How to Load test an alternator ?

[RJsfishin]

Fas idle....1200 rpm 14,4 volts 100 amp alternator

HF 500 amp carbon pile load tester connected to battery.

Using logical thinking only,........
If I gradually load the system, at what amperage should should the voltage start to drop dramatically ?

In other words, should it hold the +14 volts at an 80 amp load ?
And that would indicate a good alternator ?

Any tech help on this apprecieated

[westend]

pnichols wrote:
This is just a guess - but I'll bet most modern vehicle alternators are protected from being buned out from any excessive load(s) one may attempt to put on them.

We use our Ford 130 amp alternator to charge up the engine battery plus two deep cycle coach batteries as needed, all together at the same time if necessary, at any beginning state of charge of any of the three batteries, and at any engine speed including idling. While at engine idle, I've seen our dash mounted coach battery amp meter show around 50 amps being supplied by the alternator to just the coach batteries alone, not even counting what the other chassis loads were. FWIW, the Ford output curves for this alternator show it capable of delivering around 60 amps at engine idle speeds. So far we haven't ruined the alternator as far as I can tell. We're either lucky or Ford did something right.

I'm betting on Ford getting it right. I have the same alternator and it charged the stock battery for 10 yrs. I have had up to 100 amps of draw on the 12v system but that is for short duration.

[pianotuna]

Hi,

Best to get an external diode if you wish to run an alternator "flat out". I would not run at more than 2/3 of total capacity for long periods of time.

My hall effect ammeter maxes out at 70 amps. I have a way to limit house charging to 50 amps, but if I don't do this I do see an error display from time to time on the unit.

[time2roll]

Make sure the alternator is cool. Heat will limit output afaik.

[pnichols]

This is just a guess - but I'll bet most modern vehicle alternators are protected from being burned out from any excessive load(s) one may attempt to put on them.

We use our Ford 130 amp alternator to charge up the engine battery plus two deep cycle coach batteries as needed, all together at the same time if necessary, at any beginning state of charge of any of the three batteries, and at any engine speed including idling. While at engine idle, I've seen our dash mounted coach battery amp meter show around 50 amps being supplied by the alternator to just the coach batteries alone, not even counting what the other chassis loads were. FWIW, the Ford output curves for this alternator show it capable of delivering around 60 amps at engine idle speeds. So far we haven't ruined the alternator as far as I can tell. We're either lucky or Ford did something right.

[weasel4]

When amperage goes up, volts go down. When volts are up amperage is down.

Ohms law will tell you what you need to know.
BTC

[MrWizard]

High performance replacements are available
I put a new heavy duty hot rod replacement, in a 1982 pace arrow
It would do 90 amps at idle if the batteries would take it
Bought it from a place in Michigan over the internet and had it shipped to Calif
this was about eight years ago

[Chris_Bryant]

I don't think any alternator will put out near rated current at fast idle, I think you will be lucky to get 40 amps at that speed.

[RJsfishin]

Adjusting for specific load no problem w/ the carbon pile tester.


I know the 100 amp load is for short term testing, as it heats stuff up perdy fast.

Auto Parts testing ?? Anybody can do that,....besides that the nator is not off the vehicle.

Thanks for the tips. I think I will load it at 80 amps for 10 seconds, and expect it to hold 13.5+, and if so, I'll call it good.

Is it possible to show 14+ volts, but be drastically short on amperage ?? I'd guess maybe it is.

[More_To_See]

I'd be a little careful putting a long term heavy load on an alternator. I suspect the 100 amp rating is not a continuous rating. Maybe not even close to that.

[the_bear_II]

Step one: Go to local auto parts store and have them test the altenator

Step Two: Go back home and install the altenator

[Golden_HVAC]

Chances are that the voltage at the alternator output will be about 1 volt higher than the battery voltage where the load tester would be connected under a 100 amp load. THis is due to the #8 wire running between most alternators and the battery. The small wire will prevent alternator overloads - I guess?

So how to put a 100 amp load on a battery with a 500 amp load tester? Can you divide the load up into 100 or 50 amp sections, and only put that load on the battery?

I would expect the voltage to drop into the 13.8 volt range at 100 amps load on it. Just because of the voltage loss across the internal diodes within the alternator. Also as the alternator warms up, most manufactures claim that they will only put out about 80% of their cold rating. Yes you already realize that many require at least 2,000 alternator RPM to put out anything near the rated amperage.

Fred.