Cheap LiFePO4 BMS?

[peterboat]

6 hours ago, MoominPapa said:

 

For my sins, I've spent 20 years writing server code that is exposed to the jungle of the 21st century internet: https://thekelleys.org.uk/dnsmasq/doc.html

 

Making stuff secure and at least vaguely DoS resistant is therefore fairly second-nature. It's interesting though that one day I couldn't connect: it turned out that something/someone had made multiple TCP connections and used up all the (deliberately limited) connection slots. A quick rewrite to close the oldest connection when the limit is reached, rather than refuse new ones, fixed that problem.

 

MP.

 

Blah blah blah! 🤣 it's all over my head, yes I have BMSs but unless they are fully automatic and I only have to check them every say 10 years with the puter its all pointless

[nicknorman]

21 hours ago, peterboat said:

Blah blah blah! 🤣 it's all over my head, yes I have BMSs but unless they are fully automatic and I only have to check them every say 10 years with the puter its all pointless

But I think the point is that whilst you want a fit and forget, others enjoy tinkering and creating stuff that might not be rigorously necessary. It’s called a hobby!

[peterboat]

2 hours ago, nicknorman said:

But I think the point is that whilst you want a fit and forget, others enjoy tinkering and creating stuff that might not be rigorously necessary. It’s called a hobby!

So much to do and so little time to do it Nick, that's why I want fit and forget. 

[MtB]

5 hours ago, nicknorman said:

But I think the point is that whilst you want a fit and forget, others enjoy tinkering and creating stuff that might not be rigorously necessary. It’s called a hobby!

 

Whilst many of us (including me) are lost in admiration at your sophisticated installation, I too enjoy tinkering but prefer the opposite philosophy, make it ever simpler and easy to understand. 

 

Having mucked about with my own lithiums for a while now I still feel there must be a simpler "bare lithium cell solution" than all those I've seen described on here so far. One day I'll come up with it but every time my thoughts turn to it, one of my vintage diesels starts playing up. I think they get jealous.

 

(I discount the expensive proprietary solutions e.g. Victron. Where's the fun in those?!)

[Sea Dog]

7 hours ago, MtB said:

 

(I discount the expensive proprietary solutions e.g. Victron. Where's the fun in those?!)

Well, there's always a compromise - how's about combining a Victron for the batteries and leave the fun element to your vintage engine(s)?

 

[Tony1]

 

I hope the OP won't mind me temporarily hijacking this very useful and informative thread to ask a question relating to lithium battery charging. 

 

I sort of jury-rigged a charging system for my 400Ah lithium bank in Feb-March this year, in order to get up and cruising again asap, but with Autumn in mind, I'm starting to ponder how I can best fix the charging setup. 

 

At the moment its a right old mish mash. I have a canaline 38 with 75 and100 amp alternators. Those are connected to a Sterling A2B charger, and from that there is a feed to the engine starter battery, and another to a separate lead acid battery. This LA battery acts as the source for a Sterling 60 amp B2B, which in turn charges the lithiums. 

The B2B is nominally rated at 60 amps, but in reality usually puts about 40 amps into the lithiums, which is pretty poor. I think the reason it's charging way below par is that the source LA battery is knackered.

 

I did initially try connecting the A2B directly to the lithiums, and they were taking 100 amps of charge, but the larger 100 amp alternator overheated to 150 degrees within minutes- so my quick fix was to have a B2B installed, as a way of limiting the current draw from the alternator.  

 

I had a quick chat with Sterling support earlier, and the chap there suggested connecting both alternators directly to the engine start battery, and using that as the charge source for the B2B charger.  

This would simplify matters greatly, taking the A2B charger and the current knackered LA battery out of the system, and it would probably increase the charge from the B2B up to maybe 55 amps, which is great. 

 

So my question for any lithium/electrical guys who might have time is this: does that proposal sound ok?

I dont have any electrical knowledge worth a damn, so I didn't even know you could safely connect 175 amps of alternators directly to a LA battery, with no charger in between.

And is there any reason why I wouldnt want to use the engine starter battery as the source for the B2B charger?

(The B2B has an automatic disconnect from the source battery once the voltage falls below a user-set value, so it shouldnt put the starter battery at risk...)

 

But there is another thing I'm wondering. Assuming it is ok to do all that, will the two alternators then share the load between them in a more even way? 

