Kind of a goofy design but the only way that I can think of charging that would be with separate chargers. Problem is you wont get the full mAh out of the batteries
I assume this is what you had in mind.
Wolf
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This wouldn't work unless using two separate power supplies to power the chargers. Remember, they can't have common ground across both chargers. Isolation is required.
A 2s BMS would handle the 2s1.5P (is the configuration I'll call it for now). As long as the BMS had built in balancing, ie it would bleed off the excess power from the 1p side until the 2p side catches up. This is a huuuuge waste of power.
I thought of a few options... I could include 2 BMS, 1 each for each set. The BMS of the single cell in series would stop charging once full; while the pair in parallel continues charging. (?)
You could use 2 BMS's, sure. But you will then run into the problem of the 1p cell getting charged before the 2p side, and therefore the BMS on the 1p cell will cut power, shutting down the whole circuit. So this option is not viable.
Option 1, each BMS connected to a single charger in parallel, Same voltage, but split Amps.
This won't work because you have to have isolation. They can't share grounds, otherwise you'll have a dead-short across the first cell. You would need two power supplies.
I agree with Floyd. If you have a certain space limitation, you should look for cells that would better fit that space. There are plenty of LiPo's that come in various sizes and dimensions. These are also called "pouch cells". They generally cost about the same as 18650's or other cylindrical cells for a given capacity & discharge/charge rate.
Or as ajw22 mentions, go with a 3S configuration and then use a buck converter to run at the lower voltage of your device, and have a charge circuit that can charge at the 3S rating.
Is this a particular application that is real world, or are you just coming up with a scenario to test our knowledge? If it is real world, it would be nice to know what the actual application is as it could help us give more directed and applicable information specific to the purpose.