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I am currently designing a 12v 10000mah (really 11.1v nominal) lipo battery pack that I am going to use for camping. This will allow me to run 12v appliances from it like fans and car chargers when i'm on the go, something I couldn't do with just a standard 5v charger.

The issue I am coming across now is how do I charge the thing? I know I can charge the battery using a CV/CC fixed 12.6v output, but how am I going to balance the cells. Now, I know the way the hobby chargers work is they attach resistors on each series cell that simply "bleeds" the individual cell down when it reaches 4.2v. I want to design a circuit similar to this.

Here's what i'm thinking of doing. Feeding the battery with a CV/CC 12.6v source then have three individual seperate circuits that I can attach to each individual series cell that will simply bleed the cell down until it reaches 4.2v.

This drawing is absolutely garbage, but it should help to get the point across of what I am trying to do: enter image description here I'm thinking of using a comparator that is powered by the cell to maintain isolation that will just switch on a bunch of resistors when the cell reaches 4.2v, does anyone have a suggestion on a way to do this?

Now, you might be wondering why I am trying to build a discrete circuit for this when there are tons of 3S lipo chargers available on ebay and amazon for very cheap. The reason I am doing this is for the learning experience. The other reason I am doing this is because I can build a charger that can actually charge the cells at 4-5 amps. I could use a bigger charger like an imax b6 to charge the pack at 5 amps, but I don't want to carry around a dedicated charger just to charge some lipo cells . I want to just plug in a 12v DC jack and be on my way, and have the charging circuit be built into the device. so no, I am absolutely certain I want to DIY this.

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The easiest way is to have a shunt regulator such as a TL431B across each cell that bleeds extra voltage away. Make sure to use 0.5% resistors or better to match the required tolerance. The datasheet gives a schematic for use in high-current situations which may be required in this application.

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  • \$\begingroup\$ The datasheet gives a typical application circuit like this for a high current shunt regulator. Would something like that work? \$\endgroup\$ – Dylan 873 Sep 28 '16 at 13:26
  • \$\begingroup\$ If you mean the one labeled "High-Current Shunt Regulator", that is indeed the one I was talking about. \$\endgroup\$ – Ignacio Vazquez-Abrams Sep 28 '16 at 13:32

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