there isn't. But quite probably this effect is negligible.

 

Obviously if one can detect actual cell imbalance (SoC imbalance) before the end of the charge, there is more time to do balancing - but I accept that once initially balanced, probably only very minor tweaking of balance would be required every cycle.

 

I don't know, it was just a thought! Looks like I'll be the only one charging at 150-200A so I guess I will need to find out!

Edited by nicknorman
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On 02/12/2019 at 14:01, WotEver said:

There’s that Chinese video on YouTube where they shot them, knifed them, burned them... and the batteries just sat there like a brick. 

I’ll be surprised if they get them to do anything untoward. 

 

On 02/12/2019 at 11:28, Dr Bob said:

 I will say that it is very difficult to actually get a thermal runaway going - ie a blow lamp is not good enough (!) so we can all sleep safe in our beds - especially when our batteries are under the bed.:) They've not tried any LiFePO4s yet but I will get them to look at them.

I think we need to be a little careful when talking about safety of LiFePO4s. I've done a bit of digging and it probably warrants a separate thread on Li safety.  Bottom line is that LiFePO4s are safer  BUT there is still a risk. In this thread and others we keep talking about over or undercharging wrecking your batteries but we should bring in the risk of fire/explosion as well rather than just damage to the cells. Overcharging is the biggest cause of fires and LiFePO4s are not immune to it. I will try and pull something together later today to explain my thoughts.

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1 minute ago, Dr Bob said:

 

I think we need to be a little careful when talking about safety of LiFePO4s. I've done a bit of digging and it probably warrants a separate thread on Li safety.  Bottom line is that LiFePO4s are safer  BUT there is still a risk. In this thread and others we keep talking about over or undercharging wrecking your batteries but we should bring in the risk of fire/explosion as well rather than just damage to the cells. Overcharging is the biggest cause of fires and LiFePO4s are not immune to it. I will try and pull something together later today to explain my thoughts.

But equally LA batteries are not immune to exploding! I think if we treat the installation as a whole, LA system completely unprotected and unmonitored, vs a sensibly set up Li system with protection circuitry, I doubt the Li system would carry a higher risk.

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7 hours ago, Tom and Bex said:

Apologies about this long post, but just catching up and wanted to reply to a few points. Didn't realise when I started this post it would end up so long though!

 

 

 

 

 

Surely the ability to charge at high rates is one of the big advantages of Li over LA? Seems a shame not to be able to benefit fron thiis. 

 

 

 

 

I am not sure Tom, in my case I am only charging to 80% so I can watch my Midnite on the puter doing its stuff and also the batteries as well, the batteries stay very well balanced up to 75% after that [this is when I took them all up 100% at the beginning} the BMS is working very hard to keep the cells balanced. So in my case this now never happens. Bulk is to 13,9 absorb is 13.8 which means only small amps trickling in and float 13.6 nothing happening at all.

When I first bought the batteries I did a 100% charge on them all individually then put them all in parallel, 10 of these batteries were used to provide drive power at 60 volts so they were put in series and I monitored them carefully at the beginning but after a few months stopped bothering as they were all staying balanced, I think this is down to lower voltages and softer charging routine by solar, but as we know this is just guessing

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6 minutes ago, nicknorman said:

But equally LA batteries are not immune to exploding! I think if we treat the installation as a whole, LA system completely unprotected and unmonitored, vs a sensibly set up Li system with protection circuitry, I doubt the Li system would carry a higher risk.

The main risk with LA batteries is hydrogen explosion, which is dangerous to people close by, but very unlikely to set fire to a boat, so lower consequence (in a risk-assessment sense) than a Li battery going up.Apart from hydrogen, and the plastic case, there're not much fuel in a LA battery.

 

MP.

 

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2 minutes ago, peterboat said:

 I think this is down to lower voltages and softer charging routine by solar, but as we know this is just guessing

My best guess is that this isn't so. As we know, charge efficiency of Li batteries is effectively 100%. Charge efficiency of LA batteries is much lower and the mechanism for that is the gassing of the electrolyte causing electrons to be "lost". In order for Li batteries to have reduced charge efficiency there would have to be a similar decompositional path for the lost electrons to take - but as I understand it, there isn't. But it could well be that high charge currents cause an apparent imbalance during charge, as per my previous post. Or not!

