I’m quite sorry if this question has been asked recently. I found a topic that was last discussed in 2019, but said discussion wasn’t quite in alignment with what I was looking for.
Just curious to hear thoughts/opinions on wiring two batteries like the PRT-13854* in parallel to increase capacity.
Thanks.
You certainly can, but I’d sooner recommend just getting a higher capacity battery to begin with, like this 1500mAh Lithium Ion Battery - 1500mAh (IEC62133 Certified) (~about the capacity of 2* 850)
or others Battery - Power - Components
Thank you! I’m trying to adhere to this package size. I’d love to go with a bigger battery, however, the redesign would be a formidable effort.
Batteries in parallel are a problem since a cell’s natural self discharge, the slow decline in voltage when not significantly loaded and/or stored, feeds back to its neighbors who try to balance the voltage drop. This causes their own voltages to drop and the whole thing spirals into waste warmth in a fraction of the expected standby time.
Thanks for the input.
That isn’t to say never put batteries in parallel, just to understand the big drawback if you leave them like that.
Take a car getting jumpstarted by a second, healthy car. The cables put the two batteries in parallel which both boosts the voltage to somewhere between the two and also couples the fresh battery’s capacity when the dead car’s key is turned. The engine starts because of the parallel batteries.
The trouble would be if you parked the cars, engines off and with the batteries still in parallel. The healthy batt will try to charge the weak one all night and, more than likely, both are dead in the morning. Even when there isn’t such a large charge difference and both batteries are healthy, the balancing takes place at lower levels and causes early death.
It’s really not that concerning in this specific application.
Many Li-Po’s are constructed as 1S2P (1 series, 2 parallel or more) cells already.
The circuit board you see isn’t what we typically consider a BMS.
It’s just providing Over/Under Voltage and Current Protection for the cell or the parallel pack.
There are no cells in Series for a BMS to balance.
If both Li-Po’s are in similar healthy conditions and SOC, you can safely parallel the 2 (3.7V nominal) pouch cells. I’d say the majority of the li-po cells with decent capacity are constructed in this manner.
I would bottom balance the cells and confirm the IR’s are similar.
But it’s generally easier to find a higher capacity li-po that fits your dimensional constraints.
Totally different ballgame when dealing with lithium packs that have Series connections internally.
They both have great points above; to summarize: you’ll want to confirm the internal resistance and 0%/100% voltages are very similar…this is usually not the case when combining new and used batteries, and brow did an awesome job of explaining why combining dissimilar batteries will usually be an issue…
But in this case they’d both be new and presumedly near-identical spec…you’ll just want to confirm, as I have seen batteries in the same box (from amazon) with +/- 20% IR 
Sure, cells in parallel are accommodated for during manufacture in a production environment and careful design. But failure to do so is bad and there’s good reason we don’t do much of it outside the sandbox. Parallel power supplies in general.
In conclusion:
♫♫BMS to
balance bottom
balance li-po
lithy bottom
||
Pouch: constructed
Dimensional constraint
Over, Under, I protecc
A BMS I ain’t♫♫
TGIFriday!
Agreed. Lots of snags to avoid in general for sure 