jasonhc73
Cat herder, and dog toy tosser.
When you parallel a cell(or battery) they balance all on their own, simply by the nature of being connected.
2 BMS on a 2P4S battery?
- Thread starter jaffadog
- Start date
lexio
feel the balance 3.65
no, lifepo4 cells will not shelf balance voltage difference is to small
Nope.
Paralleling LFP cells at rest to "balance" them
Hypothesis: Paralleling cells at rest does almost nothing beneficial. While voltages may be equalized, their states of charge remain dramatically far apart. I decided to do an experiment. Take two CALB 40Ah LFP cells. Fully charge to 3.65V Fully discharge to 2.5V Charge ONE to 3.65V Let them...
diysolarforum.com
My (very limited) understanding is that what may be true in theory is not necessarily true in a practically relevant sense.
Putting cells in parallel moves them in the direction of balancing. But because of the flatness of the voltage curve for LFP, and very small differences in voltage equating to large differences in SOC, simply paralleling cells is not a reasonable way to balance in any realistic timeframe.
If I understand correctly:
[Equalization current] = [Difference in Voltage] ÷ [Total Resistance]
What this means (1) current in the middle of the curve will be very low even with a substantial difference in SOC (2) it will get lower and lower as SOC equalizes.
This is very interesting to me. So, if you have cells in parallel inside of a pack and you do an initial top balance of all the cells, would it be possible for the cells in parallel to drift apart from each other after a certain number of charge cycles on the battery pack? Or do they stay pretty balanced if you initially balance them? The reason I'm asking is because if you do lets say a 2p4s battery pack, you're only monitoring the series cells as a single cell with the BMS and you wouldn't be able to see if the cells were unbalanced. I have 8 cells coming for a 12v system and I'm still trying to decide between two battery banks and paralleling 2 cells together in a single battery bank.
Possible? Yes. Typical? No. The meager 50mA-ish BMS passive balancing current only during charges is typically enough to overcome any tendency to lose the top balance.
Parallel cells subjected to current will stay consistent provided the cells are properly connected. They will be at very nearly the same state of charge at all times.
Some concepts got intermixed. If you take two cells at very different SoC and parallel them, but just let them sit there doing nothing, their SoC will be within about 10% of each other after a week. If you apply current to them, they will rapidly equalize to the same SoC.
In my 5th wheel RV I have two 4s 12v LiFePO4 batteries in parallel. It has only been 2 months and works quite well. Pros include: 1) redundancy in case of a single battery failure. 2) with two 120a BMS(Overkill Solar) units you can monitor and balance each of the 8 cells via the Bluetooth App. 3) They stay very equal in SOC(state of charge), both when charging and discharging. And the big one is... 4) With two BMS units the load is split equally so each one does half the work and extends the life of my batteries & BMS. The only con would be if one of your batteries is lower in max SOC then it will cut-off a bit sooner than the other. However, as you stated they do balance between each other after the charger turns off. My two batteries are only off by +-.010v and pass power between each other for about 10 minutes to a final balanced SOC.
Note: I set up my system for long cell life by top balancing each battery before hooking them in parallel, then configuring the BMS to operate from 90% SOC down to 20% SOC. This conservative setup is giving me +-30-40% more run time than my previous lead-acid batteries depending on loads...
Note: I set up my system for long cell life by top balancing each battery before hooking them in parallel, then configuring the BMS to operate from 90% SOC down to 20% SOC. This conservative setup is giving me +-30-40% more run time than my previous lead-acid batteries depending on loads...
sid the seagull
Slip slop slap
- Joined
- Nov 12, 2020
- Messages
- 64
I’m interested, what was you’re lead acid capacity and your new lifepo4 capacity?
Lead acid capacity was 250ah.
sid the seagull
Slip slop slap
- Joined
- Nov 12, 2020
- Messages
- 64
What is the capacity in each of your two new lithium battery 12v battery’s ?
The old Lead acid capacity was 250ah, the new LiFePO4 batteries are 135ah each, so in parallel 270ah.
