Electronics > Projects, Designs, and Technical Stuff
Balance charging lithium cells
OM222O:
In pretty much every balance charger circuit I can find, they just use some braking resistors to discharge the cells that are charged to a higher voltage. wouldn't it be just easier to create multiple isolated supply lines and charge each cell to it's desired voltage, e.g: 4.2V and then stop charging all together? it should be more efficient too?
Miti:
--- Quote from: OM222O on April 24, 2019, 10:29:38 pm --- it should be more efficient too?
--- End quote ---
...and way more complicated. Probably you're playing with RC balanced chargers. Technically, if the cells are well matched, you'll have to balance them only once, after that they should be pretty much charging/discharging in sync without much "braking". However, if the cells aren't perfectly matched, the circuit that uses shunt resistors, if properly designed, will give you a better capacity matching than the circuit that you proposed.
OM222O:
how come? with a voltage reference and an op amp, you can create a very exact cut off voltage ??? you create the desired voltage, e.g: 4.2 or 4.1 volts and use one comparator per cell to deactivate the charging circuit and since they are all referenced to the same point, then the only error would be the input offset of your comparator, which you can select to be very small, even less than 1mv :-DD also there are plenty of DC-DC isolators available in through hole packages which do boost / buck conversion as well! they are limited to low currents (500mA max I believe) but it wouldn't be much more difficult to make small converters on your PCB either :-// you can also create small constant current / constant voltage charging circuit which adds more complexity than just charging with constant voltage and cutting off at some point, but it's presumably much better in keeping the batteries happy campers :-/O
digsys:
For serious applications, we use ICs like this - https://www.analog.com/media/en/dsp-documentation/evaluation-kit-manuals/dc2064afa.pdf
There are a few manufacturers / designs out there already, and it very popular in EVs
I NEVER have it permanently connected, always as a plug in - and only when the BMS says I need to. They are very efficient, also cascade able.
OM222O:
very interesting ... but it seems like a very specialty item?
the method I proposed can be done with jelly bean parts however :-//
or another way would be to have 1 constant current constant voltage charger with it's cut off point set to a voltage reference IC, and use mosfets to effectively swtich in / out different batteries. this way the offset would also be the same, so the cutoff point is exactly the same :-DD it would just take longer to charge the batteries which is not ideal :-//
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