At the moment, with both connected to the A2B, the 100 amp alternator is doing all the work for the lithiums, and I found that it overheats if it gives out more than 60 amps of charge. 

But if they were both connected directly to the starter battery, they would share the charging load between them more evenly, right? 

 

If that's true, it means I could increase my potential charging current from 55 amps to maybe 90+ amps, and I would then put in a second B2B to draw that extra charging capacity.

 

Sorry for the essay chaps, but there is a fair bit of detail needed for these sorts of descriptions and questions to make sense, and thanks in advance for any light that can ne shed. 

 

ETA- I looked into having a bigger 150 amp alternator installed, plus a Mastervolt regulator, and the total cost is at least £1000, so that's my 'last resort' option at the moment. 

 

 

Edited August 17, 2021 by Tony1

[peterboat]

21 minutes ago, Tony1 said:

 

I hope the OP won't mind me temporarily hijacking this very useful and informative thread to ask a question relating to lithium battery charging. 

 

I sort of jury-rigged a charging system for my 400Ah lithium bank in Feb-March this year, in order to get up and cruising again asap, but with Autumn in mind, I'm starting to ponder how I can best fix the charging setup. 

 

At the moment its a right old mish mash. I have a canaline 38 with 75 and100 amp alternators. Those are connected to a Sterling A2B charger, and from that there is a feed to the engine starter battery, and another to a separate lead acid battery. This LA battery acts as the source for a Sterling 60 amp B2B, which in turn charges the lithiums. 

The B2B is nominally rated at 60 amps, but in reality usually puts about 40 amps into the lithiums, which is pretty poor. I think the reason it's charging way below par is that the source LA battery is knackered.

 

I did initially try connecting the A2B directly to the lithiums, and they were taking 100 amps of charge, but the larger 100 amp alternator overheated to 150 degrees within minutes- so my quick fix was to have a B2B installed, as a way of limiting the current draw from the alternator.  

 

I had a quick chat with Sterling support earlier, and the chap there suggested connecting both alternators directly to the engine start battery, and using that as the charge source for the B2B charger.  

This would simplify matters greatly, taking the A2B charger and the current knackered LA battery out of the system, and it would probably increase the charge from the B2B up to maybe 55 amps, which is great. 

 

So my question for any lithium/electrical guys who might have time is this: does that proposal sound ok?

I dont have any electrical knowledge worth a damn, so I didn't even know you could safely connect 175 amps of alternators directly to a LA battery, with no charger in between.

And is there any reason why I wouldnt want to use the engine starter battery as the source for the B2B charger?

(The B2B has an automatic disconnect from the source battery once the voltage falls below a user-set value, so it shouldnt put the starter battery at risk...)

 

But there is another thing I'm wondering. Assuming it is ok to do all that, will the two alternators then share the load between them in a more even way? 

At the moment, with both connected to the A2B, the 100 amp alternator is doing all the work for the lithiums, and I found that it overheats if it gives out more than 60 amps of charge. 

But if they were both connected directly to the starter battery, they would share the charging load between them more evenly, right? 

 

If that's true, it means I could increase my potential charging current from 55 amps to maybe 90+ amps, and I would then put in a second B2B to draw that extra charging capacity.

 

Sorry for the essay chaps, but there is a fair bit of detail needed for these sorts of descriptions and questions to make sense, and thanks in advance for any light that can ne shed. 

 

ETA- I looked into having a bigger 150 amp alternator installed, plus a Mastervolt regulator, and the total cost is at least £1000, so that's my 'last resort' option at the moment. 

 

 

Try the new battery first Tony it might up your Amps immediately

[Tony1]

1 minute ago, peterboat said:

Try the new battery first Tony it might up your Amps immediately

 

Cheers Peter- just to be certain, do you mean try taking the old lead acid out of the system first, and using the engine starter battery instead? Or do you mean buy a new lead acid to replace the knackered one? 

 

Thing is, if I'm going to use the engine battery as the charge source for the B2B anyway (to simplify things), there's no point spending any money on a new lead acid, right? It never gets a good charge anyway as I only run the engine every few days, and cruise for a few hours maybe once a week, so if I replace it, the new one will start deteriorating anyway, unless I can divert some solar charge into it. 

