2.5V voltage drop normal when running a 50 amp load?

jeegro

Adventurer
OK here's an easy one...

When my air compressors kick on (with or without the car running) the voltage at the battery drops around 1-2v. With the car off, volts go down to under 12v pretty quickly. With car on, volts struggle to stay above 12.3 ish

Volts when alternator is running = 14v

Dual odyssey group 34 68aH batteries. Fully charged 12.8-13v
130amp alternator (17 years old).. I have a 200amp (or so they claim) sitting around I've been meaning to install

Wire run summary: 7 feet of 2/0 welding cable (battery to SafetyHub) + 9 feet of 4 gauge welding cable (SafetyHub to circuit breaker panel) + 2.5ft 6 gauge welding cable (from breaker to compressor)
 

john61ct

Adventurer
Sure. A bigger bank will help for short runtimes. An LFP bank more so but pricey.

Bigger alt may not be needed, but need more than idling.

Maybe an adjustable VR:

​Transpo 911-02R. Mechman, Excessive rebadge them
 

LandCruiserPhil

Expedition Leader
I have a very similar set up and a hand throttle lock running RPMs at ~1500 helps a ton. With a 50A load your vehicle should always be running.
 

dwh

Tail-End Charlie
Yea, sounds about right.

I'd want to take voltage readings at the compressor end with the compressors running (engine off, engine idle and engine 1000+ rpm) just to make sure the compressors are getting enough voltage.

Too low a voltage can overheat some stuff.

The bigger alternator won't really matter unless it puts out more at idle than the one you have now.
 

Ducky's Dad

Explorer
100% charge level on those batteries is 12.85+OCV, so their state of charge seems to be OK. Key is to not run the compressor when the engine is not running, and maybe do the hand throttle thing to keep charge rate up. I changed my factory alternator from 160amp to 270amp, but my new alt puts out 116 amps at 700rpm idle, so I can run my big Warn compressor all day at idle. Have done that for hours on end with a framing nailer on the compressor, intermittent use, no problem.

You seem to have 18.5 ft of wire between your battery and your compressor. If the compressor is physically near the battery, why not wire compressor directly to the battery with an inline fuse and a Blue Seas rotary switch for positive disconnect? That's what I did and it works fine, maybe five feet of wire from battery to compressor. Compressor is 12cfm free air flow, so it pulls a lot of amps.
 

jeegro

Adventurer
Interesting... what about winching?? My winch is capable of 380 amps.

I'll do some more measurements as suggested. I did another quick one with the engine idling, and my battery monitor was saying negative 5-25amps (15 average) coming out of the battery, which means the alternator can't produce enough juice at idle.

Someone told me that the way these alternators are rebuilt for higher amps, actually produce FEWER amps at idle. Haven't tested that yet. Can I simply take my 200a alt to o-reilly's and have them test the idle and throttle output? I'll probably just keep the 200 amp alt as a spare until the main one dies unless I can prove that it performs better all-around.

116 amps at idle is impressive!

The way my stuff is wired it's not really practical to change it up. The compressors are in the cargo area. I'm using pretty heavy cables. Upgrading the 4awg line to something bigger could help though.
 

Ducky's Dad

Explorer
Interesting... what about winching?? My winch is capable of 380 amps.

I'll do some more measurements as suggested. I did another quick one with the engine idling, and my battery monitor was saying negative 5-25amps (15 average) coming out of the battery, which means the alternator can't produce enough juice at idle.

Someone told me that the way these alternators are rebuilt for higher amps, actually produce FEWER amps at idle. Haven't tested that yet. Can I simply take my 200a alt to o-reilly's and have them test the idle and throttle output? I'll probably just keep the 200 amp alt as a spare until the main one dies unless I can prove that it performs better all-around.

116 amps at idle is impressive!

The way my stuff is wired it's not really practical to change it up. The compressors are in the cargo area. I'm using pretty heavy cables. Upgrading the 4awg line to something bigger could help though.

My winch is rated by Dodge as 12,000 lbs, but the motor is same as Warn 15,000 lbs. It pulls 440 amps at max load IIRC. I actually have three batteries with isolators and Blue Sea rotary switches so that I can include or isolate batteries as needed. Batteries are an Odyssey G31 and a pair of Optima Blue dual purpose G34s. I had a rebuilt/upgraded alternator in another truck and it fried itself, so for the Dodge I got the 270 amp as a newly manufactured piece from DC Power Engineering and it has been great. Their test sheet packed with my alternator shows actual max output at 285 amps, and we measured the actual idle amps after we installed it in the truck. I chose the 270 because that was the highest output that would run on stock pulleys and the stock serpentine belt (easy to find one in the boonies). Everything wired with 3/0, except winch on factory cables and compressor on Warn factory wire. Warn told me that upgrading the short cable on the compressor would just be a waste of money, and it seems that they were right.
 

rayra

Expedition Leader
There are relatively expensive alternators marketed to the emergency services market that are made to have a much higher Amp output at idle. That or you'll want some method of maintaining the higher RPMs needed to up your output during high load use. You aren't going to match your winch draw regardless. Not without a lot of expense and engineering. Soemthign like double high output alternators.
 

dwh

Tail-End Charlie
Are those winch amp draw ratings at FLR (full locked rotor)?

It's extremely rare to see a good sized winch loaded till it stalls.
 

Ducky's Dad

Explorer
Are those winch amp draw ratings at FLR (full locked rotor)?

I have never had to load mine to stall, and not sure one could with any reasonable electrical system. When Dodge spec'd the winch system for Warn to build, they insisted that the winch include thermal protection and a low voltage interrupt. With a stock Grp 65 battery and the original 160 amp alternator on my '05 Power Wagon, I suspect the LVI would kick in before you could stall the winch. Lots of early 3G PW owners have complained at length about the intrusion of the thermal and voltage protection. Solution seems to be more battery and bigger alternator, and some owners have bypassed the protections with no ill effects. The 440 amp max draw is from the printed specs for my truck.
 

snowblind

Adventurer
Someone told me that the way these alternators are rebuilt for higher amps, actually produce FEWER amps at idle. Haven't tested that yet. Can I simply take my 200a alt to o-reilly's and have them test the idle and throttle output? I'll probably just keep the 200 amp alt as a spare until the main one dies unless I can prove that it performs better all-around.

That might be true of low quality options. Companies like http://www.mechman.com/ list the amps at idle AND at cruising speeds. They have a number of models that will produce over 150amps at idle.


Matt
 

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