How verify charging current flowing to battery?

Randy Webb · 09-09-2004, 05:29 PM

How can I verify that the alternator hookup (which is all changed around from what it was before my engine swap/ ext. VR changed to int. VR) is sending current to the charge the battery?

The alt. light works (lights up when the key is turned on w/o engine running). And, when the engine is running the light is not on. But could that mean that some wire somewhere is not connected right, or does it necessarily mean that things are OK?

The battery has only about 11.8 volts, measured in a no load condition.

Thanks!

Joe Bob · Joe Bob's Garage

Volt meter....a fully charged battery at rest should read about 12.5volts, 13+ at idle and 13.5-14 at 2500 rpms.

falcon · falcon's Garage

I had a battery at rest measuring 12V. Found that the alternator needed a rebuild. After reinstalling I get pretty much what mike posted

Randy Webb · 09-09-2004, 08:01 PM

Thx - but that isn't really my question. Checking the battery is something I can do after driving it for a few hours - but it isn't driveable and I don't know the alt. is connected up right. And running it for a while with the alt. looking at an open circuit might damage it.

I want to see if there is any way to measure the alt. output or something instantaneously to see if it's working right??

911pcars · 09-09-2004, 09:00 PM

Not many of us have a high capacity ammeter to measure alternator output. That's why reading voltage output at the battery is usually sufficient to verify the charging rate is okay.

There's no way to verify charging output without running the engine. No voltage, no current. If the alternator wiring isn't correct, there usually won't be any output to measure.

The charge light should be ON with key ON (Engine OFF) and should go out with the engine ON. If the voltmeter reads 11.8 volts (connected to battery), then something is wrong. Do you have a wiring diagram for the charging system you have?

Sherwood

john70t · 09-10-2004, 05:21 AM

Randy, 11.8V is way too low for any battery, especially when the car is running.
I've been told that 12.2V is around 50% charged, and regular lead/acid batterys only can be fully discharged a few times max for a short time before going cuput.

The reason for the higher battery voltage while running is that the recharging current/amps indicates a flow of electricity from a higher (created by the alternator) to a lower voltage state (in the battery).
If there's voltage, there's current flow-unless the thick wire is connected by a single strand, but that resistance would quickly generate heat/fire.

What's probably happening now is that the battery is deader than a doornail, and the alternator is full-fielding trying to bring it back to the voltage level set by the regulator. Bad way to lose the alternator as well.

Harbor Freight and the like have cheap ampmeter attachments for the DVOM.