Dual alternators, dual batteries - linked??

[nickburt]

A mate of mine has fitted dual alternators.
Both alternators are identical (12v, 100amp).
In principal, one alternator charges the main battery and one charges the auxiliary battery.
But, he's also fitted a voltage sensing split charge between the two batteries.
He's wired the voltage sensing relay so the relay only energises when the auxiliary battery voltage rises above the activation threshold. Which wasn't what he intended (should have been the other way round), but, it works - or did for a while. One alternator has now stopped working - I suspect the voltage regulator has failed open circuit.

The debate we've been having is one of imbalance between the alternators.

For now, let's assume there is no resistance to each of the electrical connection.

My concern was that one alternator could end up back feeding the other, based on the batteries internal resistance and the fact that the alternators, while both the same, will not be absolutely identical in their outputs. i.e. the voltage maybe slightly different, and the current draw, depending on the state of each battery, could be different, even though the batteries will act as one accumulator.

Result - one alternator ends up try to drive the other as a motor, hence why the regulator has failed.

Thoughts ????

 

[Tim]

Interesting thought but wouldn’t that imply that you’d be doing the same thing if you put a battery on charge or had a solar panel attached?


Sent from my iPhone using OB Talk

 

[nickburt]

Interesting thought but wouldn’t that imply that you’d be doing the same thing if you put a battery on charge or had a solar panel attached?


Sent from my iPhone using OB Talk

Hmmm, but wouldn't the intelligence of the solar controller or battery charger take things into account, including preventing back feed?

 

[Tim]

Hmmm, but wouldn't the intelligence of the solar controller or battery charger take things into account, including preventing back feed?

Maybe but not sure how. The output of the alternator is connected to the battery at the same time as the solar or the charger. Effectively the output of the solar is only another type of regulator. If there’s a diode in the alternator/regulator output then you can’t back feed it.


Sent from my iPhone using OB Talk

 

[Tim]

Yes there’s a diode to stop you back feeding an alternator in the regulator pack

Sent from my iPhone using OB Talk

 

[nickburt]

Ha, was just revising the same .....
Given that most older Land Rover alternators only have one battery output and one feed to the dash warning lamp:



Negative being through the body. and the AC terminal is more often referred to as the W terminal (my rev counter is run of this connection)

Then in this case, it's probably blown a diode, or two, in the rectifier, rather than voltage regulator. Although both are usually in the same "pack" either in the alternator, or screwed to the outside of it.

 

[Megadoomer]

Maybe it's not the main question, but they make 200a alternators. Having 2 will cause more parasitic drag for no real added value.

Sent from my SM-G935V using OB Talk mobile app

 

[nickburt]

Indeed it may, just depends on what the vehicle is set up to do and what kit it has on board.

 

[Tim]

I’ve been researching what others have done to wire this up. Seen some simple solutions to the more complicated...

Sent from my iPhone using OB Talk

 

[nickburt]

I’ve been researching what others have done to wire this up. Seen some simple solutions to the more complicated...

Sent from my iPhone using OB Talk

So there's no direct connection between the alternators. Which is what I would expect.

 

[Sarab Loh]

I ran into this video last night researching how to setup up one my self. Video is an hour long, but super educational.

Sent from my SM-G950U using OB Talk mobile app

 

[Tim]

So there's no direct connection between the alternators. Which is what I would expect.

Yeah, not in this setup. Was one of the more comprehensive diagrams, it even considered separate charging lights. Would still like to know if there is a real issue with connecting alternators in parallel. It’s been interesting reading up on this but have yet to find a definitive answer.


Sent from my iPhone using OB Talk

 

[nickburt]

Yeah, not in this setup. Was one of the more comprehensive diagrams, it even considered separate charging lights. Would still like to know if there is a real issue with connecting alternators in parallel. It’s been interesting reading up on this but have yet to find a definitive answer.


Sent from my iPhone using OB Talk

Indeed, part of me is saying it should be possible, the other is saying no it isn't.

 

[nickburt]

The more I read about running alternators in parallel, the more I'm reading about isolators. But, the built in diodes should do the job, unless they're not man enough for the kind of currents that could be involved.

 

[Tim]

Even if the two alternators aren’t producing perfect DC and there’s some oscillation of the output, I’d have thought the battery would smooth this out to some degree. My only other thought right now is how does the voltage regulator work when it senses a change in voltage that it’s not actually responsible for producing (the other alternator is)?


