Joining 2x 7s21p Batterypacks

Joined
Jun 26, 2021
Messages
10
Hey Guys,


I have a question (again).

I am relatively new to the whole "build your own battery" thing and so far I enjoy it, some sparks, no burns, no fires so far....

I build a 7s21p Batterypack from used 18650cells.
I got my BMS to work and with a small 500W inverter I drew 100Watts for hours on end.

Now, I still have another 7s21p Batterypack in the same layout and I was wondering whether I can just just "extend" the existing Batterypack with the new one, since all the strands that combine the cells in parallel should technically just be "extended" , right?

I thought about charging both packs to the same voltage to minimize differences between the cells.

So...
What do you guys think? Possible? Or just a very expensive way to start a chemical fire?

Also, can I attach an active balancing board on the same battery pack that already has the BMS? I have the feeling that the DALY BMS has minuscule balancing currents.

Thanks in advance for whatever might come!

EDIT:

IMG_20211022_193700.jpg

Best,

Max
 
Parallel connection is fine.

Keep just using one BMS by connecting the balance leads in parallel as well as the main power leads.

Yes get voltage within 0.1V before connecting.
 
@john61ct

thank you for the fast reply!

Do you, by any chance, also know whether it makes sense to connect a dedicated balancing board with the strands? to improve balancing especially when under load?

Best,

Sack
 
Dedicated active balancers do indeed work faster than most cheap BMS.

The actual balancing current rate is critical

Best are ones that guarantee the same rate regardless of the delta between cells.

But, newish to quality well matched cells may not require much balancing at all

only when the start to approach EoL maybe.

So long as you can watch the cells' voltages even out as the balancing proceeds

can you make an informed judgment as to when better gear is required.

If you do go that way, then use a BMS without balancing, or one that is easy to disable that function.
 
Charge all parallel strings to the same voltage before hooking up the bms and or balance the battery before you put the BMS on. The second battery should also be balance and match the first battery.
 
No cells are truly equal. the stronger cells will support the weaker ones. If you have a group that drops faster then the balance board comes into play.

by transposon » Oct 27 2021 5:09pm

After the combination, will all of the parallel groups have roughly the same capacity?
 
Well, first of all, the merging worked without a problem, but I have not yet attached the BMS of the combined pack because I want to find a suitable case and put the whole pack into a heatshrink most likely.

Both sets were more or less identically mixed/matched from 18650 Batteries of around the same capacities.

And once everything is back together I will send an update.

Again, thank you guys again for your help so far !
 
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