Mppt as a dc to dc charger

So I've been scratching my head over this one and wondered if anyone's tried it before I go doing something daft in the yard.

Background: I've got a Victron SmartSolar 100/30 that's currently doing nothing while I wait for my new panel array to arrive. My garden office runs off a separate Fogstar 100Ah lithium setup, but I've also got an older 100Ah lead acid leisure battery in the Land Rover that I use for the electric fence controller out in the field.

Here's my thinking — if I fed 12V from the Landy's alternator output into the PV input terminals of the MPPT (obviously while it's disconnected from any actual panels), would it effectively behave as a DC-to-DC charger and top up the leisure battery properly? The alternator sits around 13.8–14.4V when running, which isn't a million miles off what a decent solar input looks like on a cloudy day.

I know dedicated DC-DC chargers like the Victron Orion-Tr Smart exist specifically for this job, and yes they handle the alternator's voltage profile much more sensibly. But I've got the MPPT sat on the shelf doing nowt, and curiosity got the better of me.

Questions I can't quite work out:

• Would the MPPT's MPPT algorithm cause any issues pulling from an alternator rather than a panel?
• Any risk of damaging the unit or the vehicle's electrics?
• Has anyone actually measured whether it works efficiently in practice?

Genuinely not sure if this is a clever bit of repurposing or a recipe for a fried controller. Anyone been down this road?

@ExFarmer90 done exactly this in my van build as a stopgap before my proper DC-DC charger arrived.

Short answer: technically it works, but it's a bodge.

The MPPT is looking for a panel with specific voltage/current characteristics to do its thing properly. Feed it a DC source instead and you're essentially tricking it — behaviour becomes unpredictable depending on your source voltage relative to your battery bank.

What actually happened in my setup: it sort of charged, but the Victron couldn't properly regulate because the input didn't behave like a panel. Bulk/Absorb cycling went a bit mad.

If you're using it for emergency backup in a yard setting, it might limp along. But for anything regular, grab a proper Victron Orion-Tr Smart — isolated version if your chassis grounds differ. Night and day difference.

What's your source voltage and battery chemistry?

Works technically but there's a catch worth knowing — the MPPT "sees" your alternator/source voltage and will try to MPPT-track it, which it can't do properly since it's not a PV array.

You'll want to disable the MPPT algorithm if your controller allows it, or just accept it'll behave oddly at the input side.

Practically speaking:

• Keep input voltage higher than battery to give it something to work with
• Watch the input current draw — it'll pull hard on startup
• Set charge profile correctly for your battery chemistry

Done it myself on the narrowboat between the engine alternator and a leisure bank as a temporary fix. Worked fine for weeks. Wouldn't rely on it long-term though — a proper Orion-Tr Smart is built for the job and handles alternator quirks (like smart alternators) properly.

Been curious about this myself for my van build — just want to make sure I understand the limitation correctly before I try anything similar.

If the MPPT is expecting a solar panel with a higher open-circuit voltage than your alternator output, does it just refuse to track properly, or does it actually throw a fault? Asking because I've got a Victron SmartSolar 75/15 sitting spare and my alternator is putting out roughly 14.4V — worried that's too close to my 12V battery voltage for the controller to see enough headroom to do anything useful.

Also worth flagging — has anyone run this through the Victron VRM portal to see what the charge curves actually look like? Curious whether it reports anything sensible or just goes haywire in the logs.

Good topic — this came up when I was wiring my motorhome's secondary battery bank a few years back.

@CaddyProject has touched on the key issue: without a proper varying-voltage input (like a solar panel), the MPPT algorithm has nothing meaningful to track. Your alternator presents a fairly fixed voltage, so the controller essentially operates in a permanent "bulk" state rather than intelligently seeking a maximum power point.

Practically speaking, it can work as a crude pass-through charger, and @SussexVanLifer's experience backs that up. But you're not getting the sophisticated charge profile you'd get from a dedicated DC-DC like a Victron Orion-Tr Smart or even a Renogy equivalent.

The bigger concern on a farm vehicle: without proper isolation, you risk feedback into your alternator's sensing circuit. A B2B charger with galvanic isolation protects against that properly.

Fine as a short-term bodge, not something I'd run permanently.