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It is easy to charge a single cell or cells connected in parallel with a single TP4056 module.

This question shows why this can't be done with cells connected in series (short circuit of the cells).

What circuit or device can be used to charge two (or more) cells in series?

Context: an 8.4v toy robot charged by USB.

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  • \$\begingroup\$ Hello & welcome. I would not connect battery cells directly in parallel under any circumstances, particularly when the state of charge of each cell is not known. Reason: the cells with higher charge will dump high current into the cells with lower charge. This could damage either the cells, or the wires of the parallel connections, or both, and may cause a fire or explosion. Suggest re-phase your question. \$\endgroup\$ Commented Sep 8, 2023 at 3:58
  • \$\begingroup\$ @FabioBarone I can't see that Mark wrote a question about cells in parallell, but a device that charges cells connected in series. I could be wrong though. \$\endgroup\$
    – MiNiMe
    Commented Sep 8, 2023 at 4:52
  • \$\begingroup\$ @FabioBarone, thanks for the comment. I just mention the parallel mode in order to make clear that I am not asking that question, but about serial charging. Basically, I know many devices have serial cells inside and are charged from a single source, and I would like to know what kind of circuit is used for that. \$\endgroup\$ Commented Sep 8, 2023 at 5:49
  • \$\begingroup\$ @MiNiMe The first line of the post tells me that the OP (Mark) intended to connect a single cell or cells connected in parallel with a single charger, that charger being the TP4056. \$\endgroup\$ Commented Sep 8, 2023 at 5:49
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    \$\begingroup\$ @MarkHarrison Thanks for the clarification, please refer to my answer, cheers. \$\endgroup\$ Commented Sep 8, 2023 at 6:55

2 Answers 2

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Battery cell re-charge must be carefully managed, each cell must receive the correct current (amps) and total charge (couloumbs, or amp-hours) to prevent any damage to the cell. When a string of cells (multiple cells connected in series) is charged, they all receive the same charging current. If this current is limited to the current required by the cell that needs the least current, then this won't cause damage but it may prevent some cells from being completely charged. On the other hand, if the charging current is set by the cell that needs the most charge, then some cells may be over-charged and damaged - or worse, an explosion or fire can occur.

So it is clear that each cell must receive a current vs time profile that is individually tailored for its own requirements. In the case of a good-quality simple charger that is designed to charge a string of cells, this is usually done by providing a separate controllable current path in parallel with each cell, this path allows the current from the charger to bypass a particular cell and continue to the next cell. The diagram below gives the basic idea for a charger of 3 cells connected in series. Each cell has a bypass circuit of a resistor and switch, if the switch is open the cell receives the full current of the charger; if the switch is closed then most of the charger current bypasses the cell. The value of the resistor must be carefully selected such that when the switch is closed the cell does not discharge, its current must reduce to a small "trickle charge".

enter image description here

The switch can take many forms: it can be a relay, or a transistor (usually a MOSFET). Charge controller ICs are available that can control the switch, here is one example from Renesas (the ISL94202) that can charge anywhere from 3 to 8 cells:

https://www.renesas.com/us/en/products/power-power-management/battery-management/battery-front-end-ics/isl94202-standalone-3-8-cell-li-ion-battery-pack-monitor

Here is an excerpt from the datasheet that shows the Current Bypass implemented with resistors and MOSFETs ("CB" = Cell Balance")

enter image description here

Instead of a fixed-value resistor, more sophisticated controllers may have a means of modulating the current in each bypass path, in response to changing charger current and/or cell State of Charge (SoC).

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    \$\begingroup\$ Very informative, and the cell balancing is what I was going to try to learn about next! \$\endgroup\$ Commented Sep 8, 2023 at 23:32
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You bascially have two options to do that:

Use isolated supplies for each charging IC:
In the other question you linked to all the TP4056 modules used the same supply. This means that the outputs are not galvanically isolated. Connecting several batteries in series and trying to charge them in that configuration will result in a short (as you already realized).
Solution: Don't supply the TP modules (or in general, the charging ICs) with the same supply, but have one isolated supply for each charger. You can use DC/DC converters to generate multiple isolated supplies from one input voltage.
diagram showing three chargers each supplied via separate DC/DC converters from the main supply

Use a single charger that is capable of charging cells in series: The other option is to use a charger IC that generates a high enough output voltage to charge all the cells in series. Preferably this solution also needs some means of cell balancing, to accommodate for variations in cell capacity between the different cells.
3S charger connected to 3 cells with balancing connections in between the cells
See the answer from Fabio Barone for more details about ICs for this solution.

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