There seems to be a fair bit of FUD surrounding 18650 cells, so I need a sense check: I have a brilliant little USB powered desk fan with a built in 18650 battery, which will run it for about an hour. I tend to leave it plugged in but I was thinking of extending the battery life by hooking up a second cell in parallel, for better portable use, as there's enough space for more batteries in the base of the fan body. That's the easy bit. Thing is, I don't know enough about 18650s to just chuck one (or more) in there. There is a little battery controller board in there, but I'm not sure if these things are capable of controlling batteries in parallel - in theory, there shouldn't be anymore load on the system as I'm only extending amp hours. Am I right in thinking that, as long as I stick to using protected cells, hooking up a couple or more in parallel will charge and operate as intended? I get that the battery array would take longer to charge. Cheers
TL;DR: If the device wasn't designed to use mutliple cells, regardless of series or parallel configuration, it's safest to avoid using multiple multiple cells. The issue you'll have is balancing the cells. Whether in series or parallel, the cells need to charge and discharge at the same rate or you risk damaging the cells. Protected cells would go some way to mitigating damage when discharging, but you would still have issues when it comes to the in-built charging circuit. Charging a LiIon/LiPo cell is an active process that needs a proper constant current/constant voltage charge controller, and if the charging circuit wasn't designed for multiple cells then you risk disrupting the charging process and damaging the cells. Whatever battery or charge management is already built in to the fan is going to have been designed for 1 cell. In theory it might be OK if you used protected cells and charged them in an 18650 charger that can charge cells independently (like the Nitecore chargers)... but personally I wouldn't want to risk it. As you're no doubt aware, the risk with these things, even protected cells, is that they burst into flames.
Yeah, the charge controller board's ability to handle multiple cells was what was concerning me. I really don't know enough about them to make an informed decision. I think the existing battery might be a fairly low mAh unit, so I'll probably just replace it with a higher rated battery and leave it at that. Cheers, B
Putting cells in parallel is fine as far as they are matched (ie, dont put new and old cell in parallel, multiple new identical ones are fine). In 99% of consumer grade, non automotive application cells in parallel are treated as a single big cell. There might be a problem, if BMS has a proper fuel gauge (adding more capacity wont do much if max capacity is set in BMS), but for desktop fan I doubt it. But if you still worried and there is enough space inside - consider using 21700 cell.
