Question on cable lengths in dual battery system

Thread starter bluey
Start date Jan 31, 2013

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bluey

Seaman

Jan 31, 2013

#1

Greetings,

This is probably a dumb question but I need to ask it anyway.

I know when working out the AWG size of battery cables, the length of the proposed circuit is a critical part of the calculation. But how is this applied in a dual system where you have a cranking battery, a house battery, and a three way battery switch.

The cranking battery's circuit runs via (+) red from the battery to the switch (1) then via (com or both) to the starter motor, and returns to the cranking battery via (-) black. The deep cycle house battery will be connected via switch (2) but won't be used for cranking the engine.

So let's suppose there and back in the cranking circuit is a run of 10 feet. Is it necessary then to also included the house battery circuit into the equation? , because the run then would be more like twenty feet.

Thanks,
bluey.

tpenfield

Moderator

Staff member

Jan 31, 2013

#2

Re: Question on cable lengths in dual battery system

Typically the AWG is the same for all battery leads and switch lead to the starter, and the extra run length of the cables is not that much more so as to make a difference in the wire gauge.

Don S

Honorary Moderator Emeritus

Jan 31, 2013

#3

Re: Question on cable lengths in dual battery system

If it's possible to use the house battery as a starting battery, then you need the proper size wiring for starting.
With a standard Off, 1-2-Both battery switch you can turn it to position 2 (house battery) and use it for starting the engine.

Silvertip

Supreme Mariner

Jan 31, 2013

#4

Re: Question on cable lengths in dual battery system

If you don't plan to start the engine from position #2 you are limiting thereason for the switch. It's main purpose is to allow you to do just that.

Grandad

Lieutenant Commander

Jan 31, 2013

#5

Re: Question on cable lengths in dual battery system

The reason for correct sizing is to ensure adequate voltage for specific components to operate properly. If you have several components and/or several switching paths, you'll have to do a calculation for each scenario, from the battery to each component and back to the battery. Don't forget that you need to do this calculation based on all the loads that could be connected at the same time, each contributing to some current flow. To save time, you might start with the worst case path/situation to give you some perspective on the need for further calculations. - Grandad

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Auger01

Petty Officer 1st Class

Jan 31, 2013

#6

Re: Question on cable lengths in dual battery system

If the house battery is deeply discharged and then paralled with a fully charged starting battery, there will be a significant current flow to the house battery as it equalizes voltage with the starting battery. If small wire ( < 6 AWG) is run to the house battery you will be blowing fuses or heating up wire pretty good. Also if you forget to turn off the house battery and try to start the engine, you will definately smoke the skinny wires.

bruceb58

Supreme Mariner

Jan 31, 2013

#7

Re: Question on cable lengths in dual battery system

Auger01 said:
If the house battery is deeply discharged and then paralled with a fully charged starting battery, there will be a significant current flow to the house battery as it equalizes voltage with the starting battery. If small wire ( < 6 AWG) is run to the house battery you will be blowing fuses or heating up wire pretty good.

Not likely but depends on how small and how long. In the case where you connect a deeply discharged battery to a fully charged one, the smaller gauge wire will likely have enough voltage drop to effectively reduce the current enough that you probably wouldn't have an issue. That's why some ACRs specify a minimum length of cable between the battery and the ACR. They want a larger voltage drop to limit the current.

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glenn property of pam

Commander

Jan 31, 2013

#8

Re: Question on cable lengths in dual battery system

rule of thumb: take the longest wire lenth to and from the starter and that is the lenth used to calculate the gauge of the wire. all battery, perko switch and motor starter connections will be the same gauge.
The wire to the fuse panel from the switch will be sized for the max load on the fuse panel. there is no need to add the load on the fuse panel to the load required for starting (in 99% of cases) as the starter is designated as a short term load.

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bluey

Seaman

Feb 1, 2013

#9

Re: Question on cable lengths in dual battery system

glenn property of pam said:
rule of thumb: take the longest wire lenth to and from the starter and that is the lenth used to calculate the gauge of the wire. all battery, perko switch and motor starter connections will be the same gauge.
The wire to the fuse panel from the switch will be sized for the max load on the fuse panel. there is no need to add the load on the fuse panel to the load required for starting (in 99% of cases) as the starter is designated as a short term load.

Sorry for the delay in responding to all the replies . Been busy doing other things for "She, who must be obeyed" .

I thank you all for the answers to my question. I suppose there could be times when I might need to use the house battery to start the engine so it's circuit will need to be included as well, as some of you advise. I was really just trying to find a way of reducing my costs.

My thanks again to everyone.

bluey.

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Auger01

Petty Officer 1st Class

Feb 1, 2013

#10

Re: Question on cable lengths in dual battery system

bruceb58 said:
Not likely but depends on how small and how long. In the case where you connect a deeply discharged battery to a fully charged one, the smaller gauge wire will likely have enough voltage drop to effectively reduce the current enough that you probably wouldn't have an issue. That's why some ACRs specify a minimum length of cable between the battery and the ACR. They want a larger voltage drop to limit the current.

That is unusual, could you show me where to find that documentation?

bruceb58

Supreme Mariner

Feb 1, 2013

#11

Re: Question on cable lengths in dual battery system

bluey said:
Sorry for the delay in responding to all the replies . Been busy doing other things for "She, who must be obeyed" .

I thank you all for the answers to my question. I suppose there could be times when I might need to use the house battery to start the engine so it's circuit will need to be included as well, as some of you advise. I was really just trying to find a way of reducing my costs.

My thanks again to everyone.

bluey.

For the best prices on battery cables, go here:
GenuineDealz.com - Marine Electrical, Boat Wire & Cable, Custom Battery Cables

B

bluey

Seaman

Feb 2, 2013

#12

Re: Question on cable lengths in dual battery system

bruceb58 said:
For the best prices on battery cables, go here:
GenuineDealz.com - Marine Electrical, Boat Wire & Cable, Custom Battery Cables

G'day bruce58,

Thanks for the link. I've been comparing prices and they do seem to be the cheapest, trouble is I'm in Australia and the international shipping cost is a killer. Genuinedealz website advertises "Free shipping across the USA", which is kinda like rubbing salt into the wound. It would be much better if it read "Free shipping across the Pacific!" : (

Cheers,
bluey.

bruceb58

Supreme Mariner

Feb 2, 2013

#13

Re: Question on cable lengths in dual battery system

jeez...didn't notice your location! Sorry about that!

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