Thread: Can I run two alternators in parallel?

Originally Posted by harry

it's easy to run two alternators, apart from the complication of fitting them and driving them.
if you really want two independant sources of power generation, remember to fit the alternators to separate drive belts, otherwise, when the belt fails so does both alternators.
but if the engine fails?

twin engined aircraft have been doing it for years,
you need a regulator for each alternator [internally regulated alternators haven't made it to aircraft yet, perhaps they aren't reliable enough yet?] and a paralling regulator,
and yes, often one takes more load than the other, that doesn't mean one will fail sooner than the other, and in a car, how often does the alternator fail?

the alternators should be switchable by turning the fields on or off, you should have ammeters for each system,
you should have circuit breakers for max output for each.

both alternators and regulators should be the same type and capacity, if not, the better one will always take the mass of the load, and the other will be lazy, because it is only a backup [might as well not have fitted it]

when the system is switchable, one can select both or, [let's call them left and right], or left or right.
when the ammeter shows one has dropped off line, switch the good one off and the other should take over,
try telling that to a pilot, as i have been for many years!

the system must have protection to prevent the good alternator feeding the failed alternator in case of an emergency, otherwise it all goes pear shaped.

forget car technology,
do a search on twin aircraft alternator systems, they work well.

i do think it is a waste of time for a car though,
how often does your alternator fail?

perhaps you should look at it more often.

we generally have the alternator of a single engined aircraft inspected each 500 or 600 hours flight time.
that's about 50,000 to 60,000 thousand miles or about 80,500km to 96,500 km in your language.

when did you last pull your alternator for an overhaul?

I don't think he is after redundancy. He is just after more capacity, so switching them in and out doesn't do that for him.

Redundancy is not the issue, just after some more charging capacity, much like fitting a bigger alternator. I'm reading a lot about 1 alternator doing all the work and the other becoming 'lazy'. At present, there is only one alternator in the car, fairly safe to say it is doing all the work, and the other alternator has been sitting in a box in my shed for 3 years- hard to see it getting much lazier than that. So the possibility of the existing 80amp alternator doing all the charging until the load exceeds 80amps, and the second alternator contributing the shortfall after that, is not a problem in my view- in fact it is exactly what I am after. I am more interested to know whether there is likely to be a problem resulting in damage to the electrical system or neither alternator charging because of some adverse interaction between the two outputs. It is sounding more and more like this is not likely to be a problem. Might just give it a go.
The twin alternator setup first got wings with me when I fitted a Chev 305c.i. V8 to an old Landcruiser. The motor came complete with a built-in-reg alternator and the mounting/drive hardware, plus I kept the original vehicle alternator to save having to rewire the car. Worked a treat as two separate systems, so I transferred it to my last RR. Just wanting to use the same mounts I fabbed for the old Rangie and wire it differently this time. The second alternator will be driven off the aircon belt and mounted on top of the compressor.

the only time "noise" becomes a real issue is if one drops a rectifying diode. That problem can be a problem with just one alternator. The compounding issue with twin alternators is the voltage spike and drop the dodgy diode causes will make the other alternator "ripple" its output and if the stars align just wrong that will stress out the regulator and it will either go KAflooey (the technical name for a lucas electronic item letting its smoke out) which will then result in no or over charging.

a couple of good chokes would prevent it but IMHO its such a low risk event about the only precaution I would take would be to put an unfiltered digital Volt meter onto the connection point of the 2 alternators outputs and spend a week learning what its readout says and if it changes checking the outputs on both alternators.

Do you have any photos of your last set up? I'm probably going to do this with my Range Rover. I've got a York aircon compressor with an extra pulley, so I might put the alternator up next to that, or I might use the slightly later compressor that went low down on the LH side of the motor and put another alternator up top.

My reason is the same as yours - big lights for roos and cattle. I don't want one great big alternator trying to power everything. I'm going with the KISS principle and having two separate systems. You'll know what each system is doing and one failing shouldn't affect the other.

Originally Posted by Davo

Do you have any photos of your last set up? I'm probably going to do this with my Range Rover. I've got a York aircon compressor with an extra pulley, so I might put the alternator up next to that, or I might use the slightly later compressor that went low down on the LH side of the motor and put another alternator up top.

My reason is the same as yours - big lights for roos and cattle. I don't want one great big alternator trying to power everything. I'm going with the KISS principle and having two separate systems. You'll know what each system is doing and one failing shouldn't affect the other.

wait, what? 2 alternators is simpler than one bigger one?

Why not just grab a larger alternator thats got 2 outputs, bolt it into place run the normal vehicle system off of one output and all the extra electrics off of the other?

charging capacity,
fitting two 80 amp alternators means max output is still 80 amps, but if both are paralled properly they will not work as hard.

if you want 160 amps, buy a bigger alternator.
not sure if you can get a 160 amp jobbie, but i have a 120 amp jigger in mine, still limited to 100 by the original fuse.

Originally Posted by Blknight.aus

wait, what? 2 alternators is simpler than one bigger one?

Why not just grab a larger alternator thats got 2 outputs, bolt it into place run the normal vehicle system off of one output and all the extra electrics off of the other?

Where do you find such a magical alternator? For those of us who aren't doing this for a living, it's hard to find things like this. Up here, it's really hard to find things like this. It's hard enough ordering a regular one for my Rangie. When I tried through one guy, I was told there was only one in the country and the price reflected that! (Instead I got a NAB900 in the end.)

Two is simpler if it means simply bolting in, wiring, and not having to worry about how the alternators will play together and what nasty surprises they'll give you later on.

Question: will a V-belt start slipping when driving a bigger alternator?

Originally Posted by harry

fitting two 80 amp alternators means max output is still 80 amps, but if both are paralled properly they will not work as hard.

If the battery voltage is dropping due to a 100A drain then each alternator will try to increase its output to restore voltage. 1 might run at full output and the other only make-up the shortfall due to slight differences in components (eg wire resistance, electronic component manufacturing differences) but overall the full current drain should be supplied by the alternators.