I have a few Li-ion batteries which I want to use in series to have max voltage. But the problem is that the charger I have will not be able to charge the batteries in series, but will be able to charge them in parallel.

So how can I connect the batteries in a way that I will be able to charge them in parallel but use them in series? I'm new to these Li-ion batteries and i have seen the damage they can do I wrongly handled, hence I am not so eager to experiment with something that can blow up in my face, literally, hence the question here.


PS: Batteries are Samsung 18650 all are working


Your cells have a capacity of 2500mAh. If for example you have 3 cells in your battery (to make 11.1V) then the total capacity when connected in parallel would be 7.5Ah. You want to use a single cell charger which has a maximum charging current of 1A. This combination will take more than 7.5 hours to put a full charge into the battery.

A higher current charger could charge the battery quicker, but you still have the problem of how to switch from series to parallel. The usual answer is to charge in series with a high voltage charger, but that requires a balancer circuit to ensure that the cells maintain equal voltages.

If you must use TP4056 modules then I suggest one module per cell, each with its own isolated 5V supply (which can be a cheap USB power adapter). This allows you to keep the cells connected in series, but charge them in 'parallel'. As an alternative to separate power supplies you could use a single transformer with several output windings (each rectified and regulated to produce 5VDC). This technique is used in some commercial chargers, eg. Turnigy E3 Compact 2S/3S Lipo Charger.

  • \$\begingroup\$ I would do this in a future iteration as I only have one charging module and its not readily available here in my country, even the site I mentioned takes two to three days so I'll go with the Battery Bus for now and wait for at least 7 hours for full charge. \$\endgroup\$ – echo_salik Aug 29 '16 at 5:59
  • \$\begingroup\$ For the sake of 2-3 days wait you would go for a dangerous and unsatisfactory solution? My advice:- do it properly, or not at all. \$\endgroup\$ – Bruce Abbott Aug 29 '16 at 7:11
  • \$\begingroup\$ yeah, I agree but i need it today. You see we are having a little electric issue, so I need my router working for me to work hence the rush, I have ordered the charging circuit so I'll be getting it soon. But for now, I might use Gareth's answer and when those arrive then I'll shift to your idea which is no doubt safer. \$\endgroup\$ – echo_salik Aug 29 '16 at 9:56
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    \$\begingroup\$ Safest temporary solution is charge each cell individually. It will take the same total time, but you must switch the charger from one cell to the next as each one becomes charged. \$\endgroup\$ – Bruce Abbott Aug 29 '16 at 16:53

To hand make it yourself, here is the following circuit:

The left end of this picutre (or top if rotating) features what i call the "batter bus" which has the four batteries that will be used/charged depending on circuit switches. The middle section or "parallel charging circuit" allows it to be charged at 1.5V. the last section right/bottom. Allows all batteries to be used as series, aka "series load"

You can try it with or without PLC board (programmable logic controller), but to save energy and not make it wasted, I suggest a push button that allows it all to run.

The three main parts to a series load and a parallel charge

  • \$\begingroup\$ With the TP4056 modules that Bruce was mentioning about, you can have the parallel charge circuit part, and turn it into four of those modules, each with their own. the four samsung batteries can benefit from this. and the 1.5V, i misread that as the 1.5V lithum ion aaa batteries :facepalm: ... Still the same princple is applied. \$\endgroup\$ – Gareth Compton Aug 28 '16 at 20:04
  • \$\begingroup\$ can both switches be on at the same time? \$\endgroup\$ – echo_salik Aug 29 '16 at 5:56
  • \$\begingroup\$ BTW any specific type of Diode? or 4001 or similar diodes would work? \$\endgroup\$ – echo_salik Aug 29 '16 at 5:59
  • \$\begingroup\$ Diodes won't work with a Lithium charger. You need a multipole switch that connects all the cells directly in parallel. The switches must not be on at the same time! \$\endgroup\$ – Bruce Abbott Aug 29 '16 at 7:08
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    \$\begingroup\$ @echo_salik the cells are 4.2V when fully charged. The charger knows this and won't put out a higher voltage, so the battery won't get fully charged. It also won't charge at full rate until the voltage goes over 2.9V, but the diodes stop it from reading battery voltage so it will probably stay in 'trickle' charge mode (= very slow charging!). \$\endgroup\$ – Bruce Abbott Aug 29 '16 at 16:45

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