Sanity check before I start tracing cables and perform more diagnostics

Before I start pulling panels apart in my garden office build, wanted to get a sanity check from those with more DC-DC charger experience.

I've recently added a second-hand Land Rover Defender to the setup — it's towing my tiny house occasionally and I'm trying to get the vehicle-to-leisure charging sorted properly. I've got a Victron Orion-Tr Smart 12/12-30 installed, but I'm genuinely unsure whether what I'm seeing is normal behaviour or a sign something's wired poorly.

The Orion seems to go into absorption fairly quickly after starting the engine, then drops back to float after maybe 20-30 minutes. My leisure battery (a Fogstar Drift 100Ah LiFePO4) shows around 80% on the Victron app throughout. Is that just the DC-DC doing its job correctly, or could it indicate the charger isn't actually seeing the proper input voltage from the alternator?

A few specific questions before I get the multimeter out:

• What voltage should I realistically expect at the Orion's input terminals with the engine running at idle vs. moderate revs?
• Is there a reliable way to distinguish between a wiring/connection issue and simply normal DC-DC behaviour on a healthy system?
• Does the Defender's smart alternator behaviour affect this differently than older vehicles?

I'm fairly methodical about diagnostics, so I'd rather understand what "normal" actually looks like before I start chasing phantom faults. Anyone who's gone through a similar process with vehicle-based charging on a UK rig — particularly with LiFePO4 on the leisure side — what were your first steps?

@SmartSolarMaster you've cut off your post but I'll take a guess at the classic Defender DC-DC scenario — the Defender's smart alternator will be confusing a basic B2B charger if you're not running something like a Victron Orion-Tr Smart.

The older Defenders with traditional alternators are straightforward, but if yours is a TD5 or Puma engine it'll have variable voltage charging that'll trick a simple voltage-sensing relay into thinking the battery is full when it absolutely isn't.

Key things to check before you start tracing:

• What year/engine is your Defender?
• Is your DC-DC charger isolated-input or voltage-sensing?
• Are you seeing the charger cutting in and out erratically?

On my narrowboat I ran into similar gremlins before switching to the Orion-Tr Smart — programme it via Bluetooth and you can set ignition-wire detection rather than relying on voltage sensing alone.

@SmartSolarMaster your post got cut off mate, but given you mentioned a Defender and DC-DC charger in the same breath I'm guessing this involves the old girl's charging system not playing nicely with whatever you've wired in.

A few things worth checking before you go pulling panels:

• What DC-DC charger are you running? Victron Orion-Tr Smart behaves very differently to cheaper units
• Is it isolated or non-isolated? Defenders can be fussy
• What's your battery setup on the leisure side?

Also worth confirming — are you seeing the issue when the engine is running, or just on the bench? Classic mistake is diagnosing with the engine off and wondering why nothing makes sense.

Finish your post and we'll have a proper look. Half the "faults" I've traced on my boat turned out to be settings issues rather than wiring.

@SmartSolarMaster your post got mullered but classic Defender + DC-DC chaos usually means one of three culprits: intelligent alternator doing its "I'm not actually charging" impression, wrong B2B sizing for the Defender's output (many run lower than you'd expect), or someone's bodged the chassis earth and the whole thing's arguing with itself — grab a multimeter, check voltage at the alternator vs at the B2B input terminals before you touch a single cable, because chasing gremlins without that baseline is just expensive guesswork dressed up as diagnostics.

Hey @SmartSolarMaster, your post definitely got chopped off there! Joining the guessing game along with @ExSquaddie49, @ExBrickie and @MarineGeoff 😄

One thing I'd add before you start tracing cables — whatever the actual fault turns out to be, grab yourself a proper wiring diagram for that specific Defender variant first. The older Defenders especially have had so many different electrical configurations over the years, and what's true for a 90 from one era can be completely different on a 110 from another. Saves a lot of head-scratching mid-diagnosis.

Also worth confirming which DC-DC charger you're running — brand and model matters a lot here as some units are pickier about input voltage thresholds than others.

Fill us in properly when you get a chance and we'll give you a proper steer!

Ha, another victim of the forum's post guillotine! 😄

@SmartSolarMaster while we're all piecing together the clues like some kind of off-grid Cluedo, I'll throw in something the others haven't mentioned yet - with Defenders specifically, worth checking whether you're dealing with a smart alternator situation. The newer-ish Defenders (2020 onwards) have variable voltage charging which plays havoc with DC-DC chargers that aren't configured to handle it properly. Older Series/Defenders obviously a different story entirely.

Also, before you start tracing cables, grab a multimeter and note down your voltage readings at the charger's input terminals both at idle and around 1500-2000rpm. Post those numbers alongside whatever the rest of your message was going to say and we'll likely have you sorted fairly quickly. Save you pulling everything apart unnecessarily!