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Differences in the behaviour of 2 lithium batteries


170968

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I have 2 supposedly identical 200AH lithium batteries installed in parallel on my boat. When I charge them together, one is only taking in approximately 2/3 the current of the other, although the voltages are similar. Also, when discharging the same one is discharging less current.

 

They are both connected to a distributor using identical cables. I have tested for voltage drop across all the cables and found none (I thought perhaps my lugs were badly done, or something similar). ETA: My measurements were wrong. The actual drop was 0.04v from battery to distributor, but the same on all of them.

 

Thinking that maybe one had a lower capacity, I isolated it, charged it, and put a 0.3C load on it. Although the voltage drop was more than I expected, once rested, it picked up to where it should be, so far indicating that the capacity is as advertised. I'll continue to drain it to be sure.

 

Does anyone know what is happening here with the current being less in and out when connected to the other and charged/discharged?

 

Perhaps I should charge them together, then isolate and test the capacity of each?

 

 

Also, is the voltage droop normal? It went from 12.9 under a 0.3C load, to 13.2 rested for ten minutes.

Edited by 170968
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Hard to answer when the only description is “lithium battery”. Do you mean a drop in LA replacement with internal BMS. Or bare cells? Or what?

 

It might just be that they are badly matched for impedance, which sounds bad but probably doesn’t matter too much unless you get close to hitting max charge or discharge current in any one battery. What will happen is that one battery will discharge more first, but eventually the other battery will take over so that they both get to “flat” at more or less the same time. Same thing with charging, they will eventually both fully charge but one will be ahead of the other until near the end.

 

I suppose the question is, why the big difference? Something in the slow one is dropping voltage and if that is a high resistance connection then high resistance + lots of current = something getting very hot. Could be a poor internal connection if it is a drop in replacement type battery.

 

I don’t think 12.9 under 0.3C drifting up to 13.2 after 10 mins is unexpected especially if it is a drop-in with built in BMS - there will be some voltage drop in the protection MOSFETs.

Edited by nicknorman
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Oh and just to say that you can’t really measure SoC and hence capacity by looking at rested voltage. The curve is too flat in the mid range. Best to discharge it until the voltage starts to go down the “knee”, say below 12v, then you know you are down to 5% -10% SoC or so.

Edited by nicknorman
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8 hours ago, nicknorman said:

Oh and just to say that you can’t really measure SoC and hence capacity by looking at rested voltage. The curve is too flat in the mid range. Best to discharge it until the voltage starts to go down the “knee”, say below 12v, then you know you are down to 5% -10% SoC or so.

 

They are drop in batteries with internal BMS. Thanks for the information; that's very helpful.

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There’re is a very good article on the 12 volt boating group on facebook which goes into detail as to why this happens and is quite common I keep looking at lithium batteries. There is a wealth of information regarding lithium batteries on this forum.

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2 hours ago, chevron said:

There’re is a very good article on the 12 volt boating group on facebook which goes into detail as to why this happens and is quite common I keep looking at lithium batteries. There is a wealth of information regarding lithium batteries on this forum.

 

I don't have Facebook, but I'd like to read it. Do you happen to have a link so I can maybe use a friends account, or get them to copy/paste it?

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1 hour ago, 170968 said:

 

I don't have Facebook, but I'd like to read it. Do you happen to have a link so I can maybe use a friends account, or get them to copy/paste it?


