# Isolated two battery load/charging system (parallel 18650)

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

• Please put a numerical value on ”each cell was pretty far away from the other one”. Commented Sep 18, 2023 at 5:57
• Yeah sure, it it 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 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.) Commented Sep 18, 2023 at 14:50
• Good. Please edit that into the question. I don't see any problem with 130 mm. Are each cell user replaceable or do they come as a pack? Commented Sep 18, 2023 at 15:05
• User-replaceable is the goal. Basically an 18650 charger/discharger on each side. A pack was considered but I am still on the fence with it. Commented Sep 18, 2023 at 17:58
• For individual replaceable cells, bare 18650? If yes, you have several problems to consider. What if both are installed backwards? Most companies build packs to avoid this situation (and raise the prices considerably compared to loose cells). Commented Sep 18, 2023 at 18:18

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.

• I appreciate the answer. For clarification, can series charging allow for protection of reverse batteries? I know @winny sorta mentioned reverse current problems in my system, but in my system I had planed on using chargers with reverse current protection or using diodes. Commented Sep 18, 2023 at 21:09
• > can series charging allow for protection of reverse batteries? -- No. A backward cell with blow up the BMS. The BMS will not work if you place a diode in series diode in series with the voltage sense lines. You can use a fuse in series with each cell holder and a reverse diode across the voltage on the other side of the fuse. If the cell is installed backward, the fuse will blow, Commented Sep 18, 2023 at 21:13
• If that is the case, wouldn't two isolated cell chargers be a better option for uh... not blowing things up? Fuses are great but a little impractical in portable electronics that don't need it. Commented Sep 18, 2023 at 21:16
• the chargers have nothing to do with the BMS. With our without the chargers, a backward cell will blow up the BMS. The BMS is permanently connected to the cell holders. Commented Sep 18, 2023 at 21:19
• So, with that said, how does that directly affect isolated parralel charging/ series charging. No BMS? Add protection? Commented Sep 18, 2023 at 22:16