Why VSRs & Solar Systems may be causing damage to your leisure battery

5 Comments23 November 2022

Installing a solar system to a leisure battery with a VSR may seem like a smart way of helping keep the leisure & starter batteries charged, but it is actually a bad idea...

First, let's explain how Voltage Sensitive Relays (VSRs) work. When a charging voltage is detected on either side of the relay it will close like a switch, meaning that current can flow through the relay. When that charging voltage is removed (or drops below a threshold), the relay breaks the connection and the current can no longer flow from one battery to the other. 

A solar system will always produce a charging voltage when light is present and as a result, this will activate the VSR and share the charge between the leisure battery and the starter battery. This can be great to help prevent the starter from self-discharging, especially in winter months when the vehicle may be static. There is, however, a downside to this. As both batteries are connected by the relay they are now operating as a bank of batteries, and this means both batteries are being used to power any loads connected to either battery.

As a result, when you start the vehicle the starter motor will draw power from both the starter battery & leisure battery. Over time, this type of usage will damage your leisure battery as it is not designed for the high draw requirements of the starter motor, so you will likely find yourself changing your leisure battery more often. 

To prevent this you could install an isolation switch and simply isolate the VSR system, start the vehicle then turn the VSR system back on. This would allow the starter motor to use only the starter battery but does mean you would need to remember to turn it off and back on to keep charging your leisure battery from the alternator whilst driving. The second option is a DC-DC Charger. As they only operate in 1 direction (starter battery > leisure battery) there is no way for the starter motor to draw power through the unit. You can use a trickle charger alongside a DC-DC or a dual output solar controller to then help keep both the starter and leisure battery fully charged. 

We hope this article helps you when looking at installing a solar system and as always, if you have any questions please get in touch with a member of our team.



Philip Hennessy
01 December 2022  |  15:56

I have an isolation switch for the VSR on my boat but you have to be very regimented to remember to switch it on to charge the house battery so I think a DC to DC charger is the way to go!!

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