# Combining start circuit with charging circut

Discussion in 'Electrical Systems' started by mikereed100, Jun 28, 2010.

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### mikereed100Junior Member

I am putting together an electrical system for my 46' cat and am learning as I go. The wire run from the engines to the house bank is as much as 33'. I would like to use the 100A alternators on the engines to charge the house batteries as well as use the house bank to start the engines in case of start battery failure while keeping the cost and weight of wiring as low as I can.

The wiring diagrams I have studied show a separate charging circuit and start circuit, but I am wondering if it would not be feasible to combine the two so that there is a single, large cable running from the starter to a paralleling relay and then to the house bank with a bypass switch for starting. I do not know how much the starters draw and I am 8,000 miles away right now so I can't check, but they are on 3cylinder diesels of 29hp. I assume they would draw about 150A, 450A peak.

Thanks,
Mike

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### CDKretired engineer

The simplest way is to use a Vetus battery switch with 4 positions: Bank1, Bank2, Both and None. But you have to think about turning the switch.

I once made a motorized switch for a guy who was afraid he'd forget to turn the switch.

The distance from the engine to the house bank requires a heavy cable to obtain the required starting current. Is relocation an option?

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### mikereed100Junior Member

Thanks CDK,

What I'm wondering though is can I combine the charging circuit from the alternator with the start circuit from the house bank as shown in the diagram? This would save a couple long runs of 1/0 cable.

Mike

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4. Joined: Aug 2007
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### CDKretired engineer

Yes Mike, you can do it that way. If you choose cables that can handle the starting current they will have no significant voltage drop while charging.

With the central and either the left or right switch closed, one starter battery is parallel to the house bank, so you draw current from both. That is not a problem as long as you know what you are doing.

You could bridge the left and right switches with a diode. That would ensure charging the house bank even when the switches are open, albeit at reduced rate.

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### murdomackNew Member

Do you need two ACR's? Would one wired across both starter battery switch terminals not do the same as you have, but would also stop you draining any high voltage from your start batteries through the distribution panel?

Regarding your question about the charging circuit, when you buy a new engine the alternator charging circuit is usually to the starter cable anyway, although we probably re-route it to a diode splitter in a lot of cases.

Bluesea.com give a lot of diagrams relating to ACR's

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### murdomackNew Member

Why not have the three switches coming off your house bank multi-terminal, one to each starter battery and one to the house distribution panel. Then just by-pass the two starter battery switches with your ACR's as before. That way you can isolate the house/or other engine when jump-starting, and all your batteries will automaticaly be getting a charge if a high enough voltage is present.
You will need isolating switches and fuses at the starter batteries as well.

Edit: You would need an isolation switch between your house batteries and your multi-terminal as well.

Last edited: Jun 30, 2010
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### mikereed100Junior Member

My thanks to both for the clarification and suggestions. This leads to part "B" of my question. Can I dispense with the negative return circuit from the engine to the house bank by grounding through the shafts at the engines and a ground plate at the house bank as drawn in the above circuit diagram, and if so, can I deduct the distance in wiring saved from my cable thickness calculations?

For example: I have 30' between house and starter and 10' between house and ground plate. Can I use 40' for my total length or do I still have to use 60'?

Mike

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### murdomackNew Member

If you are planning on having any combination of jump-starting, I would say you need all the negative battery terminals connected with full thickness cables. You don't want all these starting amps running free, lots of potential for fires and mechanical damage.

You could maybe have your house batteries split, two back at each engine with solenoid switching and ACR's, but it would be quite complicated and probably as expensive as that drum of cable. No, keep it the way you've planned as it gives you all the options.

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