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I have a 3 X 18650 battery set with bms. I would like this to be charged using a 12 volt solar panel which outputs anywhere from 15 to 20 volts. the BMS has a max voltage rating of 12.8 volts.

I was wondering if it would be possible to use a voltage regulator like 7812 to regulate the output of the solar panel to 12 volts so that it could charge the batteries?

I do not want to use a solar charge controller if it can be avoided.

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    \$\begingroup\$ Data sheet for the BMS? Charging current? A buck regulator will be better and one that can handle low drop-out too. Solar panel data sheet too? \$\endgroup\$ – Andy aka Nov 20 '20 at 10:38
  • \$\begingroup\$ Hi Andy. Bms is 5A rated. Solar panel is a small 10 watt panel. Assume max 1 A output is what you'll get. I don't have datasheet unfortunately \$\endgroup\$ – Ouroboros Nov 20 '20 at 11:01
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No. Lithium batteries need to be charged very delicately with a lithium battery charging device. A 7812 is simply a voltage regulator and not a battery charger. The BMS is also not a charger.

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  • \$\begingroup\$ What will happen if 7812 is used? I want to understand why it can't be used. When you say delicately, what is it that 7812 can't accomplish. \$\endgroup\$ – Ouroboros Nov 20 '20 at 10:59
  • \$\begingroup\$ A 7812 does not provide constant current / constant voltage profile needed to charge lithium batteries. It won't also stop charging when batteries are full. The output voltage tolerance is also 0.5V, while voltage tolerances must be 0.05V when charging lithium cells. Connecting empty 12V battery pack to 7812 output would short circuit the regulator output, it would provide as much current as it possibly can (around 2A) until it overheats (dissipating more than 10 watts) and shuts down, or just burns up in smoke. \$\endgroup\$ – Justme Nov 20 '20 at 11:13
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Things you should be aware of:

  1. power dissipation in the regulator you mentioned. for example if the solar panels are generating 20V and can charge the battery at 1A then the power dissipated will be (20-12)*1A=8Watts which will probably overheat the regulator and cause it to go into thermal shutdown.
  2. you should make sure you block any reverse current flowing into the regulator and solar panels. you may want to add a small schottky diode in series.
  3. Efficiency of the regulator. In the above example you will be wasting 8W of power for 12W of charging power. That is you lost 40% of the charging power of the solar panel. Another efficiency issue is that you will not be operating at the "maximum power point" of the solar panel. I don't believe this thing is an issue for you considering that the solar panel is somewhat small.
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