Alternator Load Test

[Rusty123]

Getting ready to install a high output alternator (140 amp, with output reduced to 120 amps or so via external regulator).

I want to thoroughly test the alternator after installation, but I don't have, nor need, a large inverter. But I can't think of any other way to generate full load (short of discharging and recharging the batteries, but that introduces several variable that are difficult to control).

Any other way to put a big load on? Worst case, I guess I could buy a "cheap" inverter from Harbor Freight or eBay.

[djmarchand]

I can't think of an easy way to hook up a 1,500 watt DC load to really test your 120 amp alternator short of an inveter and a 1,500 watt space heater attached.

But since the proof is in the battery charging, why don't you drain your batteries to 25% SOC and see how it charges them. A one time discharge below 50% followed immediately with recharging won't hurt them a bit.

David

[Stu Jackson]

You really don't need to do that to your batteries. Run them down to 75% SOC, which is usually a daysail worth of energy away with regular loads on a system, and then run your engine at cruising speed rpms. Because the basttery acceptance will be high initially, you should see alternator output. Beware you won't get 100% of your rated AO, almost ever.

If you don't have ti installed yet, take it to an alternator shop and they'll do it for you.

KISS.

What are you using for measuring the output?

[Maine Sail]

If you really want to hot load test the alt and accurately and safely dial back the current then you will want a cheap HF inverter. They work fine for this and after ruining a couple of more expensive ones, by lugging them on and off boats, getting them wet etc., I now use a cheap HFT inverter for alt testing. If I break it who cares, it was cheap.. You will also want a hair dryer or small electric heater. If all you have is a hair dryer be careful. You can place it facing the ceiling in the dry galley sink and it will be pretty safe.

Start at Belt Manager Level 3

#1 Install a remote temp probe on the alt and close up the engine bay like you normally would run it. Many DVM's offer this feature.

#2 Run the boat, away from the dock, up to engine temp for at least 30 minutes, while loading the alt with the inverter & heater. Take note of cold output and hot output.

#3 Return to the dock and allow the engine to "fast idle". This is the hardest load on a HO alt, heat wise, because they can put out a sizable % of their current even at low alt RPM but they lack the RPM for adequate fan cooling in a tight hot engine bay. Keep the alt fully loaded while you monitor alt case temp..

#4 Continue monitoring alt temp. If it exceeds 215-220F drop Belt Manager down another step to level #4. If it does not get over 190F it is often safe to go up one step to level #2...

You can probably just get away with BM Level 3...

[Rusty123]

Thanks Maine, you answered some questions I didn't know I had yet. Looks like a cheap inverter is the right answer.

My primary reason for not using the batteries for this is that it's difficult to control or vary the load. If I only get 100 ams out, is there a problem with the alt, or is battery acceptance coming into play? Etc.

Measurement will be with a clamp on ammeter.

Thanks all.

[Maine Sail]

Originally Posted by Rusty123

Thanks Maine, you answered some questions I didn't know I had yet. Looks like a cheap inverter is the right answer.

My primary reason for not using the batteries for this is that it's difficult to control or vary the load. If I only get 100 ams out, is there a problem with the alt, or is battery acceptance coming into play? Etc.

Measurement will be with a clamp on ammeter.

Thanks all.

I use this approach when trouble shooting alt "performance" issues, especially when customers insist it is not the batteries of installation. One guy paid me to keep the alt loaded for over two hours because he was sure the issue was not a battery, wiring or installation issue..

Turned out to be batteries, voltage drop & volt sensing location. Reg would stay in bulk indefinitely loaded down so the issue was not alt or reg as the owner insisted it was. The 1.1V drop between the back of the alt and battery bank was the biggest offender followed by partially sulfated batteries and an owner who claimed his batts were at 50% SOC by looking at a volt meter with a load on the bank..

The artificial load is a priceless tool for a guy like myself...

[Rusty123]

After hunting around for a while, I found a used Coleman 2000 watt modified sine wave inverter (4000 watt peak surge) on line for under $100. I wasn't expecting much, but just needed it to pull 150 amps or so to test the alternator.

When it arrived, I was a little disappointed in its appearance (scuffed and dirty - "like new" was a stretch). But I hooked it up to my battery bank (temporarily stored in my garage) for some testing.

It fired right up, and powered several tools with no issues. I then tried a space heater and a hair dryer, and was able to reach rated load with ease (hit about 200 amps peak). The inverter fan came on after a few minutes (very quiet), and easily maintained temperature (exterior of case stable at about 73 degrees). After securing the load, the fan ran a few minutes and stopped (unlike many inexpensive inverters I read about on line, where the fan runs continuously, and loudly).

No load consumption is well under 0.5 amps (when turned on).

All in all, I'm quite impressed. Even though it was not my intent, I may just install it permanently, if only so my wife can run her hair dryer, or so I can run the shop vac.