12V Design Guide - dual batteries, solar panels and inverters

[SilvaD4]

Hi All,

You may have already covered this (and my apologies if this is the case) but I found an excellent article on the web on the above topic (see link below).

Design Guide for 12V Systems (http://outbackjoe.com/macho-divertissement/macho-articles/design-guide-for-12v-systems-dual-batteries-solar-panels-and-inverters/)

The latest update is only 2 days old so even if you have read this before you might what to re-read just for the updated info.

[p38arover]

Moved, not D3/D4/RRS specific.

[drivesafe]

Hi Silva and thanks for the link but I think you should give little credence to much of the "info" in that thread.

There are so many basic errors and I don't know where he gets much of his info from.

For a starter, he goes on about alternators only charge at 13.8v.

Even with all the vehicles now operating variable voltage type charging systems, most vehicles still operate constant voltage alternators and these all ran at about 14.3+v.

And at 14.3v, no one has any problems.

His cable chart and "THEORIES" on voltage drop is off with the fairies.

6B&S cable, after derating to allow for where the cable is run through a vehicle, has a constant SAFE maximum current rating of around 100 amps. I have no idea where he comes up with 72 amps????

The 12v output of a generator is not for battery charging, it's a 12v supply for operating appliance, not for charging batteries, so I don't know why he raised that misinformation.

One of his worst "suggestions" is the use of the chassis as the earth return for dual battery setups.

The chassis in many vehicles can be used as the earth return but in many other vehicles can not be used and the new the vehicle, the less likely you can use the chassis as an earth with out causing problems.

A good example is a D3 or D4. They have great chassis earth returns, that is until they have to have the motor worked on, when the body is lifted off the chassis and any added earth connecting will be cut.

No professional dual battery installer will use the chassis of a vehicle as the earth return, and will always run both a positive and negative cable setup.

There are heaps of other errors and misinformation in that thread, so just take care.

[Homestar]

Yeah, got to agree with Drivesafe on a few things here. He has some strange calculations going on.

For instance, he claims the max current for a 3 metre cable run of 16mm cable (the same 6B&S Drivesafe is talking about) is 28 amps with a 0.2 voltage drop, but I make it over 50 amps for the same voltage drop and length.

And yes, you can easily chuck 100 amps through a cable like that.

Also, I note he says max current for a bit of 35mm is 119 amps. This would be for a bit of XLPE, not a mutilstrand like is used in automotive use. Our 35mm STI cable we use at work we run to 150 amps - that's to meet AS3000. In reality you could stick twice that through it for things like winching, etc. It would be fine.

I'd take all his info with a grain of salt. Not sure what his qualification are, but I've been a Sparky for 25 years. He seems to more theroretical than hands on IMO.