 

The B2Bs work off the engine starter battery in campervans etc, so in theory I could simplify things and use my engine starter battery in the same way they do...

 

[peterboat]

3 minutes ago, Tony1 said:

 

Cheers Peter- just to be certain, do you mean try taking the old lead acid out of the system first, and using the engine starter battery instead? Or do you mean buy a new lead acid to replace the knackered one? 

 

Thing is, if I'm going to use the engine battery as the charge source for the B2B anyway (to simplify things), there's no point spending any money on a new lead acid, right? It never gets a good charge anyway as I only run the engine every few days, and cruise for a few hours maybe once a week, so if I replace it, the new one will start deteriorating anyway, unless I can divert some solar charge into it. 

 

The B2Bs work off the engine starter battery in campervans etc, so in theory I could simplify things and use my engine starter battery in the same way they do...

 

Your B 2 B is supposed to combine both alternators isnt it? Last I heard you thought it was broke? what price a though for a good battery if it sorts the problem?

[Tony1]

2 minutes ago, peterboat said:

Your B 2 B is supposed to combine both alternators isnt it? Last I heard you thought it was broke? what price a though for a good battery if it sorts the problem?

 

I think you're right that buying a new lead acid source battery will improve the charging rate, but I'm thinking that a longer term solution would be better. A new LA battery will just get knackered in a few months, and I'll have a the same problem again. 

The B2B does works ok, its just down on charge because its source battery is rubbish I think. 

 

But the guy at Sterling said there is no need for for my A2B charger to be there at all, with the B2B in place, and it was him who recommended using the engine starter battery as the source for the B2B. 

I like the sound of simplifying the system, and it sounds like a simple rewiring job. 

 

But the big plus with his idea is that it takes out the A2B, which I think is somehow dividing up the charging load unevenly between the two alternators.

With both alternators hooked up directly to the starter battery, and without the A2B, I'm hoping they will share the charging more evenly, and that means I can up the charging from 55 amps to 90 or more. 

That sounds like a worthwhile upgrade, if all thats needed is a simple relocation of alternator cables. 

 

 

 

[TheBiscuits]

I'd say warm the engine up, then stop it, swap the LA batteries over and try it.

 

Minimal work, no cost, see if it helps.  If it does then rewire to only use the engine start battery and scrap the other one.  If it doesn't, swap the batteries back and think of another solution. 

[Tony1]

10 minutes ago, TheBiscuits said:

I'd say warm the engine up, then stop it, swap the LA batteries over and try it.

 

Minimal work, no cost, see if it helps.  If it does then rewire to only use the engine start battery and scrap the other one.  If it doesn't, swap the batteries back and think of another solution. 

 

That's a good shout, thanks.

I'm certain the LA source battery is goosed- it was one of the domestic set that were replaced by the lithiums in January, and it was rubbish even then, when it had the benefit of engine and solar charging. Now that its had a few months of very intermittent and incomplete charging (what with most power coming from the solar direct into the lithiums), I dread to think what state its in. 

 

But I am a tad concerned what effect it will have on my engine starter battery, if I start using that as the source for two B2B chargers. 

 

 

 

Edited August 17, 2021 by Tony1

[Tony Brooks]

58 minutes ago, Tony1 said:

 

That's a good shout, thanks.

I'm certain the LA source battery is goosed- it was one of the domestic set that were replaced by the lithiums in January, and it was rubbish even then, when it had the benefit of engine and solar charging. Now that its had a few months of very intermittent and incomplete charging (what with most power coming from the solar direct into the lithiums), I dread to think what state its in. 

 

But I am a tad concerned what effect it will have on my engine starter battery, if I start using that as the source for two B2B chargers.

 

Unless the B2B is drawing more current from the battery that then alternators can supply then it will have no effect on the start battery. Unless you leave it working when the engine is stopped. The alternators will be supplying the B2B.

 

Re the 150C alternator temperature. That sounds fairly typical to me for an automotive alternator but in car use the high current would be soon reducing. With lithiums it won't so I agree current limiting may be a good idea, can't the B2B be set for that?

 

 

[Tony1]

24 minutes ago, Tony Brooks said:

 

Unless the B2B is drawing more current from the battery that then alternators can supply then it will have no effect on the start battery. Unless you leave it working when the engine is stopped. The alternators will be supplying the B2B.

