What is a smart alternator and do I have one?


This article is an updated version of our hugely popular 4 step simple smart alternator test. I hope you find it useful. 

Smart alternators are 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 commonplace 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 and batteries 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 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.

Check out this video by one of our customers performing the test to give you a nice idea of how it's done. 

If you do happen to have a smart alternator this isn’t an issue. Here at Rayne Automotive Ltd, we use a CTEK D250SE 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.


We're just in the process of launching the new Victron Orion Smart DC-DC charger too, which will give an easier installation and better price point but without the solar capability.

Thank you for looking, we can't wait to hear from you with your results!