I’m designing an evolution of a product and the ME’s have proposed an enclosure (for the sake of size reduction in a particular axis) that would split the current 1S4P 18650 (3.7V) based lithium battery pack into two halves, with about 8 inches between them. I know this is rather unconventional…
Current design specs/needs related to power:
- AC input voltage 12Vdc+
- Ability to slow charge from an external USB 2.0 host (5V)
- Main system power rail is 3V3 and below so just using a buck converter
- Currently using a TI BQ24192 single cell charger and BQ27621 gas gauge
My concerns off the bat:
- Both 1S2P packs would now include their own battery protection circuit
- Extra impedance of the wires and voltage drops (use bigger gauge wire to mitigate?)
- Cell balancing (right off the bat after assembly, and long term)
- Will the fuel gauge be anywhere accurate?
- Total available energy
- Other safety concerns
My gut tells me that joining the packs in SERIES would alleviate the balancing issues, but he only reason I would want to keep the packs in PARALLEL is to avoid implementing a boost converter when charging from the external USB 5V. I rather not use some unicorn IC, especially giving the supply constraints we face right now.
Another option would be to treat the two packs as separate power sources and switch between them with an ideal diode setup of some sort, but I rather avoid that complexity.
Also, this isn’t very cost sensitive so I prefer simple & reliable versus elegant & cheap.
My availability to test equipment is limited right now so I can’t do any testing, and I’m not too well versed on simulations.
Any thoughts or suggestions? Thanks!!