Cheap LiFePO4 BMS?

[Dr Bob]

All this talk of how to avoid temps of lower than 5°C on Li's got me thinking.

Put them in the boat, not in a cold place! There are a couple of reasons why – but we dont normally talk about them.

Firstly, battery performance vs temperature. I think we all know that batteries are better at 'room temperature' than 'fridge temperature' but by how much? Earlier this year I was lucky enough to get a Tesla and have now clocked up around 4K miles. It has a range of circa 300 miles on a full charge (with 10 times the battery capacity of my boat) but it is very surprising how the range is affected by temperature. I reckon that I am getting 10% more range at 25°C than at 15°C (ie 10% more available power) even with the aircon unit blasting away! That is from my estimation of power used on long runs – although the actual data suggests it may be lower at 6% - but I am estimating from comparative runs. You are therefore talking 15-20% more capacity comparing a winter 5°C in the engine hole to 25°C in the boat. It makes sense to me therefore to keep the batteries inside where they can pick up as much heat as possible – ie under the bed where there may be hot pipes from the ebersparky thing.

Now, one issue with putting them inside is that the cabling requirements may be more ie you need longer cables. On our old boat we needed 5 meters of 50mm cable to connect the Li's to the combi box. Our new boat is similar. This means a significant voltage drop but that can be your friend. On our old boat we only had a small alternator – so saw charging currents of 30-50A. On our new boat we have a 240A alternator and a 5KV combi box so have 90mm cabling throughout but then 5 meters of 50mm to the Li's (dont forget I have LA's in parallel on the 90mm cabling). The extra length and 'slimmer' profile of the cable means it cant handle all that power and I find that even if the alternator can put out 200A into depleted LA's, it never gets above 90A to the Li's when just charging the Li's. That has a huge benefit in that I can never overheat the alternator! Ok, I still need to disconnect on overcharge but the overheating problem is avoided. Having quite long battery cables is therefore quite a benefit in my case. I am quite happy to be able to put 90Ahr into the Li's each hour. I could double up the wire size to get more but why bother.

Therefore, I would be a big supporter of putting the batteries where they can keep warm during the winter and far less likely to be in freezing conditions when you leave the boat – even if it means longer cables.

[peterboat]

20 minutes ago, Dr Bob said:

All this talk of how to avoid temps of lower than 5°C on Li's got me thinking.

Put them in the boat, not in a cold place! There are a couple of reasons why – but we dont normally talk about them.

Firstly, battery performance vs temperature. I think we all know that batteries are better at 'room temperature' than 'fridge temperature' but by how much? Earlier this year I was lucky enough to get a Tesla and have now clocked up around 4K miles. It has a range of circa 300 miles on a full charge (with 10 times the battery capacity of my boat) but it is very surprising how the range is affected by temperature. I reckon that I am getting 10% more range at 25°C than at 15°C (ie 10% more available power) even with the aircon unit blasting away! That is from my estimation of power used on long runs – although the actual data suggests it may be lower at 6% - but I am estimating from comparative runs. You are therefore talking 15-20% more capacity comparing a winter 5°C in the engine hole to 25°C in the boat. It makes sense to me therefore to keep the batteries inside where they can pick up as much heat as possible – ie under the bed where there may be hot pipes from the ebersparky thing.

Now, one issue with putting them inside is that the cabling requirements may be more ie you need longer cables. On our old boat we needed 5 meters of 50mm cable to connect the Li's to the combi box. Our new boat is similar. This means a significant voltage drop but that can be your friend. On our old boat we only had a small alternator – so saw charging currents of 30-50A. On our new boat we have a 240A alternator and a 5KV combi box so have 90mm cabling throughout but then 5 meters of 50mm to the Li's (dont forget I have LA's in parallel on the 90mm cabling). The extra length and 'slimmer' profile of the cable means it cant handle all that power and I find that even if the alternator can put out 200A into depleted LA's, it never gets above 90A to the Li's when just charging the Li's. That has a huge benefit in that I can never overheat the alternator! Ok, I still need to disconnect on overcharge but the overheating problem is avoided. Having quite long battery cables is therefore quite a benefit in my case. I am quite happy to be able to put 90Ahr into the Li's each hour. I could double up the wire size to get more but why bother.

Therefore, I would be a big supporter of putting the batteries where they can keep warm during the winter and far less likely to be in freezing conditions when you leave the boat – even if it means longer cables.

