DUAL BATTERIES
By Brian Tanner (Brisey)

This article is intended to explain the basic workings of dual battery systems.

This diagram shows the basic dual battery system such as Redarc. The basic configuration of all systems are very similar where the auxiliary battery is isolated from the main battery and charging system by means of a solenoid contact. This contact is then switched on and off by the control circuit of the system.

Depending on the make of the system you would see features including delayed switching to allow the main battery to recharge after starting the vehicle, low voltage cut out to prevent the auxiliary battery going dead flat and emergency starting if the main battery is flat.

The most common misconception of dual battery systems is that the system controls the state of charge of the auxiliary battery. It is the regulator in the vehicle alternator that controls the state of charge of all batteries connected to it by sensing the voltage across the batteries and then produces current to charge them.

Dual battery systems can be classified as two basic types. 
The first type connects the main and auxiliary batteries in parallel once the main battery is fully charged. If the auxiliary battery is less than fully charged, it will slightly lower the total battery voltage causing the alternator to start charging. The auxiliary battery will never quite reach full charge due to losses in the cables and at connections. Ideally the main and auxiliary batteries should be identical for maximum efficiency but in practice most installations have a normal crank battery for the main and a deep cycle for the auxiliary giving good results. When installing this type of system the batteries should be as close as possible and large cable should be selected. Also you must pay particular attention to the quality of the connections. This will be discussed later in the article.

The second type isolates the main battery and connects the auxiliary battery for charging once the main battery is fully charged. This system uses a changeover type of solenoid and only has one battery connected at once for charging. The auxiliary battery in this system will always charge fully as this is a more efficient configuration.

The control circuit of the dual battery system monitors the charging system of the vehicle and when it is 13.6 volts (indicating the main battery is charged) will energise the solenoid. If the main battery starts to discharge the control circuit will deenergise the solenoid at 12.7 volts allowing the main battery to charge.

Testing the installation

It is very important to ensure the connections are clean and tight to minimise volt drop losses which will cause under charging. If, for instance, there is a volt drop of 0.1 volt across each terminal then these volt drops must be added. For the typical installation in the diagram there is 10 connections (including the battery terminal clamps) equalling a total volt drop of 1 volt. Now consider the battery is connected to a charging system supplying 13.6 volts and subtract 1 volt which is dropped across the connections, there is now only 12.6 volts left across the battery so the battery will always be flat.

To test for volt drop in the installation firstly have the motor running and ensure the solenoid is energised. Now connect a voltmeter to the positive battery posts of both batteries as in the diagram. The meter should read 0 volt in an ideal world but 0.1 volt would be acceptable. Next test from one of the battery negative posts to a good earth on the vehicle. This should also be 0.1 volt or better. Repeat with the other battery negative post.

If any of the readings are high, disconnect and clean all terminals with sandpaper (not emery) and test again. If the readings are still high larger wire may be needed.

Extending battery life

To extend the usable life of batteries you should:

• Never store batteries on concrete. It causes condensation and damages the plates.
• Never short the terminals together. Rapid discharges damage the plates.
• Never let the water level in a battery get below the top of the plates and never use tap water to top up a battery, only distilled or purified water. The minerals damage the plates.
• Occasionally check all cells with a hydrometer. Specific gravity for a fully charged cell is 1.250.
• Put the battery on a charger for a few hours to give it an equalise charge.
• Keep the top of the battery clean as the dirt could cause a slight discharge.
• Excessive vibration and heat will shorten the life of a battery.