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Go Back   SailNet Community > On Board > Gear & Maintenance > Electrical Systems
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Old 05-04-2013
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Battery Selector BOTH load

I have two group 27 AGMs dual purpose. So if I drain down my one bank (let's call it the #2 house) running miscellaneous stuff at anchor or under sail... is it harder for the boat to start on BOTH or 1? Does the lesser charged battery drag the start down? Or is it a combined supply? I'd rather start on BOTH so I'm charging both, right?
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Old 05-04-2013
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Re: Battery Selector BOTH load

If one battery is very drained, then yes, it will make starting a bit harder when you set the switch to BOTH. In that case, I would just start on the charged battery and switch over to BOTH once the engine is running.

One thing you might consider is installing a battery combiner, so that once the engine is running the alternator will charge both batteries even if you leave the battery selector on only one bank.
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Re: Battery Selector BOTH load

I thought that in my current setup I cannot move the selector switch while the engine is running or the alternator could be damaged? Does that device allow you to do that?
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Battery Selector BOTH load

You are correct that the selector should not be moved while the engine is running. With that said, I think that most if us have done it from time to time.

I've had a completely dead battery and been able to start on Both.
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Re: Battery Selector BOTH load

Quote:
Originally Posted by dvuyxx View Post
I thought that in my current setup I cannot move the selector switch while the engine is running or the alternator could be damaged? Does that device allow you to do that?
Battery selectors are supposed to be "non-interrupt" switches. Thus, they let you switch from "1" to "BOTH" to "2" without completely disconnecting, or interrupting, the batteries between the settings. The thing you don't want to do is to switch the selector to "OFF" with the engine running.

Also, to clarify (OK, actually correct) my earlier answer--
If you set the selector to "BOTH" once you get the engine running the drained battery will tend to charge very slowly, since the voltage regulator will be "fooled" by the not-so-drained battery. So, it's probably best to get the engine going, top off the "good" battery for a little bit, and then switch to the drained battery to more efficiently charge it. The way most boats are wired, you're really only supposed to use the "BOTH" setting when neither battery will start the engine on its own.
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Re: Battery Selector BOTH load

Quote:
Originally Posted by SlowButSteady View Post
Battery selectors are supposed to be "non-interrupt" switches. Thus, they let you switch from "1" to "BOTH" to "2" without completely disconnecting, or interrupting, the batteries between the settings. The thing you don't want to do is to switch the selector to "OFF" with the engine running.

Also, to clarify (OK, actually correct) my earlier answer--
If you set the selector to "BOTH" once you get the engine running the drained battery will tend to charge very slowly, since the voltage regulator will be "fooled" by the not-so-drained battery. So, it's probably best to get the engine going, top off the "good" battery for a little bit, and then switch to the drained battery to more efficiently charge it. The way most boats are wired, you're really only supposed to use the "BOTH" setting when neither battery will start the engine on its own.
I agree 100% with the first part. It should be fine to move the switch while running. Just don't go to off. It can be convienient however to start on both and then there is nothing to foget.

To the original question, yes a dead battery can drain the whole system if its low enough.

But I disagree with the second half. It's best to charge at both and each battery will draw the current it can accept - the good battery can't fool the system the way you suggest. The one potential problem where you'd want to select one or the other is if the acceptance rate of your dead bank was higher than the alternator output. But even then most alternators should effectively limit current. And this should be a rare circumstance.
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