Complete electrical guide for campervan newbies

@CornishBoater raises a good point about practical measurement. I've been wrestling with this in my garden office setup—trying to understand where my voltage actually drops versus what the battery management system reports.

Has anyone found that their BMS readings differ significantly from direct multimeter measurements at the terminals? I'm wondering if it's worth investing in one of those inline shunts that gives you a proper picture of what's happening under load, rather than just poking around with a cheap meter.

Also, placement of leads matters, but what about cable length itself? I've read that longer runs to your measurement point can give dodgy readings, but I'm not clear on the threshold—is 2-3 metres of lead wire going to skew things noticeably?

Asking because I'm planning a tiny house setup next year and want to get the monitoring sorted from day one rather than bodging it and replacing it later.

The voltage sag issue is worth digging into properly because it's where most people's calculations diverge from reality. I've got a static caravan setup that taught me this the hard way.

What @RetiredSquaddie's saying about lead placement is spot on—your meter leads introduce resistance, especially over distance. In my emergency backup system, I run a dedicated pair of 6mm² tails directly from the battery bank to a remote shunt (Victron BMV-712) mounted near the main load centre. That 2-metre run costs you roughly 0.03V at 100A, which sounds trivial until you're trying to hold 12.8V under load and dropping to 11.9V instead.

The practical solution: measure voltage at the load point, not at the battery terminals. If you're running a 40A kettle 3 metres away on 4mm² cable, you're looking at 0.6V drop before you even see it at the battery. That's why proper cable sizing and shunt placement matter more than obsessing over the battery's surface voltage.

For a campervan, you haven't got

@Compo's right—voltage sag catches everyone out because the maths looks fine on paper until you're actually running the kettle and watching your inverter throttle back.

The measurement placement is critical. You need your meter leads directly at the battery terminals themselves, not at the busbar or anywhere downstream. A lot of people measure at their main switch or distribution point and wonder why their calculated losses don't match reality. Cable resistance compounds fast—even decent 25mm² cable over 2 metres can drop 0.3-0.5V under heavy load.

For campervan work specifically, I've found it worthwhile running thicker cable than the rulebooks suggest, particularly from battery to leisure battery management system or main busbar. The upfront cost is negligible compared to losing 10% of your usable capacity to volt drop.

One practical approach: load test your actual setup before committing to final cable sizing. Run whatever you plan to use simultaneously (inverter, water heater, charger) and measure the sag at the battery. If you're seeing more than 0.5V drop under full load, you've got a cable sizing issue to resolve

Spot on about voltage sag—caught me out proper when I first wired up the motorhome. Thought my Victron calc sheet was bulletproof until I ran the kettle and microwave together. Voltage dropped like a stone on the 4mm² cable run I'd chosen.

Ended up going 6mm² for the main habitation circuits, which felt overkill on paper but makes a real difference in practice. Also moved the battery bank closer to the load centre—sounds obvious but shaved another 0.3V off the sag.

Worth adding: test your actual setup before finalising it if you can. I borrowed a cheap multimeter and ran it under load for a weekend before signing off the install. Saved me ripping everything apart later.

The other thing nobody mentions is ambient temp affects it too. Cold weather = higher resistance. Winter trips have been revealing.

Voltage sag is brutal—I learned this the hard way with my cabin setup. Running 4AWG from battery to inverter over about 3 metres and the voltage drop was noticeable even on paper calcs, but in practice it hammered the efficiency worse than predicted.

The real killer is people undersizing cable thinking "it'll be fine for occasional use." Then you're running your EV charger or high-draw kettle and suddenly the system voltage drops enough that your Victron starts throttling. Not ideal when you're trying to optimise a small system.

Worth noting: if you're doing a campervan, keep your battery bank as close to your main loads as physically possible. Even moving a 200Ah LiFePO4 half a metre closer made a measurable difference on mine.

Also check what voltage your charger actually needs to operate at—some are fussier than others about sag, especially the cheaper Chinese units.

Voltage sag is where the real education happens, isn't it? I've got three separate battery banks across my setup and the difference between what the charge controller says and what actually arrives at the load is genuinely shocking the first time you measure it.

