Variable voltage alternator problems with travel trailers – how to fix


by Collyn Rivers – Updated 2020

Variable Voltage Alternators

Variable voltage alternator problems with travel trailers and motorhomes arise when charging auxiliary batteries. Here’s why and how to fix them. These alternators are, in particular, installed on many post-2013 vehicles.

Prior to the year 2000, alternators produced 14.4-14.7 volts, and a few close to 15 volts. This adequately charged travel trailer and motorhome auxiliary batteries. Furthermore, where it did not, voltage boosting assisted.

smart Alternator

Typical smart alternator – note extra-wide belt pulley. Pic: original source unknown.

Variable voltage alternator problems with travel trailers – temperature compensating

These produce about 14.2 volts when the engine is cold. This decreases to about 13.2 volts as the engine warms. Lead-acid and AGM battery charging, however, needs up to 14.4 volts. Charging such batteries directly from these alternators is thus not effective.

Dc-dc alternator charging nevertheless fixes this problem. It accepts voltage available, boosting it to the levels required. This assists high current appliances that are far from the alternator or connected via a too thin cable.

All alternators must ensure the starter battery has charge priority. This is done by a voltage-sensitive relay. The relay precludes auxiliary batteries charging until the starter battery exceeds about 13.6 volts. It also disconnects if the starter battery voltage falls below 12.6. Used as above, temperature compensating alternators are not a problem with RV batteries.

Variable voltage alternator problems with travel trailers – variable voltage alternator charging problems

Common since 2013, variable voltage alternator output is controlled whilst driving by the vehicles’ main computer. The voltage varies from 12.3 volts to plus 15 volts.

That 15 volts is too high for direct battery charging. It wrecks lead-acid deep-cycle batteries, gel cell and AGM batteries. Moreover, anything below 14 volts is of little charging use. When output falls below 12.6 volts the voltage sensing relay drops out. It consequently cuts auxiliary charging two to three minutes each time.

Some dc-dc alternator chargers will still work. How to do this varies as charger manufacturers develop solutions.

Variable voltage alternator problems with travel trailers – regenerative braking

A vehicle at speed has so-called kinetic energy. Conventional braking dissipates that energy as heat. Rather than losing energy, braking is done by increasing alternator voltage. This loads up the alternator such that it acts as a brake.

Doing so requires the main (starter) battery to be normally 80% charged. The recovered energy brings the battery to 100% charge. Alternator voltage then drops to about 12.3 volts (or zero) until battery capacity falls to 80%. This cycle repeats every time the vehicle brakes.

When alternator voltage drops below 12.6 volts, the voltage sensing relay drops out. This isolates auxiliary batteries for minutes each time. Worse, ongoing bursts at plus 15 volts quickly destroy them.

Fixing variable voltage alternator problems with travel trailers – regenerative braking

Companies tackling this include Redarc (Australia) and Sterling (UK). Both use the starter battery’s voltage to know what the alternator is doing and optimise auxiliary charging accordingly. This also protects auxiliary batteries against excess voltage.

How to know what alternator is which

Alternators used for regenerative braking are large. In addition, they may have multiple drive belts

To establish alternator type connect a multimeter across the starter battery. You may, however, need to extend the leads. Ensure they cannot be wound up by the drive belt. Have an assistant check voltage over a range of driving. Check whilst braking for a distance downhill. This may increase output to over 15 volts. (One BCDC maker suggests to do this by fitting a lighter plug to the meter lead socket. This is not a good idea. Some vehicles have them feed them at constant voltage!)

An output that drops below 12.7 volts whilst driving identifies variable voltage alternators.

Experienced auto electricians should be able to help. See also the dual battery system selector (that indicates RV alternator types etc) at

Variable voltage alternator problems with travel trailers – summary

Variable Voltage Alternators – these drop below 12.7 volts at any time whilst driving. They require a specialised BCDC unit that senses various voltage levels etc. None operate satisfactorily with a voltage sensing relay.

Fixed Voltage Alternators & Temperature Compensating Alternators. These produce above 12.7 volts whilst driving. Most dc-dc alternator chargers and voltage sensing relays should work. Contact their makers if in doubt.

Euro emission requirements (2020)

Increasingly rigid emission limits require vehicle makers to meet emission levels. For 2020 the target is 95 g/km of CO2 for all new cars. There is however a 12 month phase-in period such that 95% of new car to comply with the target during 2020 and 100%. This corresponds to fuel consumption of about 3.8 l/100 km.

The requirements do not specify how car makers achieve. It may or may not involve the alternator. These regulations may eventually preclude all alternator use for RV auxiliary needs. If/when, however, is unknown but can readily be resolved by using a fuel cell.

Note: Many vehicles with variable voltage alternators monitor load and charge via a cable between the vehicle’s chassis or, for chassis-less vehicles, from the metal bodywork and one terminal of its battery. This cable must not be altered in any way.

Our constantly updated books include Caravan & Motorhome Electrics, the Caravan & Motorhome Book, and Camper Trailer Book. Solar That Really Works is for cabins and RVs). Solar Success is for home and property systems.

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Collyn Rivers is a semi-retired automobile research engineer.  He is the author of seven books, five of which are about making RVs and solar work optimally.

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