Simple battery charger

Hi everyone,

to charge the 12V - 1,2Ah lead acid battery I found this scheme very simple and unpretentious ... what do you think?



with the V1 trimmer I adjusted the charge voltage to 13.7V as indicated on the battery ....

3 questions:

- does the battery have to be disconnected when I adjust the charge voltage?

- the L2 led lights up when the battery is charged or discharged?

- is a 15V power supply for LM317 okay?

thank you.

I would check this link. https://circuitdigest.com/calculators/lm317-resistor-voltage-calculator It's just a LM317 calculator.
I think 10k potentiometer is a little much. Max for the potentiometer should be 2400 ohms to get 13.75 volts.
Or just replace R2 with 1K ohm and it should be fine.

I don't think it matters either way to have the battery disconnected or not, might help to be on the safe side though and have a Volt or Amp meter alongside this circuit.

The zener's breakdown is rated for 11 volts that's when the Led will come on.

I believe 15 should be fine, what is the amperage?

There are better circuits out there, but this one will get the job done if the values are correct.

Add a heatsink to the Lm317.

billybob said:

Or just replace R2 with 1K ohm and it should be fine.

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No. The value of R2 must be low enough for the LM317 minimum load current which is a max of 10mA. Therefore 1.25V/10mA= 125 ohms, use 120 ohms. The datasheet shows a 240 ohms resistor for the more expensive LM117 that has a minimum load current of 5mA.

I don't think it matters either way to have the battery disconnected or not, might help to be on the safe side though and have a Volt or Amp meter alongside this circuit.

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A discharged battery will probably reduce the voltage of this circuit but the circuit has an output diode that does not conduct much without a load.

The zener's breakdown is rated for 11 volts that's when the Led will come on.

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No. The 2V LED is in seroes with the 11v zener so the LED will begin to glow at 13.0V.

I believe 15 should be fine.

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No. The datasheet for the LM317 says its input must be at least 2V higher than its output. The battery label said charge at 13.7V and there is a diode and resistor at the charger output so the output of the LM317 is 15V then its minimum input must be 17V.

thanks for the answers ...
but I have a doubt that the zenner diode D2 must not be connected after the diode D1?

tonytnnelectrotech said:

thanks for the answers ...
but I have a doubt that the zenner diode D2 must not be connected after the diode D1?

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That would discharge the battery if the power was unplugged.