2 alternators fried; what’s going on?

[GrowleyMonster]

Originally Posted by rgleason

I think you'll find that with a battery bank that big (800-670ah) assuming flooded deep cycle batteries, that getting smaller and smaller alternators does not help. 25% acceptance rate= 200 to 150ah alternator.
The smaller internally regulated alternators with no alt or batt temp sensors might be running at full for too long, thus overheating and causing the electrical smells. I would probably want a larger alternator that was externally regulated with temp sensors.

It could be just that simple. Easy to sorta check without getting another alternator... disconnect all but one battery and see if it still overheats. Then try charging the others one at a time, then two together, etc.

[rgleason]

Good idea.

[N4585K]

Good morning,

GrowlyMonster: I isolated the solar panels which had no effect.

rgleason: my battery bank is 4x140=560Ah. Each battery is rated at 800 cold cranking amps but an electrician measured each battery as being able to produce close to 670 cca. I know that cca and Ah are two different things and that the loss of cca due to age is not linearly related to the loss of Ah but when I have the Victron BMV installed on Friday I intend to enter my (3 year old) bank capacity as 480Ah. Since I’m out cruising there’s no possibility to have my batteries lab checked in a temperature-controlled environment. It is not a small bank but not big by any means. A 90A alternator should have no problem with that right? Additionally, this alternator only charges the house bank. Engine start battery and bow thruster batteries are charged by the smaller no.1 engine alternator.

[transmitterdan]

Originally Posted by N4585K

A 90A alternator should have no problem with that right?

A 90A alternator will probably burn up charging that big bank. If it doesn't burn up it will take days to charge the bank from 20% to 100% because it will throttle back charging current to about 25A due to overheating. Alternator vendors give a max current rating that only lasts until the alternator gets hot (about 15 minutes) then they have to reduce current else they burn up.

You need to be able to deliver about 15-20% of the AH rating for several hours. For your bank that is at least 100A for up to 5 hours to recharge from 20% to 100%. No small case alternator can do that I don't care what the nameplate rating says.

[a64pilot]

A couple of things.
An alternator should be able to output its max power continuously because it’s thermally protected.
Secondly due to the batteries charge acceptance rate as long as you don’t have a huge electrical load like trying to run an Airconditioner off of an inverter of something, the charge acceptance rate should drop so that after at most an hour or so the alternator isn’t having to max its output.

I get 90 amps or so out of my 165 alt continuously, I could get higher but I have the thermal protection tuned down so that 90C is max it will allow, and if I forced air cooled it, I believe I could get more as it will make 115 until engine room temp get high, then it will maintain 90C, but the power backs down to 90 amps.

I have a shunt mounted right at the alternators output, so I know my output, I’m not having to infer it.

[Jammer]

Originally Posted by a64pilot

A couple of things.
An alternator should be able to output its max power continuously because it’s thermally protected.

Many, if not most, are neither thermally protected or current limited in any sort of active way. (The series resistance of the winding goes up as it gets hot, which reduces output current, but that isn't current limiting as such)


More sophisticated regulators may monitor the alternator temperature and the current output. While common in aftermarket high-performance regulators, these features are rare in stock systems.

[transmitterdan]

Originally Posted by a64pilot

A couple of things.
An alternator should be able to output its max power continuously because it’s thermally protected.
Secondly due to the batteries charge acceptance rate as long as you don’t have a huge electrical load like trying to run an Airconditioner off of an inverter of something, the charge acceptance rate should drop so that after at most an hour or so the alternator isn’t having to max its output.

I get 90 amps or so out of my 165 alt continuously, I could get higher but I have the thermal protection tuned down so that 90C is max it will allow, and if I forced air cooled it, I believe I could get more as it will make 115 until engine room temp get high, then it will maintain 90C, but the power backs down to 90 amps.

I have a shunt mounted right at the alternators output, so I know my output, I’m not having to infer it.

Which reinforces my point that small case alternator don’t produce their name plate rated amps. Most do good to get 50% of name plate amps continuously.

If a 500AH bank is at 20% SOC it will draw as much current as the alternator can produce. Current won’t start falling much until >50% SOC so that requires 175AH of charge. No small case alternator can do that in an hour without flames. Probably would take more than 6 hours if actively temp regulated.

The OP has just installed an internally regulated 90A alternator and wonders why it is smoking. The reason is the bank is too big for that alternator/regulator.