4 Step Simple Smart Alternator Test

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What is a smart alternator and do I have one?


This is something that’s coming up more and more frequently these days as vehicle manufacturers seek to get their emissions down to meet Euro 5 & 6 regulations.  These are common place in T6s, Transit customs, Renault Masters and Mercedes Sprinters, amongst most other new vehicles.

A smart alternator, also referred to as a variable voltage or ECU controlled charging system, runs at a lower voltage to a traditional alternator and then increases the voltage dramatically when the driver lifts off, this high voltage (around 17VDC) quickly charges the battery. This is similar in principle to the regenerative breaking used in F1 cars.

The problem this causes for us self-converters is that the lower voltages stop us from using voltage sensing split charge relays which are only activated at 13.8V. Now you might be thinking, well I can use an ignition triggered relay rather than a VSR. This, would link your batteries, yes. However, if you try charging a leisure battery at a lower voltage (around 12.5V) it will charge about as quickly as a glacier moves, or in other words, you might as well not bother. Using conventional relays on smart alternators also poses a risk as the relays won’t be rated to take the higher voltage bursts.


So how do I know if I have one and what’s the solution?

The test, mercifully, is much simpler than the science behind the smart alternators’ use. You will need a multimeter capable of reading 12V DC.

1.      Locate your main vehicle battery

2.      Start the engine, ensuring any fans, radio, lights etc. are turned off.

3.      Take a voltage reading across the main vehicle battery

4.      Leave running for around 5/10 minutes, then repeat step 3


If your readings are around 14.4V DC then then you most likely have the traditional, non-smart alternator.

If your reading is closer to 12.5-13.5V then it’s likely that you have a smart alternator.


If you do happen to have a smart alternator this isn’t an issue. Here at Rayne Automotive Ltd, we use a CTEK D250SA DC DC charger specifically designed and wired to charge leisure batteries from smart alternators. It’s also great because it doubles as an intelligent 20A Solar charge controller.  Check out our range of products including the CTEK chargers here.


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