Updated 20 July 2016
Variable voltage alternator problems with caravans and motor homes arise when charging auxiliary batteries. Here’s why and how to fix them. These alternators are installed on many post-2013 European vehicles that accord with obligatory Euro 3, 5, 6 + etc emissions requirements.
Prior to 2000 or so, alternators had varying voltage. Most produced 14.4-14.7 volts and a few close to 15 volts). If connected by heavy enough cable, this adequately charged caravan and motor home auxiliary batteries. Where it did not – mostly because of too small cable – voltage boosting units assisted.
Typical smart alternator – note extra-wide belt pulley. Pic: original source unknown.
Variable voltage alternator problems with caravans – temperature compensating
Introduced around 2000, these semi-smart alternators typically produce about 14.1 volts when the engine is cold. The voltage then decreases to about 13.2 volts as the engine warms up. Lead acid and AGM battery charging, however, needs up to 14.4 volts. Charging such batteries directly from these alternators is not effective.
This problem was fixed by the advent of dc-dc alternator charging. This accepts whatever voltage is available and brings it up the levels and regimes required for optimum auxiliary battery charging. Such charging works particularly well where high energy-using devices (particularly fridges) are many conductor metres away from the alternator. They are often connected via far too small cable.
These, as well as pre-2000 alternators, must have a way of ensuring the starter battery has charging priority. This is done by a voltage sensitive relay. This relay connects the auxiliary battery/s for charging only after the starter battery charging voltage reaches the 13.6 or so volts for reliable engine starting. It also disconnects the auxiliary battery if the starter battery falls below about 12.5-12.6 volts.
By and large, temperature compensating alternators are not a problem for RV battery charging if used as above.
Variable voltage alternator problems with caravans – variable voltage alternator charging problems
In increasingly common use since 2013, these alternators have a voltage output varying from 12.3 volts to plus 15 volts. That voltage is controlled at all times (whilst driving) by the vehicles’ Central Processing Unit computer.
This presents problems as 15 volts is too high for direct battery charging. That upper voltage wrecks lead acid deep-cycle batteries, gel cell and AGM batteries. That below 14 volts is of little use for charging. Further, if/when the voltage falls below 12.6-12.7 volts the voltage sensing relay drops out. This relay may remain out for two or more minutes. Each time this cuts off charging the RV auxiliary battery.
Most dc-dc alternator chargers will, however, still work given a work-around with that relay. How to do this varies, but a few charger manufacturers are providing (or developing) solutions.
Variable voltage alternator problems with caravans – regenerative braking
Here’s where variable voltage alternator problems with caravans and motor homes really start. The mass of a vehicle at speed has a great deal of so-called kinetic energy. Conventional (friction) braking dissipates that energy in the form of heat. So rather than accepting that loss, the kinetic energy is utilised via a high current output variable voltage alternator. When the brake pedal is pressed alternator voltage is instantly increased.(This also happens when the accelerator pedal is released whilst (say) decending a hill.) So doing charges the main (starter) battery at 200 amps or more at 15 volts plus (some at 15.4 volts).
The above can only work by having main (starter) battery only 80% or so charged most of the time. The energy recovered by regenerative braking rapidly brings this battery to 100% charge. Whilst so doing, the alternator voltage is cut right back. This is to little over 12 volts. Some alternators turn off altogether. This happens until battery capacity falls to 80%. This cycle is repeated every time the brakes are used. It happens too when the accelerator is released for more than a few seconds.
This presents serious problems if charging an auxiliary battery.
The sudden availabilty of plus 15 volts (and a typically 200 amps or more) will quickly destroy deep cycle lead-acid, gell cell and AGM batteries. This is particularly those only partly charged.
That alternator voltage also drops to a typical 12.3 volts every time the vehicle brakes or decelerates causes the voltage sensing relay to drop out. This not only isolates the auxiliary battery from the alternator. It keeps it isolated for two to three minutes each time.
Variable voltage charging makes it impossible to charge an auxiliary battery to more than about 50% much of the time unless the new and diferent form of control described below is used.
Fixing variable voltage alternator problems with caravans – regenerative braking
Both have developed a variable voltage alternator charging technique (known currently as battery to battery charging). This uses the main (starter) battery voltage as a guide to what the alternator is doing. It optimises battery charging accordingly. It also protect the auxiliary batteries from excess feed from that alternator.
These units are in production and available with various outputs.
How to know what alternator is which
Alternators used for regenerative braking are larger and may have multiple drive belts
The alternator type can be established for certain by connecting a multimeter across the main battery. You may need to extent the leads. Make 100% sure they do not get wound up by the fan or drive belt. Have an assistant check the voltage over a range of driving. In particular check whilst braking for a distance down a hill. This may increase alternator output to over 15 volts. If at any time it drops below 12.7 volts it’s a sure indication there’s regenerative braking. (One BCDC maker suggests doing this by fitting a cigarette lighter plug to the meter lead socket. This is not a good idea. Some vehicles have them fed at constant voltage.)
The author is currently compiling a list vehicles known to need BCDC units. Information would be appreciated. I cannot assist re vehicles that are not listed. You can ask the alternator charger unit suppliers – but they cannot always advise. Experienced auto electricians should be able to help. See also the dual battery system selector (that indicates RV alternator types etc) at https://www.redarc.com.au/calculator/dual-battery-calculator .
Variable voltage alternator problems with caravans – summary
Fixed Voltage Alternators & Temperature Compensating Alternators – 12.7 volts and above whilst driving. Most conventional dc-dc alternator chargers and voltage sensing relays should work (contact their makers if in doubt).
Controlled Variable Voltage Alternators – less than 12.7 volts at any time whilst driving. Variable voltage alternator problems with caravans and motor homes require the use of a specialised BCDC unit that senses various voltage levels etc. None will operate satisfactorily with a voltage sensing relay still in place.
The Euro emission requirements
These increasingly rigid requirements require that vehicle makers must not exceed specific emission levels. They do not however specify how they achieve it. This may or may not thus include the alternator. It is common to find the variable voltage unit on a diesel engine car because its higher compression ratio provides more efficient compression braking. It may not be used on its lower compression petrol-engine equivalent. And so on.
The Euro Emission regulations change again in 2017 but details are not yet known. These regulations may eventually preclude all alternator use for RVs – but if/when is unknown.
Note: In vehicles with variable voltage alternators, the main cable from the chassis to the engine and/or starter battery doubles as a so-called current shunt. This provides load current indication. All negative return leads from RV appliances etc MUST be taken to the chassis side of that cable. That cable must not be altered in any way.
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