Electronic – Connecting tp4056 in parallel

1. I don't think this is by definition a wasted exercise to your developing interest in battery technologies. I do think there are better ways. There are charger ICs on the market for nearly any number of cells in series. But, if it's for testing your idea, this should be fine.

2. Yes, but they will regardless of how you charge, discharge or store them. Just if you let them discharge below a certain level or charge over another level, they will degrade much faster. With an intelligent charger system, which I think your PCBs are, overcharging should be protected.

3. In charging: No, in fact, they all get charged to full. In discharging yes. This can cause increased deterioration, which then makes them get out of balance faster, which causes a snowball effect. How can you prevent that? When they are in use, make sure they are protected from over-discharge. By this mean, each cell individually. Just checking if the total gets below 7V or 8V is not going to help, you need to have something that protects each cell from going below 2.5V. Here I note that some of these charge managers I mentioned also do this for you; they are a complete battery pack protection solution.

4. You will need a set of isolated DC-DC converters. Which types are the cheapest depends on how and where you get them and where you are having them delivered, but many different manufacturers make DC-DC converters with 5V output at 100mA to over 20A. Just make sure they are fully isolated.

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.