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I have been working on a small DIY project. Reference for this project is the following video from YouTube. DIY - 12V Mini UPS

While I was able to put together everything and it works for most part, I see one issue and need some help there.

The device works from the backup perspective. So, when I have power from the mains, the router works on mains but when power goes off, it works on the batteries from this device.

So, when I started, the batteries has full charge so I was getting 8.4 volts out of them. To check the working I turned off the main power. After sometime I checked and as the voltage had dropped to 7.5 volts I switched on the main power. While the router did not have any interruptions due to this, I noticed that the batteries are not getting charged even when I have main power available.

Question - Does this type of BMS charge the batteries? This is one of those cheaper BMS available online through a number of sites and from the description of it I had the impression that it charges the batteries as well. The actual link from where I bought this BMS is mentioned below. 2S 10A 18650 7.4V-8.4V Lithium Battery Protection Board

Any help will be appreciated.

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    \$\begingroup\$ The BMS is only a BMS and not a charger. I've not watched the video (a bad way to convey information to question answerers) but its not obvious how you are supplying charging energy. A circuit is extremely desirable \$\endgroup\$
    – Russell McMahon ♦
    Aug 27, 2020 at 11:45
  • \$\begingroup\$ Hi Russell, thanks or the comment. Apologies for providing the direct link to the video. I thought that it would be better than I am trying to explain it in text. To provide more information, the power supply is a AC to DC convert which is connected to the input terminals of the BMS. But, I understand from both you and Jay that I may be missing a charging circuit here. I will check this further to confirm and probably add the missing part. \$\endgroup\$
    –  Almoral
    Aug 27, 2020 at 12:30
  • \$\begingroup\$ A link to a video is fine, but supporting it with a description of the main points made or any circuitry is always a good idea. Most people will not spend the time watching videos as with so many questions it would take more time than question answering. (eg your alone is over 12 minutes). Even if applying the raw power supply to the batteries did charge them it would be an extremely bad idea as, while it is relatively easy to make a well behaved LiIon charger, it is even easier to make a fatally bad one. \$\endgroup\$
    – Russell McMahon ♦
    Aug 28, 2020 at 2:32

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In the linked video, the designer provides an external charger that match the cells he used (~8.4Vdc). This is the reason why it works with its own design.

The BMS he used is over-dimensioned and adding fuses in the B+/B- lines might mislead the voltage detection provided by the BMS (there is necessarily a small voltage drop across the fuses).

The BMS doesn't provide current regulation for charging (and this model just limit the current to a huge 10A, a 3A model will be more adequate). So you might check your own power supply adapter that could run in protection mode due to the inrush charging current.

As stated by other people, a charge management module is highly recommended (if not provided by the power supply adapter) - A TP5100 module could be used (with some adaptation to lower the charging current, the default 2A is way too much for a 24/7 application)

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  • \$\begingroup\$ Thanks @Alpha01 for additional points to consider on the charging circuit. After some looking around, I too arrived at the TP5100 module as a solution. I will try it along with the recommendations you have given. \$\endgroup\$
    –  Almoral
    Oct 6, 2020 at 11:43
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The BMS you linked doesn't look to include a battery charger. And that's true with most BMS circuits you find online. The purpose of this board is more to protect against Over Charge, Over Discharge, and Over Current. So for your project, you'll either need to buy some type of charger board that works with your UPS, or you'll have to design one.

Also, I would encourage you to either find a BMS that has balance charging for the cells, or use some type of battery management IC for balance charging. Cause I don't think the BMS you included has that feature either.

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  • \$\begingroup\$ Thanks for the reply. I will check this further and yes will check for balanced charging option for the cells. \$\endgroup\$
    –  Almoral
    Aug 26, 2020 at 19:06

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