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14 minutes ago, Dr Bob said:

 

I think we need to be a little careful when talking about safety of LiFePO4s. I've done a bit of digging and it probably warrants a separate thread on Li safety.  Bottom line is that LiFePO4s are safer  BUT there is still a risk. In this thread and others we keep talking about over or undercharging wrecking your batteries but we should bring in the risk of fire/explosion as well rather than just damage to the cells. Overcharging is the biggest cause of fires and LiFePO4s are not immune to it. I will try and pull something together later today to explain my thoughts.

From Wikipedia (yes, I know) I found the following:

 

One important advantage over other lithium-ion chemistries is thermal and chemical stability, which improves battery safety. LiFePO is an intrinsically safer cathode material than LiCoO and manganese spinel, through omission of the cobalt, with its negative temperature coefficient of resistance that can encourage thermal runaway. The PObond in the (POion is stronger than the CoO bond in the (CoO) ion, so that when abused (short-circuited, overheated, etc.), the oxygen atoms are released more slowly. This stabilization of the redox energies also promotes faster ion migration.

As lithium migrates out of the cathode in a LiCoO cell, the CoO undergoes non-linear expansion that affects the structural integrity of the cell. The fully lithiated and unlithiated states of LiFePO are structurally similar which means that LiFePO cells are more structurally stable than LiCoO cells.

No lithium remains in the cathode of a fully charged LiFePO cell. (In a LiCoO cell, approximately 50% remains.) LiFePO is highly resilient during oxygen loss, which typically results in an exothermic reaction in other lithium cells. As a result, LiFePO cells are harder to ignite in the event of mishandling (especially during charge). The LiFePO battery does not decompose at high temperatures.

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1 hour ago, nicknorman said:

My best guess is that this isn't so. As we know, charge efficiency of Li batteries is effectively 100%. Charge efficiency of LA batteries is much lower and the mechanism for that is the gassing of the electrolyte causing electrons to be "lost". In order for Li batteries to have reduced charge efficiency there would have to be a similar decompositional path for the lost electrons to take - but as I understand it, there isn't. But it could well be that high charge currents cause an apparent imbalance during charge, as per my previous post. Or not!

What I can see is that at high charging currents towards 100% is cell imbalance with the BMS struggling to stop cells going beyond safe voltages, at my charging regime this just doesnt happen Richard has seen the same, however Nick you do what you want I believe what Richard and I see on the screen  and honestly it doesnt matter a jot as these batteries only lose a little by restricting the voltage to 13.9, its only.7 of a volt off maximum for these batteries, so it really doesnt matter in the real world of boating

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2 hours ago, WotEver said:

From Wikipedia (yes, I know) I found the following:

 

One important advantage over other lithium-ion chemistries is thermal and chemical stability, which improves battery safety. LiFePO is an intrinsically safer cathode material than LiCoO and manganese spinel, .........

Yes...... all good, but what about the andode and electrolyte. I'll start a new thread!

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2 hours ago, peterboat said:

What I can see is that at high charging currents towards 100% is cell imbalance with the BMS struggling to stop cells going beyond safe voltages, at my charging regime this just doesnt happen Richard has seen the same, however Nick you do what you want I believe what Richard and I see on the screen  and honestly it doesnt matter a jot as these batteries only lose a little by restricting the voltage to 13.9, its only.7 of a volt off maximum for these batteries, so it really doesnt matter in the real world of boating

I think 13.9v will get the batteries fairly well charged as you say, however it will take a while with current tailing off over a relatively long period (nothing like as long as LA, of course). My proposed strategy would be to fast charge at max current until the highest cell voltage reaches 3.6v, then gradually reduce charge current (by reducing target alternator or charger voltage) to maintain 3.6v on the highest cell until current falls off to 10% capacity or whatever.

 

That would be for a charge to 100%, if charging for 80% then lower voltage would be used obviously.