 

Re the 150C alternator temperature. That sounds fairly typical to me for an automotive alternator but in car use the high current would be soon reducing. With lithiums it won't so I agree current limiting may be a good idea, can't the B2B be set for that?

 

 

 

Thanks Tony- the B2B is a Sterling BB1260. Its current output is set at 60 amps, although you can set it to half power mode, so 30 amps. 

 

There is a user-defined charge profile which I tweaked to suit the lithiums, but current is not one of the configurable parameters- you set the voltages and time durations for each phase. 

 

Re the temp issue, I checked the temp of the alternators before the lithiums were installed, and both ran at about 50 degrees C (on the casings). 

I lost my nerve and stopped the test at 150 degrees with the lithiums- for all I know it may have increased much higher if I'd carried on. 

 

This is one of the tests I found on youtube- these folks are getting concerned at around 150 degrees. 

 

My thinking is that if I get rid of the A2B and it results in the combined alternators putting out more charge safely (i.e. not overheating), I could maybe get another B2B and run them in parallel, which would draw about 100- 110 amps. 

If that's too much and the alternators get hot again, I can turn one of them to half power, which will still deliver a decent 85 amps of charge- I'd be happy with that, as an alternative to spending over £1000 on a new alternator and a regulator.

 

It just depends on whether the alternators behave differently when directly connected to the starter battery than they do when connected to the A2B. Its worth a try, I think... 

 

 

 

 

Edited August 17, 2021 by Tony1

[nicknorman]

2 hours ago, Tony1 said:

 

I hope the OP won't mind me temporarily hijacking this very useful and informative thread to ask a question relating to lithium battery charging. 

 

I sort of jury-rigged a charging system for my 400Ah lithium bank in Feb-March this year, in order to get up and cruising again asap, but with Autumn in mind, I'm starting to ponder how I can best fix the charging setup. 

 

At the moment its a right old mish mash. I have a canaline 38 with 75 and100 amp alternators. Those are connected to a Sterling A2B charger, and from that there is a feed to the engine starter battery, and another to a separate lead acid battery. This LA battery acts as the source for a Sterling 60 amp B2B, which in turn charges the lithiums. 

The B2B is nominally rated at 60 amps, but in reality usually puts about 40 amps into the lithiums, which is pretty poor. I think the reason it's charging way below par is that the source LA battery is knackered.

 

I did initially try connecting the A2B directly to the lithiums, and they were taking 100 amps of charge, but the larger 100 amp alternator overheated to 150 degrees within minutes- so my quick fix was to have a B2B installed, as a way of limiting the current draw from the alternator.  

 

I had a quick chat with Sterling support earlier, and the chap there suggested connecting both alternators directly to the engine start battery, and using that as the charge source for the B2B charger.  

This would simplify matters greatly, taking the A2B charger and the current knackered LA battery out of the system, and it would probably increase the charge from the B2B up to maybe 55 amps, which is great. 

 

So my question for any lithium/electrical guys who might have time is this: does that proposal sound ok?

I dont have any electrical knowledge worth a damn, so I didn't even know you could safely connect 175 amps of alternators directly to a LA battery, with no charger in between.

And is there any reason why I wouldnt want to use the engine starter battery as the source for the B2B charger?

(The B2B has an automatic disconnect from the source battery once the voltage falls below a user-set value, so it shouldnt put the starter battery at risk...)

 

But there is another thing I'm wondering. Assuming it is ok to do all that, will the two alternators then share the load between them in a more even way? 

At the moment, with both connected to the A2B, the 100 amp alternator is doing all the work for the lithiums, and I found that it overheats if it gives out more than 60 amps of charge. 

But if they were both connected directly to the starter battery, they would share the charging load between them more evenly, right? 

 

If that's true, it means I could increase my potential charging current from 55 amps to maybe 90+ amps, and I would then put in a second B2B to draw that extra charging capacity.

 

Sorry for the essay chaps, but there is a fair bit of detail needed for these sorts of descriptions and questions to make sense, and thanks in advance for any light that can ne shed. 

 

ETA- I looked into having a bigger 150 amp alternator installed, plus a Mastervolt regulator, and the total cost is at least £1000, so that's my 'last resort' option at the moment. 

