I got given a semi-professional battery charger, big unit with 2/40/60/200 amp settings and a timer, because it was supposedly faulty and out of warranty. It was a pretty expensive unit to start with so I'd like to figure out what's wrong. I did a simple test by hooking up a multimeter and got 10V on the 2 amp setting, nothing on 40 amps and 19V on 60/200 amp setting. For the readings I did get, does that mean it's working OK on those settings? Clearly the 40 amp isn't working at all, but the 2 amp is only putting out 10V.

I've never really understood how battery chargers work. Do they simply supply a voltage and check the current drawn to determine if the battery is charged or not? How do you recommend I go about testing this charger and fixing it if I can figure out what's wrong?

Thanks!

A battery charge works by supplying a higher voltage to the battery than the battery is currently at, causing current to go back in the battery. ok that was the simple explanation.

Now for the detailed one.


if your battery is at 12V, and you have a power supply (say a charger) hooked up to it where + goes to +, and - goes to -, and both the charger and battery are at the same voltage, no current moves. (remember you have to have a difference in voltage potentials for current to move.) Now if you bump the charger up to 13V (battery still at 12), then because the + on the charger is 1V higher than the battery, 1V worth of current will flow back into the battery (current is based on the internal resistance of the battery and charger along with the voltage differential). In this case (just say for the sack of example) total resistance of the battery and the charger is 2ohms. Using Ohm's law, 1V of voltage, through 2 ohms is .5Amps. So in this case you would charge the battery @ .5Amps. Now here's the tricky part. Because most batteries have pretty low internal resistance, the battery will actually pull down the voltage on the charger. Meaning it may read say 20VDC on the charger when it isnt connected to anything, but it may read 13VDC when it's actually charging a battery.

Now if the charger is at a lower voltage than the battery, current will be traveling from the battery to the charger, which is no good.

FIY: If you ever wondered, those 18V DeWalt speed chargers that can charge a battery in a matter of 30 min or so, those actually run around +50VDC when the battery isnt connected!

Thanks for the info!! Now I have a starting point for further examination.