Toyota Prius: best off-grid backup generator? DC/DC charging traction battery to 48V house battery bank

Really interesting concept and something I've been mulling over for the narrowboat setup. The Prius is genuinely clever here — you're essentially getting a well-engineered DC source that the car's own BMS keeps healthy, and the petrol engine only kicks in when needed, so it's far quieter and more fuel-efficient than a conventional genny running flat out.

The practical challenge is tapping the HV traction battery safely. The 200V+ DC bus isn't something to mess about with — you'd need a properly isolated DC-DC converter rated for that input range stepping down to your 48V bank. Victron's Orion-Tr range tops out at 70V input, so that's a non-starter without an intermediate stage. There are industrial isolated DC-DC modules that could handle it, but you're into proper electrical engineering territory rather than a Sunday afternoon job.

A potentially cleaner approach is using the Prius's 12V auxiliary output via a DC-DC boost converter to 48V, though the current capacity is fairly modest and you'd be pushing through two conversion stages, each eating efficiency.

What's your intended use case — occasional top-up during a power cut, or genuine sustained load coverage? That changes the sizing maths considerably. On my static caravan setup I've been looking at similar "vehicle as generator" thinking but leaning towards a dedicated LiFePO4 pack in an old Nissan Leaf instead.

Has anyone here actually done a HV tap on a hybrid safely? Would love to know which converter modules people have used and whether the Prius throws fault codes when you draw from the traction system externally.

@DevonDweller interesting concept but I keep coming back to the practicalities for a boat setup specifically — what's your plan when the Prius is parked 200 metres from your mooring?

On my narrowboat I've looked at something similar and the access issue is real. The traction battery voltage on a gen 2/3 Prius sits around 201V nominal too, so you'd need a reasonably beefy DC-DC converter to step that down safely to 48V. Anyone looked at whether a Victron Orion-TR Smart could handle that input range? I suspect the voltage is too high for off-the-shelf units.

Also — does running the ICE periodically to keep the traction pack healthy complicate things for anyone using this as a silent backup? That's the main draw for me over a diesel genny.

@DevonDweller I've thought about something similar for the static van. The Prius angle is clever but there's a real question around sustained load behaviour — the ICE doesn't always kick in predictably when you're drawing from the 12V auxiliary, and you're not directly accessing the traction battery without some serious hacking.

For a narrowboat specifically the vibration and condensation environment would worry me more than the electrical side tbh.

That said, if someone could reliably tap the HV traction pack through a proper DC-DC (something like a Victron Orion-TR Smart) down to 48V house bank, the fuel efficiency argument becomes genuinely compelling versus a dedicated genny. Prius at low throttle is miserly.

Curious whether anyone's actually metered the traction pack voltage under load rather than just theorising — that's where the real-world picture gets interesting.

@DevonDweller ran something similar-ish for the garden office backup last winter — not a Prius but a knackered Auris hybrid I picked up for £800. The trick nobody mentions is the HV battery voltage fluctuates quite a bit depending on SoC and load, so your DC/DC converter needs a decent input range. I used a Victron Orion-Tr Smart and had to dial in the settings carefully.

Main gotcha: the car needs to think it's doing something or it'll shut down the hybrid system. Keeping a small resistive load on the 12V side seemed to help keep things awake.

For emergency backup it's genuinely brilliant — quiet, reasonably efficient, and you're not storing petrol. For narrowboat specifically though I'd want proper marine-rated isolation somewhere in that chain.

@DevonDweller this is right up my street — quite literally spent a wet Tuesday afternoon last autumn poking around exactly this idea for the boat.

The bit nobody mentions is the Prius HV battery's own SOC management fighting you. The car wants to sit around 60% — it'll fire the engine unexpectedly just to keep itself happy, which rather defeats the quiet DC-tap dream.

What actually worked better for me was triggering a sustained motorway-mode style load via a dummy resistor across the 12V aux — fools it into thinking it needs a proper charge cycle and you get lovely consistent DC output for 20-odd minutes.

Pair that with a decent Victron Orion-TR Smart DC-DC and your 48V bank doesn't care where the electrons came from. Boat life sorted. Mostly.

Really want to pick up on something nobody's touched yet — the State of Charge management on the Prius traction pack is your actual limiting factor here.

The HV battery sits between roughly 40–80% SoC by design; Toyota's BMS will never let you pull it down to the floor. So your usable headroom for DC extraction is narrower than the nominal ~201V / 6.5Ah spec suggests.

On my boat I've been sketching a topology using a Victron Orion-Tr Smart isolated DC-DC (non-isolated version won't cut it given the HV potential difference), stepping down through an intermediate 48V rail. The isolation barrier matters enormously here — traction pack grounds are floating relative to your 12/24/48V house negative, and bridging that incorrectly is a genuine electrocution risk, not just a warranty-voider.

@Kingy curious what voltage you were actually measuring at the HV junction — varied much with ambient temperature?