1) Can I use this to balance 3S or 4S? Absolutely. Yes.
If you're prepared to rework / redesign it. If not, maybe.
2) When balancing 5S, the maximum voltage needed is I suppose 5 * 4.2 = 21 V. That's a poor assumption. The voltage you need will depend on the exact chemistry of the cell. Look in the cell datasheet.
2a) Will this charge the cells to 4.2 V each? Yes. if you're prepared to rework / redesign it. If not, maybe. You said it was a balancer, not a charger.
2b)What happens if the power supply delivers 25 V constant voltage. Will the board attenuate that?I can state with some vague certainty that it either will, or will not burn with the light of a billion suns.
3) What happens if the power supply can only deliver 19 V? 19/5 = 3.8 V. In this eventuality, with only 19V on this exact PCB from China, I would expect Steve Hogarth to leave Marillion.
3a) Will all five cells balance exactly at 3.8 V or will they be random with a total of 19 V? I understand that the cells will not be fully charged. Balancers balance. The trick is recognising that the pronoun is also a verb. To be fair, I think this is considered post-grad stuff these days.
4) The board also protects the cells from under voltage, over current, and short circuit. Does this mean that during discharge if the cells are at 4 V each for a total of 20 V, the output of the board will only show 19.4 volts or less? Yes, Steve Hogarth's potential will have dropped by 0.3V, Steve Rothery, Ian Mosley and Fish each take 0.1V. The other instrumentalists are not entitled to a drop in potential, but may try to wangle something. This latter effect will be difficult to measure with an oscilloscope. Please use personal protection when probing Marillion.
4a) Would I need a heat sink @ 5A? If there is no drop in output voltage, then how does the board break the circuit?P=IV. Even in China, except I suppose they may use squiggly characters.
5) When you say 4S3P, is that 4 series of 3P or 3 parallels of 4S? Ah, a sensible question, deserves a sensible answer. It's ambiguous.
Hi - I am new to this forum, and thank you for reading my post.
Hi Trancistor. Welcome to the forum. If you'd like more accurate details, please post a link to the technical spec. or schematics of the board you have ordered. I only say this as, without more details to back up your questions some users might be tempted to poke a little fun out of you. Please ignore them if they do, I'm sure they mean no offence.
And on a very serious note:
Please do not use this board with unprotected cells. There is a real risk of fire*. You will not be able to extinguish a lithium fire.
*Actually it's more like a slow explosion.