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I have feared running out of battery voltage to start the boat engine after running the electronics with the engine off. Would something as simple as this figure work? Obviously both are 12V. Main is deep cycle lead acid and aux battery would be smaller lead-acid. When the electronics die, i would know to start the engine and flick the switch on and start charging both again.diagram

I have seen battery isolators - but that seems overly complicated for this application? I suppose it would eliminate the need for a manual switch and remembering that. If i dont flick the switch off when the engine is off, then i would drain both batteries.

Thanks for any ideas guys. Neil

n c
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  • Remotely related thread: http://mechanics.stackexchange.com/a/16692 The advantage of a battery isolator is that you don't have to worry about forgetting to flip the switch open and letting the radio (and other appliances) drain the starter battery. – Nick Alexeev Jul 26 '15 at 21:56
  • Works in principle but has issues as WRB says. Placing eg a low voltage light bulb in series with the switch can help, or a low R resistor. This limits surge current but allows aux bat to charge fully in time. – Russell McMahon Jul 26 '15 at 23:05

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It is common practice on "simple" boats to have two batteries with a "1-both-2" switch to select one or both batteries as needed. It is usually suggested that you put the switch on "both" while the engine is running, to charge both batteries, then switch to "1" or "2" when the engine is off, so only one battery is discharged - usually, you should alternate which battery is used with the engine off to equalize wear on the batteries. I haven't heard of any surge problems as suggested by WhatRoughBeast when using this system.

A more complex system would have dedicated "start" and "house" batteries, with the start battery only used for engine related things, and the house battery powering all non-engine loads (lights, electronics, fridge, etc.) There are various battery combiner or isolater devices to ensure that both batteries are charged when the engine is running, without user intervention. (On my boat, I have two alternators, one to charge the start battery, and the other for the house battery.)

Peter Bennett
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  • Thanks Peter. Will look more into 'simple' option since is only a small boat - no galley/head. just 12v electronics- load might even be so small engine would have to be off for a long time before discharging the battery too low. I should do the load calculation to come up with rough allowable time of operation without the engine running. Cant imagine radio/depth sounder draw much. – n c Jul 27 '15 at 13:37
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This is a very bad idea, particularly for the lifetime of the aux battery. If the aux is discharged, and the main is charged, when you close the switch you will get an enormous current surge - basically, as much as the main battery can produce. This will probably weld your switch shut or burn it out. Neither is good. Additionally, the surge won't be good for the aux battery, either, and you may wind up damaging it, particularly if you do it often.

WhatRoughBeast
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  • OK. Thanks WRB! Definitely NOT simple! Will not attempt. So on boats with the A or B and A+B battery type switch there must be more to it than a simple switch I guess or this same surge would happen there too. Appreciate the answer. – n c Jul 27 '15 at 01:20