Installing a Voltage Sensitive Relay (VSR)

DISCLAIMER: Working with electricity can be dangerous. I am not a professional electrician, nor do I have an extensive history with electrics. All the information provided has been gathered through advice and my own research and is specific to my electrical system.


TOTAL: £30.34

Voltage sensitive relay installed under the passenger seat

The next component to install in my electrical system was a Voltage Sensitive Relay (VSR).

When driving the van, the voltage of the battery will jump to around 14V. The VSR has a cut in voltage of 13.7V, so when the voltage exceeds that, the VSR will start sending power to the leisure batteries as well, allowing you to charge them.

The VSR also has a cut out voltage of 12.8V so when the voltage of the starter battery falls below that, the VSR will cut out.

It’s worth mentioning that newer vans will have a Euro 6 engine that includes a ‘smart alternator’. In an effort to be more economical, this alternator will reduce the voltage when you brake or sit idle with the engine running. Because VSRs rely on a constant voltage to be able to function, they won’t work with Euro 6 engines. Instead you would need to purchase a B2B charger which has a different wiring method.

Click here to see my full wiring diagram and section being described in this post.


VSRs have two positive terminals and a negative cable that will need to be grounded to the vehicle’s chassis. One of the positive terminals needs to be connected to the starter battery’s positive terminal and then the other positive terminal is connected to the positive busbar of the electrical system.

My VSR is installed underneath the passenger seat, however this means that the wire leading to my positive busbar needs to be fed through the floor of the van, along the bottom of the van and back up through another hole in the van’s floor, to where my electrical system sits. Alternatively, depending on how far through your van build you are, you could feed the wire through the interior of the van.

It’s also a good idea to place a fuse on the cable between the VSR and the busbar.

Fortunately for me, my van already had a VSR installed under the passenger seat, along with a 200A fuse, then the cable was already fed under the van to where my electrical system is housed. This saved me from drilling holes into the floor of the van again. Instead I just did a couple of things to tidy everything up.

Tidying Up

First of all, the end of the cable attached to my starter battery didn’t have a lug on it, so it was just exposed cable fibers wrapped around a bolt. I removed this connection and crimped, then heat shrunk a cable lug to the end of the wire to make sure I had a safe and secure connection.

The second thing I did was to fit a 200A circuit breaker between the starter battery and the VSR in case I need to isolate the system at any point. The cables were all 35mm, which has an amp rating of 240A, so I chose a 200A circuit breaker as this will trip before any damage can be done to the cables.

After everything had been connected up, a great way to test it was working was to open up my Victron app and view my battery monitor then go for a drive. You can clearly see the voltage increase on the app when the battery is being charged.

If you have any questions at all, feel free to ping me an email or drop me a message on Instagram!