View Full Version : Battery chargers

01-22-2006, 11:07 PM
What is the correct way using a multimeter to check if a battery charger is working correctly. I had a friend fix two of mine and ill be darned if they seem to do anything but make the battery worse. Im amazed he screwed them up i thought they were a relatively simple device. Thanx

J Tiers
01-22-2006, 11:28 PM
Depends on the charger.

The stupid-simple types can be checked out easily by reading voltages.

The fancier types must have a battery on to work right, and then you need to know the details of how they work to fix them.

The "charge profile" varies by type of battery, so that is in the mix too.

What kind of charger, for what kind of battery?

For car battery? Cordless drill? Small NiCad? something else?

01-22-2006, 11:31 PM
I had a computer supply recently kick my ass.

I thought it was bad. Then I found out you can turn them on by inserting a resistor in two places on the plug that goes to the motherboard.

I remember when the computer switch was on the right back side. It turned the 120 vac off.

I got this one so hung up once with one of my "Poking around in it's guts Visual basic" program I had to unplug it. The button on the front had no effect. Hell I was just reading the video card/laser line and a typo had selectd the wrong base address.

01-23-2006, 12:39 AM
One way of doing it is to hook up the charger to the battery, then read the voltage on it, then turn on the charger. The voltage should rise to something more than the battery's fully charged voltage. In other words, if it's a 12 v battery, fully charged might be around 13.6 volts after a few minutes of being removed from the charger. If your charger doesn't get the voltage up to at least that point, it isn't going to fully charge the battery. If the measured voltage doesn't rise when the charger is turned on, the charger is either bad or it's responding to a battery with shorted cells. Some chargers need to see a minumum battery voltage before they will cut in. If, when you measure the battery's voltage bafore the charger is turned on and it doean't read at least 11 volts (for a vehicle battery), then this could be what's happening.

One thing this won't tell you is if the battery is bad, open-circuit wise. Normally there's a meter on the charger that reads current. There should be some flowing. If the voltage reads high enough, but no current flows, the battery needs attention. It may be shot or need water.

01-23-2006, 12:40 AM
"I thought it was bad. Then I found out you can turn them on by inserting a resistor in two places on the plug that goes to the motherboard."


Just ground the green wire (to black) to turn on an ATX supply.

"The button on the front had no effect"

Press and hold it for five seconds. It will turn off.

[This message has been edited by Evan (edited 01-23-2006).]

01-23-2006, 12:51 AM
Hi Madman,

One thing to watch out for on really simple chargers: They don't put out DC, just rectified AC. So you have a pulsed waveform (actually a half sine-wave) going to the battery, and that will screw up the readings from any meter that thinks it's looking at real DC.

Leigh W3NLB