 

If that doesn’t work well I’ll devise a different strategy!

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2 hours ago, nicknorman said:

I think 13.9v will get the batteries fairly well charged as you say, however it will take a while with current tailing off over a relatively long period (nothing like as long as LA, of course). My proposed strategy would be to fast charge at max current until the highest cell voltage reaches 3.6v, then gradually reduce charge current (by reducing target alternator or charger voltage) to maintain 3.6v on the highest cell until current falls off to 10% capacity or whatever.

 

That would be for a charge to 100%, if charging for 80% then lower voltage would be used obviously.

 

If that doesn’t work well I’ll devise a different strategy!

Nick it doesn't take that long on tail voltage in fact its not something I even notice or care about anymore. Same with the drive batteries I store them at 50% and when needed just put them on charge for a couple of days job done 

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  • 2 weeks later...
On 03/12/2019 at 11:39, Dr Bob said:

It looks like the balancing is only happening when you are into the voltage knee ie above 3.45V. The 30mV delta is typical of what you will see at 3.45V ish - the test is to take them back up to 3.5V+ and see whether the delta goes back up to 200+mV.

 

Be careful about 'float'. If you have a cell up to 3.8V then that is full. Maybe the internal BMS will disconnect at that point but the general wisdom is you stop charging when full and not float at anything over 13.5V total bank voltage. Be ultra careful when you are charging at that high voltage and going so high in cell voltage. You are a whisker away from wrecking your bank.

 

Taken a while to reply.

 

I read the Valence XP manual cover to cover before taking the charging steps related. "Normal Range" of the cells is stated to be between 2.8V and 3.9V. I dont think the internal BMS takes any actual action.  The "recommended" external BMS if attached, (but which we don't have), would sound a warning, but take no action, if a cell reaches 3.9V. If a cell reaches 4.0V the BMS "requests" a shut down and, at 4.2V, the BMS shuts things down without permission. It therefore seems that taking a cell to 3.8V is fine, and 3.9V is not a major issue. Above 3.9V, steps need to be taken.

 

I therefore wasn't worried that a cell reached just over 3.8V, and was ready to shut things down long before 3.9V.

 

At the discharge end of the scale, a warning is given at 2.8V, a request to shut down is sent at 2.3V, and shutdown without permission is instructed at 2.0V. It therefore seems that discharging to 11.2V is not an issue - I dont think I'll be going anywhere near that.

 

Unless reality differs from the manual, I am hoping I'm not missing anything here, with respect to over and under voltages.

 

It also seemed that it is advised to charge to the fullest before use, as well as to reset the SOC counter to 100%. The other 3 batteries seem to be better balanced, such that no cell actually reached 3.8V, but all reset their SOC counters to 100%. 

 

I've now got them in position, with a 75A Blue Sea terminal fuse, a Victron Battery Protect set to cut the supply to the load, (domestic), when voltage drops to 12.5V, and a Victron BMV712, (bluetooth), for monitoring and alarms when I work out how to connect something to it.

 

Flicked the switch and started using them on Saturday evening, all full at 13.4V. I set the bank size in the BMV at 450Ah, on the assumption that 13 year old batteries will have lost some capacity. I've set the low SOC alarm at 20%, and this should be reached when I've used about 360Ah. I'm watching what is happening quite regularly.

 

48hrs on, I've used 143Ah, percentage SOC is showing 71% on the BMV, and voltage is 13.13V while drawing 3.6A. The voltage drops a bit when the fridge comes on and volts drawn rise to 7 or 8A.

 

I think the BMV calculates SOC based upon the bank size entered by the user, and Ah used to date..... so it's not an accurate measure and depends upon knowledge of capacity by the user. The batteries internals are all currently showing 99% as SOC, and the manual states that there need to be a few full cycles to below 20% and back to fully charged, before the batteries work out capacity and SOC. (They must have some kind of Smartguage in their internal BMS thing).

