Electrical Gurus Please Check My Calculations

Submitted: Thursday, May 08, 2008 at 13:49
ThreadID: 57384 Views:1938 Replies:4 FollowUps:2
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I have been reading everything I can on this and other forums with respect to charging batteries and running a 12 Volt absorption fridge in a caravan from the tow vehicle's alternator.
Would some of the electrical wizards care to comment on my assumptions and calculations please?
Assuming my alternator puts out 13.69 Volts after the engine has been running for 20 minutes or so (measured at the battery terminals) and that there will be say, a 10 metre run to the caravan batteries (in the front boot) and a 15 metre run to the fridge (toward the back of the van)
Firstly the caravan batteries.
Using Tycab Battery Cable ABC119203 13.56 sq mm (approx 6 B&S) which has a resistance of 1.4 ohms/km @20 deg C, if I assume 20 A and 10 metres of cable I will drop 0.28 V and therefore have about 13.41 V at the terminals of the caravan battery.
Secondly the fridge.
Using twin sheath two core flat AWC26503 which is equivalent to 6 mm Auto and which has a resistance of 4.1 ohms/km at 13 A and over 15 metres, I'll drop 0.85V by the time I get to the fridge terminals. So at the fridge terminals there will be 12.84 V

These calculations do not take into account any other resistances such as an Anderson plug, trailer connector or any terminations or relays that might be in the circuit.

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Reply By: Notso - Thursday, May 08, 2008 at 14:15

Thursday, May 08, 2008 at 14:15
I reckon you are over estimating the length of cable you need and the Amp draw of the fridge (I think mine takes about 12 amps), aside from that your assumptions appear sound enough.

My fridge using a slightly different charging method for the van battery gets about 13.4 volts.

AnswerID: 302688

Follow Up By: Ircon - Thursday, May 08, 2008 at 14:29

Thursday, May 08, 2008 at 14:29
Hi Notso,

You're probably right about my assumptions but I decided to be on the consevative side.

Also I know that the caravan manufacture is not using these size wires from the front of the van.

FollowupID: 568795

Reply By: Robin Miller - Thursday, May 08, 2008 at 16:08

Thursday, May 08, 2008 at 16:08
Hi Rosscoe

Looks about right but one of those specs isn't.

The ratio of resistance should be the same as the copper cross section ratio and its not.

E.G. 13.56/6 = 2.26 whereas 4.1/1.4 = 2.93

Good rule is use no less than 8mm square anyway.
Robin Miller

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AnswerID: 302704

Follow Up By: Ircon - Thursday, May 08, 2008 at 16:37

Thursday, May 08, 2008 at 16:37

Sorry. Mixing standards!
6 mm Auto = 4.59 sq mm

13.56/4.59 = 2.95

FollowupID: 568832

Reply By: Star Bug - Thursday, May 08, 2008 at 22:00

Thursday, May 08, 2008 at 22:00
Close..... A common mistake in calculations is people forget the return loss of the cable run.

IE on a 10m run or twin core the total current path is 20m. So you drop 0.28v in each leg. Total volt drop is 0.56v.

Hope this helps
AnswerID: 302794

Reply By: Ray - Friday, May 09, 2008 at 08:30

Friday, May 09, 2008 at 08:30
I have wired my adsorption fridge up from the two batteries (140amphr) that are mounted on the draw bar of my caravan using 6g (B&S) I now have 13.8vollts at the fridge terminal rather than the 10v I had previously. I also are using a couple of solenoid switches that isolate the fridge from the batteries when the ignition is turned off so that the fridge is only operative when the car engine is running. When stationary for any length of time I either run off gas or 240 mains.
AnswerID: 302825

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