Because they cant be isolated from the cells they are in parallel with. A bad cell in parallel with a good cell will drag the good cell down to its level.
parallel = balanced 
not with the cell in series with them though.
Hi everyone, I really hope someone can help me with my easy question because I’m getting very confused.
I have a 6S2P Lipo pack connected to a bms, everything worked great but recently I the cells were dicharged to much so 2 cells in parallel broke so I replaced them today.
The other cells are fully charged 4,2V but the 2 other cells are 3,7V. If I charge them with my normal 25,2V charger all the other cells charged until 4,25V so I tought damn they will be overcharged so I stopped charging while the ‘‘replaced’’ 2 cells are 3,9V.
So now I’m charging them seperate but my question is does a bms always balance the cells? Also when the charger isn’t plugged in? I thought a bms also balances the pack when no charger is connected but after 10h without the charger the batteries were still the same 4,25V and 3,9V.
Just stumbled on this thread, hope this information’s still useful.
A BMS has a very limited ability to balance cells. In the specs, it will give a rated “balance current”. This is the current it can use to charge or discharge individual cells when charging or in use. It’s usually around 500mA. To charge a 10000mah cell at this current, it would take 20 hours.
That’s why BMS’s should be installed with equalized cells, they can only account for very little variation in battery voltage. The 500mA is barely anything in comparison to the 2 or 3 amps we output from the chargers. The charge current just affects the overall current on an individual cells level. So when charging a pack from a 2A charger, the cells will charge at 2A if they are in sync with the other cells, 1.5A if they are near full, and 2.5A when undercharged. When discharging the same effect is achieved to keep cells in sync. The small amount of current variation is why a bms could possibly overcharge a cell if it started with too high a voltage.
Sorry if this is a rant, formatting is hard on mobile 
4 Likes
Ooh thanks man! Very useful for me 
I was just a bit concerned that I broke my bms due the undervoltage of a few cells.
1 Like
Thank you for this, very useful information that I had yet to find. I tried multiple bms’s with no luck. But I was trying to balance a pack of cells that where 3.8 and 4 volts. Must have just been too much.
My only concern is do you know why it would keep charging a cell past 4.2 volts? My issue with the bms’s I tried was it took that 3.8 cell to 4.2 and that 4 to a 4.4, ecensially, I saw no difference.
Every one keeps using (including enertion now) the bms in parallel with the bms, but so far, it seems like it just bypasses the bms and doesn’t actually go through it.
Only reason I can see is that your BMS isn’t hooked up correctly or is flat out defective. FYI 3.8 and 4v isn’t too much. I’ve had cells with a .2-.3 voltage differential and it was able to balance all cells to 4.22v just fine.
Your BMS should have a voltage cutoff celing where, once it triggers, charging for that cell will be disconnected until it discharges to the rated voltage release. Pick up one of the small bestech BMS’s I have. I can assist in making sure it’s hooked up correctly.
Could you link me to the one your using?
The two I haven’t aren’t available in 12s, but just take a look here and find one that fits your size requirements.
http://bestechpower.com/444v12spcmbmspcbforli-ionli-polymerbatterypack/
I have 2 of these: http://www.batterysupports.com/36v-30a-12s-12x-32v-lifepo4-battery-bms-lfp-pcm-smt-pcb-system-p-249.html
How should I modify this so it’s in parallel?
You plan to use LIFEPO cells? If so, not sure what you mean.
No, wrong one, they all look alike, cause they take 1 picture and use it for all, haha.
I think there are parallel adapters for the balance connectors. Otherwise you can combine then yourself by soldering it together.
Honestly with my latest build, I just took the L and got a high amp rated bms. Being able to charge and discharge through the bms will make it balance cells faster, as opposed to only balancing cells when charging.
From my experience cells don’t drift at all during discharge, so I doubt it’ll make much difference, for me anyway.
Most BMS will balance cells all the time, not only when charging, even if its wired to bypass the bms for discharge. Most cheapo bms (bestech, supower) will overcharge the batteries and balance drain, so balancing only happens when cells are 4.2v or higher.
No, most BMS’s balance at the balance voltage which is near 4.2v. There is no passive balancing besides with individual parallel groups, only when charging.
@Jinra i’ve measured it, numerous times. I have a supower, still balancing when the charger is not connected. And the only way to balance to 4.2 is to overcharge and drain with these types of bms.
