I have 3 of 18650 lithium battery cells and one heat element and one TP4056 charging module. I want to connect the cells in parallel to the charging module to charge them and at the same time I want to connect them in series to power the heat element with 11.1 V. how can I do that? and what are the alternatives if that's not possible?


You can't connect them at the same time for charging in parallel and discharging in series.

You can connect them with relays, to either charge in parallel, or discharge in series.

But it's generally just not worth it. The cost and power consumption of relays would be better spent on simply being able to balance charge a series set of cells.

  • \$\begingroup\$ can you please explain how to balance charge a series set of cells? do you mean using a 12 V charging unit? if so, any recommended module? \$\endgroup\$ – user3926604 Mar 11 at 14:32
  • \$\begingroup\$ @user3926604 you buy a charger that has a built-in balancer. I've got a Turnigy Accucell, but other makes are available. \$\endgroup\$ – Neil_UK Mar 11 at 16:59
  • \$\begingroup\$ I need a small module that I can use in a small DIY project if there is any \$\endgroup\$ – user3926604 Mar 12 at 8:46

It's possible, but not usual.

If there was some extremely persuasive reason to do so it could be achieved with 6 SPST switches. These could be relays or FETs .
Or 3 x SPDT switches / relays.
Or a single 3PDT switch or relay.

Break before make is essential if switches or relays are used.

One "persuasive reason" may be the ability to use a 5V supply for charging with no boost converter :-).

Early AM hours here, work & sleep calls so no diagrams for now BUT you should be able to work it out from the above.

Here is a word picture for an FET based arrangement.

Cells A B C in series line A at top, NOT connected.
A+ to V+
C- to ground. Series connect FETs A- to B+, B- to C+

Parallel connect FETs A- to ground, B- to ground, B+ to V+, A+ to V+.

Series to parallel switch: Series FETS off, THEN Parallel FETs on.
Parallel to series switch: Parallel FETS off, THEN Series FETs on.
Do it wrong = magic smoke.



3 cells can be connected to V+/ground in series or parallel

Initial state : Series connection
All switches SSx closed. All switches SPx open.

Change from serial to parallel:
Open all SSx switches then Close all SPx switches

Change from parallel to serial Open all SPx switches then Close all SSx switches

Changing switches in the wrong sequence will lead to magic smoke being released.
All SS switches may be ganged together. All SP switches may be ganged together.

IF "break before make" can be guaranteed a single 4PDT switch could be used.


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ I'm not a smoker so... I'll wait for diagram :) \$\endgroup\$ – user3926604 Mar 11 at 14:33
  • \$\begingroup\$ @user3926604 Up briefly. Draw the arrangement that I gave you above. It's easy if you simply follow the text and place a line with an x between the points I noted as switches. If you can't do that you may have trouble building it. Not being a smoker is commendable :-). \$\endgroup\$ – Russell McMahon Mar 11 at 18:10

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