BigBattery 12v 170AH in series balance issues.

[McRod]

I have purchased 4 - 12v 170AH BigBatteries and have them connected in series (24v) / parallel.

They don't seem to keep the balance between 12v batteries. When charging, one battery will hit ~13.6v and stop charging and the rest of the charge will go into the other battery at 15.3v (13.6+15.3=28.9v).

I don't want to break the warranty seal to check each of the cells. I sent one back to big battery and their shipping department said it was fine, but they didn't test it as a series configuration...

Just a follow up on my original post.

I am still waiting to get the 12v battery I returned to BigBattery back to me. It was returned 1 month ago, just fyi if anyone is curious on the warranty's timeline.

They claim there is no issue with the battery, but they would not test it in series with another battery. Discussing it with manager, stated the series issues didn't matter as long as the single battery performed correctly. He wasn't willing to acknowledge their could be issues with each bms not being able to communicate with the other causing the balancing issues.

When I get it back, I will go through each cell and check for balance and then balance to the other battery in the series.

They were not willing to swap/trade for 24v batteries that I could run in parallel.

If I continue to have balance issues while in series, I may purchase the 24v 8s BMS here on Wills site and use the one BMS to handle two batteries.

I'll do another follow up.

 

[MisterSandals]

I may purchase the 24v 8s BMS here on Wills site and use the one BMS to handle two batteries.

Each battery needs its own BMS.

 

[McRod]

Each battery needs its own BMS.

I am pretty sure I can take the Lishen cells from two 12v batteries and combine them to make one 24v battery.

 

[niktak11]

This is the reason why I would never use packs with separate BMSs in series

 

[NwCali]

stated the series issues didn't matter as long as the single battery performed correctly.

Makes me glad I didn't "troubleshoot" the one that I got from them that was DOD and just sent it back...

 

[rmaddy]

Just to add a data point. I have two 12V 206Ah SOK batteries connected in series. When I first got them I charged each one separately and then connected them in series. When I then charged them through my inverter/charger, the two battery voltages started to diverge a lot during the absorption phase.

The problem was resolved by removing the two batteries from my system and connecting the two batteries together in parallel. I then trickle charged them overnight to 100% SOC and then I let them sit, in parallel, for several hours. At that point I hooked them back up in series in my system. Since then I've never seen more than a 0.2V difference between the two batteries and that's only briefly, during absorption, when the batteries are above 99.8% SOC. As they do their post-charge settle, the two voltages go back to exactly the same value.

So I suggest connecting your four batteries in parallel and charge them together to 100% SOC and then let them settle together for a few hours, still in parallel. Do make sure you use a proper parallel battery connection so all four are truly getting to the exact same SOC.

 

[McRod]

Just to add a data point. I have two 12V 206Ah SOK batteries connected in series. When I first got them I charged each one separately and then connected them in series. When I then charged them through my inverter/charger, the two battery voltages started to diverge a lot during the absorption phase.

The problem was resolved by removing the two batteries from my system and connecting the two batteries together in parallel. I then trickle charged them overnight to 100% SOC and then I let them sit, in parallel, for several hours. At that point I hooked them back up in series in my system. Since then I've never seen more than a 0.2V difference between the two batteries and that's only briefly, during absorption, when the batteries are above 99.8% SOC. As they do their post-charge settle, the two voltages go back to exactly the same value.

So I suggest connecting your four batteries in parallel and charge them together to 100% SOC and then let them settle together for a few hours, still in parallel. Do make sure you use a proper parallel battery connection so all four are truly getting to the exact same SOC.

I plan to do exactly that, when I get the battery back from them.

One thought I had was about cable length. I custom cut the Anderson cables to the needed length to reach the parallel bus bar, but that means each cable is a different length. (All of them are within 18" of each other).

I remember someone saying that mattered with lead acid batteries. Does it impact Lithium the same way?

 

[rmaddy]

I plan to do exactly that, when I get the battery back from them.

One thought I had was about cable length. I custom cut the Anderson cables to the needed length to reach the parallel bus bar, but that means each cable is a different length. (All of them are within 18" of each other).

I remember someone saying that mattered with lead acid batteries. Does it impact Lithium the same way?

Please see:

Wiring Unlimited

Wiring Unlimited is a great resource that covers many many common topics and questions relevant to DIYers. It is a free E-book from Victron Written by Margreet Leeftink. Check it out! Download

diysolarforum.com

and read, at a minimum, chapter 3 about hooking up batteries in parallel. Basically, all of the wires between the batteries and the bus bars should be exactly the same. Otherwise each battery sees different resistance and they won't charge/discharge evenly.

 

[McRod]

Please see:

Wiring Unlimited

Wiring Unlimited is a great resource that covers many many common topics and questions relevant to DIYers. It is a free E-book from Victron Written by Margreet Leeftink. Check it out! Download

diysolarforum.com

and read, at a minimum, chapter 3 about hooking up batteries in parallel. Basically, all of the wires between the batteries and the bus bars should be exactly the same. Otherwise each battery sees different resistance and they won't charge/discharge evenly.

That's a good read.

Here are a few of my thoughts.

When I upgraded to lithium, I made several other changes to my system. The biggest change was going from 12v to 24v system. (This is why my lithiums are 12v, not 24v).

1. The increase in voltage should help lower the overall resistance.
2. I shortened the factory Andersen cables, which should also lower resistance. Although the lengths were based on the batteries proximity to the busbar. But all of them are within 18" of each other.
3. Lithium batteries have a lower internal resistance than my previous Lead Acid batteries. But lithium batteries, like all batteries, can have different internal resistance's to each other

In my mind I was improving any resistance issues, but I can still see the challenges identified in the paper with series connected batteries eventually coming out of balance due to different internal resistance's. The paper mentions a battery balancer, which I didn't even know existed. When I was speaking with Bigbattery I was asking about a "master BMS", that could keep the batteries in balance. This appears to simply be a battery balancer. They should have the technical expertise to recommend this to me.

So my take away is this:

1. Check each battery cell for balance within each 12v battery.
2. Charge fully each 12v battery before adding it to the system.
3. Put all fully charged 12v batteries in parallel to allow balancing before putting into the system.
4. Configure the 12v in series to 24v.
5. Wire in battery balancer to maintain balance in series connected batteries.

Thanks for the link, I will contact Victron to get the battery balancer.

I appreciate the helpful responses in this forum.

 

[McRod]

This is the reason why I would never use packs with separate BMSs in series

Yes, unlike dumb lead acid batteries, which can be equalized using a simple "equalization" voltage, smart lithium BMS's don't communicate with each other and operate independently of each other in series.

Live and learn, I guess. Looks like there is a solution that can help.