 

There is no indication in the manual as to how to identify what resting voltage indicates a 20% SOC. Everything I find online suggests 12.9V for normal LiFePo4 batteries, but these are a bit different as they have some magnesium in them, which seems to allow higher and lower voltages without damage. I therefore wonder if the SOC range may be a bit different. I haven't been able to find anything online which compares SOC with voltage for Valence XP batteries

 

I'm tempted to go down to 12.5/12.6V then charge to 100%, and see how the SOC readings change, and how everything else looks. Maybe a few full cycles to get the balance and SOC readings in order, then perhaps relax into using them between about 85% and 20%, or so.

 

So all is good. I haven't had to run the genny for 2 days, and may not have to run it until Thursday. Previously it was a 3 hour event every day on board, (between October and March). 

 

It will be interesting to see how long it takes at 60A to fully charge the batteries - 6 hours plus, so maybe over two days.

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Maybe I am the only one but I think you are bonkers charging an individual cell to 3.9V regardless of what the manual says. That is just asking for problems and risk of something going wrong is much higher. Maybe every now and again take it to full to re sync the BMV but that is charging until you get to the CV stage and tail current drops to 4-5% NOT a cell voltage of 3.9V  or wotever. Be very very careful when you get up to those high cell voltages. 

On your SoC measurements, I would just not bother. They are never going to be accurate. Are the settings on your BMV accurate? Do you know the actual capacity? If not, you are just guessing. What is the temp of you batteries?

Measure at rest voltages as you discharge (or near at rest) and record the bank voltage and amp hrs used. Plot a graph of that data and use that to estimate how far from full you are. A glance at the voltage tells you then the state of your batteries. I do look at the Ahrs used on my BMV but after 2-3 months since the last resycn it is a load of cobblers. 

There is a temperature effect on the SoC as lower temps give lower capacity. This last week mine have been down to 10degC and looking at the literature that means maybe 10% capacity less. Reading just the voltage is better. My system has 3 things drawing power which are not connected via the BMV shunt (the cell monitor, the auto disconnect switch and the BMS) all taking power ...maybe 20Ahr per month. Your batteries will be doing something like that internally so that will confuse the BMV SoC reading. Just looking at voltage is better.

Edited by Dr Bob
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Richard, the internal BMS just balances the cells, if you take one to pieces you will see that it cant disconnect, the master BMS would connect to the charger and discharge source to do that bit.  I had a high voltage BMS for my 36 volt batteries it also controlled heating and cooling.

You are pretty much the same as me, as soon as the batteries go off charge they drop to 13.4 volts and then gradually drop until the morning and the sun shines again,they really are the easiest batteries to get on with. As Bob says dont go to high per cell,  stick to max charge of 13.9 bulk 13.8 absorb and 13.6 float like I do and all really will be Bobs your uncle

Edited by peterboat
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37 minutes ago, Dr Bob said:

Maybe I am the only one but I think you are bonkers charging an individual cell to 3.9V regardless of what the manual says. That is just asking for problems and risk of something going wrong is much higher. Maybe every now and again take it to full to re sync the BMV but that is charging until you get to the CV stage and tail current drops to 4-5% NOT a cell voltage of 3.9V  or wotever. Be very very careful when you get up to those high cell voltages. 

On your SoC measurements, I would just not bother. They are never going to be accurate. Are the settings on your BMV accurate? Do you know the actual capacity? If not, you are just guessing. What is the temp of you batteries?

Measure at rest voltages as you discharge (or near at rest) and record the bank voltage and amp hrs used. Plot a graph of that data and use that to estimate how far from full you are. A glance at the voltage tells you then the state of your batteries. I do look at the Ahrs used on my BMV but after 2-3 months since the last resycn it is a load of cobblers. 

There is a temperature effect on the SoC as lower temps give lower capacity. This last week mine have been down to 10degC and looking at the literature that means maybe 10% capacity less. Reading just the voltage is better. My system has 3 things drawing power which are not connected via the BMV shunt (the cell monitor, the auto disconnect switch and the BMS) all taking power ...maybe 20Ahr per month. Your batteries will be doing something like that internally so that will confuse the BMV SoC reading. Just looking at voltage is better.