This is a note for those of you with two lithiums connected in parallel with internal BMSs whether hybrid or not. Some have been noticing that one battery discharges while the other one doesn’t despite the discharge ‘switch’ on both the BMSs being turned on and this doesn’t change until the charge switch turns back on on the second battery. After some experimentation and research on Mark at Lifebatteries’ part and a bit of muppetry of my own to confirm and explain it we have an explanation:
Because all discharge has to take place by current flowing from the battery and ‘backwards’ through the diode in the turned off charge switch before it gets to the discharge switch there is a small voltage drop in the turned off charge switch. Now if you have two batteries in parallel and they both have their charge switches off as they have reached full charge both will deliver power together briefly until one of them turns its charge switch back on. Even though they are set the same there will be small tolerances that means one will switch first. As soon as that one switches the charge back on its voltage will rise by the amount of the voltage drop that was across its diode and this could be over 0.3 volts. This means that it will now take the load current and so the other battery voltage won’t decrease as there is no load so it won’t switch back on. Eventually, under a light load, the battery taking the load will decrease in voltage and then the other battery will start delivering load which will drop its voltage and allow its charge switch to turn on. They will now deliver power together and, to a certain extent, equalise between themselves however, at the next full charge they will become fully equalised. 
Now that this mechanism is understood it is fairly safe to say that it actually isn’t a problem, it won’t reduce capacity and both batteries are actually turned on so there is no worry about only being able to use one. Of course another way to get the other battery to turn its charge switch back on is to give the system a heavy load. 
One way to possibly reduce this effect a bit would be to raise the charge reconnect voltage from 13.5 to 13.55, you may say ‘why not 13.6’ and the reason is to keep the batteries away from being held at 13.6 for long periods which would effectively be at full charge. No it might not do much harm but then nor does a bit of imbalance between two batteries.
Just for fun and for those interested here is a diagram of the relevant bits of the BMS with the mosfets being CFET and DFET for charge and discharge respectively.

 

This is what the answer was from Phil who runs the site who is very clued up on all thing electric I would try getting on facebook and join the group as it’s a fantastic source of information regarding set ups plus have a link for buying Lithium batteries with good customer support and back up. When i finally take the plunge.

 

 

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14 minutes ago, chevron said:

This is what the answer was from Phil who runs the site who is very clued up on all thing electric I would try getting on facebook and join the group as it’s a fantastic source of information regarding set ups plus have a link for buying Lithium batteries with good customer support and back up. When i finally take the plunge.

 

This isn't actually the problem I have, but it was interesting anyway. Thanks!

 

(Mine are both charging and discharging, just at different rates)

Edited by 170968
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23 minutes ago, 170968 said:

 

This isn't actually the problem I have, but it was interesting anyway. Thanks!

 

(Mine are both charging and discharging, just at different rates)

 

Just a thought, didn't @nicknorman have a problem with a connection between cells in one of his batteries that caused a high resistance? If I recall nothing visible but it was there just the same.

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5 hours ago, Tony Brooks said:

 

Just a thought, didn't @nicknorman have a problem with a connection between cells in one of his batteries that caused a high resistance? If I recall nothing visible but it was there just the same.

Yes I did, but it was at cell level. Hard for the OP to check that sort of thing without dismantling his “drop in” battery.

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6 hours ago, chevron said:

 

This is what the answer was from Phil who runs the site who is very clued up on all thing electric I would try getting on facebook and join the group as it’s a fantastic source of information regarding …

 


Well, he certainly likes to think he is. But having read his article on Li batteries in one of the canal mags, I’d say “he doesn’t know what he doesn’t know” and certainly he doesn’t tolerate any differing opinions on his closed Facebook group empire. He banned me just for pointing out that a refractometer was in many ways a better way of measuring battery specific gravity that a float thingy. Nope, only his way is acceptable, everything else is heresy and you will be banned for even thinking it. Which is one of the reason I hate Facebook - little cliques of groupthink people who don’t tolerate any alternative viewpoints.

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24 minutes ago, nicknorman said:

Yes I did, but it was at cell level. Hard for the OP to check that sort of thing without dismantling his “drop in” battery.

 

 

Agreed, and it is one thing that concerns me about such batteries in view of what you found.. Extrapolating from LA batteries with faulty cell interlinks, which may well be wrong, the symptom sounds like a cell interlink problem to me.

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I made a mistake in my original post. The battery that was only taking 2/3 of the charge (battery 1), actually discharged more than the other for the first 40% of the drain (I had said that it discharged less). Is the advice still the same in this case?

