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Edward, Consultant
Category: General
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Experience:  I am the #3 General expert on Just Answer.
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how do i reduce dc current in a simple circuit

Customer Question

How do I reduce the current from a 12 Volt car accessory socket down to 500mA in a simple circuit that could easily be constructed at home?
Submitted: 10 years ago.
Category: General
Expert:  Edward replied 10 years ago.

HiCustomer Thank you for asking your question on Just Answer. The other experts and I are working on your answer. By the way, it would help us to know:

-Could you explain your situation a little more?

-Could you tell me why you need to reduce the current to 500mA?

-What will you be using this circuit for?

Thank you again for trusting us with your problem. Please reply as soon as possible so that we can finish answering your question.

Customer: replied 10 years ago.
Reply to Edward's Post: I want to make a suitable cable to charge a 12 Volt Ni-Cad battery from a 12 Volt DC car supply.

The original battery charger is 240Volts AC - 12Volts DC @ 500mA.

I want to be able to charge this Ni-Cad battery from the car accessory socket, which will be 12 Volts DC at presumably a much greater current than 500mA, by connecting an accessory plug which will then go to the Ni-Cad battery (to be charged).

The manufacturer of this 12 Volt DC Ni-Cad battery recommends that the charging current is not more than 500mA @ 12 Volts DC!

At the end of the day I want to make a cable suitable for charging the 12 Volt DC Ni-Cad battery from the 12 Volt DC supply in the car.

I want a simple electronic circuit, showing how to reduce the original current of the 12 Volt DC car supply down to 12 Volts DC @ 500mA, that I can easily solder up into a SAFE charging cable for this battery.

i.e. I want to reduce the 12 Volt DC current of the car battery down to 12 Volts DC @ 500mA!
Expert:  Edward replied 10 years ago.

The amps applied to the battery would depend on the state of charge of the battery, and the voltage being applied.

A simple circuit would be a rheostat and a amp meter to detect the charge rate. Adjust the rheostat to limit the charge rate.

For a more automatic solution, fIrst you would need a amp meter and volt meter up to determine the voltage needed to charge the fully discharged battery at 500mA. Once this voltage is established, you will know the max voltage that can be applied to the battery, to limit it's charge rate to 500mA.

Since the voltage from a automotive battery will vary, you would need to regulated the automotive power source to the established voltage, to keep it from charging more that the 500mA.

Check out for several simple voltage regulator circuits.