# Homework Help: Design a circuit to recharge a battery

1. Sep 29, 2011

### VitaX

1. The problem statement, all variables and given/known data

Design a circuit to recharge a 1.2-V Ni-Cd battery from a car battery with a current of approximately 10 mA.

2. Relevant equations

3. The attempt at a solution

I was looking in the back of the book on this one and it had a simple circuit devised with a 12-V battery and 1.2-V battery and a resistor of 1.1 k-Ohms. I know a loop equation is needed here, but shouldn't there be at least one more known in the problem? I don't understand how to find the voltage of the car battery as well as the resistance of the resistor, but if the voltage of the car battery is known, it becomes pretty simple to find the resistance of the resistor within the circuit. How should I go about approaching this problem?

2. Sep 29, 2011

### Staff: Mentor

Modern car batteries are generally taken to be 12V. There was a time when 6V batteries were common, too, and even today some trucks have 24V batteries. But today it's almost always 12V for a car battery.

Note that a car battery in a running car is being recharged by the alternator, and as a result can read something like 13.7V or so. But you shouldn't have to worry about this here -- no mention was made of a car in the problem, just the car battery.

If bonus points were available, you might add some voltage regulation after the car battery to ensure a constant voltage for the charging circuit

3. Sep 29, 2011

### VitaX

Wow I feel dumb now. I get 1080 Ohms, while the book gets 1.1 k-Ohms, guess they rounded but easy problem. Thanks for that.