I suspect that the loss of capacity with low temperature thing is exactly the same principle as for LA. In other words, no capacity is lost at low temperature. Well, it depends on how you measure capacity! Yes for sure if you carry out a capacity check by steadily discharging at a fairly high rate until the terminating low voltage is reached, you will get fewer AH at low temperature. But those “lost” AH are not actually lost, they are just temporarily unavailable due to the slow reaction time at low temperature. I suspect that if you then rested the battery and or increased its temperature, the remaining AH would be available.

 

As an example, take a LiFePO4 battery soaked to 0C and discharge quite fast down to roughly 50% SoC. Then warm soak the battery up to 25C. Then carry on with the discharge to the terminating low voltage. I strongly suspect that you will get the same total AH out of the battery that you would have done if the entire discharge had been done at 25C.

 

In other words, discharging at low temperature doesn’t waste capacity, it’s just that some of that capacity is temporarily unavailable / slow to be available.

 

Of course it does waste energy, because at low temperature the terminal voltage will be lower, but that is not the same thing as capacity.

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It’s quite interesting that there is a “80%” cult on here. The members are categorically saying “oooh don’t go above 80% / don’t charge to 3.7v per cell” etc. But where is the evidence that this is a bad thing? Is it just an old wives’ (or spouses’, perhaps!) tale thing? We all know that if the voltage gets too high, the cells will be damaged. But the zeitgeist seems to be that not only must the max voltage not be exceeded, one must also leave a (randomly decided) large safety margin. Where is the evidence e for this strategy?

 

Surely one either has effective protection systems, or not. If one has, why leave a large safety margin? If not ... well one should do!

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19 minutes ago, nicknorman said:

I suspect that the loss of capacity with low temperature thing is exactly the same principle as for LA. In other words, no capacity is lost at low temperature. Well, it depends on how you measure capacity! Yes for sure if you carry out a capacity check by steadily discharging at a fairly high rate until the terminating low voltage is reached, you will get fewer AH at low temperature. But those “lost” AH are not actually lost, they are just temporarily unavailable due to the slow reaction time at low temperature. I suspect that if you then rested the battery and or increased its temperature, the remaining AH would be available.

 

 

Agreed. The point I was trying to make to Richard is that as temp goes down, you have less in the tank to use. We've been in a marina for the past 2 weeks on shore power with the Li's isolated and I can see the voltage (at rest...well nearly, maybe a 500mA draw) decreasing with temp.

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19 minutes ago, nicknorman said:

I suspect that the loss of capacity with low temperature thing is exactly the same principle as for LA. In other words, no capacity is lost at low temperature. Well, it depends on how you measure capacity! Yes for sure if you carry out a capacity check by steadily discharging at a fairly high rate until the terminating low voltage is reached, you will get fewer AH at low temperature. But those “lost” AH are not actually lost, they are just temporarily unavailable due to the slow reaction time at low temperature. I suspect that if you then rested the battery and or increased its temperature, the remaining AH would be available.

 

As an example, take a LiFePO4 battery soaked to 0C and discharge quite fast down to roughly 50% SoC. Then warm soak the battery up to 25C. Then carry on with the discharge to the terminating low voltage. I strongly suspect that you will get the same total AH out of the battery that you would have done if the entire discharge had been done at 25C.

 

In other words, discharging at low temperature doesn’t waste capacity, it’s just that some of that capacity is temporarily unavailable / slow to be available.

 

Of course it does waste energy, because at low temperature the terminal voltage will be lower, but that is not the same thing as capacity.

 

12 minutes ago, nicknorman said:

It’s quite interesting that there is a “80%” cult on here. The members are categorically saying “oooh don’t go above 80% / don’t charge to 3.7v per cell” etc. But where is the evidence that this is a bad thing? Is it just an old wives’ (or spouses’, perhaps!) tale thing? We all know that if the voltage gets too high, the cells will be damaged. But the zeitgeist seems to be that not only must the max voltage not be exceeded, one must also leave a (randomly decided) large safety margin. Where is the evidence e for this strategy?

 

Surely one either has effective protection systems, or not. If one has, why leave a large safety margin? If not ... well one should do!