Their is another reason to put them inside Bob the cost of them! I have at retail 15ks worth of domestic batteries and about 60ks worth of drive batteries my leisure ones are in the boat the drive live in engine hole with lock down covers!

[nicknorman]

32 minutes ago, Dr Bob said:

All this talk of how to avoid temps of lower than 5°C on Li's got me thinking.

Put them in the boat, not in a cold place! There are a couple of reasons why – but we dont normally talk about them.

Firstly, battery performance vs temperature. I think we all know that batteries are better at 'room temperature' than 'fridge temperature' but by how much? Earlier this year I was lucky enough to get a Tesla and have now clocked up around 4K miles. It has a range of circa 300 miles on a full charge (with 10 times the battery capacity of my boat) but it is very surprising how the range is affected by temperature. I reckon that I am getting 10% more range at 25°C than at 15°C (ie 10% more available power) even with the aircon unit blasting away! That is from my estimation of power used on long runs – although the actual data suggests it may be lower at 6% - but I am estimating from comparative runs. You are therefore talking 15-20% more capacity comparing a winter 5°C in the engine hole to 25°C in the boat. It makes sense to me therefore to keep the batteries inside where they can pick up as much heat as possible – ie under the bed where there may be hot pipes from the ebersparky thing.

Now, one issue with putting them inside is that the cabling requirements may be more ie you need longer cables. On our old boat we needed 5 meters of 50mm cable to connect the Li's to the combi box. Our new boat is similar. This means a significant voltage drop but that can be your friend. On our old boat we only had a small alternator – so saw charging currents of 30-50A. On our new boat we have a 240A alternator and a 5KV combi box so have 90mm cabling throughout but then 5 meters of 50mm to the Li's (dont forget I have LA's in parallel on the 90mm cabling). The extra length and 'slimmer' profile of the cable means it cant handle all that power and I find that even if the alternator can put out 200A into depleted LA's, it never gets above 90A to the Li's when just charging the Li's. That has a huge benefit in that I can never overheat the alternator! Ok, I still need to disconnect on overcharge but the overheating problem is avoided. Having quite long battery cables is therefore quite a benefit in my case. I am quite happy to be able to put 90Ahr into the Li's each hour. I could double up the wire size to get more but why bother.

Therefore, I would be a big supporter of putting the batteries where they can keep warm during the winter and far less likely to be in freezing conditions when you leave the boat – even if it means longer cables.

I would expect that there is no difference in ultimate capacity at 5 degC vs 25 degC, provided the batteries are discharged very slowly.  But we have to bear in mind that capacity, as in Ah, is not a unit of energy. You have to multiply by voltage to get energy. And so under a relatively high load, the voltage will be lower with cold batteries, than with warmer batteries. So in terms of energy, the batteries can supply a lot less at cold temperature and high load - the car will require more Ah to achieve the same thing when batteries cold than when warm.

 

I think in terms of boats where the discharge rate is generally slow compared to a car, there will be minimal difference to the usable energy at 5 degC vs 25degC.

 

Of course in terms of charging, one needs to avoid fast charging at low temperatures and any charging at all below zero, but I can’t recall ever seeing our existing battery temperature showing below 5 degC. The batteries are in the “engine room” ie below boards in the trad stern area, so not exactly outside nor inside the heated space. But they would be warmed by the engine or the Mikuni. Not sure whether I will ever need heated mats because needing to charge the batteries will surely coincide with running the engine to go cruising, and worst case scenario is that charging has to be delayed (by my integrated control system) until the batteries have warmed up. Hopefully just a few lines of code to do that.

[Dr Bob]

5 hours ago, nicknorman said:

I would expect that there is no difference in ultimate capacity at 5 degC vs 25 degC, provided the batteries are discharged very slowly.  But we have to bear in mind that capacity, as in Ah, is not a unit of energy. You have to multiply by voltage to get energy. And so under a relatively high load, the voltage will be lower with cold batteries, than with warmer batteries. So in terms of energy, the batteries can supply a lot less at cold temperature and high load - the car will require more Ah to achieve the same thing when batteries cold than when warm.

 

I think in terms of boats where the discharge rate is generally slow compared to a car, there will be minimal difference to the usable energy at 5 degC vs 25degC.

