Okay to cross alternators?

Hello all,

Attached is a diagram of the switch system I have created for my 31’ Chris Craft Commander (1969) with two 1987 Crusader 270s. I just bought the boat and discovered one alternator was blown and the wiring a mess. I have separated the wiring so as to separate the two engines. I do not want to use an isolator or any type of automatic switch, I want complete manual control over the system at all times.

My question is, if I put the Start 1/Start 2 Switch on the “Both” setting will it damage my new alternators (Motorola 8MR2022K).

Thanks in advance.

Steve

The posted schematic is cropped too much to see the alt. Mot hasn't made alternators since the late '70's. They are worth rebuilding, however.

The schematic is only meant to show the 1-2-Both switch setup. As for the term NEW, I suppose I should have said newly rebuilt.

Steve, I'm quite fussy, and I do quite a bit of Marine Electrical and have for years..... but your battery schematic is rather odd to me. I think that you can do much better with your battery configuration and MBSS placement.

By the way... it is very helpful to show the rear terminals of the MBSS schematically. I.E., #1 terminal, #2 terminal, and the "common" terminal.


The short answer (even though I'm not understanding your configuration), is yes..... one engine alternator can and will contribute the charge rate to two (2) battery banks simultaneously via the ALL/BOTH MBSS selection. But the question is perhaps "why" when this could be so easy and simple?

MBSS = Main Battery Selector Switch
SLBB = start load battery bank
HLBB = house load battery bank
ACR = auto charge relay... which may also serve as a combining unit

Here are two alternatives for a twin engine hull.
One scenario calls out 2 SLBB, and 1 HLBB.
The other scenario calls out ony 1 SLBB, and a single shared large HLBB.

Dual station is optional.

I have been using the second scenario for years.... and I have had no issues to date.

Rick, thanks for taking the time with a very detailed reply. I saw some other wiring diagrams that you contributed to another thread and they were the model for what I came up with (for a 45 year old boat with little in the way of voltage assessment and control). I did label one switch 1-2-Common, the other was S-H-Common, S being 1 and H being 2. I understood my own drawing but should have made it more clear for everyone else I suppose. The upload demanded that I crop the image to allow uploading and I could not shrink the image without losing a lot of resolution, so I cut a lot out/off the original.

Anyway, my question is still the same. If 1=port engine/alternator/starting battery, and 2=starboard engine/alternator/battery, if I should switch to BOTH while one or both engines are running, or perhaps because of a low starting battery on one engine, will it damage one or both of my alternators?

Steve, the answer is still NO.
Alternators can be combined... as well as battery banks being combined during charging.

The down side to this is.... a battery charger, or an engine alternator, performs best when it can sense the battery bank independently.... and then predicate the charge rate accordingly.

Toss two Delco 10si alternators in there.... and and go have fun!


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Thanks Rick. I've been doing a bit of searching and have seen various comments, all of which only served to confuse. I figured that if I can jump off an engine from another and the alternators are not damaged then it stands to reason that could be done on a boat too (I'm new to powerboats and have only had sailboats with single engines). I don't intend to switch to BOTH, I just want to make sure that if I screw up and turn it to both that I don't junk a couple of alternators.

The short answer to the original question is No.

I'd emphasize it only applies to the "static situation" posed in that question.

I'd also encourage NOT changing any switch settings with the engines running as too many unknowns still exist.

Thanks makomark. I just want to make sure that if it does happen, nothing will self-destruct. The control panel is actually in the head (just forward of the helm) and I don't want a guest to make a mistake. Thanks for all who replied and offered the wisdom of their experience!

Alternator diodes get killed, usually shorted, when a spinning alternator on a battery suddenly is presented with an open circuit on the B+ terminal. THat is, a switch disconnects it, even for a very short time.

Okay, easy enough, I can run them on BOTH but don't switch anything while the engines are running...simple enough. Thanks all!

Steve

Are we talking about a BS (battery switch)......, or are we talking about an MBSS (main battery selector switch)?
An MBSS of the more recent 25 years or so, have all been "make-before-break" style contacts.
With this style MBSS, there should be no issue when switching back-and-forth between battery banks

They're new battery selector switches. But there should not be a reason (short of an emergency) that I would turn a switch with the engines running so it's no big deal. Thanks.

Steve, understood!
But should you find yourself switching from one bank to another while the engine is running....., it will be safe to do!

Now..... that said..., you DO NOT want to pass through the OFF position while the engine is running!
Doing that could damage an alternator.




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