Sent from my iPhone using OB Talk

 

[nickburt]

As you say @Tim, you would think the battery would smooth the voltage difference if there is any.
I think there's something in the voltage difference even a small difference.

 

[smritte]

"Result - one alternator ends up try to drive the other as a motor, hence why the regulator has failed."
Not really possible.

The diagram that Tim posted is the better one. It show's the layout of the average charging system.

Alternators are just electromagnets surrounded by a coil. Put simply the Stator picks up the magnetic field produced by the rotor. The Stator puts out AC voltage, the Bridge converts to DC. The regulator controls the rotor magnetic field and senses the output through the diode trio.

In the past, people ran dual alternators for more output. The large high output alternators were just too big to fit things smaller than a medium duty truck. In the 80's higher output in a smaller case started showing up. These gave people the option of not needing two alternators. There were a few who still preferred to run two but, I don't see the need.
I use to build custom alternators for people on the side. I've done a few dual setups. The style of alternator will depend on how it senses the systems voltage. If set up properly they wont interfere with each other.
The question is, how large was the second alternator? Most alternators don't like being run more than 70-80% of their output for long periods of time. To figure out what happened you would have to test the pieces and see what failed.
Burned stator(most common) means overloaded for whatever reason. Shorted battery, high draw for a long time.
Regulator: bad regulator, shorted rotor (not common). Regulators only control the rotor and the rotor is just a magnet.
In the late 70's early 80's vehicles were drawing more and all it took was a shorted lead acid battery to cook one. What normally went was the stator and or the bridge. Modern alternators are harder to kill. Things still happen though.

Most of the failures I see with custom charging set up's is not hooking things up correctly and wrong combinations of parts. Throw in a weak battery or a poor connection and you get failure.

Hope this helps some.

Scott

 

[nickburt]

"Result - one alternator ends up try to drive the other as a motor, hence why the regulator has failed."
Not really possible.

The diagram that Tim posted is the better one. It show's the layout of the average charging system.

Alternators are just electromagnets surrounded by a coil. Put simply the Stator picks up the magnetic field produced by the rotor. The Stator puts out AC voltage, the Bridge converts to DC. The regulator controls the rotor magnetic field and senses the output through the diode trio.

In the past, people ran dual alternators for more output. The large high output alternators were just too big to fit things smaller than a medium duty truck. In the 80's higher output in a smaller case started showing up. These gave people the option of not needing two alternators. There were a few who still preferred to run two but, I don't see the need.
I use to build custom alternators for people on the side. I've done a few dual setups. The style of alternator will depend on how it senses the systems voltage. If set up properly they wont interfere with each other.
The question is, how large was the second alternator? Most alternators don't like being run more than 70-80% of their output for long periods of time. To figure out what happened you would have to test the pieces and see what failed.
Burned stator(most common) means overloaded for whatever reason. Shorted battery, high draw for a long time.
Regulator: bad regulator, shorted rotor (not common). Regulators only control the rotor and the rotor is just a magnet.
In the late 70's early 80's vehicles were drawing more and all it took was a shorted lead acid battery to cook one. What normally went was the stator and or the bridge. Modern alternators are harder to kill. Things still happen though.

Most of the failures I see with custom charging set up's is not hooking things up correctly and wrong combinations of parts. Throw in a weak battery or a poor connection and you get failure.

Hope this helps some.

Scott

Many thanks @smritte
Yes, my comment about motoring one of the alternators is wrong.

A mate of mine has fitted dual alternators.
Both alternators are identical (12v, 100amp). [.......snip..........]

As far as I'm aware, no big loads were applied at any time, with the exception of normal starting and, for a very short period, jump starting a Disco TD5.
Haven't stripped the failed alternator yet (they are both still under warranty - both bought from the same supplier at the same time and are standard type alternator fitted to a 200 TDI Defender), but tested it's output and there's nothing, so regulator pack is probably what's failed. Would need to strip and test for AC input to the pack to determine if stator OK or not.
Suspect he's going to try a warranty claim, so won't get chance to strip and test.

 

[smritte]

Get them warrantied. Recheck your wiring. As I said, the newer alternators are fairly hard to kill. On some of the delco SI series alternators (not sure what you have) The control had 2 terminals. One was designed to go through a bulb and the other was 12v switched. If you hooked 12 volts to the wrong one you cooked the regulator. The sensing wire can be critical on some, be sure to check that also.

Good luck

Scott

 

[nickburt]

Bog standard, B+, WL and W, can't really get it wrong.