Isolated two battery load/charging system (parallel 18650)

Question

I am working on a project and wanted it to be very robust and user-friendly. To power said device, I had planned on using two 18650 in series (li-ion), like everyone else. The problem I had was that each cell was pretty far away from the other one, and making a series circuit at that distance seemed odd. To counter this, I had the idea of overcomplicating things and making each cell have its own charger and regulator, and that there could be a load-sharing device that could discharge each cell at around the same time. So in practice, parallel charging, but not. Each regulator would make sure that the battery is fine, so no voltage balancing is needed ( in theory ) and only one battery would be needed to run the load. Maybe a Diode Oring setup?

As dumb as it sounds, I am wondering if there could be any traction with the idea. Maybe I am overlooking something... I don't know. I figured the experts know best. I am willing to make changes and fixes based on feedback.

Edit : it is about 5 inches apart (130mm to be exact ). The idea is, if one cell stops working ( or if you want to add in another cell of a different capacity) you don't need to match them up to the other 18650 for voltage. That's why they are so far apart, due to the isolation of circuits ( and the fact that the main circuitry is in the middle and it looks better with the 18650 on the sides.)

I'll build a diagram to explain both ideas

Answer 1

Cell holders for more than one Li-ion cell are a bad idea for a few reasons. But it appears that you are already aware of the issues and have begun addressing them.

All and all, you're making things more complicated than they need to be. You say: "very robust and user-friendly". Then, the only reliable solution is to connect the cells permanently. Once you do, you don't need to twist yourself into a pretzel to make it safe. "if one cell stops working" is not an issue with a battery protected by a correctly-installed protector BMS. Maybe your approach is a solution in search of a problem.

If you must have a removable solution, use a larger cell (e.g., 26650) instead of two cells. Then you don't have to solve some of the problems introduced when using two removable cells. You still have to deal with someone installing a cell backward.

If you absolutely must use two 18650 cells in holders, then placing them in series is most effective. Charging each cell independently does overcome the imbalance problem that occurs if the user is free to replace cells willy nilly. Discharging in series does not present any particular problems, as long as you use a protector BMS. Diode ORing is not required, because they are in series.