The technical guide to smart alternators

By: Bill Savidan

MCD gives the low-down on what’s new and interesting in the world of RV technology. This month, we're talking smart alternators.

To achieve compliance with stricter emission controls, European vehicle manufacturers are adopting novel systems to lower emissions and improve the number of kilometre per litre vehicles travel.

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One example is the ‘smart’ alternator. When an alternator is generating power, it imposes a load on the vehicle’s engine, using fuel and increasing engine emissions. If a vehicle has a smart alternator, the vehicle computer management system can be programmed to switch the alternator off or on.

For example, it could be programmed to switch the alternator off whenever the vehicle engine is under load, say climbing a hill, allowing all of the engine’s power to be directed towards getting the vehicle up the hill. With the alternator switched off, the engine would rely on battery power for its electrical needs, but there’s more to it than that. When used in conjunction with a ‘regenerative braking’ (RB) system, the smart alternator contributes even more.

Every time a motor vehicle coasts downhill or when brakes are applied, energy is wasted. Hot braking systems after a hill descent illustrate this point. RB systems capture this energy as electricity and store it in the vehicle’s battery(s). This allows the smart alternator to remain dormant for longer periods of time, thus saving fuel and reducing emissions.

For RB to operate successfully, the vehicle battery must have room to accept the large amounts of energy the RB system is likely to deliver. Ideally, the battery should be around 20 percent depleted, still with sufficient charge in the battery to restart the engine while leaving enough capacity to accept RB energy ‘dumps’.

When RB is ‘dumping’ energy, the alternator voltage can rise to upwards of 15 volts, fast charging the battery in the process. Once charged, the battery voltage drops to around 12.4 volts, and the vehicle uses the stored battery capacity ‘bonus’ instead of using alternator power.

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While the technology fitted by the vehicle manufacturer achieves its objectives of emission reduction and saves fuel for the vehicle, it does pose problems if the vehicle alternator is used to charge the house batteries of the motorhome mounted on the vehicle.

First, the vehicle charge system is designed to charge batteries to 80 percent capacity, leaving 20 percent for RB charging. This limitation means the house batteries would never be fully charged using this regime. The vehicle battery can cope with the high volts and amps generated by the RB, topping up the final 20 percent.

House batteries, however, are usually depleted below 20 percent, often down to 50 percent, and at that level, they could be seriously damaged by the high currents and 15 plus voltage produced by the vehicle alternator. The more depleted the batteries are, the more likely they are to be damaged.

A battery-to-battery charger

A solution to these problems is to use a battery-to-battery charger, sometimes referred to as a DC-to-DC charger. This device is connected between the starter battery and the house battery(s). The unit can be programmed to deliver the correct maximum voltage depending on the battery type, lead acid, gel, or AGM. It will then deliver the correct voltage to the house battery(s).

If the voltage from the alternator or start battery is too low, it will raise the voltage delivered on to the house battery. If the voltage it receives is too high, it will lower it before delivering it on. It also protects the house battery from high current surges.

Most importantly, a battery-to-battery charger is not restricted to the 80 percent maximum charge limit placed on the vehicle’s battery, and the system isn’t reliant on activation once the alternator voltage is above 13 volts. The house batteries can be fully charged and are protected from voltage variations by a safer and faster charging system.

Two brands of battery-to-battery chargers currently available are CTEK D250SA/SmartPass 120 ( and Sterling Power (

Thanks to John Wightman of Element82 Ltd in Auckland for his expert guidance on this subject. 

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