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Hello StackExchange community. So i have been for a long time wanting to build someything portable and recharchable with the 18650 batteries. the only problem is i cant find any good guide on how to actually charge them in series.

The charger im looking at right now (LINK 1) migth be what i need but i cant find someone using it. It shows how to connect the batteries in series but i dont know how i am supposed to charge it. i have a external 5v source i can plug into the input but i am unsure how to get the 3,7*4 volts after they are charged

LINK 1: https://www.ebay.com/itm/4S-20A-Li-ion-Lithium-Battery-18650-Charger-BMS-Protection-PCB-Board-12-6V-Cell/332212250493?epid=2176455939&hash=item4d5964a77d:g:7VYAAOSwfRdZENaz

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closed as off-topic by Olin Lathrop, PeterJ, Dmitry Grigoryev, Elliot Alderson, Maple Sep 29 '18 at 22:40

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – PeterJ, Elliot Alderson, Maple
If this question can be reworded to fit the rules in the help center, please edit the question.

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    \$\begingroup\$ -1 for the contempt exhibited towards the volunteers here with the sloppy writing. Closing since I'm not going to read past the first paragraph. \$\endgroup\$ – Olin Lathrop Sep 17 '18 at 12:38
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    \$\begingroup\$ Please don't link to ebay, instead provide a link to the actual manufacturer's data sheet. If you can't find the data sheet don't buy the product. \$\endgroup\$ – Elliot Alderson Sep 17 '18 at 14:40
  • \$\begingroup\$ @ElliotAlderson, Sorry for linking the ebay link, but i actually was not able to find the manufacturer's data sheet anywhere. \$\endgroup\$ – Niklas Jensen Sep 20 '18 at 12:13
  • \$\begingroup\$ Most of the cheap protection boards have "charger" in the listing title even though they have nothing to do with charging except basic over-voltage protection. Yours is one of those. \$\endgroup\$ – Maple Sep 29 '18 at 22:40
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As Marko says you have to charge the cells in series with a CC/CV charger. This charger first pumps CC (constant current) into the battery until its voltage reaches 4.2V per cell, then it switches to CV (constant voltage) to finish topping up the charge. End of charge is detected when the current drawn by the pack goes lower than a threshold. You'll also need a safety timer.

You need to ensure the batteries' specifications are met: maximum charging current they can take, end of charge voltage, acceptable temperature range for charging, etc. Read the docs.

And, also very important, the cells need to be balanced. One of the cells will inevitably have lower capacity than the others, so when charging it will reach its max voltage first. At this point, the other cells are not fully charged, but letting the CC phase go on will result in overcharging the lowest capacity cell, which can shorten its life or start a fire. Thus, you need a balancing circuit, which will detect and prevent this. This is usually done by placing a shunt element across each cell, in order to shunt away the full charging current from the cell that has already reached maximum voltage, while still charging the other cells.

There is no way to know if the BMS you link implements this feature, so I cannot tell if it will do what you want. It does seem to monitor the voltage on each cell, but this may only be to prevent over-discharge of the lowest capacity cell.

Since Li batteries are expensive and can be dangerous, buying a $2 BMS without docs is maybe not the best idea. I'd recommend paying more for something that has documentation so you know it'll be safe.

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    \$\begingroup\$ The ad for the charger does specify a max current of 20A☺ That could be exciting. But I like blowing things up. \$\endgroup\$ – Misunderstood Sep 17 '18 at 21:22
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You need to charge them with constant current constant voltage charger CC/CV. The cahrger has to have a current limiter to limit the charging, once the voltage on cells is rising, then a voltage limit is applied.

Therfore you have to charge them with 4x cell charge voltage, which is 4.2V, so 16.8V. It is important to match the voltage source and cells, as well to use the correct BMS you have linked.

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  • \$\begingroup\$ I think you meant to say "... once the voltage on cells has risen to the maximum charging voltage then the charger switches to a constant voltage". \$\endgroup\$ – Elliot Alderson Sep 17 '18 at 14:39
  • \$\begingroup\$ @ElliotAlderson Exactly. It's much easier for someone who speaks english since is born to make such poetical words.. \$\endgroup\$ – Marko Buršič Sep 18 '18 at 12:52

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