The Truck Alternator is Not an Effective Charging source

Two effects combine to render the truck alternator ineffective as a charge source for the camper battery pack.

  • The Unimog alternator is capable of delivering around 150 amps at 14.0 volts. However there is a voltage loss between the output terminal on the alternator and the starting batteries. At the starting batteries the voltage from the alternator is approximately 13.5-13.7 volts.

  • Between the truck starting batteries and the camper batteries charging current from the alternator must pass through the diode battery isolator. In the process a further voltage drop occurs so that at the camper batteries the alternator is only applying a voltage in the range 12.7 to 13.0. This is too low a voltage to have any meaningful charging effect.

Proposed Solution 1 - Smart battery Isolator

I have experimented by connecting the camper batteries directly to the truck batteries so that the diode voltage drop is eliminated. The effect is that the alternator can apply up to 13.5 volts to the camper batteries. While this is far from ideal the result is that the alternator does make a charging contribution in the order of up to 70 amps during the bulk phase when the batteries are heavily discharged. However it makes no contribution during the absorption phase.

I have ordered the following device which should connect the camper and truck batteries only when the alternator is running relieving me of the need to manually connect and disconnect the two battery packs.

The Smart Isolator at best will only be a partial solution because even if it works perfectly it will only allow the alternator to make a contribution to charging the camper batteries in the bulk phase but will not make any contribution to the absorption phase.

Proposed Solution 2 - Battery-Battery Charger, Alternator-Battery Charger

The alternator on our truck can put out up to 150 amps and there is an upgrade that can put out 260 amps. If this could be harnessed it would represents a very capable charging source.

The key challenge in harnessing the alternator is in molding the output into a 3 stage charging profile.

The usual way of harnessing an engine alternator involves modifying the alternator to accept an external regulator that implements a 3 stage charging program.

However I have recently discovered a devices from that holds promise of utilizing the alternator without any modification to the standard truck alternator or charging system. These devices are:

  • Battery-battery charger,, a device that connects between the truck staring battery and camper battery pack, draws on the truck alternator and applies a 3 stage charging profile to the camper batteries.

    See later post on this device

  • Alternator to battery charger This is a variation on the previous device. It connects directly to the vehicle alternator and feeds both the starting batteries and the camper batteries. It applies a typical vehicle charging profile to the starting batteries and a 3 stage AGM charging profile to the camper batteries. Because it has a direct connection to the alternator it has more control over alternator output and hence can produce much higher charging rates for the camper batteries.

    This device is not recommended for modern vehicles electronically controlled engines.