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Solenoid vs. Battery switch


Solenoid vs. Battery switch

Old 05-10-2002, 03:06 PM
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Default Solenoid vs. Battery switch

Has anyone installed a normally open relay (solenoid) to replace the 1/both/2/off battery switch?

Here's my though, insert a normally open relay on the starting battery and energize it off an ignition switched source.

Run the deep cycle hardwired in parallel to the starter and everything else. That way the radio, bilge etc is always connected, and when you turn the key the starting battery and the deep cycle turn over the engine and are connected to the alternator for charging. When the keys off the starting battery is isolated.

It seems like a foolproof solution to always having to manually switch the batteries...... but there has to be a catch it seems too easy.

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Old 05-10-2002, 03:50 PM
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catch? maybe.

If your "house" battery is extremely low, and your "starting" battery is hot as a firecracker then when you go to turn the ignition on, the solenoid will energize and will throw 1000 amps into your dead battery. Deep cycle or not, this repetitive jolting of massive amounts of current is not hospitable to long life of the "house" battery. It will not last very long in this environment. It can also blow up the dead battery if it gets a shorted cell (nasty, very nasty). Also, an 8-volt charge on the house battery will pull your voltage at the starter down significantly. If you have a hard-cranking motor, this may be too much of a voltage drop to start your motor if it has vapor locked or is otherwise finicky. Without the switch, you have no way to disconnect the dead battery from the loop (besides taking the cable loose from it).

Secondly, in the case of a dead house and hot start battery, you will pass lots of amps through the cables and solenoid until the two batteries equalize out (which may take ten minutes or more). This may overheat the battery cables or the solenoid (fire and sparks and boats do not mix).

Your idea has merit, but I would prefer that you use one of the following methods:

1) CHEAPEST: Battery switch Off-A-Both-B. Swap to B when sitting. Swap to A to start if B is dead. After starting, swap back to B to charge with the 35-to-55 amp alternator/regulator.

2) Starter and motor wired to A. House and access wired to B. Battery isolator hooked to alternator to charge both banks when running. Solenoid that will strap A and B together when neccessary activated by a manual switch on the dash (all diesel boats have manually activated battery strapping solenoids).

various combinations of configurations can be used with Battery Switches, Battery Buddies, Isolators, Solenoids, etc.

Have fun.
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Old 05-10-2002, 05:10 PM
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The solution is to run 2 batteries and use a isolator. The isolator will charge both batteries when the engine is running and your starting and house batteries will be totally seperate. If you have an ( OFF- A-BOTH-B ) switch leave it in the A position unless you need the extra power.
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Old 05-13-2002, 12:26 PM
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Default It will work great!!


Your idea will work just fine. I have rigged many boats and motor homes this way and it works great. I have my own boat rigged this same way and noooo problems. A solid state isolator will work too except one problem: if it fails (opens up) you will have two dead batteries and possibly ruin your alternator. If your relay fails, just your auxilary battery will be effected. Make sure you use a continuos duty solenoid. You cannot use one made for a starter. I prefer Cole Hersee part # 24117. It has a plasticized housing made for marine use. I have them If you can't find one close to home. Any questions just holler.

Steve Hixson

Art's Auto Electric & Marine

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