 

Of course in terms of charging, one needs to avoid fast charging at low temperatures and any charging at all below zero, but I can’t recall ever seeing our existing battery temperature showing below 5 degC. The batteries are in the “engine room” ie below boards in the trad stern area, so not exactly outside nor inside the heated space. But they would be warmed by the engine or the Mikuni. Not sure whether I will ever need heated mats because needing to charge the batteries will surely coincide with running the engine to go cruising, and worst case scenario is that charging has to be delayed (by my integrated control system) until the batteries have warmed up. Hopefully just a few lines of code to do that.

I know what you are saying about  high current draw but it is interesting on long runs when cruising at 50mph in the Tesla, the draw spends a lot of the time at fairly low values and can be charging at fairly low values so the overall capacity of the battery to charge and discharge is governed by temperature. I am really surprised at the boost in range given high temperature. On our boat we find in the evening, we are discharging at say 0.04C which cant be far off the idling current when cruising in the Tesla at 50 mph on the flat.

I wonder if Li's are more temperature dependant than LAs under this lighter discharge.

[nicknorman]

1 hour ago, Dr Bob said:

I know what you are saying about  high current draw but it is interesting on long runs when cruising at 50mph in the Tesla, the draw spends a lot of the time at fairly low values and can be charging at fairly low values so the overall capacity of the battery to charge and discharge is governed by temperature. I am really surprised at the boost in range given high temperature. On our boat we find in the evening, we are discharging at say 0.04C which cant be far off the idling current when cruising in the Tesla at 50 mph on the flat.

I wonder if Li's are more temperature dependant than LAs under this lighter discharge.

I don’t really know exactly what the chemical reaction in Li batteries is, but surely the principle must be the same in that there is a certain amount of chemicals to react, and each molecule of chemical mobilises a specific number of electrons. Each electron has a specific charge (4.450576349637 x 10^-23 Ah to be precise!) There is simply a precise and linear relationship between the amount of chemical and the amount of charge. Electrons are preserved around a circuit, they can’t just disappear or be “dropped”. Kirchov’s first law.
 

What is variable is the voltage at which that charge is pumped out, and that variable voltage results in variable energy. And of course at high current drains, the reaction speed might not be fast enough at low temperatures hence the voltage drops below a critical shut-off value whilst there is still in fact a fair bit of chemicals left to react.

 

0.04C would mean 25 hours run time. At 50mph that would be 1250 miles range. It can’t do that, by a factor of around 4.

[Craig Shelley]

I always find the term ”loss of capacity at low temperature" a bit of a curious conundrum. If the capacity of a charged battery goes down, where does the energy go?

Even if the voltage at the terminals is lower at low temperatures, the energy stored cannot simply disappear.

 

The main thing I have observed at low temperatures is the apparent increase in internal resistance. I.e. pulling a heavy load causes the terminal voltage to drop more when the batteries are cold. Likewise, our charger which charges until a voltage threshold is crossed, trips out notably earlier when the batteries are cold.

In an EV with long strings of cells I could imagine this internal resistance increase being problematic as the load would have to be stopped as soon as the weakest cell voltage reached its minimum allowed value. This would definitely cause an apparent loss of range.

I theorise that if the batteries were to be warned up, the the lost EV range would be recovered, hence energy conserved.

 

One other observation worthy of note is that the internal resistance resistance of the cells is highly dependent on the state of charge. The effect of temperature and SOC seem to add together to influence the overall internal resistance.

[nicknorman]

27 minutes ago, Craig Shelley said:

I always find the term ”loss of capacity at low temperature" a bit of a curious conundrum. If the capacity of a charged battery goes down, where does the energy go?

Even if the voltage at the terminals is lower at low temperatures, the energy stored cannot simply disappear.

 

Yes good point, conservation of energy is always a great guiding principle! I am tempted to say the lost energy must be converted to heat - simply put, I^2R losses with R higher at low temperatures. But then the battery wouldn’t be cold any more!

Edited November 5, 2020 by nicknorman

[MoominPapa]

My understanding is that most of the loss of range in electric cars in cold weather is down to use of electricity for lights, windscreen wipers, demisting, and (biggest) heating. It's quite common to find electric cars with heated seats just so that you can avoid freezing whilst not burning energy on space heating in the cabin. Better electric cars come with aircon that can be reversed to run as a heat pump in winter, for more heat from fewer joules of precious charge.

 

MP.

