Connected battery tach problem?

[Roger Long]

We have an old and simple trawler with twin engines and minimal 12 volt loads. There is no house bank, just an 8D for each engine and the master switch can draw house loads from either or both. We have a generator so I have always left the switch on “Both” on the theory that I’m cutting battery draw down for cabin and anchor lights in half for better battery life. If I should draw them down too far for the engines to start, I can just run the generator for a while. Stove and refrigerator are 120 volt so I start the generator first thing every morning and starting is never and issue.

Recently, I’ve had some mysterious problems with the tachs on one side. Both fly bridge and lower helm behave identically. For a couple days, I would have to run the engine up to nearly full rpm in neutral to get them to come alive. Then, they died with the needles just barely flickering near zero. Alternators were just bench tested and check out. The tachs for the other side are fine.

It’s been suggested elsewhere that a slight drift in the voltage regulator set point or tiny resistance difference in the wiring is causing the voltage from one tach to fool the other into thinking it doesn’t need to start producing power. I’m away from the boat for a month so I can’t experiment but I’d appreciate some thoughts. If I put the master switch on 1 or 2 to isolate the systems and the tach comes alive, the theory will have been proven. If that doesn’t work, I’ve got to plan a day of wiring trouble shooting.

Any other issues with running two alternators to two connected batteries that I’m overlooking? No other problems in the last 4 years. I do know that a bad cell on one battery can take out the other but I keep a close eye on them to be sure the voltage of each remains the same.

 

[High Wire]

Common to see this on a boat run at dockside with a shore powered battery charger. Shutdown shore power. Put a small DC load on that both engines see. A spotlight, a fan, or the refrigerator compressor by opening the door until it starts. I bet the tachs come right up. When the voltage regulator sees no loads, the alternator output goes to zero. The tach has no pulses to count.

 

[diver dave]

Yep: the alternator “W” terminal that drives the tach is one of the 3 phase windings. An AC source. But, if the field winding is totally shut down, then there is no ac drive for the tach. Fortunately, it doesnt take much dc load to make the regulator drive some field current.
I suspect that charging isolators could also be involved. If the setpoint on port is different than stb, the alternator with the lower setpoint will want to shutdown.

 

[sbu22]

Thanks, High Wire! Not the same tach problem as Roger's (one tach would indicate 1 - 2k higher than actual at idle), but I tried securing shore power today before engine start. Tach is spot on.

 

[jungpeter]

Hi Roger,

Any other issues with running two alternators to two connected batteries that I’m overlooking? No other problems in the last 4 years. I do know that a bad cell on one battery can take out the other but I keep a close eye on them to be sure the voltage of each remains the same.

Given you've been operating this way for four years without apparent problems, it may be moot, but you're probably only drawing apreciable charge current on one alternator. Upon energizing, the first alternator in your battery system that gets direction from it's regulator to begin charging will take on 100% of the charge duties for both batteries. The second alternator on the other engine will simply loaf along. It is likely that the 2nd alternator will NEVER do any work.

Again, this may not be a "problem", but it's not ideal. Multi-engine boats gain appreciable electrical robustness with the addition of external voltage regulation with something like the Wakespeed WS500 regulators on each alternator.

Regards,

Pete