Dual Battry system for 4 x 4

Looking for a diagram to buildt my own dual system

12 volt? Is it for a truck or ATV? What is the second battery going to be used for?

This is what I have in my 4WD:
Battery 2 pos. is connected to battery 1 pos. via a 60Amp relay. Relay coil is connected to ignition circuit. Both batteries neg. connect to chassis.

Operation: engine (diesel) off; battery 2 supplies in car fridge, 2 way radio's & fluoro light in the back. I have a battery voltage monitor fitted and also wiring to a charger for external charging if mains power is available.

Engine on, relay connects both batteries in parallel.

It does not get any simpler than this.

There are fancy battery isolators available but most suffer from a voltage drop, meaning battery 2 never gets fully charged - unless some booster device is also used.
Klaus

Klaus said:

This is what I have in my 4WD:
Battery 2 pos. is connected to battery 1 pos. via a 60Amp relay. Relay coil is connected to ignition circuit. Both batteries neg. connect to chassis.

Operation: engine (diesel) off; battery 2 supplies in car fridge, 2 way radio's & fluoro light in the back. I have a battery voltage monitor fitted and also wiring to a charger for external charging if mains power is available.

Engine on, relay connects both batteries in parallel.

It does not get any simpler than this.

There are fancy battery isolators available but most suffer from a voltage drop, meaning battery 2 never gets fully charged - unless some booster device is also used.
Klaus

Click to expand...

A battery isolator does have a diode drop in it, at least 0.7v and often over 1v. But, if your alternator uses external voltage sensing, this is not really a problem because an isolator creates a similar drop on both sides. So the alternator will generate that much higher of a voltage.

The flaw is that the voltage drop is not perfectly equal on the isolator, the alternator will only guaratee the proper voltage on the main battery. The deep cycle can get a bit too much or a bit less than optimum.

The flaw of the relay is it takes a decent amount of current in its coil. This is not a real problem while driving, if you're idling and you have an alternator with a low idling output, charge current may suffer slightly. If you turn the key to listen to the radio without starting the car, this may well turn on the relay's coil unless you can find a "run" wire.

Also the relay contacts have a somewhat limited life, especially when large currents are involved.