We had a bit of a nightmare working out how to charge our leisure batteries from our vehicle starter battery. Any vehicles with a Euro 6 engine (such as our 2013 Sprinter) cannot use a standard VSR (voltage sensing relay), as they have what's referred to as a 'smart' alternator. A traditional alternator will have its voltage controlled by an internal voltage regulator, and will maintain a voltage of around 14V while the van is on. A standard VSR relies on a high voltage like this to charge your leisure batteries. A 'smart' alternator instead has its voltage controlled by the Engine Control Unit (ECU). This means that after the van has been on for a short amount of time, and if the starter battery is close to fully charged, the voltage will drop well below 14V, which reduces fuel consumption and lowers emissions. However, this also means that any vehicle with a Euro 6 engine can't use a standard VSR that relies on a constant high voltage whilst driving.
The other difference you experience in a van with a Euro 6 engine is regenerative breaking. As you brake, the smart alternator uses the kinetic energy from braking to charge the starter battery. This will have a much higher voltage than you would typically see (15V+ rather than ~14V).
The standard method of charging a leisure battery from a starter battery with a Euro 6 engine is using a battery to DC to DC (battery to battery) charger. The two most popular we've found are the CTEK D250SE (or the CTEK Smartpass 120 if you have batteries over 300Ah) and the Sterling Power Procharge Ultra. These are the safest way of charging leisure batteries if you have a Euro 6 engine, as they will take the variable voltage from the starter battery and output a stable voltage to your leisure battery(ies). The downside is that these cost £200+, whereas a standard VSR is around £40, and they are also much larger.
There are a couple of downsides to not using a DC to DC charger with a Euro 6 engine that arise from regenerative braking. Firstly, regenerative braking is only useful if the excess charge can be stored. This means that the ECU maintains the starter battery at around 80% of its full charge, so that it can use the charge created by the regenerative braking. If a leisure battery is above 80% charge and it is connected to the starter battery, its charge will also be dropped to 80%. The second issue is that AGM and gel batteries are sensitive to overcharging and voltages above 14.4V, such as the high voltages seen from regenerative braking, can cause gas bubbles to form which damages the battery.
To avoid the issue of dropping the leisure battery charge to 80%, we use our kill switch to turn off the connection between the VSR and our starter battery when our leisure batteries are above 80%, and allow them to charge the remaining 20% via our solar panels. As we have flooded lead acid batteries, and we are only using our VSR to charge our batteries up to 80%, we didn't need to worry about overcharging the batteries and damaging them. If you have AGM or gel batteries, it may be advisable to purchase a DC to DC charger instead.
Installing the Durite Smart Relay
Unfortunately there isn't much information about the Durite Smart Relay online, which is why we wanted to give some information about how we installed it. When we ordered the relay, it arrived with a small 2 sided leaflet explaining how to wire it in, and the different programmes available, and there is currently no information other than what is on the leaflet available online. We have written below a more detailed explanation of how to wire in the Durite Smart VSR (including a wiring diagram and photos connecting it to a Sprinter starter battery), and how to change the programmes.
Connecting to your starter battery
Before we laid our insulation and the wooden floor in our van, we routed some wires through a conduit tube from our electrics box to the battery. In a 2013 Mercedes Sprinter van, the starter battery can be found under the passenger side footwell. Below is a photograph of how we connected the VSR to our starter battery. We used a fuse on the positive wire, as close to the starter battery as possible.
Wiring instructions To install the VSR, simply connect the input (terminal 30) to the starter battery, the output (terminal 87) to the leisure battery(ies), and terminal 85 to the ground. We used 25mm cable to connect our batteries to our VSR. We put a fuse between the starter battery and the smart relay, between the smart relay and the leisure batteries, and between the leisure batteries and the starter battery. This protects both the VSR and the leisure batteries from a high current surge. Once your VSR is connected to the batteries, the small LED on top of the smart relay will light up.
There are nine programmes available to use on the smart VSR, and the factory setting is programme 6. As we didn't want to wait 5 minutes to check if the smart VSR was working, and we wanted a low switch on voltage (due to the nature of smart alternators), we changed the setting to programme 3.
To enter programming mode, your van will need to be off so there is no power going to the input (terminal 30). Connect terminal 86 to the input (terminal 30) via a momentary switch. We wired up a light switch to use for this. Turn your switch on, to connect terminals 86 and 30, and then turn on your van to supply power to the input (terminal 30). The LED on the VSR should flash a number of times, to indicate which programme you're currently on (this should be programme 6).
To change the programme, switch off and back on the connection between terminals 30 and 86 within 40 seconds. The LED will start to cycle through the programme numbers. Once on the programme you wish to change to, turn the switch off. The LED will flash to show the chosen programme. If this is correct, turn the switch back on. Now you can turn your van off, and re-wire the smart relay as per the wiring instructions.
Once the VSR is wired in, you can test that it's working by turning on your van, waiting the delay period (either 5 seconds or 5 minutes depending on the programme you're on), and seeing if the voltage of your leisure battery(ies) increases via your battery monitor. We have a Victron BMV-712 battery monitor, so we used the app to check the leisure batteries once the smart relay had kicked in. We saw readings of 350W and 35A with the engine running, so our 360Ah batteries should charge from 50% to 100% in about 5 hours.
In summary, the suggested method for charging your batteries with a Euro 6 engine to prolong the life of your leisure batteries is using a DC to DC charger, however if you don't wish to spend £200+ and you don't have AGM or gel batteries, the Durite Smart VSR is a good alternative, which we have found to work in charging our leisure batteries.
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