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# 24V dc to 12V dc battery charger

#### energuy

Apr 5, 2016
7
Hi all,

I am in need of some help with my battery charger circuit.
I am using a 24Vdc permanent magnet motor as a power source, and will be charging a 12V, 100Ah battery.
My main question is what i can do to increase or decrease the charging current in this circuit?
Also I would like to add another lm338 in parallel to allow for a max charging current of 10A. Can i just connect all three pins ? or do i need a resistor placed in there as well.
Let me know if you need any more info!
Thanks

#### Attachments

• Schematic.pdf
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#### Gryd3

Jun 25, 2014
4,098
Hi all,

I am in need of some help with my battery charger circuit.
I am using a 24Vdc permanent magnet motor as a power source, and will be charging a 12V, 100Ah battery.
My main question is what i can do to increase or decrease the charging current in this circuit?
Also I would like to add another lm338 in parallel to allow for a max charging current of 10A. Can i just connect all three pins ? or do i need a resistor placed in there as well.
Let me know if you need any more info!
Thanks
To increase or decrease charge current, you need to vary your output voltage.
Adding more than one supply in parallel usually requires that:
A) They have identical output characteristics.
B) They have a low value resistor in series with the output to help in cases where the outputs may be slightly off.

If they are NOT matched properly, you will end up forcing current backward through the weaker supply. You can use diodes, but will suffer from a 0.7V loss in the supply voltage.

#### energuy

Apr 5, 2016
7
To increase or decrease charge current, you need to vary your output voltage.
Adding more than one supply in parallel usually requires that:
A) They have identical output characteristics.
B) They have a low value resistor in series with the output to help in cases where the outputs may be slightly off.

If they are NOT matched properly, you will end up forcing current backward through the weaker supply. You can use diodes, but will suffer from a 0.7V loss in the supply voltage.

What i meant was not to add another supply, but add another voltage regulator to split the supply current (half to one regulator, half to the other). I have added a diode to prevent discharge of the battery as well, thanks.

#### Gryd3

Jun 25, 2014
4,098
What i meant was not to add another supply, but add another voltage regulator to split the supply current (half to one regulator, half to the other). I have added a diode to prevent discharge of the battery as well, thanks.
Same difference. You are using two regulators/supplies.
If they don't regulate exactly the same way, one regulator may over-power the other.
You need to balance the outputs, usually by using a resistor as mentioned above AND carefully using matched parts.

#### Colin Mitchell

Aug 31, 2014
1,416
You need to know the current capability of the motor you are using as the source.
The first thing to do is connect the motor to the battery via a diode and 0.47 ohm 10 watt wire wound resistor and measure the voltage across the resistor.

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