 

Connecting 2 alternators to one LA battery is fine. However we never really got to the bottom of why your existing setup doesn’t share the load between the alternators, and I can’t see why removing the A2B and connecting to a different battery would make any difference in that respect. I would try disconnecting the larger alternator and making sure the smaller one is actually working properly. Because that seem to be the nub of the problem. And as TonyB says,, I can’t see that using a different LA battery is going to make much difference to the B2B since the current is provided by the alternators, not the battery. Well unless it’s something to do with the alternator ripple I suppose.

[Tony1]

3 minutes ago, nicknorman said:

Connecting 2 alternators to one LA battery is fine. However we never really got to the bottom of why your existing setup doesn’t share the load between the alternators, and I can’t see why removing the A2B and connecting to a different battery would make any difference in that respect. I would try disconnecting the larger alternator and making sure the smaller one is actually working properly. Because that seem to be the nub of the problem. And as TonyB says,, I can’t see that using a different LA battery is going to make much difference to the B2B since the current is provided by the alternators, not the battery. Well unless it’s something to do with the alternator ripple I suppose.

 

I must admit Nick, I've not made a priority of investigating the reason why the 75 amp alternator is not doing its fair share, but the approach of Autumn- and the likely increasing need for engine charging- has brought it to mind again.

I have discussed the charging situation with a couple of electricians, but they are more focused on 'supply-and-fit' solutions than they are on investigation. 

The small alternator certainly spins, and gets up to its normal temp of 50 degrees or so, so its doing something.

 

The last self-styled electrician whom I let near the alternators removed a section of wiring from the A2B, explaining that the alternators had been 'working against each other'.

This pseudo-technical patter did not sound convincing, and indeed his ministrations made no difference. Well, at least he didnt make it any worse. 

At the moment I'm not sure whether the alternators and A2B are wired incorrectly, and I dont understand it well enough to know for sure, so I'm arranging for a local electrician to come and look it over, after which we can discuss possible solutions. 

My only sketchy theory at the moment is that the A2B is the cause- that it is directing the charge current from the two alternators in such a way that it the 75 amp units is not fully used, or shared- whether due to incorrect wiring or a fault. 

 

I'm impressed that Sterling themselves recommended dispensing with the A2B, and the dedicated source battery. That means step 1 of any upgrade solution is to do as they suggest, and connect the alternators directly to the starter battery. That will itself reveal if there is going to be a resulting charging upgrade, so I'll know soon enough.  

If I were a betting man, I'd bet that getting rid of the A2B will allow the 75 amp unit to give more charge. 

I dont think you can get more than 60% of their rated charge anyway with these sorts of cheaper alternators (without them overheating), so the most I can safely expect is maybe 100-110 amps, but that would be great as far as I'm concerned. 

 

Even if I dont get a hoped-for charging increase to 100 amps, at least the wiring reconfiguration should allow the B2B to deliver 50-55 amps, so thats a step forward.

The next step might be to get the small alternator tested out somehow.

 

 

[nicknorman]

Just for the suckers still using LA batteries, here is the last 10 mins or so of a full charge with Li batteries. Started logging at about 88% SoC with alternator controller on slow rate, so current is around 70-95A depending on engine rpm (dips are when slowing past moored boats. Regulated voltage is set to 14.3 which is a bit lower than it could be, but barely any more charge is rammed in by having it at the max allowed of 14.6v. You can see how the voltage rockets up as 100% SoC is approached, but the current doesn't start to decrease until about 99% SoC. No more hours spent wastefully running the engine to get LA up to 100%!

Alternator controller waits a few minutes after current drops below 5% (30A) then goes to float mode. Amps  and SoC on LH axis, voltage on RH axis.

 

[Tony1]

7 minutes ago, nicknorman said:

Just for the suckers still using LA batteries, here is the last 10 mins or so of a full charge with Li batteries. Started logging at about 88% SoC with alternator controller on slow rate, so current is around 70-95A depending on engine rpm (dips are when slowing past moored boats. Regulated voltage is set to 14.3 which is a bit lower than it could be, but barely any more charge is rammed in by having it at the max allowed of 14.6v. You can see how the voltage rockets up as 100% SoC is approached, but the current doesn't start to decrease until about 99% SoC. No more hours spent wastefully running the engine to get LA up to 100%!

