How doyou test alternator current output ??

[Boothy]

I usually test an alternator by checking voltage of the battery before starting and watching the voltage rise upon starting to somewhere around 13.8 ish, also making sure that the exite terminal is also powered up 12 volts or so, before cranking to exite the field coils.

Now when its started and producing 13.8 ish volts thats usually been enoiugh to satisfy me that the alternator is charging the batteries, but this Sunday whilst on the Howlin Wolf challenge the winch batteries went flat, these are 2 big yellow top optima's in parrallel fed of a seperate alternator to the vehicle wiring, purely so they don't affect anything to do with running of the engine etc.

The batteries are parrallelled in 75mm copper and the cable supply from the alternator is multi stranded 25mm copper, bolted to the alternator terminal, all crimps have been properly crimped by a Hypress tool with the correct dyes fitted and are clean and secure, the alternator also has a seperate earth run to the chassis and to the alternator.

Problem was there was 14 volts being indicated coming out of the alternator but the batteries were hardly charging, it a new reconditioned 100A alternator and was superb last round of the H/Wolf all day, the way round it for the day was to link temporary the vehicle alternator across to the winch alternator cable and let it charge that way, which it did.

Problem being that with electric waterpump, 4 electric fans, fuel pump and Megasquirt, etc there wasn't much spare to charge the extra batteries.

Question is how do I tell what is the current output from a 100A alternator without lots of extra wiring and gauges and what would prevent an alternator from giving its maximum current output to the batteries, I know there is a simple answer of carrying as pare dual handed alty but how to check it full possible output is being produced is puzzleing me, also if it is 100A output is that constant to a full load and could that be measure by using a clamp on ammeter which I've got but am not sure what I should be measuring. By the way the red charge light was being extinguished.

Technical help please boys.

[stuck]

I have a D.C. clip on ammeter you could borrow, guess you could leave it under the bonnet on "peak hold" for a quick check (as long as you don't burn or drown it!)

You say you already have one but are you sure it will do DC as most wont.

Other than that I could design you a permanent install if you like...

HTH's

Mick.

[yella 90]

i always thought that 14.4v was what a good alternator should pump out

[Boothy]

I've got some clamp on ones we use at work thanks Mick, the problemis I don't want to get caught out again with a duffer, you put a recon on and expect it to work as it should, put your meter on and see 14 volts so your off elsewhere testing looking for a fault, and after an hour your back to the alternator, in the middle of a competition this is, and now you've also got flat winch batteries as well.

See the problem, thanks very much for the offer.

Chris

[RangeyRover]

I've got some clamp on ones we use at work thanks Mick, the problemis I don't want to get caught out again with a duffer, you put a recon on and expect it to work as it should, put your meter on and see 14 volts so your off elsewhere testing looking for a fault, and after an hour your back to the alternator, in the middle of a competition this is, and now you've also got flat winch batteries as well.

See the problem, thanks very much for the offer.

Chris

Chris,

Create a drain on the battery pack by winching against a load. Use the clamp ammeter to measure more than 100A on the winch cable supply. Then change to the output of the alternator, while still winching. It should be delivering its max available supply into the batteries. I KNOW you have monster cables on, but check for voltage drop between output of alternator and arrival at batteries. I2R is the loss formula, the more current flow, the more voltage drop. Basics first.

Give me a bell if you like

Mark