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Temporarily connect starter and domestic batteries


PaulG

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My engine can be a bit of a s*d to start in the winter. I'm thinking of adding a switch so that I can temporarily connect the domestic bank in parallel with the starter battery.

Assuming that I use adequate cable size and a decent isolation switch, does anyone see any problems with this?

(Apart from forgetting to put the switch back and flattening the starter battery, of course!)

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13 minutes ago, PaulG said:

My engine can be a bit of a s*d to start in the winter. I'm thinking of adding a switch so that I can temporarily connect the domestic bank in parallel with the starter battery.

Assuming that I use adequate cable size and a decent isolation switch, does anyone see any problems with this?

(Apart from forgetting to put the switch back and flattening the starter battery, of course!)

It's quite a common feature and built into quite a few proper marine battery switches.   Another method of your start needs charging in winter via lack of use would be to get A Voltage sensible relay instead of a split charge relay if you charge your main batteries using another method that's not the engine.   (Although if you describe your current setup we can advise better)

Edited by Robbo
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31 minutes ago, PaulG said:

Assuming that I use adequate cable size and a decent isolation switch, does anyone see any problems with this?

(Apart from forgetting to put the switch back and flattening the starter battery, of course!)

If you use an equally decent relay, powered via the ignition key, instead of the manual isolation switch, the problem of forgetting goes away (as long as you remember to turn the key off..)

Edited by eggpie
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Or, instead of repeatedly calling out the emergency services in the form of your domestic bank, you could just resolve the actual issues which are your engine being a dog to start and/or inefficient starter system. Maybe start by ensuring your starter battery alternator belt isn't slipping and all electrical connections are clean and tight.

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11 minutes ago, PaulG said:

My engine can be a bit of a s*d to start in the winter. I'm thinking of adding a switch so that I can temporarily connect the domestic bank in parallel with the starter battery.

Assuming that I use adequate cable size and a decent isolation switch, does anyone see any problems with this?

(Apart from forgetting to put the switch back and flattening the starter battery, of course!)

Well, the main issue is that you engage the switch when the domestic bank is depleted, and it actually drags the starter down.

Not sure that it would, but it is a thought.

Perhaps done with big latching relay and circuitry that will;

1) not engage when domestic is depleted

2) unlatch when the voltage rises above 13 volts

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3 hours ago, PaulG said:

My engine can be a bit of a s*d to start in the winter. I'm thinking of adding a switch so that I can temporarily connect the domestic bank in parallel with the starter battery.

Assuming that I use adequate cable size and a decent isolation switch, does anyone see any problems with this?

(Apart from forgetting to put the switch back and flattening the starter battery, of course!)

When I needed to parallel both batteries on my last shareboat, having flattened the starter battery trying to bleed the fuel system after removing gallons of water from the diesel tank (turned out the boat's base had never heard of condensation forming in their bulk tank and we're selling water branded as diesel :(), I improvised and used a stout screwdriver across the terminals.

Only do this ifor you have a suitable screwdriver you are prepared to ruin and know what you are doing. Definately don't try it into the batteries have recently been charged. 

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For the odd occasion it is needed, a simple jump lead will suffice. If it needs to be done regularly, then you are better off sorting out the starting problem.

I carry one in my battery box for just such an occasion.

Edited by Ex Brummie
Too hasty with my mouse on the enter button
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Thanks for the input, people. I'll have a think about it.

I've been using a jump lead on occasion, and it works OK, but was thinking of a more convenient method.

Mr. Cuthound, I think I'll pass on your screwdriver method if you don't mind.  :D

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8 minutes ago, PaulG said:

I've been using a jump lead on occasion, and it works OK, but was thinking of a more convenient method.

I'd suggest that the most convenient would be to do as Sea Dog suggests:

 

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I use a short jump lead in this situation.

I've also used it when (half of) the blocking diode burnt out, and the ammeter was showing nil on the domestic battery. Used it to temporarily connect the central (alternator) terminal with the domestic battery terminal.

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Jump lead or a big spanner.   Contrary to what a lot of people think, the starter doesn't draw a huge current.   A typical 1.5 - 1.7 litre 4 cylinder will draw about 120 - 140 amps.  Might be a bit more on a four cylinder Gardner!

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