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Constant current battery charger


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I have a doubt.

If I charge a 12 volt battery with a constant current regulator (120mA) using lm317,  The CC regulator is powered from a 15 volt 2 Amp power supply. The battery is a SMF 12v/1.2Ah type.

will the current reduce if the voltage in the battery increases ? I have attached a diagram for clarity on the configuration.


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The battery voltage will rise to about 14V when fully charged but the LM317 current regulator needs a voltage across the LM317 of at least 2V plus 1.25V for the current-setting resistor so you need an input of at least 17.25V.

Since your input voltage is too low then your battery will never be fully charged and might not have any charging.

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Your very simple circuit is simply a constant current source, not a battery charger.
A battery charger circuit will limit the voltage to whatever is required.

Another LM317 can be added after the constant current LM317 to limit the voltage but it also needs an additional minimum voltage to work.

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Hello Pier.
Charging a battery can be tricky, but as a rule of thumb standard charge of a battery is achieved with a constant current of a 10% of battery full capacity to its +20% of nominal voltage. Example: a 12V battery of 36A is charged with 3.6A until it reaches 14.4V.
So, you will need basically two things: a current limiter (your 317, by the way it could be even a light bulb) to set a constant current, and a voltage comparator to stop the charge process when the battery has reached its nominal voltage plus 20%.


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