I've been following a thread over on RVnet about using the vehicle's alternator to charge the house batteries while you drive. Somebody there claimed that demanding heavy output from an alternator like that could lead to damaging the diodes enough that it would lead to increased ac ripple. Said ac ripple could damage or destroy engine control computers and similar advanced stuff. There was also a claim that you could measure ac ripple by setting a good digital multi-meter to the ac setting and touch it to the positive and negative battery posts.

Of course, the debate over there has become somewhat contentious. But of course, over there, you can find people who will argue with you if you claim the sun rises in the east.

Anyway, I'd never heard of this before, and I was wondering what the people over here know about it? Thanks.

 

Your alternator is really a 3 phase AC generator. There are three wires coming off of it internally that generate all 3 phases of A/C. In case you did not realize, AC or alternating current means the little electron dudes in the wires are running forward, turning around and running backwards in the wire. Your house wiring is like this, the current flows one direction, turns around, and flows the opposite direction. In your house it does this at a steady 60 times per second. On the alternator in your car, the frequency that it alternates is determined by how fast the alternator is turning.

Inside the alternator they have a network of 6 diodes. Diodes are like electrical check valves, they only let the current run one direction, and stop it when it tries to go the opposite direction. They have the diodes organized so when one circuit tries to go the opposite direction and the diode stops it, another circuit will contribute during that time.

When all six diodes are working, you still get ripple, but it is a small bumpy looking dc voltage. If one of the diodes burns out, they usually open up and that disconnects that part of the circuit, so you get a "hole" of sorts in your bumpy dc voltage, giving you a larger bump, more ripple.

They are correct, you can put a meter on AC and read this ripple. You will always have some. There is a spec for the alternator for a maximum ripple voltage that is allowed. The alternator testors in the stores test for this, and this how they can determine if there is one or more bad diodes in the alternator.

I have never seen excess ripple damage the electronics in the vehicle. It will make it go crazy and not run correctly, but I have never experience permanent damage. You can prevent damage to the alternator and it's diodes by controlling the amount of charge current that can be drawn from the alternator.

If you have a simple 10 gauge or so charge wire going to the house batteries with a relay to turn it off when the ignition is off, this usually will not pull excessive current from the alternator. #1, you will put a 30 amp circuit breaker in this line. If this line draws over 30 amps, the circuit breaker will open up and stop it. When it cools off it will close and run a little while longer till it heats up and trips again. It will keep doing this till the batteries charge enough to stop pulling excessive current. You really should not charge the batteries at a very high rate in the first place, 30 amps is plenty. #2, having a smaller 10 gauge wire running some distance from the front of the rv to the location of the house batteries adds some protection. As the current draw increases, the resistance in this wire will increase the voltage drop across this wire, self regulating the charge current. I know this sounds strange to some people, but the 30 amp circuit breaker will keep the smaller wire from getting too hot, and the resistance of the wire is extra protection for this charge circuit.

 

Thank you for responding. What I've quoted above is exactly what I was looking for.

BTW, I have a ham radio technician license, and a background as an amateur soundman for local rock bands, so I do understand the fundamentals. But this thread is liable to be seen by lots of people who know nothing about it, so it was probably good to go into the basic background.

As for 10 gauge wire and 30 amp circuit breakers, uh, the overlanding and expedition crowd usually run 4 gauge and 80 or 100 amp breakers between the house and the engine, along with ACRs from Blue Sea Marine and similar. Wonder if you could comment on how THAT much current is liable to affect the alternator and the diodes?

Again, thank you for responding.

 

I think a very large wire like that, connecting a alternator and good battery to a very low highly discharged battery or bank of batteries is asking for trouble. There is no way to control the current exchange that is going to happen when the charged battery and alternator see the dead batteries, both circuits will try to equalize and a very high rate.