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Cable size for dual battery system

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Hi,

I am having a dual battery system fitted to my new SUV. The battery will be 102Ah and this will feed to the rear of the vehicle then into my caravan and a 175W 12V heating element for the absorption fridge to operate whilst enroute.

Some questions;

a) As the cables are connected to the battery the control circuit will "see" the alternator charging voltage of circa. 14.4V?

b) This also means that the 12V element will be subject to a higher voltage when the alternator is charging and a lower when nor charging. Is this a problem?

c) The cable being used is 50mm2, I get a 1.2% voltdrop for the total length of 10m? Is this acceptable.

I don't see any issues with the above other than having good regulation at the control board to eliminate the fluctuating voltage.

Any thoughts

Cheers
Andrew
 

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Chippie

Member
Andrew,

The way it is done in the UK is to use a 'split charging system'..

A feed is taken from the vehicle battery +ve and feeds a voltage sensing relay. Then this feed is switched to a supplementary socket mounted on the tow bar and appropriately wired...the caravan cable plugs into the said socket...

http://www.western-towing.co.uk/acatalog/Caravan_12S_wiring_diagram.html

Operation:

When the vehicle engine is not running the feed to the van is disconnected, preventing the vehicle's battery becoming discharged...
When the engine is started and running, the voltage rises and is sensed by the relay which then reconnects the caravan...Simple and cheap without the need for a spearate battery...

http://www.western-towing.co.uk/acatalog/info_ELSCR1.html

You could do the same and include you 102Ah battery in the same way...it just means that the fridge is always on when connected to the SUV...

I used 4mmsq single to get from the engine bay to the rear of my car...You only need sufficient current to maintaint the charge in the battery and that size was adequate for my fridge..
 
Last edited:

MikeMl

Well-Known Member
Most Helpful Member
Make sure you use a ground connection across the trailer's ball and hitch. The voltage drop calculation between the front of the SUV and the trailer's fridge must include the voltage drops along the vehicle frame, the hitch, and the trailer frame. You might be surprised to find that the ground drops are higher than what you calculated along the power wire feed.

For a dual battery system, your vehicle should be fitted with a battery isolator
 

chemelec

Well-Known Member
Besides using a Relay (or Solenoid) to Disconnect the second battery, When the car is Not Running, I Also recommend a 30 seconds or even a few minutes, Delay Circuit on the Relay.
I use a Simple 555 Circuit to do this on my vehicle.

This gives a bit of time for the main battery to recover from a Hard Start, before attempting to charge the Spare battery.
Charging Both Batteries Immediately on starting can be VERY Hard on the Alternator. Especially if your batteries are LOW in charge.

Hi,

I am having a dual battery system fitted to my new SUV. The battery will be 102Ah and this will feed to the rear of the vehicle then into my caravan and a 175W 12V heating element for the absorption fridge to operate whilst enroute.

Some questions;

a) As the cables are connected to the battery the control circuit will "see" the alternator charging voltage of circa. 14.4V?

b) This also means that the 12V element will be subject to a higher voltage when the alternator is charging and a lower when nor charging. Is this a problem?

c) The cable being used is 50mm2, I get a 1.2% voltdrop for the total length of 10m? Is this acceptable.

I don't see any issues with the above other than having good regulation at the control board to eliminate the fluctuating voltage.

Any thoughts

Cheers
Andrew
 
Make sure you use a ground connection across the trailer's ball and hitch. The voltage drop calculation between the front of the SUV and the trailer's fridge must include the voltage drops along the vehicle frame, the hitch, and the trailer frame. You might be surprised to find that the ground drops are higher than what you calculated along the power wire feed.

For a dual battery system, your vehicle should be fitted with a battery isolator

Thanks Mike,

not using the hitch or frame as a path for the current. Running 35 or 50mm2 cable direct from the battery to rear of vehicle into special plug (Brad Harris). Special plug connects to same size cable running to the caravan so the only drops could come at the connections which are being specially crimped for me and the cable.

Cheers
Andrew
 
Last edited:
Besides using a Relay (or Solenoid) to Disconnect the second battery, When the car is Not Running, I Also recommend a 30 seconds or even a few minutes, Delay Circuit on the Relay.
I use a Simple 555 Circuit to do this on my vehicle.

This gives a bit of time for the main battery to recover from a Hard Start, before attempting to charge the Spare battery.
Charging Both Batteries Immediately on starting can be VERY Hard on the Alternator. Especially if your batteries are LOW in charge.

Hi Gary,

thanks.

The unit being fitted automatically does all of what you say. On start up the unit will first provide the primary battery until fully charged, it then switches to the secondary battery. I need the secondary battery to run when the car is off, an hour or two while stopped for lunch in ambients of 35°C plus and the fridge looses far too much cool. The unit allows the fridge to draw current from the secondary battery until a low battery alarm point is reached and it then shuts of the secondary battery. At no time is the primary battery used other than for starting the vehicle.

Cheers
Andrew
 
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