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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?

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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:

schematic

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:

schematic

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.

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  • \$\begingroup\$ Yes, my batteries are single cell (3.7-4.2 V). I am aware of cell balancing issues with serial connections, and I will not do that (my stack will be 3.7V with more mA*h). Thanks for a detailed answer. \$\endgroup\$ – Dmitry Grigoryev Aug 5 '15 at 6:17
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    \$\begingroup\$ This is actually a very wrong answer on many levels. First, it mostly discusses how to connect multiple batteries with respect to balancing issues which according to the question the OP already took into consideration. It only marginally touches the question of how to connect the NTC pins of multiple batteries, and there it gives a very dangerous advice. Continued in next comment... \$\endgroup\$ – ultimA Jan 25 '18 at 15:07
  • \$\begingroup\$ The adviced method by this answer answer is to just pick a single NTC lead and connect that instead of both. This is dangerous because a high temperature on one battery is likely to arise exactly in the case when the batteries get out of balance (can happen even if they were in balance in the beginning), and in that case only one of the batteries will get hot. What if the user decided to connect the other (cold) battery while assembling the pack? See how dangerous this is? The temperature protection has been basically eliminated with a 50% statistical chance! See my answer for solution. \$\endgroup\$ – ultimA Jan 25 '18 at 15:13
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The accepted answer is very dangerous for reasons as I described in the comments of the wrong answer.

To correctly and safely handle the NTC (=TH) pin, ideally the charger should always see the hottest battery, whichever it is. The problem is, at start we don't know which battery will get hot. This unfortunately requires a small circuit that takes both NTCs (one for each battery), and selects the correct one to forward to the charger dynamically. This boils down to selecting either the lead with the higher or smaller voltage of both.

Unfortunately, the "hotter" NTC lead can be either the lead with the smaller or higher voltage, depending on how the temperature measuring circuit inside the charger works.

If you cannot figure out how the charger measures temperature, or you cannot build the circuit necessary to connect both NTC leads, you should remember that with at least 50% probability you have no temperature protection at all, so you should not leave the batteries unattended during charging.

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  • \$\begingroup\$ How to build that small circuit that takes both NTCs and select the correct one to forward to the charger? \$\endgroup\$ – Defozo Apr 14 '18 at 16:40

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