Hot alternator

[Tooeez]

I have had a 2,000 watt inverter in the boat for years, it runs the fridge, microwave, vacuum when I make a mess, and so forth. It is hooked up to 2 group 31 deep cycle batteries in parallel, with the option to switch in a third battery if necessary. Never had an issue with charging with a standard 55 amp alternator, even after 2 days on the hook if the battery bank was flat--once the engine started it would charge about 20 amps for a while, then cut back to 10-12 amps for a few hours more. I added a small a/c, and after the addition of a soft starter I got it to run off the batteries with no problem (uses about 750 watts total when the fridge is running). I don't know how long I could run it that way, but I intend to use the a/c almost exclusively when the boat is underway, so that isn't a concern. Today I tried the a/c out while underway--ammeter showed about 20 amp charge, which stayed steady for about 40 minutes. Then the charge went to zero, so I assumed the a/c had cycled off, but when I went below to check the a/c was still running, and I could smell that unique smell that overheated electrical equipment makes coming from the engine compartment. I shut the a/c down, and when I got underway again the ammeter showed about an 8-10 amp charge on the ride back. At the dock the alternator was so hot I could not hold my hand in it. The inverter, cables, batteries, connections, etc. were all just slightly warm to the touch.
I assumed that when underway the alternator would supplement the batteries to provide power to the inverter, making for less drain then the batteries alone. Where did I go wrong? Is there a way to run this load underway, or is trying to run an a/c without a genset a fool's errand?

[Charlie J]

sounds like you need a bigger alternator
I had 140 amp in mine but was running nothing
the guy
I bought the boat from put them in, he saved the 55 amp
and eventually switched back to the 55 amp

[Rawleigh]

I have a 200 amp on my pickup truck. I would get a bigger alternator for what you are doing. I think the little one was overworked.

[Stephan]

Also consider the pulley size (make sure the alternator is operating in the correct RPM range for efficiency at your cruise power setting) and perhaps double belts for the higher output alternators.
Best,
Stephan

[Bruce]

P=IE. P, power in watts, I, amperage, E, voltage.

Using this formula with known values using your 750 watts at 13vdc, your ampere draw from battery to run the inverter would be approx 57 amps of current plus or minus depending on efficiency of the inverter.

Standard alternators have a power curve and if rated at 55 amps would have to spin at about 5500 rpm to put out its max output. This speed depends on pulley ratios and engine rpm.

Battery condition also has a lot to do with current draw. One weak cell can draw more than usual.

But with a 55amp, you would have to be running max rpm if spinning proper rpm and still 2 amps behind. Standard altenator cases are small and don't have enough surface area to dissipate the heat from the diodes and can burn out.

Btw an altenator is nothing more than a 3 phase motor wired either y or delta with a rectifier to convert to dc and a regulator to keep dc voltage in range.

[Tooeez]

Thanks all for the input. When I did the repower a few years ago I installed a 140 amp alternator on that engine (this was before the a/c). On the way back from our first overnight trip the high output smoked the 45 amp main fuse and shut down everything on that side. I did not want to tackle installing heavier wire to the alt. and a bigger fuse, so I put the 55 amp back on.
Bruce, if you don't mind let me know what you think of this idea:
My understanding is that whatever the rating of an alt. may be, it isn't designed to maintain that output for very long. Realistically they can maintain about half of whatever they are rated at. The max rmp I run at is around 3200, and I have the standard pulley setup. The batteries are less than a year old, and don't seem to have any bad cells. If I install a 105 amp alt. it should put out around 40-50 amps max at the speeds I operate, which is about half of its' rating. The existing wiring can handle that load. In round numbers that would leave a shortfall of about 15 amps, which the batteries should supply, correct? The house bank is 315 amp hours, so I have a lot of capacity.
I installed this system mainly to give the crew a place to get out of the heat when fishing (the wife has a tough time with the heat, but is a great fisherman) so the use on a typical trip would go like this: 15 minutes at idle out of the harbor, 30-40 minute cruiseing speed to the grounds, trolling for 3-5 hours, then back home. Do you thingk the 105 amp. unit will do the job?