Thread: Can I run two alternators in parallel?

Originally Posted by POD

I have a Bosch 80 amp IC reg built-in alternator that I want to set up as a second alternator on my Rangie. I have used this alternator as an auxilliary charging system on 2 previous vehicles, but my plan is to do it a bit differently this time and I need to know if it will work.
On my old Rangie (and on a Toyota before that), I had this alternator set up to charge the aux battery only, so that the two charging systems were completely separate. I now have one of Drivesafe's SC70 split charging systems and want to take advantage of the ability to run a fridge etc off both batteries down to a cutoff voltage, but also want to take advantage of the extra charge available from a second alternator when running multiple driving lights etc.
What I want to do this time is to set up the second alternator to run in parallel to the main alternator, in effect just increasing the available charge current in the one charging system. Will two alternators work together like this, or would this arrangement cause some sort of problem? Mounting the second alternator is no problem, just need to know how two alternators, with separate regulators, will interact in parallel.

Being that both will be paralleled after their regulators, both will be dc and fine to join I'd be reckoning, if it ac they'd have to synchronize

I was interested in this also as I have 2 alternators.
Apparently it is a bad idea according to lots of marine sites as one alternator becomes "lazy"

here is a sample
Two Engine/Alternator System

Regards Philip A

Originally Posted by PhilipA

I was interested in this also as I have 2 alternators.
Apparently it is a bad idea according to lots of marine sites as one alternator becomes "lazy"

here is a sample
Two Engine/Alternator System

Regards Philip A

Yes I would agree that it is not a good idea unless certain precautions are taken.

Regulators have smarts built into them, simple things like temperature compensation to name but one. The insides are all achieved with silicon diodes and other things like transistors and voltage references/ A slight mismatch in any one of those things and you will end up with one alternator taking more of the load. It will not be until you are running the alternator with the lowest set point to it's maximum and it can no longer support the load before the second one will kick in.

You might get lucky and they might play nicely. I would keep them on different circuits or use a single regulator for both.

Just to be contrary , here is another really good article that suggests that in practice it is OK.
Alternitive method of battery charge-second alternator - Boat Design Forums
Regards Philip A

Hmm, sounds like just mounting the second alternator, energising the field off a relay and running the charge wire straight into the existing system, would give me a better than even chance of releasing all the smoke out of my electrical system one way or another. I really don't want to get into much more technical setting up than that, balancing phases, voltages and amps etc.
Another option would be to run the charging systems separately, as I have done in the past, and connect the batteries via the SC70 whenever I want to make camp for a few days. I have a set of bull lights mounted on my roof rack, that goes onto the car for outback trips- great for avoiding roos. This means I'm running 6x100w driving lamps plus the standard headlights, not much chance of recharging batteries whilst running all that off the one 80A alternator. With separate systems, I can run 4 driving lights off the aux system so can recharge both systems when night driving. Has worked well in the past.

yes, easily done BUT...

make sure that the maximum regulated voltage is the same on both alternators prior to joining them or you will wind up with one alternator being a little lazy and the other will burn out brushes early. I would also reccomend fiting a hefty blocking diode on the output of each and tapping the voltage sense after the diode so that in the event of one alternator failing the other can still do work without having the dead alternator suck all the power back.

very common to use 2 alternators.we did lots with rally cars.
the idea was to run a normal 55-80 amp and a delta wound hi output.

the hi output often has to run at very hi rpm to work.so you run it on a seporate belt with different gearing-also in case it self destructs.we used to run external regs set at different voltages or switchable.this was to prevent the regs fighting each other.we used the smaller alt with lower setting as it was intended as a race car.for a offroad crawler you probably set it up the other way.

simple fuse links are OK.

fire trucks an other vehicles often use aux engine[pump] with a second alternator.the isuzu we had troubles with setting of warning lights.
the fix was 2 large diods in series on the second alternator as a voltage drophigher loads more V drop..i though it was one for each but that doesnt work as well.

been playing this week with a hilux alt.
has a variable output signal.ecu driven duty cycle pulse to adjust the maximum voltage.

what the storey with residual magnitism exciting alt.ive had them burst into life without the reg conected.

we used to fire up an alt with small battery on the feild to get a flat battery charged.