Van battery: how many days of power can you actually expect?

The right question isn't "how many amp-hours?" but "how many days?". And the answer depends mostly on one appliance: your fridge.

On van conversion forums, the question comes up every week: "which battery for my van?". The answers arrive in amp-hours — 100, 200, 300 — and almost nobody asks the only question that matters: how many days do you want to last, and with what running?

Because between two vans fitted with the same battery, one will last five days and the other a single night. The difference isn't the battery.

The fridge decides everything

This is the most common mistake, and the most expensive.

There are two families of fridge in a converted van. A compressor fridge works like the one in your kitchen: the compressor starts, cools, stops. Over 24 hours it uses about 380 Wh.

An absorption fridge — the ones that take gas, 12 V and 230 V — has no compressor. It heats continuously to produce cold through a chemical cycle. On 12 V, that means roughly 100 watts continuously, day and night, never stopping.

It's written in the manuals. Nobody reads them. And plenty of people replace a battery that was perfectly fine.

What everything else actually uses

Here are figures observed on common equipment, for a full day:

• Four LED lights, three hours: about 60 Wh
• Roof fan, four hours: about 100 Wh
• One hour of television: about 30 Wh
• Two phones charged: about 35 Wh
• Water pump, normal use: about 15 Wh
• Standby draw (battery electronics, inverter): about 25 Wh

Add it up: all of that together comes to about 265 Wh. That is nine times less than an absorption fridge left on 12 V.

In other words: before buying a bigger battery, check which mode your fridge is running on. That check costs nothing and changes everything.

A battery doesn't give you everything it holds

The second source of disappointment: the stated capacity is never the usable capacity.

On a lead-acid battery (AGM, gel, deep-cycle), you can't go below half without wearing it out very quickly. A 200 Ah lead battery is really 100 Ah usable.

On a LiFePO4 lithium battery, you can go much lower without harm. We always calculate on 80% of capacity — we *could* claim more, but a battery emptied completely every day doesn't last, and autonomy stated on 100% is autonomy on paper.

A 314 Ah battery at 12.8 V therefore holds about 4 kWh, of which we count 3.2 kWh genuinely available.

The table that answers the question

For a consumption of 648 Wh per day — compressor fridge, four LEDs for three hours, fan for four hours, one hour of television, two phones and standby:

• 105 Ah — about 1.3 kWh — nearly 2 days
• 150 Ah — about 1.9 kWh — nearly 2.5 days
• 200 Ah — about 2.6 kWh — about 3 days
• 280 Ah — about 3.6 kWh — about 4.5 days
• 314 Ah — about 4.0 kWh — 5 days

Those five days aren't a back-of-envelope figure: it's an actual reading, taken in a van, with exactly that equipment.

The two parts that decide how long it lasts

A lithium battery comes down to two things that matter: the cells and the management system, called the BMS. The rest is a box.

The cells determine real capacity and cycle count. Serious manufacturers state over 8,000 charge and discharge cycles, with 70% of capacity still available at that point. To picture what that means: even emptying and refilling the battery completely every day, that is over twenty years. In a van used a few weeks a year, it will outlast the conversion around it.

But that figure only means something if the cells really are the ones stated. Downgraded cells will never reach it — and nobody could tell you, since nobody told you what was inside.

The BMS protects each cell and keeps them level. A BMS with active balancing genuinely moves energy between cells, instead of burning the excess off as heat. That is what keeps a battery healthy over time.

Seeing your battery, not just using it

Most batteries show four bars in an app. Better than nothing, but it won't tell you whether one cell is starting to drift — that is, whether your battery is ageing faster than it should.

An open BMS lets you read, at home and without going through the internet: each cell's voltage, the spread between highest and lowest, temperatures, and accumulated cycle count. That information feeds straight into a home automation system like Home Assistant, with no account and no remote server.

The spread between cells is the real health indicator. While it stays small, all is well. When it grows, you see it coming — instead of discovering it one winter morning.

In short

• The useful question is "how many days?", not "how many amp-hours?"
• Check your fridge first: an absorption model left on 12 V empties any battery in a day
• Count on 80% of a lithium battery's capacity, 50% of a lead-acid one
• Insist on the cell brand and the BMS model — those two parts decide everything
• Prefer a BMS you can read locally, without depending on an app

Run the numbers, and let's talk

The autonomy calculator answers in seconds, free and without sign-up.

We also build 12 V LiFePO4 batteries by hand, with branded cells and a BMS that can be read entirely locally. But if your problem is the fridge, we'll say so — it will cost us a sale and save you several hundred euros.

Write to us with a list of what you run.