# Safely detecting lead-acid battey charger output?

How can I safely detect output of a car battery charger?

It has a OS/MP switch and for output it lists 12 V and 10.1 A. Unfortunately I have no idea what OS/MP means and I of course have no manual. The charger also has a single 5x20 glass slow fuse rated at 2 A and a 10 A standard household circuit breaker.

I'm hoping to use the charger as a power supply for a project. Either constant current or constant voltage would be OK, but I can't determine right now which one the charger is. It looks low-tech, so I doubt that it has both output types.

On the equipment front, I have two multimeters and I could easily obtain various 5 W and 11 W ceramic resistors. I have several 10 $\Omega$ resistors available right now.

The charger circuitry is usually not as extensively built as a power supply circuit. Things such as filtering, and well regulated circuits for both current and voltage. I don't know what you are trying to do with it, so it may satisfy your basic needs for devices such as motors, etc...

To test the charger you can connect an ammeter in series with the output, and monitor the voltage output with a dvm. Then apply different loads to the output while monitoring the current and voltage meters. If the voltage deviates a lot, it shows poor or no voltage regulation. You mentioned you have 10 ohm resistors, but you did not say what the power rating in watts were. Using ohms law I=E/R tells you that 12 volts/ 10 ohms = 1.2 amps. Them P = I*E tells us that 1.2*12 = 14.4 watts that the resistor must be rated at. Providing the resistors are 15 watts or greater, you could use these as a load test. Adding other 10 ohm resistors to the load will increase the current additive, so to achieve 10A you could get close with 8 resistors 8*1.2=9.6.

• Actually, the resistors are just 5 W. I've had success with tests previously by building resistor networks, so that load on each resistor is lower than 5 W. I don't need regulation for my idea, so the supply should work. I tested it as mentioned, but it seems that the supply isn't outputting anything at all. Could it be that it needs voltage form the battery to start working? Looks like I'll have to find the manual. – AndrejaKo Mar 24 '11 at 21:13
• I would not think it would need power from the battery. I've never seen that before. That would mean it could not charge a dead battery. The only reason I can think of no voltage is that you would have to give it a load, but you have already done that. Sounds as if there is a problem with the unit. Can you get into the unit? Start with checking the basics; switch, line cord, circuit breaker, etc... – SteveR Mar 24 '11 at 22:20
• OK. I'll start with the basics. – AndrejaKo Mar 24 '11 at 23:07