Tesla is the evidence they charge car batteries to 80% like most of the other EV makers, now I know these EV makers know their stuff because they do it for a living and want the people in the car to be safe they also dont want people claiming for new batteries, so whilst the 80% can be overridden I am sure their will be a clause saying do it at your own peril. As for batteries being cold and not handing out the goods, my electric truck loses range when its cold a lot of range because it cant heat the batteries. in my boat the batteries are heated so that they will charge and discharge correctly and it works

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10 minutes ago, peterboat said:

 

Tesla is the evidence they charge car batteries to 80% like most of the other EV makers, now I know these EV makers know their stuff because they do it for a living and want the people in the car to be safe they also dont want people claiming for new batteries, so whilst the 80% can be overridden I am sure their will be a clause saying do it at your own peril. As for batteries being cold and not handing out the goods, my electric truck loses range when its cold a lot of range because it cant heat the batteries. in my boat the batteries are heated so that they will charge and discharge correctly and it works

Reading through ALL the Tesla stuff is really interesting about battery life and temperature effects. Note: there is a lot of Tesla stuff on this if you are prepared to look hard.

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1 hour ago, Dr Bob said:

Agreed. The point I was trying to make to Richard is that as temp goes down, you have less in the tank to use. We've been in a marina for the past 2 weeks on shore power with the Li's isolated and I can see the voltage (at rest...well nearly, maybe a 500mA draw) decreasing with temp.

At a very low discharge rate (small fractional C) I suggest that actually no capacity is lost / you don’t have “less in the tank”. I think what you are seeing is simply the temperature coefficient of the relationship between voltage and SoC. So, yes the voltage is less, but no, this does not mean fewer AH are available at low discharge rates. Yes, fewer WH are available but not fewer AH.

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1 hour ago, peterboat said:

 

Tesla is the evidence they charge car batteries to 80% like most of the other EV makers, now I know these EV makers know their stuff because they do it for a living and want the people in the car to be safe they also dont want people claiming for new batteries, so whilst the 80% can be overridden I am sure their will be a clause saying do it at your own peril. As for batteries being cold and not handing out the goods, my electric truck loses range when its cold a lot of range because it cant heat the batteries. in my boat the batteries are heated so that they will charge and discharge correctly and it works

However one should bear in mind that the performance demands on EVs are extreme compared to providing domestic power for a boat.

 

Yes your truck will definitely lose range in the cold, for 2 reasons. The first being that the batteries need to be able to supply quite a large current as a proportion of capacity, maybe 1C or so, more under hard acceleration. When cold, the battery voltage will dip very low even when there is a fair bit of AH remaining.

And secondly because range is a function of available WH, rather than AH. The ultimately available AH will be the same if, eg, the truck made a lot of short trips with resting in between, but of course the WH will be less due to the reduced voltage.

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31 minutes ago, nicknorman said:

However one should bear in mind that the performance demands on EVs are extreme compared to providing domestic power for a boat.

 

Yes your truck will definitely lose range in the cold, for 2 reasons. The first being that the batteries need to be able to supply quite a large current as a proportion of capacity, maybe 1C or so, more under hard acceleration. When cold, the battery voltage will dip very low even when there is a fair bit of AH remaining.

And secondly because range is a function of available WH, rather than AH. The ultimately available AH will be the same if, eg, the truck made a lot of short trips with resting in between, but of course the WH will be less due to the reduced voltage.

Yes as soon as I stop the voltage rises very quickly,  I would say I have lost two thirds of my range, in the summer it would do 30 miles between resets now its 10 miles! I have actually stopped using it for the moment mainly because its cold and the chances of damaging the batteries is very real for no good reason 

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38 minutes ago, nicknorman said:

 

And secondly because range is a function of available WH, rather than AH. The ultimately available AH will be the same if, eg, the truck made a lot of short trips with resting in between, but of course the WH will be less due to the reduced voltage.

What is a WH and  an AH?  A Watt Henry and an Ampere Henry? ?

Wh and Ah I understand, but many do not, as clearly demonstrated in many, many battery theads.

  

N

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