I have a Li-ion charger with 3 contacts: (+) (-) and (TH), based on TP4056 chip. I also have several batteries with corresponding contacts, and the charger does a good job charging them.
Now I would like to increase the capacity by connecting a stack of batteries in parallel. TP4056 is able to handle the increased load (according to the datasheet it can even handle a continuous short in the load circuit) and I don't mind the increased charge time. I'm taking all the precautions I can think of before connecting the batteries together, like only using similar batteries and charging them to the same level before connecting.
While connecting (+) and (-) terminals of all batteries in parallel is obvious, I don't quite understand what to do with (TH) terminals. Should I connect them in parallel as well, or should I only connect a terminal of one battery (supposedly somewhere in the middle of the stack) to report the temperature to TP4056?
Best Answer
This is either dangerous, not advisable or doable depending on...
It comes down to what the "cells" are made up of. Since a bare single cell doesn't have a TH, you are talking about a pack. It may be a pack of 1 single 3.7V cell, in which case it's effectively a cell and you can, to a limit, connect in parallel to your heart's content.
I'd advise you to balance them out to each other with an extra resistance before you hard connect them, though. If they're large cells and 0.5V apart that will mean large currents will flow until they "agree", which is good for neither the low one, nor the high one. It would look a bit like this:
simulate this circuit – Schematic created using CircuitLab
You should pick a TH contact, since it is a temperature sensitive resistance that will be measured, if you connect multiple in parallel it will measure a low resistance, compared to what's expected. Since usually they are NTC 10k, that means the charger will think your batteries are hot when you first connect them, which means it will not charge.
If you have a pack of more than 3.7V, such as 7.2V or 11.1V, then connecting multiple in parallel without any "internal cross connections" will increase the speed at which the first cell will die. If the pack is multiple cells in series without any balancing connections, it can be debated whether that's advisable to start with. But anyway.
If you have a 2 cell in series pack, you will want to connect them like this:
simulate this circuit
This way the odd batteries will join force as will the even batteries and that will severely decrease the statistical chance of a dangerous defect in the pack.
((Of course it's better to start out with resistors for the parallel connections for the first hours to cross-balance again))
If you have a multi cell pack and can't make the cross connections as drawn above, I'd say, on balance, you're better off not connecting anything in parallel at all. Especially for charging purposes.