 

They are behaving as @nicknorman said they would so far. After the 40% drain, they equalised their output and now the other battery (battery 2) is discharging slightly more. Battery 1 has currently discharged about 10% more. I'll continue to drain them down and then confirm their behaviour while charging. Then I'll go back to the vendor with this information.

 

Thank you everyone for your input.

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1 hour ago, 170968 said:

I made a mistake in my original post. The battery that was only taking 2/3 of the charge (battery 1), actually discharged more than the other for the first 40% of the drain (I had said that it discharged less). Is the advice still the same in this case?

 

They are behaving as @nicknorman said they would so far. After the 40% drain, they equalised their output and now the other battery (battery 2) is discharging slightly more. Battery 1 has currently discharged about 10% more. I'll continue to drain them down and then confirm their behaviour while charging. Then I'll go back to the vendor with this information.

 

Thank you everyone for your input.

 

 

One thing I don't think has been covered in this thread is how you are determining SoC. As I understand it, measuring a change of 40% is not possible using voltage. Maybe you (or your BMSs) are coulomb counting instead, but unless you know, I'd have thought this calls into question the whole premise of the thread. 

 

 

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8 minutes ago, MtB said:

 

 

One thing I don't think has been covered in this thread is how you are determining SoC. As I understand it, measuring a change of 40% is not possible using voltage. Maybe you (or your BMSs) are coulomb counting instead, but unless you know, I'd have thought this calls into question the whole premise of the thread. 

 

 

 

 

Yes, coulomb counting. The BMS' and the battery monitor are doing it and approximately match each other.

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1 hour ago, MtB said:

 

 

One thing I don't think has been covered in this thread is how you are determining SoC. As I understand it, measuring a change of 40% is not possible using voltage. Maybe you (or your BMSs) are coulomb counting instead, but unless you know, I'd have thought this calls into question the whole premise of the thread. 

 

 

 

It's quite possible I'm kidding myself with this, but I did find a voltage vs SoC graph on a solar power forum, and one of the members tweaked it to add more data points. 

 

As we know, the graph of resting voltage vs SoC doesn't go up in a straight diagonal line, and there are large sections that are quite flat, but looking at the section between say 30% SoC and 40% SoC, it seems a bit easier to derive the SoC from the voltage, so I use that section to do an occasional reality check that my measured SoC (using the BMV712) is consistent with the voltage.

The graph tells me that the resting voltage at 30% SoC will be about 13.00v, and the voltage at 40% will be about 13.10.

So what I do personally is to assume that 31% SoC is equivalent to 13.01, 32% SoC is 13.02v, and so on up to 13.10v. 

What I should do is to plug in the laptop and use the valence software to get more accurate voltage measurements (I think it goes to 5 decimal places), but on a day to day basis I cant be arsed with that, so I'm looking for a quick and dirty method, but one that is still accurate enough to monitor the batteries well enough for my needs, and to prevent me damaging them. 

Tbh the real challenge in knowing the true battery voltage for me seems to be that they are very rarely at rest for any length of time. So the resting voltage is a thing that never really happens in day to day use, which makes these graphs of limited use - without some further messing about.

My solution, in order to estimate what the resting voltage would be at any given time, is to compensate for the voltage drops and rises that are caused by the loads and the charges that are constantly being applied to the batteries. 

After many months of observing how the voltage changes when the batteries are subjected to a wide variety of loads and charging currents, I think I have an idea of how to estimate what the resting voltage would be. 

And its not even that simple. For example, just as lead acid batteries do, for several hour after a high-current charge (in my case 100 amps or more), the lithium battery voltage seems to stay 'falsely' high. 

There are times when I know the batteries have been charged to say 80% SoC, but their voltage stays at 13.3v or higher for several hours, even under a moderate load. 

This sort of voltage would normally correspond to a SoC of 90% or more, but within a few hours it seems to subside and the voltage appears to correspond to the SoC. 

This 'surface charge' type of behaviour  may be a specific feature of the valence batteries with their magnesium, or it may just be the electrical pixies at work again.

Some things I'm just better off not knowing.

 

Lithium voltage chart.jpg

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