 

[peterboat]

40 minutes ago, MoominPapa said:

My understanding is that most of the loss of range in electric cars in cold weather is down to use of electricity for lights, windscreen wipers, demisting, and (biggest) heating. It's quite common to find electric cars with heated seats just so that you can avoid freezing whilst not burning energy on space heating in the cabin. Better electric cars come with aircon that can be reversed to run as a heat pump in winter, for more heat from fewer joules of precious charge.

 

MP.

 

Heated steering wheel as well, plus the heat created by the motor and battery pack is used to heat the cabin via the mentioned heat pump 

[Dr Bob]

50 minutes ago, MoominPapa said:

My understanding is that most of the loss of range in electric cars in cold weather is down to use of electricity for lights, windscreen wipers, demisting, and (biggest) heating. It's quite common to find electric cars with heated seats just so that you can avoid freezing whilst not burning energy on space heating in the cabin. Better electric cars come with aircon that can be reversed to run as a heat pump in winter, for more heat from fewer joules of precious charge.

 

MP.

 

I'm talking about the improvement from 15 -20 deg  C up to 25 -30deg. At the 15-20 the is very little on in the car whereas at 25 the 'american' type aircon is going full blast. No, in this case it's the batteries that are more efficient at high temps. I take Nick's point about the laws of physics but chemical reactions double in speed for each 10degC rise (good rule of thumb ) so I just wonder if the Li chemistry is doing some thing different to lead acids and working better at higher temps to free up each electron.

 

[Col_T]

 

12 hours ago, Dr Bob said:

Now, one issue with putting them inside is that the cabling requirements may be more ie you need longer cables. On our old boat we needed 5 meters of 50mm cable to connect the Li's to the combi box. Our new boat is similar. This means a significant voltage drop but that can be your friend. On our old boat we only had a small alternator – so saw charging currents of 30-50A. On our new boat we have a 240A alternator and a 5KV combi box so have 90mm cabling throughout but then 5 meters of 50mm to the Li's (dont forget I have LA's in parallel on the 90mm cabling). The extra length and 'slimmer' profile of the cable means it cant handle all that power and I find that even if the alternator can put out 200A into depleted LA's, it never gets above 90A to the Li's when just charging the Li's. That has a huge benefit in that I can never overheat the alternator!

With apologies asking a really basic question - isn't the alternator still trying to output the maximum that the Li's can take regardless of the voltage drop? If so, surely the alternator will still, potentially, overheat just as much with the Li's being fed by 50mm cable as it would being fed by 90mm as the alternator would be trying to output as much as it can?

[nicknorman]

16 minutes ago, Col_T said:

 

With apologies asking a really basic question - isn't the alternator still trying to output the maximum that the Li's can take regardless of the voltage drop? If so, surely the alternator will still, potentially, overheat just as much with the Li's being fed by 50mm cable as it would being fed by 90mm as the alternator would be trying to output as much as it can?

No, the alternator output (as controlled by its regulator) is a function of how far below the regulated voltage the output terminals actually are. So if there is say a volt dropped in the wiring, that means the alternator terminal voltage will be 1v more than the battery voltage and thus the output current will be much less.

 

It’s just the way basic alternator regulators work - (ignoring the effect of rpm for the exercise) they set a field current that is roughly proportional to the difference between actual terminal voltage and the nominal regulated voltage.

 

This is of course one reason why alternators are not good at fast charging lead acid batteries - as the battery voltage rises so the alternator current decreases. Yes I know this is contrary to the forum mantra that “the battery controls the current” but sorry, that is simplistic rubbish, as I have discovered now I have a modern digital regulator with PID control that allows the alternator to produce maximum output virtually right up to the regulated voltage. Chalk and cheese from an old fashioned analogue regulator.

Edited November 5, 2020 by nicknorman

[Murflynn]

13 hours ago, nicknorman said:

 Each electron has a specific charge (4.450576349637 x 10^-23 Ah to be precise!)

 

precise?         not quite - that's only an approximation to thirteen significant figures.  

 

as teacher would have said :  "needs to try harder".

 

 

 

coat    ..................................    

[jetzi]

On 05/11/2020 at 10:16, Dr Bob said:

All this talk of how to avoid temps of lower than 5°C on Li's got me thinking.

Put them in the boat, not in a cold place!

Practically rather than theoretically though I'm constrained by the realities of living on the boat. We have a 65' boat that goes saloon - galley - bedroom - bathroom - office. The Li's are in the office to be near the alternator, MPPT and all the other stuff.

