24 to 12Vdc battery charger circuit

Thread Starter


Joined Apr 10, 2016
Hi all,
I am using the attached circuit as a 12V deep cycle SLA battery charger. My input will be a 24V DC permanent magnet motor being powered by a bicycle.
I have added a diode(with a voltage drop of 1V) at the positive output terminal to prevent the battery from unnecessarily discharging. I was hoping someone could help me change the resistor network in order to change Vout to accommodate the voltage drop of the diode? (around 13.5-14V plus 1V drop)

Any other suggestions would be appreciated!


Thread Starter


Joined Apr 10, 2016
do you mind explaining how this voltage is calculated? i understand the operation of the circuit for the most part, but not how to calc Vout.


Joined Sep 17, 2013
One or two changes perhaps :-
Swap the inputs to pins 2 and 3 of the opamp.
Reduce the 15k to 3k3.
Lose the diode (optional) and the 'start' switch.
Last edited:


Joined Nov 4, 2008
I think because you are using a bicycle to run the generator, the voltage may be 24V or less depending on the shaft speed of the bicycle, and the current may be fairly low. I guess it may reach 2A. All you will need is a couple of resistors to set the charge voltage for the battery.


Joined May 19, 2014

Are you really going to have a person "pedal" charge a deep cycle battery? If you are going to have a person pushing the pedals you do not need an electronic regulator, because you will already have a built in control system; the person pushing the pedals. All you need to do is put two display in front of the "peddler", battery voltage and current.

Then simply tell the peddler to:
1. keep the current at X number of amps until the voltage reaches Y number of volts (bulk charge).
2. hold the voltage at Y volts until the current drops to below X amps (absorption charge).

Be aware that this should take about 6 hours.

There you have it, an intelligent battery charger. An electronic linear regulator will waste energy. The peddler will produce the correct amount of energy all the time, no waste. Cool!

This is not a joke. Think about it.

Oh yea, Welcome to the forum.