Alternator controller waits a few minutes after current drops below 5% (30A) then goes to float mode. Amps  and SoC on LH axis, voltage on RH axis.

 

 

Thats the sort of graph we all want in our lives! 

 

In an ideal world,  you don't want to get to 100% every day (only once every couple of weeks for calibration, right?), so routinely you'd only want to go to 80 or 85%.

That way you get a longer service life from them. 

Sadly the BMV712 doesn't have a high SoC alarm. But is this something you're thinking of automating, or are you just manually keeping an eye on it? 

You have plenty of time to spot that the SoC is getting too high, I guess, but I cant help wondering if there's an easy way I could automate it, just in case I let other people go cruising for a day or two without me being there.

 

 

 

 

 

[nicknorman]

1 minute ago, Tony1 said:

 

Thats the sort of graph we all want in our lives! 

 

In an ideal world,  you don't want to get to 100% every day (only once every couple of weeks for calibration, right?), so routinely you'd only want to go to 80 or 85%.

That way you get a longer service life from them. 

Sadly the BMV712 doesn't have a high SoC alarm. But is this something you're thinking of automating, or are you just manually keeping an eye on it? 

You have plenty of time to spot that the SoC is getting too high, I guess, but I cant help wondering if there's an easy way I could automate it, just in case I let other people go cruising for a day or two without me being there.

 

My alternator controller has selectable target SoC. I normally leave it on 80% - it goes to float when the BMV gets to 80%. But as you say, you need to take it to 100% from time to time to synchronise the BMV which is what I’ve done here.

[nicknorman]

4 hours ago, Tony1 said:

 

Thats the sort of graph we all want in our lives! 

 

In an ideal world,  you don't want to get to 100% every day (only once every couple of weeks for calibration, right?), so routinely you'd only want to go to 80 or 85%.

That way you get a longer service life from them. 

Sadly the BMV712 doesn't have a high SoC alarm. But is this something you're thinking of automating, or are you just manually keeping an eye on it? 

You have plenty of time to spot that the SoC is getting too high, I guess, but I cant help wondering if there's an easy way I could automate it, just in case I let other people go cruising for a day or two without me being there.

 

You could perhaps use the SoC relay function in reverse to turn off the B2B at a specified SoC, but I think you are using the relay for emergency disconnect at the moment, so it would mean another BMV which seems overkill!

[BEngo]

Nick,

 

You use the term 'float'.  As I understood it, LiFePO4 batteries should not be floated,  (except when nackered when a float test may be appropriate!) but instead the charge should stop completely.  I assume you are using the term float to mean "reduced the alternator voltage to below the battery voltage"?

 

This might be a bit pedantic, but there are still those out there selling drop-in solutions,  where a LA style float is still included  to the detriment of the Li batteries.

N

[peterboat]

9 minutes ago, BEngo said:

Nick,

 

You use the term 'float'.  As I understood it, LiFePO4 batteries should not be floated,  (except when nackered when a float test may be appropriate!) but instead the charge should stop completely.  I assume you are using the term float to mean "reduced the alternator voltage to below the battery voltage"?

 

This might be a bit pedantic, but there are still those out there selling drop-in solutions,  where a LA style float is still included  to the detriment of the Li batteries.

N

My solar goes into float but doesn't put any amps into the batteries 4 years with no issues 

[Tony1]

17 minutes ago, nicknorman said:

You could perhaps use the SoC relay function in reverse to turn off the B2B at a specified SoC, but I think you are using the relay for emergency disconnect at the moment, so it would mean another BMV which seems overkill!

 

Its a very good idea, if it will work. 

Thing is, there is a low-voltage disconnect setting, and a low voltage will usually mean a low SoC, so I sort of have that end covered. 

What I would like is to make it a bit more automated at the 'top end', because in my case a high SoC does not correspond with a high voltage. 

My charger only puts 40-45 amps into the lithiums, and at such a low charging current their voltage stays around 13.4 to 13.6, right up to 90% SoC and beyond. It does eventually go up to 13.8, but by then they are nearly 100% full. 

 

I think in this scenario I don't want to use the relay and disconnect the batteries. 

In this case an alarm would be enough, so I can pop in and turn off the B2B at the next convenient opportunity.

Disconnecting seems an overly drastic action for SoC going over 85%. 

