Disclosure: I paid for my Victron Cyrix unit myself and have no relationship with Victron or its UK suppliers. However, links marked with (eBay⇒) or (Amazon⇒) are affiliate links. This means I get paid a small commission if you buy something after clicking on the links. This money helps to pay for the running of the website.
As I mentioned in my 18 month review of our DIY van conversion, we replaced the Sterling Battery to Battery Charger we originally installed with a a voltage-sensing relay at the end of 2009.
The reason for this was that I had decided that the B2B charger was overkill for our simple conversion with its modest power requirements (excluding a fridge). At the same time, I wanted something better than a standard split-charge relay. I came across the Victron Cyrix-i Battery Combiner. Like Sterling, Victron is a maker of high-quality 12/24V electric power equipment that is aimed at the marine market and at the top end of the motorhome market.
What Is A Victron Cyrix-i Battery Combiner?
The battery combiner is a small box that contains a microprocessor-controlled relay. It is installed between the starter and leisure batteries and is effectively an intelligent, voltage-sensing relay. Unlike standard relays, no connection to the vehicle’s alternator or ignition is required – the Cyrix detects whether the engine is running and charge is available from changes to the battery voltages:
- If the voltage across the starter battery rises sufficiently, it will connect the leisure battery so that it gets charged too.
- If the voltage across the leisure battery rises, it will detect that a battery charger has been connected and will connect the starter battery so that it too gets charged by the battery charger.
- It notices voltage trends – the combiner notices how the battery voltages are changing – upwards or downwards – and acts accordingly to protect you from the risk of either going flat unnecessarily.
- It offers an optional bypass switch that allows you to connect both batteries for 30 seconds to start the engine if the starter battery is flat.
There is almost no voltage loss across the Cyrix, so the leisure battery gets charged at virtually the same voltage as the starter battery, helping to ensure it receives a reasonable charge.
The battery combiner is essentially a passive device – it is not a charger and thus it will not charge batteries to maximum capacity in the way that a multi-stage battery charger will. The model I chose was the 100A version – 100A is far more current than will ever be used to charge my leisure battery!
Installing & Using the Cyrix-i
The Cyrix-i is an extremely simple piece of equipment to install. Victron provides lots of information and example wiring diagrams on its website, but essentially, you just have to connect it between the positive terminal of the starter battery and the positive terminal of your leisure battery. It needs a single earth connection, too.
Here’s a picture of mine, installed:
In use, the Cyrix-i is a fit-and-forget piece of equipment. It automatically connects and disconnects depending on whether the engine is running or whether you have a battery charger connected and requires no maintenance or user input at all.
We’ve had no electrical problems at all since installing the Cyrix-i and our leisure battery has always been kept adequately charged from driving alone. I am very pleased with it.
Update: 5 years on from this review and the Cyrix unit is still in place and working perfectly. A great piece of kit.
The Cyrix-i is pretty affordable – prices are around £35 at the time of writing. This is a little more than a standard split charge relay (which is controlled by the alternator/vehicle ignition feed), but the Cyrix is far more sophisticated and easier to install and does not require any connection to the vehicle’s electrical system.
I also like the bi-directional functionality of the Cyrix-i – when on a campsite for a period of time, you can connect up your mains-powered leisure battery charger and know that your van’s starter battery will automatically be kept topped up, too.
Overall, a good piece of equipment, thoroughly recommended.