 

The solid fuel stove is at the bow doors and this supplies the bulk of our heating. When the saloon is warm, the bedroom is comfortable for sleeping, the bathroom is chilly and the office may as well be outside. I'm in the process of reinstalling our Eberspacher based central heating and also a second solid fuel stove in the office and I have a temporary diesel heater set up in the office. Even so though, this is mostly to make it comfortable for me during the workday. If I'm not working then I'd rather not be heating that entire space just for the benefit of my Lithiums. That's the case especially if I'm cruising and charging the battery, but also the case if I'm getting a bit of winter solar power.

 

In any event it seems to make sense to automate a battery warmer so it's something you don't have to think about. I'm going to try the mats connected to a thermostat and a main switch that I can turn off outside of December - March. Thanks for the tip on the mats Craig. If anyone could advise whether this thermostat would be appropriate or could recommend one I'd be grateful.

 

On 04/11/2020 at 23:13, Craig Shelley said:

This enables the heaters to be left on without any means of control. Fuses provide basic protection for over current, and the switch is a guarded and illuminates to give a very clear indication that the heaters are powered.

As the new monitoring system gives us better visibility of battery temperature, it is theoretically possible to automatically control the heaters. I doubt we'll ever get around to making that happen.

Do you mean to say that you just leave it on all the time? Or that you manually turn it on before charging (say, half an hour before a cruise or sunrise?)

The former seems rather a lot of energy to spend on warming the three batteries, if the latter then rather a lot of effort. I get that you have the temperature monitoring but wouldn't a simple thermostat in the heating circuit do the trick for you?

 

 

On 05/11/2020 at 10:16, Dr Bob said:

On our new boat we have a 240A alternator and a 5KV combi box so have 90mm cabling throughout but then 5 meters of 50mm to the Li's (dont forget I have LA's in parallel on the 90mm cabling). The extra length and 'slimmer' profile of the cable means it cant handle all that power and I find that even if the alternator can put out 200A into depleted LA's, it never gets above 90A to the Li's when just charging the Li's. That has a huge benefit in that I can never overheat the alternator! Ok, I still need to disconnect on overcharge but the overheating problem is avoided. Having quite long battery cables is therefore quite a benefit in my case. I am quite happy to be able to put 90Ahr into the Li's each hour.

 

11 hours ago, nicknorman said:

It’s just the way basic alternator regulators work - (ignoring the effect of rpm for the exercise) they set a field current that is roughly proportional to the difference between actual terminal voltage and the nominal regulated voltage.

 

This is of course one reason why alternators are not good at fast charging lead acid batteries - as the battery voltage rises so the alternator current decreases.

Currently my alternator charges my Li's at around 30A constant, and I put this down to the fact that I (intentionally) used a very thin 25mm^2 cable to connect them (to prevent overheating). I noticed yesterday with a flatter than usual battery (around 12.7V) the alternator put out 50A for a short while and I could hear more labouring from the engine.

 

My plan for today was to try out a piece of 70mm^2 cable because at 30A I'm still running my engine too much.

 

Nick, you've made me think - is my alternator not just regulating itself down because it's charging at 13.4V and it "thinks the LA it's connected to is getting close to full?"

I think I'll still try the thicker cable, but I have been quite happy that my alternator is happy...

[jetzi]

Follow up, the thicker cable does seem to have made a marginal difference, but not by a lot - around 5A or so. So perhaps it really is the field current limiting the voltage of what it thinks is a full LA battery.

 

I'm increasingly reaching the conclusion that an alternator controller like Nick's is really what is required. For now I guess I'm going to be charging at 35A max.

 

Ideally you want to be able to turn off charging when any one of the cells is full, or when the cells are below 5 degrees, or when the alternator is above say 70 degrees.

 

I like his ability to switch between fast/slow charge, but in my view it's only relevant if you have a decent sized alternator - my little 70A (ish) one is never really going to abuse the battery.

[Richard10002]

1 hour ago, ivan&alice said:

Nick, you've made me think - is my alternator not just regulating itself down because it's charging at 13.4V and it "thinks the LA it's connected to is getting close to full?"

I thought modern alternators charged at 14.4V or so? I think mine runs at 14.2V, whereas I used to have a boat with one that ran at 13.6V, and I never got around to changing it.

 

If you can either get it to run at 14V+, or get one that does, you would likely see a decent increase in Amps.

 

Unless I'm missing something, or maybe a typo?

