cable size

ebygum32 · Post by **ebygum32** » Wed Jan 24, 2007 6:59 pm

I am wiring a leisure battery in and am abit confused as to the size of the cable and relay i need,

i have seen three different ideas,

1st uses a standard 30amp fused relay with 35amp cable running from relay to each battery pos+ this came from http://www.showmesome.info/hilux/workshop/battery.htm

2nd uses 70amp cable with 70 amp relay in the same way as the first

3rd uses 30amp relay with 5 amp cable to run to battey pos+ with a 10 amp inline fuse added
http://www.Kampenwagen.co.uk/split%20charge.htm

i think 70 amp cable and relay would be best, not running too much just tv/dvd player etc so no real current there also gonna wire 150w inverter into it so can use laptop or other small 240v items.

dandywarhol · Post by **dandywarhol** » Wed Jan 24, 2007 9:19 pm

Really the heavier the better (within reason) a 150W invertor will draw 150/12 = 12.5 amps. max.

The 70A cable will allow for the leisure battery to be charged without frying the cables if th battery is very discharged.

moonshine · Post by **moonshine** » Thu Jan 25, 2007 12:15 am

A 35 amp relay would soon burn out as it would not handle the full output of the alternator, which it would have to if the leisure battery was nearly flat.

The third option is positively dangerous, in my opinion. If you use 5 amp cable with 10 amp fuses to "protect" it, the cable could melt and leave the fuses intact! Also, how is 5 amp cable supposed to handle the output from an alternator which is capable of producing at least 40 amps?

Option 2 is the onle safe and reliable one, in my opinion.

dandywarhol · Post by **dandywarhol** » Thu Jan 25, 2007 12:22 am

moonshine wrote:A 35 amp relay would soon burn out as it would not handle the full output of the alternator, which it would have to if the leisure battery was nearly flat.

The third option is positively dangerous, in my opinion. If you use 5 amp cable with 10 amp fuses to "protect" it, the cable could melt and leave the fuses intact! Also, how is 5 amp cable supposed to handle the output from an alternator which is capable of producing at least 40 amps?

Option 2 is the onle safe and reliable one, in my opinion.

I'm not so sure about that moonshine because with a voltage sensed relay (not directly from the alternator) the alternator supplies the discharged (through cranking the engine) battery first and doesn't necessarily give full output to the leisure one.
I have only a 35A line fuse in mine and it's never given a problem.

moonshine · Post by **moonshine** » Thu Jan 25, 2007 9:30 am

Hi Dandy,

Even with a simple relay energised through the ignition switch, the alternator will still supply current to the lowest discharged battery first. This would normally be the leisure battery, unless you have had a problem starting and have drained the main battery by an extended cranking period. A normal start would not drain a good battery very much, but prolonged overnight use of the leisure battery could.

It's quite possible you have had no problem with your 35 amp fuses because you have never completely drained your battery, so the alternator has never had to give its full output. My main point though is that you should always use a fuse with a lower rating than that of the cable it is to protect, other wise the cable may give out first. The fuse is supposed to be the weakest link.

pippin · Post by **pippin** » Thu Jan 25, 2007 11:20 am

So, as I have no stamina and always give up first, that makes me the weakest link, I am a fuse!

Most people have this common misconception that a fuse is there to protect the appliance, it is not, it is to protect the wire feeding the appliance.

What is a fuse? A thin piece of wire.
Ergo, what is a piece of wire that is too thin? A fuse.

They can both burn out, but a proper fuse does it in controlled circumstances - eg a glass tube.
A piece of wire that is too thin burns out and can start a fire.

dandywarhol · Post by **dandywarhol** » Thu Jan 25, 2007 12:29 pm

moonshine wrote:Hi Dandy,

Even with a simple relay energised through the ignition switch, the alternator will still supply current to the lowest discharged battery first. This would normally be the leisure battery, unless you have had a problem starting and have drained the main battery by an extended cranking period. A normal start would not drain a good battery very much, but prolonged overnight use of the leisure battery could.

It's quite possible you have had no problem with your 35 amp fuses because you have never completely drained your battery, so the alternator has never had to give its full output. My main point though is that you should always use a fuse with a lower rating than that of the cable it is to protect, other wise the cable may give out first. The fuse is supposed to be the weakest link.

Agreed moonie/pippin............the fuse protects 60A wire on mine.

grumpo · Post by **grumpo** » Thu Jan 25, 2007 1:45 pm

As dandy said previously, there are "intelligent" systems available which
always charge the traction battery first to around 95% before charging
the leisure battery. You can also set an upper limit for the leisure charge
current.

dandywarhol · Post by **dandywarhol** » Thu Jan 25, 2007 8:36 pm

moonshine wrote:Hi Dandy,

Even with a simple relay energised through the ignition switch, the alternator will still supply current to the lowest discharged battery first. This would normally be the leisure battery, unless you have had a problem starting and have drained the main battery by an extended cranking period. A normal start would not drain a good battery very much, but prolonged overnight use of the leisure battery could.

It's quite possible you have had no problem with your 35 amp fuses because you have never completely drained your battery, so the alternator has never had to give its full output. My main point though is that you should always use a fuse with a lower rating than that of the cable it is to protect, other wise the cable may give out first. The fuse is supposed to be the weakest link.

I'm not going to argue the toss over this one because there are educated folks contributing but I have run my leisure battery very low by cooking too many microwave meals with the invertor and it still recharged via the Towsure (GREEN) split charger and a 35a fuse.So the charger must be intelligent enough at £11.95 to do the job.
I also tested the durability of the split charger by putting an ammeter in place of the 35a fuse to see what was recorded when the 700W microwave was on full chat and over 60Amps were recorded. The charger still works fine