Swapped out my split charge relay for a Victron BIM — was it worth the extra cost?

After two seasons running a basic split charge relay on the van I finally caved and bought a Victron Battery Isolator Manager (the 90A version) to keep my 200Ah leisure battery topped up from the alternator. Cost me about £85 fitted versus maybe £15 for the old relay setup, so a fairly significant jump in outlay for what looks like a similar job on the surface.

The main thing that pushed me towards it was the BIM being "voltage sensing" rather than just switching on at a fixed voltage. With the old relay I kept getting the leisure battery dragged down on long motorway runs because the relay was cutting in and out based on dodgy voltage readings. The BIM seems to handle that much more cleanly and my 200Ah Battle Born equivalent (a Fogstar Drift 200Ah LiFePO4 I fitted last spring) is consistently hitting 100% after a couple of hours on the road now.

What I'm less sure about is whether I'm genuinely getting more out of the alternator than before, or whether the improvement I'm seeing is mostly down to switching from AGM to lithium at the same time. The two changes happened pretty close together so it's hard to isolate the variables properly.

Has anyone else run a direct comparison between a standard relay and a BIM, ideally on the same battery chemistry? Curious whether the BIM is doing real work here or if I've just paid Victron premium for peace of mind.

Reply by LauraCole

@Neil1962 Great upgrade choice! One thing worth mentioning that often gets overlooked — the BIM responds to actual alternator output voltage rather than just switching on a timer or simple threshold, so you'll notice it's much smarter about not pulling the starter battery down when the engine's at idle or during stop-start driving.

We did the same swap on our Ducato-based van last year and the difference in winter charging was noticeable — shorter runs actually contributed something meaningful to the leisure bank rather than being almost pointless like before.

Also worth checking your cable sizing is matched to that 90A rating properly if you haven't already — it's easy to inadvertently create a bottleneck there and never realise you're not getting the full benefit. What alternator are you running?

Reply by WonkyBuilder

@Neil1962 Good shout on the upgrade. One thing I'd add that @LauraCole has probably already touched on — make sure your alternator can actually handle the sustained load. Older vehicles with smaller alternators (sub-70A) can struggle a bit once the BIM gets stuck into a deeply discharged lithium bank, since unlike a relay it'll genuinely push proper current rather than just dribbling charge through. Worth keeping an eye on your alternator temperature for the first few longer runs. Also, if you're running LiFePO4 rather than AGM, double-check the BIM firmware version — some earlier units needed updating to play nicely with lithium charge profiles. Not a dealbreaker but worth knowing before you start scratching your head over odd behaviour. Overall though, the voltage-sensing and proper isolation alone make it worth the premium over a basic relay in my experience.

Reply by WezWhite72

@Neil1962 Worth every penny in my experience. The key thing neither @LauraCole nor @WonkyBuilder seem to have touched on yet — the BIM uses voltage sensing to determine when the alternator is actually running, rather than just responding to ignition-on. This matters because a standard relay will sometimes close on a high resting voltage and you end up with a brief period of battery-to-battery discharge before the alternator spins up properly.

Running a 280Ah LiFePO4 bank (Fogstar Drift cells) in my off-grid cabin setup via a dedicated charging circuit, and the voltage-based detection has been rock solid.

One practical tip: make sure your alternator wiring is properly rated for sustained 90A draw. A lot of older vans have undersized runs that cause nuisance voltage drops and confuse the BIM's detection thresholds.

Running the BIM on my boat setup for 18 months now and the difference in charge quality is noticeable — particularly on shorter runs where a traditional relay would barely put anything meaningful back.

One thing nobody's touched on yet: the BIM plays nicely if you ever decide to add a Victron MPPT solar controller later. Having everything talk the same language through a SmartShunt and the VictronConnect app makes monitoring genuinely useful rather than just guesswork.

@Neil1962 with 200Ah of leisure battery you'll really appreciate it once the colder months hit — alternator charging becomes more critical when solar input drops off. The 90A version was the right call, the 30A unit would've been a bottleneck.

Reply by MoorCamper

Mainly use my setup for emergency backup rather than full-time living, so the BIM doesn't get hammered daily — but when I do need it, I really need it. The voltage-sensing charge profile means my Fogstar lithium bank is actually at proper capacity when a power cut hits, not just "sort of charged" like the old relay left it.

One thing worth mentioning that nobody's flagged yet: the BIM plays nicely with Victron's GX ecosystem if you ever want monitoring via VictronConnect. Overkill for some, but if you're already in that world it's a bonus.

@MarineMike — good to hear the marine side holds up too. Same core benefit really, charge quality when it counts.

For @Neil1962 — short answer, yes, worth it.

Reply by ExFarmer21

Ran a split charge relay for one winter in the van and my leisure battery aged about ten years — the BIM basically stopped the alternator from bullying it into an early grave. @MarineMike is right about charge quality, and if you've got a lithium bank like my Fogstar 200Ah it's non-negotiable since the relay will just hammer it at full voltage like a tractor going through a hedge.

Worth noting that the BIM plays really nicely if you ever expand your setup — I added a Victron SmartSolar MPPT to my cabin later on and having the Victron ecosystem already in place made integration much simpler. The Bluetooth monitoring alone justified a chunk of the cost for me.

@ExFarmer21 makes a fair point about relay damage. I ran a dumb relay on a static setup years back and the voltage drop meant my Fogstar lithiums never hit a proper full charge. Wasted capacity I was paying to maintain.

If you're running decent batteries, the BIM is worth it — don't pair a smart battery with a dumb charger.