This page explains broadly some of the problems associated with charging an auxiliary lead acid battery. Particularly remote (from power source) batteries, such as that in a camper or caravan.

To fully charge a lead acid battery, the charge voltage needs to be around 14.8 Volt. This figure is dependent on a few factors, including battery type and actual battery temperature. However as the alternator output from most modern engines is around 13.8 to 14.1 Volt, a direct (no matter how large the cable) connection will NEVER fully charge the second (remote) battery. Some vehicles (eg. many common rail diesels) even reduce this voltage further (to near 13V), when the ECU thinks its cranking battery is charged and the engine bay is hot ! Please see our "More Information" for a graphical example.

One reason why a vehicle charging system is set at this voltage, is that a lead acid battery will reach a suitable level of charge for engine cranking etc at about 14 volts. It also can be continually held near this level without causing damage. eg a trip from Perth to Sydney. This gives a charge level of approximately 70%, with one standard fixed voltage. There is some temperature compensation, based upon alternator temperature, not actual battery temperature, except for latest ECU controlled systems, which have complex algorithms (but LOWER voltage).

If the battery is to be charged to its rated capacity (ideally approaching 100%), a higher "boost" voltage is needed, with a method of monitoring the state-of-charge. This "boost" is then reduced to a float voltage, once this charging level has been reached, as further "boost" charging WILL cause damage, possibly permanent, to your battery. A well-designed multi-stage (at least three) charger is ideal for this operation. Sophisticated three-stage mains chargers are now available. These are gradually being introduced into the better-designed caravans/campers. However, this does not help whilst on-the-road, or when camping away from powered sites.

A further problem with campers/caravans is the distance between the charge source (alternator) and the second battery. As every cable has resistance, when charging a discharged battery at a rate greater than a few amps, there is a significant voltage drop through this cable. As the current increases so does the voltage drop, further reducing the battery charging voltage. Therefore, even charge levels of 70% are not possible. eg using quality low resistance cable (10 metre of 4 square mm.), fuses, relay and connectors (total resistance of 0.25 ohms);  will have a voltage drop of about 2.5 Volt at 10 Amp. Deduct this from your supply of say 14 Volt, leaves only 11.5 Volt. This voltage is comparable with a battery charge level of 25% capacity. This low charge also severely reduces the battery life, adding more expense.

Also as most campers/caravans simply parallel the battery and fridge circuits, if you also run a heat cycle fridge (typically 12-15 amps) on the same circuit, this will actually discharge your battery en route ! This low voltage also severely effects the efficiency of the fridge. Ideally these high current devices need their own circuit.

With a dedicated charging circuit, (without boosting) as your battery accepts a charge over time, the charging current will reduce, so the cabling voltage drop will also reduce. It is possible over a very long period of driving for this second battery to reach a charge level approaching that of the main battery (about 65%). Importantly, an absorption fridge connected to the charge circuit will prevent the voltage from rising enough to EVER charge the battery. 

The RanOx™ microprocessor controlled battery booster overcomes these problems, providing up to 25 amps output and charges in three stages; constant current, constant boost voltage, then constant float voltage. All parameters are user selectable. Temperature compensation and protection for the battery is provided. It delivers the best mobile voltage and current feed to a Caravan, Camper or other auxiliary battery available today. In addition, it can protect this battery against excessive discharge.