Address
Blyth, Northumberland
U.K. NE24 2QW
Telephone
+44(0)1670 336766
Opening Hours
Monday to Friday: 08:00 - 17:30
VSR vs B2B charger — it’s the split charge question that comes up on almost every campervan and leisure vehicle build, and the answer has changed significantly in the last decade.
There’s a device fitted to the majority of campervan and leisure vehicle builds in the UK. It’s cheap, simple, widely available, and sold by almost every auto electrical supplier and online marketplace that serves the van conversion market. It does the job it was designed for adequately, in the conditions it was designed for.
The problem is that the conditions it was designed for are increasingly not the conditions it’s being used in.
That device is the voltage sensing relay. And if you’ve fitted one recently to a vehicle built in the last five to eight years, there’s a reasonable chance it’s either not charging your leisure battery properly, or it’s quietly annoying your van’s engine management system, or both.
What a VSR actually is and what it does
A voltage sensing relay — VSR, split charge relay, battery isolator relay, smart relay, call it what you like — is a relay that monitors the voltage on your vehicle’s starter battery circuit. When it sees a voltage above a threshold (typically around 13.3–13.8V, depending on the unit), it interprets that as “the alternator is running” and closes a contact that connects your leisure battery in parallel with the starter battery. Current flows from the higher voltage source to the lower, the leisure battery charges, everyone’s happy.
When the engine stops, voltage drops, the relay opens, and the two batteries are isolated again. Simple. Elegant. Cheap to make. Requires no configuration, no settings, no wiring beyond the relay itself and a suitably sized cable between the two batteries.
For the best part of thirty years, this worked well enough. Vehicles had conventional alternators that charged at a predictable voltage — typically 13.8–14.4V when running, dropping to resting voltage when off. The VSR saw a high voltage, closed, charged the battery, opened when the engine stopped. The leisure battery got a reasonable charge on longer drives. People were mostly happy.
Then smart alternators happened.
The smart alternator problem
Modern vehicles — and we’re talking anything from roughly 2012 onwards, with the list getting longer every year — increasingly use variable voltage or smart alternator systems. The reasons are fuel economy and emissions. A conventional alternator runs continuously at full output, consuming engine power and burning fuel whether the battery needs charging or not. A smart alternator modulates its output based on what the vehicle’s battery management system actually needs.
In practice this means the alternator voltage bounces around. It might charge at 14.8V during engine braking when the system is harvesting energy. It might drop to 12.8V or lower during steady motorway cruising when the battery is full and the BCU is reducing the load on the engine. It might pulse. It might shut off entirely for periods.
A VSR cannot distinguish between “alternator is running normally at low output” and “engine has stopped.” Both look the same to a relay monitoring voltage. The result is a VSR that opens and closes repeatedly during a journey — sometimes multiple times per minute — as the smart alternator modulates its output up and down across the relay’s threshold voltage.
This has two consequences. First, your leisure battery gets an erratic, interrupted charge that may be significantly less than you’re expecting. Second — and this is the one that makes workshops nervous — the repeated switching can create voltage spikes and electrical noise on the vehicle’s charging circuit. Some BCUs and engine management systems on modern vehicles are not happy about this. The fault codes that result range from annoying to genuinely disruptive.
We find VSRs on smart alternators regularly in the vehicles we diagnose. The symptom presentation varies — sometimes it’s a leisure battery that never seems to get properly charged despite long drives, sometimes it’s phantom BCU faults, sometimes it’s just an owner who’s noticed their starter battery seems to be working harder than it should. The common thread is always the same: a device designed for a dumb alternator fitted to a smart one.
What a B2B DC-DC charger is and why it solves the problem
A battery to battery charger — B2B, DC-DC charger, DC-DC converter, again there are several names in common use — is a fundamentally different piece of kit. Where a VSR is passive (it’s just a relay that closes when it sees enough voltage), a B2B charger is an active device with its own electronics, its own charge algorithm, and its own regulated output.
The B2B charger takes power from your starter battery circuit — at whatever voltage the smart alternator happens to be producing at that moment — and converts it to a stable, regulated charging voltage appropriate for your leisure battery. It doesn’t care whether the input voltage is 12.8V or 14.8V or bouncing between the two. It takes what it’s given and produces a controlled output.
That controlled output means your leisure battery gets a proper multi-stage charge — bulk, absorption, float — rather than whatever the VSR happened to push through during a two-hour drive. It means the input draw from the starter battery circuit is smooth and predictable, which modern BCUs are considerably happier about than the switching noise a VSR produces. And it means the two battery systems are properly isolated from each other — there is no direct parallel connection between your starter and leisure batteries at any point.
The B2B charger also gives you a defined output current. A 20A B2B charger puts 20A into your leisure battery. A 30A unit puts 30A. You know what you’re getting. With a VSR, the charge current is whatever the voltage differential between the two batteries happens to produce, which varies throughout the journey and is difficult to calculate in advance.
How to choose between them
Here is the honest answer, which some suppliers would rather not give you because VSRs are cheaper and easier to sell.
If your vehicle has a conventional alternator — older vehicles, many commercial vans, some fleet vehicles — a VSR is a perfectly adequate solution for AGM or lead-acid leisure batteries. It’s cheap, it works, it’s easy to install, and you don’t need to overthink it.
If your vehicle has a smart or variable voltage alternator — which means most modern passenger vehicles and an increasing number of commercial vehicles — fit a B2B charger. Not because VSRs are categorically terrible, but because the conditions they were designed for no longer reliably apply, and the failure modes when they don’t are expensive and annoying to diagnose.
If you have a lithium leisure battery — we’ll do that conversation justice in a separate post, but the short version is: fit a B2B charger. Full stop.
If you’re not sure what kind of alternator your vehicle has, the quickest diagnostic is to look up the base vehicle and check whether it has a battery management system or smart charging. If it’s a Euro 6 emissions vehicle it almost certainly does. If it was built after 2015 there’s a very good chance it does. If you’re in any doubt, ask — it’s a ten-second check and it changes the answer entirely.
A note on cable sizing
Whichever route you go, the cable between your starter battery circuit and your leisure battery system needs to be sized correctly. A VSR connection carries sustained alternator output current — potentially 80–100A on a modern alternator — and the cable needs to be sized for continuous duty, not peak. A B2B charger connection carries the charger’s rated input current, which is typically lower and defined by the charger’s specification sheet.
Either way, check the cable size, check the run length, check the volt drop. A correctly specified cable for a VSR installation is typically 16–25mm² depending on the run. Getting this wrong is one of the most common installation errors we see, and it’s one of the least glamorous faults to diagnose.
See our 12V Cable Sizing Guide for the reference data.
If you’ve got a vehicle with a smart alternator and a VSR and you’re not sure whether it’s working correctly, that’s a diagnostic conversation we’re set up for. Tell us what you’ve got and we’ll tell you what we think.
Alternator image: Manuel E Sankitts on Unsplash. Campervan aerial: Erik Fabian on Unsplash.