alternator failure - do both batteries get charged?

How are most boats setup (with twins) if an alternator fails? Will the engine ultimately die once the battery runs down or will one alternator continue to run both engines? I know I can toggle back and forth for starting reasons.

My alternators are both original and 24 years old. I'm wondering if it's worth changing one and keeping the old one on board as a spare or just buying a new spare.


Bob

If you got 24 years out of them you did extremly well, why not take them to a shop and have both rebuilt?

Concur with the suggestion to take them in for a rebuild.

The short answer is "It depends". If you can connect the batteries for starting, you can connect them for running. Once running, gasoline engines only require a power source for ignition to keep running. Whether you can/should do that depends upon how the alternator fails. Say the belt breaks - no problem; on the other hand, if a diode fails and shorts to ground (possible but not common), connecting the two systems could disbale bothe engines.

just to clarify, my boat doesn't have a battery switch (1/2/ALL). I have an emergency start button so I can connect to the other battery for starting but it's momentary. I can't leave it in that position. Belt is no problem, I have a couple on board.

I'll put a couple of rebuilds on the winter list. I'm the anti "don't fix it if it ain't broke" guy but I seem to have a knack for thinking of what might fail and then have it happen.

Do the rebuild parts need to be marine specific or is that function built into the alternator? In other words, can an automotive shop do the rebuild?

Bob

Bob, you may want to consider something like this, using MBSS (main battery selector switch). I think having the ability to manage our batteries (whether manual or automatic) is important.

Notice that this shows the Stbd engine cranking on the House Bank. I've been set up this way for years..... no issues to date.
My Stbd engine has no dedicated cranking bank..... just a large House Bank.
(it helps to have quick firing engines, and HTGR/PMGR starter motors)
And with the combing feature, I have my back-up.

Note the optional momentary combining solenoid switch.... (sounds like you have something similar).
Note the connection points for the O/B charger..... (it's permanent, and no battery "rat's nest").



What is not shown in the above, is an Un-Interruptible power source circuit for bilge pump float switch, as shown in this image. (if you moore this boat, it's not a bad idea)


Not suggesting that you make these changes, nor that it be done exactly like this..... just food for thought!

I'd sure want to have MBSS's in this system.


Oh, one more thought. I'd recommend that we not keep used parts on board for spares. When we need them most..... we don't want a failure.
.

For the most part, very few replacement electrical parts are marine specific.

An auto electric shop should be capable of doing the rebuild.

The key is to make sure they know they are marine units and the don't do anything that would de-marinize them. They should be able to show you where the marine provisions are before they take it apart. If there is any hesitation on the shop's part, I'd find another shop. Same goes if they tell you there isn't a "marine grade" (USCG approved for safety) alternator.

If you regularly go offshore (too far to call for a tow) and/or spend overnights away from the dock you may want to think about doing a modernization to your charging system. If you just day trip, then you are pretty much OK as you are.

Fully agree!

Note that in my first schematic image, there is a combine mode offered via using ALL/BOTH selections of the two MBSS's.
This would be considered "Manual" setting of a combine mode.

Blueseas has a great little unit called an ACR that will control/direct alternator or O/B charger current ...... (usually unecessary with a multiple bank o/b charger.... but great for alternator charging).



Several ways to wire one, but again....... IMO, best to take these leads to the # 1 and # 2 terminals of the MBSS, rather than further any "Rat's Nest" caused by direct battery connections.
(also helps avoid small terminal corrosion from a direct battery connection)



Point being; if you were to look at this, schematically it is correct!
Physically, the two battery connections could easily be eliminated by using the MBSS terminals.
If so, we now have cables ONLY connected to our batteries.
It cleans it up, and reduces the likihood of a mistake when replacing batteries.

Your call on that!

.

BTW, I think that your original question "alternator failure....do both batteries get charged?" was answered by Mark.
Agree....... it would depend entirely on how your system was configured, and/or the combining ability.

I am a fan of NO battery switches. My system is completely automatic with virtually no need for me to do anything to it. Both start and house bank are charged from either the alternator when under way or thru a Charles charger on shore power.
I have a single, but twins are pretty much the same way.

o2batsea, the Bluesea automatic systems are great, if that's what you have.
With twins, and in older boats, often the Stbd side supplies house loads as well.... similar to a single engine.
If we have both cranking and house bank on the Stbd side, this too can be configured with a Bluesea system.
It gets somewhat complex during the install, but once done,.... it works well.

For ME, I like having the ability to mechanically turn all 12 vdc power off when not attending the boat (minus anything that requires un-interrupted power).

Rick,

4 years ago we had a completly new set of plans drawn up by a gentlemen in Tampa for both AC and DC systems and looking at your first drawing, this is exactly what we are using. Pleased to know it is recommended.... We have had ZERO trouble with it wired thid way Not an inexpensive proposition to re wire a boat