16s w/ bluetooth suggestions

[whatismouse]

I have two 24V a124 batteries from bigbattery in series in my RV. They are these ones:

Each has its own 8s BMS. non-descript, from china.

Maybe because they are used cells and different, I can't seem to keep the two batteries in balance. I couldn't find a battery equalizer for 24V, but I had the thought that I could just remove the two 8s Bms' and add a single 16s bms. Would this solve all of my problems?

Also because the balance leads are all nicely soldered on to the pouch cells already, I thought maybe the easiest would be to cut the bms side of the balance leads and join all of them to the new 16s bms balance lead cable. Or I can just solder on to the cell terminals like Will does in the video and ignore the pre-soldered balance leads. I've never soldered before so i'm intimidated.

I was thinking the overkill 48V 100amp w/ bluetooth, but open to other suggestions.

 

[sunshine_eggo]

What you want is possible, but there may be no benefit. BMS passive balancing uses very little current. If they can't be kept in balance separately, it's unlikely a 16S BMS can either.

Can you be more specific about "can't seem to keep the two batteries in balance"?
What are you seeing?
Have you top balanced them individually?
Have you top balanced them in parallel?

 

[whatismouse]

What i mean is the two batteries don't stay in balance with each other. Even after connecting them in parallel and top-balancing the two together, they drift apart. One of the batteries goes higher on the top end of charging as well as lower on the bottom end of discharging.

edit: no, one of the batteries was lower at the top & the bottom.

 

[whatismouse]

I think even gaining Bluetooth monitoring functionality would be worth the hassle of a new bms. Just because the cells are used and I'd like to see what they're doing individually too

 

[MisterSandals]

One of the batteries goes higher on the top end of charging as well as lower on the bottom end of discharging.

How are the batteries wired? Is the positive main wire on one battery and negative on the other? Or both on the battery with the wild voltage swings?

Happen to have a good pic of your wiring?

 

[whatismouse]

They are in series, so the main negative is on the negative of the one that fluctuates more and the main positive on the positive of the other one.

 

[whatismouse]

Hmm I just had a small epiphany though. I had to get one of them warrantied and they sent me the upgraded model, same battery but in an enclosure with a LCD voltage display and a circuit breaker. Which could be enough of a draw to cause a drift...

 

[MisterSandals]

They are in series,

Ah, of course. sorry.

Which could be enough of a draw to cause a drift...

That’s a good catch. It’s surprising how even tiny loads add up. Even powering the BMS from one cell (I think they all do this) makes a difference. Can you put a switch on the display?

 

[sunshine_eggo]

What i mean is the two batteries don't stay in balance with each other. Even after connecting them in parallel and top-balancing the two together, they drift apart. One of the batteries goes higher on the top end of charging as well as lower on the bottom end of discharging.

Unlikely that a single 16S BMS with passive balancing will improve things.

I think even gaining Bluetooth monitoring functionality would be worth the hassle of a new bms. Just because the cells are used and I'd like to see what they're doing individually too

If you just want monitoring, these might be a cheaper option, and they can do a little bit of balancing:

ISDT BG-8S BattGo Battery Meter, LCD Display Digital Battery Capacity Checker Battery Balancer Battery Tester for NiMH,LiPo,Life,Li-ion,Nicd: Amazon.com: Electronics

ISDT BG-8S BattGo Battery Meter, LCD Display Digital Battery Capacity Checker Battery Balancer Battery Tester for NiMH,LiPo,Life,Li-ion,Nicd: Amazon.com: Electronics

www.amazon.com

Hmm I just had a small epiphany though. I had to get one of them warrantied and they sent me the upgraded model, same battery but in an enclosure with a LCD voltage display and a circuit breaker. Which could be enough of a draw to cause a drift...

If they are different capacity and have different loads, they will likely always drift.

 

[whatismouse]

oh, i am mistaken. it is the battery with the lcd screen that is lower on the top and the bottom, which completely makes sense. So maybe all i need to do is remove the screen & top-balance again and they might stay together now.

I am guessing the breaker is just wired into either the positive or the negative and doesnt actually draw power from the 24V battery?