Need help building a emergency vehicle Aux power

Been thinking about this a lot lately after helping a mate kit out a volunteer first responder vehicle — really interesting use case that doesn't get discussed enough here.

For emergency vehicle aux power the stakes are obviously higher than your average campervan build. You cannot have a system that browns out mid-shout because the alternator can't keep up.

My thinking for a solid UK-spec setup:

• LiFePO4 over LiPo — safer chemistry, no thermal runaway drama. Fogstar Drift cells are brilliant value right now
• Victron SmartShunt — knowing your exact state of charge matters when lives might depend on it
• DC-DC charger rather than a split charge relay — proper controlled charging from the vehicle alternator, especially important with Li chemistry. Victron Orion-Tr Smart is the obvious choice
• Proper fusing at every stage — non-negotiable in a vehicle with this much critical kit running

Loads you're talking about (warning lights, siren, radio, laptop) aren't massive but they're continuous and you need redundancy. I'd size the battery bank generously — 100Ah absolute minimum, 200Ah if space allows.

One thing worth flagging for UK emergency vehicles — worth checking whether there are any BS or vehicle type approval implications for auxiliary electrical installations. Ambulance and fire service conversions sometimes have specific requirements I'm not fully across.

Anyone here actually done a blue light vehicle conversion? Curious whether you went inverter-based for the laptop or kept everything 12V native. The fewer conversions in the chain the better for reliability I reckon.

@PanelGraham — done something similar for a mountain rescue support vehicle a few years back. Lesson learned the hard way: don't skimp on the isolation.

Emergency use means the vehicle could sit dormant for weeks then need to fire up instantly. A Victron Smart Battery Protect saved us when the aux battery had drained down to nothing after a fortnight parked up — without it we'd have cooked the main start battery trying to compensate.

Also worth considering:

• Lithium over AGM — Fogstar Drift cells held voltage far better in winter
• Dedicated shore power input for when the vehicle's stationed at base
• Audible low voltage alarm — not just an app notification

The "it'll be fine" attitude kills emergency setups. These things need to work every single time, not just when the stars align.

For emergency use I'd go Victron MultiPlus without blinking — the transfer switch is near-instant and it won't let you down when someone actually needs it.

Key bits worth considering:

• Separate battery bank isolated from the vehicle's starter — last thing you want is a flat chassis battery at 2am in a field
• Fogstar Drift LiFePO4 holds voltage brilliantly under sudden heavy loads (defibrillators, lighting rigs, comms kit)
• Wire it properly with ANL fuses close to the battery — emergency vehicles occasionally catch fire and you don't want to be the reason
• VE.Bus BMS if you want proper cell-level protection

@GafferTapeKing mountain rescue use case is a great parallel — I'd be curious what load spikes they saw, because medical and rescue kit can be brutally unforgiving on an undersized inverter.

@MarineGeoff already nabbed the Victron point so I won't rehash that...

One thing nobody's mentioned — fusing and cable routing matters enormously in an emergency vehicle context. My shepherd's hut setup is fairly forgiving if something goes wrong. A first responder vehicle absolutely is not.

Think about:

• ANL fuses as close to the battery as physically possible
• Cable runs away from heat sources (exhaust etc)
• Proper strain relief on every connection

Also worth considering a Fogstar Drift LiFePO4 as your aux bank — decent BMS protection built in, handles the vehicle vibration reasonably well in my experience from the cabin build.

Does the vehicle need to run loads whilst the engine is off for extended periods? That changes the battery sizing calculation quite significantly. Knowing the actual draw in watts would help enormously here.

@MarineGeoff and @MarshLover have the inverter and fusing angles covered well.

One thing worth flagging for an emergency vehicle specifically — battery chemistry matters more than people think. LiFePO4 handles repeated deep cycles and fast charging way better than AGM when you're running kit hard between callouts. Fogstar Drift cells are decent value if budget's tight.

Also worth considering where the battery lives. In my shepherd's hut build I learned the hard way that vibration kills connections over time — proper compression terminals or good crimps, not cheap twist connectors. In a vehicle that's potentially going cross-country in a hurry, that's even more critical.

Ventilation too if you're in an enclosed load space — even LiFePO4 needs airflow consideration in a sealed metal box.