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.
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 http://ourworld.compuserve.com/homepages/Bill_Bowden/page12.htm for several simple voltage regulator circuits.