Alternator Charging Problems

Prior to 2000 or so, vehicle alternators produced between 14.4 and 14.7 volts, and a few produced close to 15 volts. Their output was adequate for rapid and deep battery charging. Almost all charging problems were due to often using far too light cabling. This however changed as it was realised that limiting alternator voltage (in vehicles in normal use) helped to reduce emissions.  It did however introduce problems where an alternator was also used to charge RV batteries.

There are two main types of such alternators. Both can cause RV battery charging problems, but the more recent (variable voltage) alternators are causing many issues for RV owners who don't realise how and why these problems are caused.

Starter battery charge protection - ensuring the engine can start

All vehicle alternators are set up such that they first replace the energy needed for engine starting. This, to many people’s surprise, is very little – it depletes a starter battery (in good condition) by a
mere 2% or so and is typically replaced within two to three minutes of starting.

To ensure engine starting receives priority, a voltage sensing relay (in effect a switch) only allows RV auxiliary battery charging when the starter battery exceeds about 12.6 volts, and disconnects RV battery charging if it drops below 12.7 volts. 

Temperature compensating alternators

Introduced around the year 2000, these produce about 14.2 volts when the engine is cold. This decreases to about 13.2 volts after 5 to10 km as the engine warms. Effective lead acid and AGM battery charging, however, requires up to 14.4 volts. These alternators will charge the RV battery bank well enough to about 50% of full charge, but very slowly thereafter.

A method called ‘dc-dc alternator charging’ fixes this problem. It accepts whatever voltage is available and boosts it to the levels required. This usually fixes charging. If it does not, the cause is
usually interconnecting cable that is too thin.

To be truly effective, dc-dc chargers must be located as close as possible to the RV auxiliary battery bank. If that is in a caravan, the interconnecting cable needs to be at least 10 square millimetres
(ideally 13.5 square millimetres) and taken via an Anderson plug and socket. This cable is costly but really necessary.

Variable voltage alternators

As vehicle engines became increasing computer controlled, it was realised that it made sense to have the computer sense the vehicle’s alternator output need and control its voltage accordingly. Variable voltage alternators were introduced around 2010 and are now (2018) used in most new vehicles.

Variable voltage alternator output can and does vary from a typical 12.3 volts to 15 volts or more. With some systems, however, it may at times be zero. Unless owners are aware of this, this can cause havoc with RV battery charging. An output of over 15 volts plus is too high for AGM batteries – and anything below 14 volts is of little direct charging use.

Further, when output falls below 12.6 volts the voltage-sensing relay drops out and consequently cuts auxiliary battery charging for two to three minutes each time.

Some dc-dc alternator chargers will still work but need the voltage-sensing relay to be connected in another manner. How to do this varies as dc-dc charger manufacturers develop solutions. If issues are encountered, contact the charger manufacturer directly as not all auto electricians are aware of the various solutions.

Variable voltage alternators and regenerative braking

This is where alternator-charging issues escalate!

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 to well over 15 volts. This
instantly loads up the alternator such that it acts as a brake. Enabling this requires the starter battery (called the ‘main’ battery in such systems) to be normally only 80% charged – requiring well over 13 volts to retain that.

During prolonged braking, that recovered energy (at 15 volts plus via a substantially sized alternator) brings the battery to 100% charge. Alternator voltage then drops to about 12.3 volts (or with a few
vehicles to zero) until battery capacity falls to 80%. This cycle repeats every time the vehicle brakes.

The above presents additional problems. For much of the time, the alternator is generating only the voltage required for the vehicle’s normal needs – and that is far too low for charging auxiliary RV
batteries. A dc-dc alternator charger is thus essential – but needs to be of a type made specifically for this usage. Either there will also be a change in the voltage sensing relay connection – or else that function is built into the special- dc-dc alternator charger.

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

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 way around electrics, you can identity alternator type by measuring its output voltage across a range of driving conditions with a multimeter. You are likely to need to extend the leads but must 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.

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

(One dc-dc charger maker suggests doing this by fitting a lighter plug to the meter lead socket. This is not a good thing to do. Some 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.

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.

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.

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