A large depleted battery bank asking for a lot of current over wiring capable of passing it can certainly overheat an alternator, and even a single additional depleted battery bank adds tremendously to its work load, especially if it is a high$$ AGM battery.
Longer Alternator life can be obtained by keeping it cooler.
A cold air intake feeding the alternator ambient air instead of hot underhood engine air can do a lot. This has been on my to do list for years, but it is a want, not need type of thing and always has been put on back burner.
Reduction in the cabling diameter to the depleted house battery will also limit what the battery can ask for, and keep the alternator cooler. The voltage drop on the thinner cable will lower the voltage delta between source and load, and voltage is electrical pressure so the effectively limits what the alternator has to make, but at the expense of faster recharging.
Reduction in field current would also reduce output and temperatures. But shutting off loads entirely on a spinning alternator can indeed cause Diode destroying voltage spikes, and these voltage spikes might also damage other electronics, such as the engine computer.
DO NOT remove a battery cable with engine running to test alternator on my modern vehicle. This 'test' of an alternator could very well destroy it, and perhaps other things. This is one of grandpa's holdovers that should be forgotten about.
Even if the engine battery is still on the system and one switches off the house battery bank which was asking for 50 amps, there could be a very short duration voltage spike that could damage electronices or the Diodes in the alternator and even if it does not instantly allow the magic blue smoke to escape, the damage is cumulative.
I am actually set up now to collect some data on alternator heating, but have not yet collected a lot of data.
What i have collected so far is a bit dismaying.
I drove around running errands, the engine was upto full temperature indicated by coolant and Oil pressure. My battery was requiring about 24 amps to be held at 14.4 absorption voltage.
The final relay of the errand trip was about 1.5 miles of 25 to 40mph with headlamps on and blower motor on medium speed for an additional ~21 amp draw , then a 100 yard 10mph slow idle to my parking spot.
When parked and idling, lights and blower now turned off, I checked the alternator temperature by turning on my digital multimeter and setting mode to fahrenheit, 119F. As I continued to idle, with 24 amps into the battery, about 8 amps to run the engine, the alternator temp rose to 139.5f in a minute or two. When I turned lights back onto high beams, and my 2 sets of fog lights, and my blower motor on high, this ~ an additional 60 amp load on the alternator, and at idle speed my alternator cannot keep up, the battery was now discharging to make up for what the alternator could not. It was maxed out. In 30 seconds of this alternator temperature had risen to 157 degrees.
So preliminary conclusions based on limited data, is that on my Dodge van, the airflow under the hood at speed makes a HUGE difference in alternator temperature compared to just Idling. My engine also idles hot at 525 rpm according to my aftermarket tachometer and cannot produce more than about 40 amps at that speed, but add a few hundred rpm and output easily climbs into the 60amp range.
So When i was driving with headlights on and blower motor on, It was around this 60 amp total level, and likely under 119f.
When parked idling and about half that total output, temperature increased 20F.
Maxing out the alternator at hot idle, not moving, increased it another ~20F in less than 30 seconds.
So More data is really needed, and how applicable it will be to other alternators in other vehicles is an unknown variable, but it is safe to say that alternator temperature is directly related to both Load on the alternator, and vehicle speed, as the alternator's fan pulling in 65F ambient air is much different than 120F ambient air.
One other data point I have is with a 2013 Ram powerwagon and a 160 amp alternator. I depleted a new Lifeline GPL-31xt battery to 50%, and then hooked this depleted battery to a large winch connector that ran directly to alternator over ~12 feet one way of 2 awg cable. When I flipped the switch to on, even the 5.7 liter Hemi engine note changed as the battery was demanding over 100 amps and the consuming ~ 4 engine horsepower to make that.
Alternator temperature quickly heated upto 220F, and then amps tapered to 86. The battery voltage was still in the low 13's indicating the alternator was being limited in output, as more engine rpm did not increase the output. It never rose above 86 amps again or 220 F searching for the hotspot with an IR temp gun, so I believe this alternator was temperature protected and 220F was where it said "Whoaaaa ho ho , Nellie, calm the Eff down"
I am not sure how these temperature protections are employed, or if this particular alternator is internally or externally regulated.
I have recently bypassed the voltage regulator in my engine computer, and have employed an adjustable voltage regulator instead. Detail of this can be read here:
https://vanlivingforum.com/Thread-Your-Vehicles-voltage-regulator
With my dashboard mounted voltage potentiometer i can easily reduce alternator output by twisting the dial downward reducing the voltage delta. If the VR is seeking to bring system voltage upto 13.4 volts, perhaps the alternator only needs to make 13 amps into the depleted battery, but if it was seeking 14.7v perhaps 45 amps additional amperage would be required to get the system voltage up that high with the depleted battery gobbling up a huge percentage of total output.
I need to do more testing with a better temperature display I can safely see whilst driving, and do it with a depleted battery which can out a larger load on the alternator at any given rpm.
Another nice option of my adjustable voltage regulator is when i set it to the same absorption voltage as my solar controller, they work in conjunction to meet the electrical demand of accessories and depleted battery. Before my VR mod, the ECM's VR would often choose 14.9v, and the second 14.5v was achieved the solar controller would go open circuit and not add anything, even if 12 amps were available and the battery would have appreciated it, and the alternator too would have enjoyed having to make 12 less amps. If the battery is fully charged and can;t accept more than 0.4 amps current, then I can lower voltage and the Solar can actually meet the electrical demands of the engine ignition and fuel delivery is enough sun is available.
BTW my temperature sensor is my digital multimeter that uses a K type thermocouple. It came with a 3 foot length one, but I have purchased additional 3 6 and 10 foot versions as they are rather inexpensive. I used Arctic silver 5 minute thermal epoxy to hold sensor tip to the prepped alternator casing. i will be doing the same for my battery and perhaps thermostat housing.
Here is a 4 input Ktype thermocouple for a reasonable price.
It should be interesting to see 4 different temperature displays at once:
http://www.ebay.com/itm/4-Channel-K...527730?hash=item46374d5d32:g:QMoAAOSw7FRWVX7l
I'd love to see some data from another vehicle's alternator at various load levels and ambient temperatures, if anybody reading is as much of a nerd when it comes to this stuff.
I've always said Idling to recharge is a waste of cas in most circumstances, but my data has proven, at least on my vehicle that it significantly increases alternator temperature, as the alternator fan is not spinning fast enough, and the engine's fan is only spinning fast enough to keep the engine coolant at 195f. This heat is effectively sent into the engine compartment surrounding the alternator.
So An Alternator that has a the ability to produce half of its maximum rating at Idle speed, and is asked for everything it can make at idle speed by a depleted battery bank, could be a smoking hot alternator with a short lifespan. So drive to recharge whenever possible, as Idling to recharge a depleted bank might seriously knock the alternator lifespan downward due to excessive heating.
Whether opening the hood will mitigate alternator temperatures at idle, would seem to be very vehicle specific as if the engine fan is able to direct a lot of airflow over the alternator with hood closed, it might run cooler than with hood open.
I am considering a ducted 3 inch bilge blower aimed at my alternator to bathe it in ambient air temp instead of underhood air temps. Me thinks this would significantly extend alternator lifespan, and likely increase total alternator output, even accounting for the 3 to 5 amp load of the bilge blower.
If one is seeking an automatic way of prolonging alternator life, having a fan operated on a temperature acivated snap switch blowing ambient air at the alternator is likely the easiest way of reducing its temperature and prolonging its life.
Whether there is room for the fan and ducting is another matter.