 

As you know, the BMV712 has a low SoC alarm and a low SoC relay trigger point , so could I use the alarm function to warn me of high SoC, by entering values of say, 100% for the 'set' value, and 85% for the 'clear' value?  Or would it be vice versa?

 

Would that have the effect of making it act as a high SoC alarm?

 

 

 

[nicknorman]

49 minutes ago, BEngo said:

Nick,

 

You use the term 'float'.  As I understood it, LiFePO4 batteries should not be floated,  (except when nackered when a float test may be appropriate!) but instead the charge should stop completely.  I assume you are using the term float to mean "reduced the alternator voltage to below the battery voltage"?

 

This might be a bit pedantic, but there are still those out there selling drop-in solutions,  where a LA style float is still included  to the detriment of the Li batteries.

N

As you say, it depends on what you mean by "float". You are right that Li batteries should not be floated in the same way as LA batteries are. However if the float voltage is at or below the resting voltage of the battery at that SoC then no current flows into the battery so it has no idea whether it is being "floated" or not. The reason for putting my Li batteries on to float is so that if some large electrical load is turned on, eg electric kettle via the inverter, the current is supplied by the alternator not by the batteries. The regulator I'm using is very precise so the system voltage only has to be pulled down by about 0.1 - 0.2v to get full output from the alternator. So with the float voltage set at the battery resting voltage, no current flows into the batteries but when a significant load is applied that would pull the battery voltage down, most of the current is supplied by the alternator.

 

My only problem is that I can only set the regulated voltage in steps of 0.1v which is quite coarse because of the very low internal resistance of Li batteries. So I have to have the regulated voltage slightly below the resting voltage at the 3 selectable SoCs (50%, 80%, 100%) which means that with a small load (the fridge etc) the SoC will decline over time with the engine running. But the code flips it back to charge mode when the SoC has dropped 1% for the 50% and 80% SoCs so after a long day's boating the SoC should still be within 1% of the selected SoC.

For the 100% SoC I didn't want the batteries kept at 100%, so when it goes to float it will stay at float until the engine is switched off and on again or the switch is moved to one of the other settings and back to 100%. But the aim is mostly to hit 100% to synchronise the SoC meters and so it doesn't matter too much that after a long day's cruise, the SoC will have fallen below 100% due to the float voltage being below the 100% SoC rested voltage, in fact it's a good thing.

 

Out of interest I'm using 13.1v for the float for 50% SoC, 13.2v for 80% and 13.3 for 100%. This last is of course way below the 14.3v that the system voltage hits at 100% SoC but the voltage drops rapidly down to around 13.4v after just a few % of SoC is extracted, in the same way that the graph above shows the voltage rising rapidly just for the last few % of SoC during charge.

Edited August 18, 2021 by nicknorman

[nicknorman]

39 minutes ago, Tony1 said:

 

Its a very good idea, if it will work. 

Thing is, there is a low-voltage disconnect setting, and a low voltage will usually mean a low SoC, so I sort of have that end covered. 

What I would like is to make it a bit more automated at the 'top end', because in my case a high SoC does not correspond with a high voltage. 

My charger only puts 40-45 amps into the lithiums, and at such a low charging current their voltage stays around 13.4 to 13.6, right up to 90% SoC and beyond. It does eventually go up to 13.8, but by then they are nearly 100% full. 

 

I think in this scenario I don't want to use the relay and disconnect the batteries. 

In this case an alarm would be enough, so I can pop in and turn off the B2B at the next convenient opportunity.

Disconnecting seems an overly drastic action for SoC going over 85%. 

 

As you know, the BMV712 has a low SoC alarm and a low SoC relay trigger point , so could I use the alarm function to warn me of high SoC, by entering values of say, 100% for the 'set' value, and 85% for the 'clear' value?  Or would it be vice versa?

 

Would that have the effect of making it act as a high SoC alarm?

 

 

I wasn't proposing to disconnect the Li batteries at the desired SoC, I was proposing to use the ignition connection on the B2B to turn off the B2B whilst leaving the batteries in circuit. But that would use the BMV's relay which is already in use for the low voltage disconnect. Hence my point about needing another BMV.

Unfortunately you can't use the SoC alarm as you propose, the set value can only be less than the clear value, so it can only be used as a low SoC alarm not a high SoC alarm.