How to size a 12V system for camping (so the fridge lasts the night)

Intro

G'day. In this one we're sizing a 12V system from a blank sheet: the three numbers, what you actually run, the hidden loads, how much of a battery you can use, how big it needs to be, and how to put the power back. Most people do it backwards. Buy a battery, chuck on a panel that looked about right, and find out on day three whether it was enough. Done properly, it's three steps in order. What you run. What holds it. And what puts it back. Let's get into it.

Why start with what you run

The battery and the charging only exist to serve the stuff you plug in. Get the daily number right and the rest follows. Victron make the same point in their book Energy Unlimited: it's the daily energy need that sets the size of everything else.

Watts, watt-hours and amp-hours

Three numbers. Watts is how hard something pulls right now. A kettle pulls a lot, a phone charger a little. Watt-hours is energy over time: watts times hours. A battery stores energy, not power, so this is the number that matters. Amp-hours is how 12V batteries are rated. On a 12V system, Ah is roughly Wh divided by 12. Time matters more than size. Victron's example: 2 kW for an hour drains about 167 Ah from a 12V battery. 2 kW for one minute is only about 2.7 Ah. A big load for a short time can matter less than a small one that never switches off.

Step 1: the load audit

Grab a notepad and go through a normal day. For each item, write down its watts, from the label or the manual, and how many hours a day it really runs. Think in three groups. Always on: the fridge, a router, anything on standby. Long runs: lights, a diesel heater's fan, charging laptops. Short bursts: the water pump, a coffee machine through an inverter. Almost everyone underestimates the first two and overestimates the third. The fridge eats more than you think and the kettle less. The fridge deserves its own line. Victron's worked example is a compressor drawing about 50 W while it runs. Half the day running is 50 Ah; a quarter of the day, 25 Ah. Hot weather, warm drinks and opening the lid all push it up. Here's a worked example. Illustrative figures, not any particular product. Compressor fridge, 50 W, about 8 hours of running: 400 Wh. LED lights, 10 W for 4 hours: 40. Phones and a laptop, 60 W for 2 hours: 120. Water pump, 60 W for a quarter of an hour: 15. Diesel heater running, 20 W for 6 hours: 120. Total: 695 Wh, about 58 Ah. Round it up. Call it 60 Ah a day, and add a bit if summer trips are the plan.

Step 2: hidden loads

The loads that catch people out sit there quietly, day and night. An inverter left on. Victron list the MultiPlus-II 12/3000 at 13 W with no load at all. Over 24 hours that's about 312 Wh, roughly 26 Ah, for doing nothing. Its search mode drops that to 3 W. If you have an inverter, switch it off when you're not using 230V, or use its low-power mode. Standby stuff counts too: USB outlets, alarms, chargers left plugged in. Small standing loads matter most when the rig is parked. Victron's lithium manual puts it bluntly: "a residual current of just 10mA can damage a 200Ah battery if the system is left discharged for more than 8 days." If the van sits in the shed between trips, fit an isolator or make sure something keeps the battery topped up.

Step 3: usable capacity (AGM vs lithium)

Size the battery for the usable part, not the sticker. How much of the rated Ah you can use depends on the type. Lead-acid, that's AGM, gel and flooded: Victron say limit discharge to 50% if you want it to last. Their AGM datasheet shows why: 400 cycles if you take 80% out each time, 1500 cycles at 30%. Lithium: Victron put usable capacity at 80 to 100%. Their Lithium Battery Smart is rated at 2500 cycles at 80% depth of discharge, and 5000 at 50%. Exotronic recommend running their Alpha batteries between 10 and 90% state of charge, which works out at about 80% usable. So, as a working figure: AGM, half the sticker. Lithium, about 80% of it.

How big a battery

Now decide how many days you want to go without much charging. Cloudy days, a few days parked in the shade. Say two. 60 Ah a day, two days: 120 Ah you need to be able to take out. Lithium at about 80% usable: 120 divided by 0.8 is 150 Ah. So a 150 Ah or 200 Ah lithium battery. AGM at 50% usable: 120 divided by 0.5 is 240 Ah of AGM. Which is a lot of weight. That gap is the main reason people go lithium. Victron's rule of thumb is a 400 Ah lead-acid bank to match the usable energy of a 200 Ah lithium one. It isn't the only thing to weigh up. Our lithium vs AGM guide goes through the rest, including when AGM still makes sense.

Step 4: charging

A battery only buys you time. What keeps you going is putting back what you used. Hours of charging equals daily use divided by what the charger puts in. For our 60 Ah a day: a DC-DC charger putting in 25 A needs roughly 60 divided by 25, about 2.5 hours of driving, at best. Chargers taper as the battery fills, so real life is longer. Solar depends on the panels, the season, where you are and how much shade you park in. There's a guide for that. Most touring setups use both: the car does the heavy lifting on travel days and the panels hold you when you stop.

Step 5: check it with a shunt

Your load list is a best guess. A shunt-based battery monitor tells you what's really going in and out, so after a trip or two you'll know. A shunt beats reading the battery voltage. Our battery monitor guide explains why.

Is bigger always better?

No. A battery twice the size you need costs more, weighs more, and takes longer to charge back up from the same panels. If your setup is a fridge and some lights for weekends, a single 100 Ah lithium battery and a decent charger might be the whole job. If you live in it, you'll want more. The van we use runs three 300 Ah lithium batteries: overkill for a weekender, about right for full-time. Do the sums first, then shop. You'll spend less, and you'll stop wondering whether the fridge will last the night.

The guide and the parts

The full guide has the table to copy with your own numbers, and the parts are grouped by type: batteries, DC-DC chargers and solar. nbcampingequipment.com.au, guides.