 

[peterboat]

1 minute ago, Richard10002 said:

I thought modern alternators charged at 14.4V or so? I think mine runs at 14.2V, whereas I used to have a boat with one that ran at 13.6V, and I never got around to changing it.

 

If you can either get it to run at 14V+, or get one that does, you would likely see a decent increase in Amps.

 

Unless I'm missing something, or maybe a typo?

 

Richard a lower charging alternator is a joy with lithium batteries, it's your first protection against overcharging, it's what John's boat does 

[nicknorman]

1 hour ago, ivan&alice said:

Follow up, the thicker cable does seem to have made a marginal difference, but not by a lot - around 5A or so. So perhaps it really is the field current limiting the voltage of what it thinks is a full LA battery.

 

I'm increasingly reaching the conclusion that an alternator controller like Nick's is really what is required. For now I guess I'm going to be charging at 35A max.

 

Ideally you want to be able to turn off charging when any one of the cells is full, or when the cells are below 5 degrees, or when the alternator is above say 70 degrees.

 

I like his ability to switch between fast/slow charge, but in my view it's only relevant if you have a decent sized alternator - my little 70A (ish) one is never really going to abuse the battery.

30A does seem pitifully low! It could be that a diode(s) has blown inside the alternator so you are only getting partial operation. What is the voltage actually on the alternator output terminal when it is producing 30A. And what is the voltage on the alternator when the batteries are fully charged and not much current is being taken?

 

I would have thought that a 70A alternator with 14.4v regulator and working properly and producing only 30A would have a terminal voltage up around 14v.

Edited November 6, 2020 by nicknorman

[nicknorman]

1 hour ago, ivan&alice said:

I'm increasingly reaching the conclusion that an alternator controller like Nick's is really what is required. For now I guess I'm going to be charging at 35A max.

 

Ideally you want to be able to turn off charging when any one of the cells is full, or when the cells are below 5 degrees, or when the alternator is above say 70 degrees.

 

I like his ability to switch between fast/slow charge, but in my view it's only relevant if you have a decent sized alternator - my little 70A (ish) one is never really going to abuse the battery.

The ability to switch to slow charge is more about protecting the alternator - electrically and mechanically - than about protecting the batteries. A 70A alternator running at 70A for more than a few minutes is going to get extremely hot and have a short life. Also at lower rpms there will be a lot of load on the drive belt which will likely slip and wear unless it is very tight - which shortens the bearing life. It is about avoiding running a device at its design maximum capacity, whether it be a 70A alternator with a V belt, or a 175A alternator with a polyvee belt. Running it at say 75% output is going to make it last a lot longer than running it at 100% output. This is why I think there is a good chance that your alternator is not working as it should.

 

Yes you can of course reduce the current when the alternator gets hot, my device does that, but it seems better to charge it slowly when one is cruising for 8 hours, rather than trying to ram all the charging into the first couple of hours.

Edited November 6, 2020 by nicknorman

[Dr Bob]

1 hour ago, ivan&alice said:

 

I'm increasingly reaching the conclusion that an alternator controller like Nick's is really what is required.

 

 

I fully agree with you.

Tom & Bex had a good solution with the guy in the US making an alternator controller but that has now gone commercial and is £500 a pop.

Being able to control the alternator is the ideal way forward and it looks like Nick has found even more benefit in being able to maximise current out. I would have pressed Nick hard to do one for me if I had not been getting a new boat as I am nervous of even changing an alternator over (ie buying a spare on and modifying as Nick has done). For me it is a step too  far and I can avoid doing it by buying off the shelf stuff but it is a compromise and takes a bit more time and thought to operate. This however is still far far short of what I would need to do to manage a set of LA's.

There really is a hole in the market for this type of device but the market is just too small for anyone to contemplate filling it - the hoops you would have to jump through to get there are just too big to make an idiot proof device. The fact that the unit T&B use is now £500 a pop demonstrates that .... and I think Nick's is far better.

I've always wondered if the Balmar controller would be an option but that is over £300 and requires internal surgery on the alternator.

[nicknorman]

7 minutes ago, Dr Bob said:

... as I am nervous of even changing an alternator over (ie buying a spare on and modifying as Nick has done).

