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I'm looking for a good and space-saving solution to charge two 18650 batteries in series.

The project is a small boombox, using an amplifier board that works with voltages between 4.5V to 25V. The battery will be fixed inside and might be rechargeable from a 5V wall charger. I already have a few projects working with one battery and the TP4056, but I've been reading that this will not work for series charging.

Does anyone have a solution?

Be lenient, this is my first post. Thanks in advance.

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    \$\begingroup\$ Hi Kiniko, welcome to EE.se. Lithium batteries cannot be charged in series. But you could bring out all three wires and use two chargers - one for each cell. They will need galvanic isolation (not grounded to each other) for that to work. \$\endgroup\$ – rdtsc Nov 20 '18 at 22:50
  • \$\begingroup\$ thx, i see your idea, but for a portable speaker, it look a complicated approach at this point. \$\endgroup\$ – Kiniko Dec 4 '18 at 23:01
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You'll need a module designed to charge your two batteries in series (commonly referred to as a 2S arrangement). Fortunately, these modules are available at low cost. Here is one example (ebay has many more): https://www.mouser.com/new/dfrobot/dfrobot-dfr0564-usb-charger/

Note that you will need very similar cells, with the same capacity and health (ideally from the same manufacturing batch), to make sure the cells share the voltage fairly evenly. In addition, you might consider a battery management circuit (BMS) to ensure they stay balanced. You'll also need a circuit to avoid over-discharge, which can lead to a dangerous condition (vent with flame) next time you try to charge it. Most BMSs will include this functionality as well, or you can build it into your boombox.

It's also worth mentioning that three batteries in series (3S) is a somewhat more commonly-used arrangement, and it's often easier to find chargers and protection circuits for this configuration.

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  • \$\begingroup\$ thx nate , i start getting the idea behind. i think this could be a good option indeed. i'm gonna dig a lil further with the way of Rednez using 2x tp4056 with Two power isolator. This concept look more safe for me if i'm gatting the idea right. Thx a lot. i totally skip all yours answers here... :/ \$\endgroup\$ – Kiniko Dec 4 '18 at 23:06
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You can use 2 of the TP4056 with a 5V power isolator like the B0505S at the input voltage of the board or use the TP5100 which can charge 2S battery at 8.4v but not balanced so the batteries should be the same or use a 2S BMS for balanced charging, battery protection and other features

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  • \$\begingroup\$ thx for this, would it be a good vue of the concept? youtube.com/watch?v=rpRYNMrVCk0 ? \$\endgroup\$ – Kiniko Dec 4 '18 at 23:10
  • \$\begingroup\$ yes, someting like this but i think the TP5100 or even better a 2S BMS is the best choice \$\endgroup\$ – RedNez Dec 6 '18 at 23:39
  • \$\begingroup\$ i'v been checking the BMS but i'm still confuse with the voltage requirement. considering i'm gonna a use 2x18650 in serie providing +/- 7.4 v how can use a 5v wall power.. Most of the video i'v seen required between 8 /12v... i'm a bit lost now. \$\endgroup\$ – Kiniko Dec 8 '18 at 21:02
  • \$\begingroup\$ in that case the TP5100 doesn't work, it needs 10-15v for 2S charging, i think the best solution for 5V charging is the 2*TP4056 with the isolators but i haven't tried that yet so i can' guarantee anything also those isolators are only 1W so the batteries will charge very slowly \$\endgroup\$ – RedNez Dec 11 '18 at 4:22
  • \$\begingroup\$ the isolator also exist in 2W actually. would this help? bit.ly/2QyOsaj \$\endgroup\$ – Kiniko Dec 17 '18 at 6:28

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