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I am using two Lithium ion tool batteries (18V) to get a 36 V line. When fully charged it is greater then 40V. As they are tool batteries, undervoltage protection is already built in.

I want to be able to charge these batteries with a very high voltage solar panel (>50V). I am not interested in efficiency. I am only worried about overvoltage protection for the batteries.

Is there a cheap way to have charge protection with this system. The application is audio, so violent audio band waveforms (< 24 kHz) is not acceptable on the power lines.

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  • \$\begingroup\$ Lion want to be charged with a constant current, then topped off with constant voltage. Solar power varies a lot. I'd look into a solar charge controller. \$\endgroup\$ – Aaron May 5 '20 at 23:47
  • \$\begingroup\$ How much current are we talking? \$\endgroup\$ – Peter Green May 6 '20 at 0:27
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    \$\begingroup\$ You'll need overvoltage protection for each battery. \$\endgroup\$ – DrMoishe Pippik May 6 '20 at 1:25
  • \$\begingroup\$ Not just each battery, but each cell in the battery. This is not a simple project at all and the difference between your starting point today and what you would need to even think about doing it safely is far, far beyond the scope of a question and answer here. Probably a modern solution will involve switching conversion and sufficient filtering. If you are adamant about avoiding that you may need to look at a lead acid battery bank and some fairly wasteful linear charging solutions... \$\endgroup\$ – Chris Stratton May 6 '20 at 4:24
  • \$\begingroup\$ 18/3.6V = 5S cells so you propose 10S = 36V nominal 42V max. Ideally use an off the shelf solution for charging - either a commercial product or a charge IC.|| The batteries will usually have had cell balancing - either withing the battery pack or the charger. I have seen LiIon batteries (Dyson vacuum!) with NO specific balancing and they failed after a while entirely as you'd expect. || I'd recommend 4.0V full charge per cell. Longer cycle life and slightly more forgiving of unbalanced charging. ... \$\endgroup\$ – Russell McMahon May 6 '20 at 8:09
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18/3.6V = 5S cells so you propose 10S = 36V nominal 42V max.

Ideally use an off the shelf solution for charging - either a commercial product or a charger IC.

The batteries will usually have had cell balancing - either within the battery pack or the charger. I have seen (now own) LiIon batteries (Dyson vacuum!) with NO specific balancing and they failed after a while entirely as you'd expect.

I'd recommend 4.0V full charge per cell. Longer cycle life and slightly more forgiving of unbalanced charging. Somewhat per cycle reduction in capacit. | You can make per cell dissipative top balancers with a TL431 and a few resistors per cell or add a transistor for as much balance load as you wish.

You can buy Chinese N cell balance boards at OK prices and that is liable to be the easiest and probably cheapest path.

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  • \$\begingroup\$ This makes a lot of sense thanks for the information. The Charge line and discharge lines are different, which makes me wonder what is this difference ? I think I will have to open it up and look. \$\endgroup\$ – Matt May 6 '20 at 21:44

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