RV Alternator Charging Problems - Dec 2018

Updated December 2018

Prior to 2000 or so, RV alternators problems were mainly due to too thin connecting cable and unrealistic expectations. Most produced between 14.4 and 14.7 volts, and a few close to 15 volts.  This, however, changed as reducing alternator voltage usefully reduces emissions - but introduces problems where an alternator is used also to charge RV batteries.

There are two main types of such alternators. Both can cause RV battery charging problems, but those used in many post 2014 vehicles cause many problems for RV owners. This article by Collyn Rivers explains how and why these problems are caused - and how to fix them.

RV Alternator Charging Problems - starter battery charge protection - ensuring the engine can start

All RV vehicle alternators are set up such that they first replace the energy needed for engine starting. This is actually little. It depletes a starter battery (in good condition) by only 2% or so. That energy is typically replaced within two to three minutes.

To ensure this in earlier systems, a voltage sensing relay (a switch) allows RV auxiliary battery charging only when the starter battery exceeds about 12.6 volts It disconnects RV battery charging if it drops below 12.7 volts. Some present-day systems have variants of this - that protects that starter battery. 

RV Alternator Charging Problems - temperature compensating alternators

Introduced around 2000, these produce about 14.2 volts when the engine is cold. This decreases to about 13.2 volts after 5 to 10 km as the engine heats up. Lead acid and AGM battery charging, however, requires up to 14.4 volts. Temperature compensating alternators charge an RV battery bank well enough to about 50% of full charge, but slow to a crawl thereafter.

So-called ‘dc-dc alternator charging’ fixes these alternator RV issues. It accepts whatever voltage the alternator puts out and boosts it if required. This usually fixes the problem. If not, the cause is usually interconnecting cable that is far too thin.

Dc-dc chargers must be located close to the RV's battery bank. If that is in a caravan, the interconnecting cable needs to be 10 square millimetres (ideally 13.5 square millimetres) and taken via an Anderson plug and socket. This cable is costly but really necessary.

RV Alternator Charging Problems - variable voltage alternators

As vehicle engines became computer controlled, it made sense for the computer to sense alternator output need and control its voltage accordingly. These alternators were introduced around 2010, fitted to many new vehicles from 2014 on. They are now (Dec 2018) used in most new vehicles.

Variable voltage alternator output varies from 12.3 volts to 15 volts or more and some at times drop to zero (see below). This causes havoc with RV battery charging as 15 or more volts plus can wreck batteries and that below 14 volts is of little direct charging use.

Further, when (not if) output falls below 12.6 volts, that voltage-sensing relay drops out. This cuts auxiliary battery charging for two to three minutes each time it happens - and that can be many times an hour.

Some dc-dc alternator chargers will work with these alternators but need the voltage-sensing relay to be connected in another manner. If issues are encountered, contact the dc-dc charger manufacturer directly as not all auto electricians are yet aware of the various solutions.

RV Alternator Charging Problems - variable voltage alternators and regenerative braking

This is where RV alternator charging problems escalate! The issues affect many vehicles made after 2014 and even more now. If you have such a vehicle, a traditional battery RV charging system will barely work. There is also a major risk of wrecking batteries through over-voltage charging.

A vehicle at speed has so-called kinetic energy. Normal braking dissipates that energy as heat. Rather than losing that energy, braking (with such systems) is also done by increasing alternator voltage to well over 15 volts. As this requires more energy to drive it, the alternator assists braking and particularly on long descents on free-ways etc. and also in suburban and city traffic.

To enable this to work, the starter battery (now called the ‘main’ battery in such systems) is normally charged to only 80% or so. This requires the alternator to generate only 12.6 to 13 volts much of the time (far too low for direct RV battery charging. 

During prolonged braking, however, that recovered energy (at 15 volts plus - via a substantially sized alternator) rapidly boosts the battery to 100%. The alternator voltage is then cut back to about 12.3 volts (or with some vehicles, to zero) until battery capacity falls to 80%. This cycle repeats every time the vehicle brakes.

For much of the time, such alternators thus generate only the voltage needed for the vehicle’s normal needs. It is fine for that, but far too low for charging auxiliary RV batteries.

A dc-dc alternator charger is thus essential – but it must be one designed to cope with such usage. This also necessitates a change to the voltage sensing relay connection – (with some dc-dc alternator chargers, that function is inbuilt).

Typical variable voltage alternator used with regenerative braking systems - note extra-wide belt pulley. Pic: original source unknown

Typical variable voltage alternator used with regenerative braking systems - note extra-wide belt pulley. Pic: original source unknown

RV Alternator Charging Problems - how to identify alternator type

Alternators used for regenerative braking are large and likely to have multiple or very wide drive belts as shown above.

If you know your electrics, you can identity alternator type by measuring its output across a range of driving conditions. You are likely to need to extend the meter leads. Keep them well clear of the alternator drive belt and radiator fan. Have an assistant check voltage over a range of driving - including braking for some distance downhill. This may increase output to over 15 volts.

If output ever drops below 12.7 volts whilst driving - that is a variable voltage alternator.

One dc-dc charger maker (that should know better) suggests doing this by measuring the voltage at a lighter plug's socket. This may not work as some such vehicles have these outlets fed at constant voltage!

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

RV Alternator Charging Problems - variable voltage alternator problems - summary

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.

Variable Voltage Alternators

These drop below 12.7 volts at any time whilst driving. They require a specialised dc-dc unit and will need some attention to the voltage-sensing relay.

Companies tackling these issues include Redarc (Australia/New Zealand) and Sterling (UK). Both use the main (starter battery) voltage to know what the alternator is doing and optimise auxiliary charging accordingly. The units also protect auxiliary batteries against excess voltage.

Important Note:

Vehicles with variable voltage alternators monitor both load and charge via a chassis to starter battery cable – or a Y-shaped cable. The cable must not be altered in any way. This is because it acts a so-called 'current shunt' that assists measure all that's going on electrically. If changed in any way that may cause the alternator to be fed with incorrect data that may drastically affects charging.

When adding RV appliances in RVs that use the chassis as a negative lead return – all such negative returns from RV appliances MUST be to the chassis side of this cable – not directly to the battery negative terminal. Unless this is done, charging cannot work correctly.


If the main battery has more than two leads to either terminal it is odd-on that the system has been wrongly installed.

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