Auxiliary Battery Wiring

by Bob Wallace


The purpose of the second battery is to provide a power supply for lights TVs, radios, computers etc while the vehicle is parked, while leaving an untouched supply for starting the engine.

The second battery should ideally be a LEISURE battery as that will provide 12 volts further into its discharge cycle. Choose a battery that has a least the same cold cranking amps as your driving battery and you will be able to jump start yourself.

On a split or loaf the favorite position for the second battery is on the left of the engine bay (those with a gas heater there will have to fit the second battery elsewhere). On a Vanagon favorite is probably under the front seat on the other side to the driving battery.

Where ever you put it, make sure its firmly secured as we don't want it to fall over and throw acid around. Use a chunky earth strap too - same size as on the driving battery will be fine.

Electrically the installation should meet the following criteria:

1. Power must not be consumed from the driving battery while stationary. This allows the second battery to be completely drained but the camper will still start.

2. Power must not be consumed from the driving battery when the dynamo/alternator is not producing electricity (during cranking or low rpm idling). This prevents power being dumped from a fully charged driving battery to a flat auxiliary battery as soon as the ignition key is turned.

3. If a fridge is fitted it is desirable that because it consumes a lot of current (so will quickly run an auxiliary battery flat) that it should be isolated from both batteries when stationary or idling. When camping the fridge can be used on its mains electricity or propane/butane gas settings.

Some of these criteria are meet by the fitting of manual change over switches but forgetting to flip the switch just once will leave you stranded.

So to connect the battery meeting all the above criteria.

Go to a camping/caravaning center, or look in the towing accessories at your FLAPS, and buy a special relay for charging a second battery. They will come with instructions and will fulfill all of the above criteria - fit it and forget it, no faffing around with manual switches.

If you can not find one of the special relays then its easy to make up your own with a couple of relays and a plastic 'project' box from Radio Shack (or other electronics store).

Use 2 double pole double throw (DPDT) relays.

Relay 1 is triggered by turning the ignition on and allows the current from the driving battery to be passed to relay 2

Relay 2 is triggered by the generator warning light and prevents current going to the fridge/second battery when the generator is not producing any charge

So here's the wiring diagram (in its various states):-


Ignition OFF                      =  Fridge OFF
Generator warning light OFF          Second battery ISOLATED

                                      ______fridge
                   /                 /
                  /                 /
             ____/| _______________/|
             |    |                 | _____________second
driving      |    |/                |/             battery
battery      |    /                 /              positive
positive_____|___/| _______________/|
                  |                 |
                -----             -----
ignition________| 1 |___       ___| 2 |___
controlled      -----  |       |  -----  |
live                   |       |         |
                     earth     |       earth
                               |
                       dynamo/alternator
                       warning light










......................................................................

Ignition ON                      =  Fridge OFF
Generator warning light ON          Second battery ISOLATED

                                      ______fridge


             _________________________
             |    |                 | _____________second
driving      |    |                 |              battery
battery      |    |                 |              positive
positive_____|________________________
                  |                 |
                -----             -----
ignition________| 1 |___       ___| 2 |___
controlled      -----  |       |  -----  |
live                   |       |         |
                     earth     |       earth
                               |
                       dynamo/alternator
                       warning light

......................................................................

Ignition ON                      =  Fridge ON
Generator warning light OFF         Second battery CHARGING

                                      ______fridge
                                     /
                                    /
             ______________________/|
             |    |                 | _____________second
driving      |    |                 |/             battery
battery      |    |                 /              positive
positive_____|_____________________/|
                  |                 |
                -----             -----
ignition________| 1 |___       ___| 2 |___
controlled      -----  |       |  -----  |
live                   |       |         |
                     earth     |       earth
                               |
                       dynamo/alternator
                       warning light




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