The trick nobody mentions is that it gets exponentially worse with longer runs. I made the mistake of running my original cabling through the floor cavity to keep things tidy—looked lovely, cost me roughly 0.8V under load. Rerouted everything with proper gauge copper (bit of a pain, admittedly) and the difference was night and day.

What really changed things for me was investing in a quality DC voltmeter on the actual load rather than trusting the battery monitor. Fogstar's DC meter is honestly brilliant for this—cheap enough that you can put one at source and one at the inverter, see exactly where you're losing it.

@Davo83 and @PeakExplorer are spot on—the maths does look fine until you're actually running the kettle at half voltage wondering why your Victron's screaming. Lesson learned: cable size matters more than

Brilliant thread, @DefenderAdventure. You've hit the nail on the head with loads first—it's the foundation everything else hangs on.

Worth adding that folks often overlook the sustained vs peak distinction. Your fridge might draw 15A continuous, but your compressor startup could spike to 40A for a few seconds. Size your cables and breakers for sustained loads, but make sure your battery and inverter can handle those peaks without collapsing.

One thing I'd emphasise for campervan newcomers: keep your battery bank as close to your inverter as physically possible. Even a couple of metres of cable run makes a noticeable difference. I've seen people lose 2-3 volts across poorly planned wiring—that's the difference between your system running smoothly and everything throttling back.

Also, labelling everything as you go saves absolute headaches later. Future you (and anyone who buys the van after you) will be grateful.

Cheers for putting this together—these fundamentals separate a reliable setup from one that'll give you grief on every damp winter morning.

Right, I've got to add something @PeakExplorer and @VanJim have danced around: voltage sag is also a brilliant argument for why your mate's "I'll just use some old car battery cables I've got lying about" approach absolutely will not work.

I learned this the hard way when I first rigged up my emergency backup system.

Spot on about loads first—saved me countless headaches when I spec'd out my boat's system. Though worth flagging: most people massively underestimate phantom draws. Had to trace everything with a clamp meter before I believed how much the fridge and inverter were pulling at night.

The voltage sag point is crucial. I've got LiFePO4 on the boat now, but when I was running lead-acid in the van, even a decent leisure battery would sag to 10.5V under load. Made the difference between a laptop charging at normal speed or crawling along. Catch-22 is you need bigger batteries or better cables to solve it, and both cost.

One thing the thread hasn't touched: battery chemistry massively changes your approach. LiFePO4 lets you pull harder and deeper, so your load calculations shift. Lead-acid forces you to be more conservative with your daily cycles, which honestly keeps you honest about consumption.

@DefenderAdventure's absolutely right that it's the foundation. Get this wrong and you're chucking money at expensive fixes later.

I've seen plenty of folk get this backwards in shepherd's hut forums too. What @DefenderAdventure's nailed is that undersizing your battery bank costs you twice—once upfront when you upgrade, again in frustration. My Victron setup taught me that: calculated my actual winter draw first, then built around it. Saves the false economy trap every time.

Has anyone actually sized their battery bank based on real-world usage rather than just guessing? I'm trying to work backwards from my daily consumption—roughly 3kWh in summer, 5kWh in winter—but I'm unclear whether to spec for worst case or use some kind of average. Are folks here using a safety margin on top of calculated figures?

@Cleggy—absolutely. I tracked my narrowboat consumption for a full month before sizing anything. Kettle, fridge, heating, lights, laptop charging—the lot. Real data beats guesswork every time. Once you've got actual figures, work backwards from your daily amp-hours to battery capacity. Game changer for avoiding over/undersizing.

Been there with the van conversion. Tracking usage is spot on—I logged mine for three weeks and realised the fridge pulls way more than expected. Also factor in seasonal differences; winter heating and poor solar make a massive difference. Once you've got proper data, everything else clicks into place.

@Cleggy @OffGridJack really nailed it there. I'd add—don't forget phantom loads from converters and control systems. They're sneaky! Also, seasonal variation matters loads. My winter consumption is nearly double summer because of heating and shorter daylight. Worth logging across different seasons if you can, even just a couple months.

Has anyone actually measured their fridge current draw with a clamp meter before sizing batteries? I'm getting wildly different figures depending on ambient temp and compressor cycles—wondering if I should be averaging the peaks or the continuous draw for my calcs? Using a Victron shunt to log everything but the data's all over the place.