Cell 0 = GND
Cell 1 = 4.2V (When full)
Cell 2 = 8.4V
Cell 3 = 12.6V
I have corrected the schematics (balancing part), so here it is again.
So lets say i want to charge cell 0 to cell 1 (0V to 4.2V). I would turn on Q8 and Q5, and turn off Q4 and Q9. Is this correct?
Hi,
I think you would do well to learn how to charge just ONE single cell all by itself first before you dive into charging a pack of four cells that are wired in series. You first need to understand how a single cell gets charged before you can move on to a more complicated charger.
The single cell has a max voltage spec of 4.250v and a max charge current 330ma to 1000ma typical depending on the actual manufacturer's recommendation.
Second, when you have to charge cells in series and you have access to the tie points you have to monitor each tie point and provide a current bypass for any cell that gets near the max charge voltage before the others. The current bypass is necessary so that the target cell charges normally too even though the others are still getting the full current.
So for example when a single cell charges it starts at say 1 amp and 3.6v, and then gradually the voltage rises to 4.0, then 4.05, then 4.10, then 4.15, and pretty soon the current starts to drop down to 0.9 amps, 0.8 amps, 0.6 amps, 0.4 amps, down to maybe 0.050 amps at which time you want to stop the charge process for that cell. But the other cells may still be at only 4.0 volts, which means they still need the full 1 amp to charge effectively.
The current bypass will therefore bypass the higher voltage cell so that it does not overcharge, while allowing the remaining cells to get their full charge current. As each cell nears completion (around 4.20 volts) each cell gets 'bypassed' a little at a time until they are all charged and then the charger can shut off completely, probably also lighting an indicator light.
So the bypass circuit might look like a voltage regulator, except that it looks more like a SHUNT regulator than a SERIES regulator.
And just to note, with all of the schematics you have provided so far, they all look like you are attempting to pulse width modulate the cells from 12v down to 4.2 volts, and that is not acceptable at all. That is actually a good way to blow up all of the cells.
The only way to do that is either with a linear circuit or with a buck type regulator. For this kind of circuit though the buck regulator would have to be specially designed such that it is able to sink current as well as source it, and that's not the typical buck.
I would suggest that you at the very least study a few reference designs for this kind of charger, and that you work on charging a single cell first before you go to try doing a charger for a series connected pack. You will be much happier in the end