EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: firworks on May 01, 2016, 09:08:30 pm
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This might be a really stupid idea but I can't actually find much information on this. There's tons on li-ion charging 18650s but it's almost always one charger board with one cell. I've never seen anyone charging multiple in series without a RC car balancing charger module and that's not really something you could or would build into a project case. I was wondering if I could join together two or more of the very cheap single cell Li-Ion charger boards in order to charge and use say 7.4V for a project instead of the 3.7V from one cell without using a DC booster. I recognize my input current is limited due to USB and so it may not charge very quickly but would it work?
Taking something like this:
http://www.amazon.com/gp/product/B00QGVP944 (http://www.amazon.com/gp/product/B00QGVP944)
Running a wire from IN+ on board 1 to IN+ on board 2 and same with IN-. That way plugging in MicroUSB to one would provide power to the second board. Then wiring the cells in series, and having charger board wired to cell 1, and charger board 2 to cell 2. They should each only see the voltage across a single cell and so should be able to balance their cell and charge it appropriately... I think. I'm not sure if it will actually work though. It could be a really stupid idea. I know there are balancing chargers that can do this but I was hoping to build it into a small project box so they could be charged easily and not have to remove the 18650 cells each time to charge them.
(http://i.imgur.com/l8w2r6G.png)
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B- and IN- (and OUT-) are typically all connected together, so if you do that you're shorting a cell.
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B- and IN- (and OUT-) are typically all connected together, so if you do that you're shorting a cell.
Yeah that does seem probable. B- and Out- I think are connected/disconnected by a pair of MOSFETs in the discharge protection IC, but IN- is probably connected to B-. I wish I had one of the boards to check that. Maybe I can deduce it from the datasheet for the charge IC.
Does anyone have any other suggestions for a way to accomplish this without using an external RC battery charger?
EDIT: Yep, TP4056 datasheet shows the BAT- being connected to input GND. Dang.
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You should be able to make this work if you use two isolated power supplies.