Usually, the objective of a second or auxiliary battery is to provide energy to operate additional electrical loads like fridges and TV sets when the engine is switched off. The auxiliary battery is connected in parallel with the starter battery in order to maintain the system voltage, usually 12V. It has to be used in a manner that will not jeopardise the working of the starter battery. Even with cellphones, and rapid response teams, failure to start a vehicle in a remote area is definitely an event to be avoided.
A number of methods have been devised for managing this arrangement, and these should be approached with a basic understanding of how alternators behave toward batteries in parallel (connected positive to positive and negative to negative), and how batteries in parallel behave toward each other.
Batteries in differing states-of-charge, and therefore with differing open-circuit voltages, will rapidly attain the same voltage when connected in parallel; this remains true even when the batteries are charging or discharging. This is so because voltage is the driving force in this situation, and a 40Ah battery will discharge happily into a 100Ah battery with a lower voltage, until their voltages are equal. Expressed in another way, current will flow as long as a voltage difference exists.
The rate in amps at which the alternator will charge a battery is determined by three factors: the difference between alternator and battery voltage, the internal resistance of the battery, and the temperature of the battery.
Internal resistance the main factor
When charging two batteries in parallel, the alternator will be faced with two batteries having the same voltage. Internal resistance now becomes the main factor and the charging current is divided between the two batteries roughly in the ratio of their respective Ah capacities, the larger current being taken by the bigger battery. This happens simply because the bigger battery has a lower internal resistance than a smaller battery of similar design.
The implications of the above rules are twofold. Firstly, a discharged auxiliary battery will draw current from the starter battery if inadvertently connected to it. Should the auxiliary battery have been deeply discharged, the voltage of the starter battery may be depressed to a level where it will not start the vehicle (less than 12.0V) and this fact must be taken into account when installing a second battery.
Avoid thin cables
Secondly, the auxiliary battery is invariably bigger than the starter battery and will have a lower internal resistance. It will, therefore, receive preferential treatment from the alternator. This is convenient since the starter battery is usually less in need of a charge than the auxiliary battery.
Parallel configurations are in worldwide use, even in installations such as nuclear power stations, and provide excellent service. The only special requirement is that no unnecessary resistance be introduced into the circuit in the form of thin cables or poor electrical connections. As far as the alternator is concerned, the unit fitted to the car will in most instances cope with the two batteries. Even calcium batteries being charged by present-day alternators at 14.4V can be maintained in a satisfactory state-of-charge. Nevertheless, First National Battery recommends an annual charge at a Battery Centre, at which the driver must insist on the use of a constant-current charger which has no voltage limitations.
When battery-powered winches being used, batteries of the recreational type are ideal, as this is an arduous application which cycles the battery to some extent. This battery type serves well as an auxiliary battery because it withstands cycling while at the same time providing the high current for winches. Management of the parallel arrangement by a simple system, using one or two automotive relays, has been found to work well. This isolates the starter battery when the ignition is switched off.