Just for clarity I didn’t buy a spare alternator, that would have been pretty expensive. I just bought a spare brush/regulator unit which was around £15 and is held in by 3 screws so easy to swap between the standard unit and the modified unit. But since the modified unit is working well, I hopefully will never need to swap back. I will carry a spare PCB, the component cost for that is around £50 (at a guess - never actually added it up) so if for some unlikely reason it stops working, I will just plug in the spare PCB.

[jetzi]

4 hours ago, Richard10002 said:

I thought modern alternators charged at 14.4V or so? I think mine runs at 14.2V, whereas I used to have a boat with one that ran at 13.6V, and I never got around to changing it.

The voltage sits at around 14.4V / 14.5V if it's only connected to my LA. If it's connected to my Li's the voltage sits at around 13.4 - 13.5, slowly rising and if it ever got to 13.8V I shut it off (manually right now - because I don't have an alternator controller - but so far I haven't taken it above 13.7V when charging via alternator - if any one cell got above 3.6V then my emergency BMS should kick in, which implies a battery voltage of 14.4V but in reality is closer to 14.0 since my cells aren't that well balanced).

 

Note that these voltages are taken at the load side of the system, but there isn't that much in it compared to the voltages at the alternator.

 

4 hours ago, nicknorman said:

The ability to switch to slow charge is more about protecting the alternator - electrically and mechanically - than about protecting the batteries. A 70A alternator running at 70A for more than a few minutes is going to get extremely hot and have a short life. Also at lower rpms there will be a lot of load on the drive belt which will likely slip and wear unless it is very tight - which shortens the bearing life. It is about avoiding running a device at its design maximum capacity, whether it be a 70A alternator with a V belt, or a 175A alternator with a polyvee belt. Running it at say 75% output is going to make it last a lot longer than running it at 100% output. This is why I think there is a good chance that your alternator is not working as it should.

 

Yes you can of course reduce the current when the alternator gets hot, my device does that, but it seems better to charge it slowly when one is cruising for 8 hours, rather than trying to ram all the charging into the first couple of hours.

Ok sure that makes sense. Slower charge will also be better for battery capacity - either way I understand that you'd rather slow down the charge on an 8 hour cruise. Alright you convinced me that the slow charge switch is valuable enough to make the effort!

 

My alternator never gets very hot (most I've ever measured is 50 something degrees).  And I've never seen it produce more than 52A or so, but it does produce 50ish A  sometimes so I don't think it could be the case that it's broken. I'm not even sure it is a 70A alternator, could be a 60A. So I honestly don't think it's not working.

[MoominPapa]

I have 70A A127, but it only every produces that when cold. As soon as it warms up, the maximum output drops to around 60A. I think the problem is that field coil in the rotor heats up and increases in  resistance which limits the maximum excitation current when fed 12v. This is a difficult design trade off. You can't get a bigger magnetic field with more turns because the wire diameter would have to reduce to fit, and that would reduce the current. You can't increase field by increasing the wire size for more current because that reduces the number of turns that will fit and hence the field from a given current. You can only make  the whole rotor bigger to fit more turns or fatter wire, and that's means upping the size of the whole alternator. I suspect that's why the biggest conventional A127 is 70A, and it's only 70A when cold - the design has run up against the limits of the form factor.

 

You might be able do better by using a DC-DC converter to feed the rotor with more than 12v to get a bigger field from the existing rotor.

 

MP.

 

[TheBiscuits]

20 hours ago, nicknorman said:

This is of course one reason why alternators are not good at fast charging lead acid batteries - as the battery voltage rises so the alternator current decreases. Yes I know this is contrary to the forum mantra that “the battery controls the current” but sorry, that is simplistic rubbish, as I have discovered now I have a modern digital regulator with PID control that allows the alternator to produce maximum output virtually right up to the regulated voltage. Chalk and cheese from an old fashioned analogue regulator.

 

That's an excellent argument for an A2B controller or a controllable regulator for Lead Acid batteries, never mind LiFePOs

[jetzi]

Hang on, something doesn't make sense.

 

Why does the alternator output 14.4V to a LA battery but 13.4V to a Li battery?

 

Isn't it simply that Li can take more current than the alternator is giving so it dragged the voltage down, whereas with LA the battery is limiting the current and so the alternator is regulating itself down to 14.4V? In fact, doesn't anything less than 14.4V mean that the alternator is working flat-out?

 

Doesn't this mean then that there must be something else in my system - a bit of thin wire, or component - that is somehow preventing the alternator from running at full strength? Either that or - as Nick suggested - there's something wrong with my alternator?