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hns1934 09-27-2008 04:45 PM

Switching battery banks with engine running
This is basically an alternator question. I have the typical battery switch OFF, 1, BOTH, 2. This is a mechanical switch designed to make contact with the BOTH position while still making contact with Bat 1 or Bat 2. So the question is will the alternator be damaged if I switch from Bat 1 to BOTH and then to Bat 2?? The alternator will always see a battery when doing this but there may be change in voltage from one battery to the other and also when in the BOTH position. I called Balmar and could not get a straight ans....they recommended a dual batter isolator. I still want to know if the alternator will be damaged and if so why? Thanks for your help

SteveInMD 09-27-2008 04:58 PM

I can't see how it would be damaged. All modern battery switches are make-before-break switches. As long as you don't pass the off position while switching, go for it. If you want to be sure, turn on a light with the engine off, then slowly rotate the switch and see when the light goes out.

ebs001 09-27-2008 05:26 PM

DO NOT DO THIS. You may only switch from 1 battery bank to the other or both if the switch has alternator field disconnect. You will burn out your alternator with even a momentary break in connection from your batteries.

SEMIJim 09-27-2008 05:32 PM

The answer is: It depends.

As SteveInMD noted: If it's a modern switch: It should be make-before-break. Problem is: Switch contacts wear. Also, as Steve noted: If you accidentally go past "1," to "OFF,": *poof*. I accidentally did that the other day, but we were not motoring, so all that happened was we lost our instuments and so-on. My recommendation is: Don't switch the switch whilst motoring.

I plan to make things fool-proof (insofar as that's possible) by going with the Blue Sea Systems e-Series Battery Switch Dual Circuit Plus [tm]

coupled with the Xantrex Echo-Charge

Switch from "OFF" to "ON" and the starter battery's connected to the starting system--the house bank to the rest. The echo charger will ensure both get charged, regardless of switch position.


Giulietta 09-27-2008 05:37 PM

the OP made a -good question. I am curious about it too

I've had the red wire coming out of my alternator disconnected once, with the engine runing, for over 1 hour, when I realized the batteries were not charging.

I looked around and found the wire disconnected at the alternator.

Nothing was damaged..the bateries, or alternator.

I have on other occasions, maybe once or twice, switched from Bat 1 to Bat 2 with the engine runing and nothing happened..

was I lucky? or is my engine good?

So one should not have any thing that charges the bateries when selectin 1, 2 or both??

I never did that in my whole life an nothing ever hapened.

SteveInMD 09-27-2008 05:52 PM

I've done this on maybe 25 different boats over many years and never once have I damaged an alternator. I rarely do it any more since I always run on both batteries all of the time now.

Giulietta 09-27-2008 06:06 PM

So do I..

I also run on both allways. I only have 2 75amp car batteries, (one for the engine one for the house)..allways used car batteries, they're lighter and cheap. And if damaged before the warranty of 2 years expire, I take them to the supermarket where I bought them and get a new one.

I have a 25 amp for the VHF (in europe the VHF needs a separate battery by law), and a 50 amp for the anchor windlass in the front.

One day we forgot the lights on and left, and when we returned we had no battery to start the engine.

So I brought the windlass bat and started the engine..then charged the 2 main ones...

ebs001 09-27-2008 06:07 PM

I'm just quoting from West Marine and from my alternators manual. It may not happen but a momentary break between batteries and alternator will burnout your alternator and you cannot be sure when swithing between batteries that you will not disconnect the alternator from the batteries. The field must have a charge in order to excite the field. So Gui it's possible that you could run the alternator and because the field was not excited your alternator did not burn out. There are proper switches which allow you to switch while the engine is running, Semi pointed out one. But they are far more expensive and it's just bad practice to switch unless you have one of these switches. Murphy's law.

SteveInMD 09-27-2008 06:57 PM

I disagree that it's bad practice. I don't think your going to come up with one right answer on this one.

ebs001 09-27-2008 07:14 PM

Steve, maybe Captain Ron's law applies - if it's going to happen it's going to happen out there. If there is a possibility of frying your alternator - that's bad practice.

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