Leisure battery: how to work out the capacity your campervan needs

The method: watt-hours per day

285 Ah on the label, 96 Ah you can actually use. Those figures do not come from a lead-acid sceptic, but from a post on the blog of Victron, a maker of batteries and chargers: applied to the AGM batteries of a nearly new motorhome, the 50% discharge rule, the effect of a high current and an ageing factor bring 3.4 kWh stored down to 1.15 kWh truly available [1]. Before choosing a battery, then, you need to know what you will ask of it, and what it can really give back.

The method comes down to three steps. First, the budget: for each appliance, its power multiplied by the time it runs each day, in watt-hours. Next, the share of the battery you can really use, which depends on its chemistry. Finally, the number of days you want to last without charging. The result converts into amp-hours when you divide it by the voltage, 12 V in most installations.

The energy budget, appliance by appliance

The fridge is the biggest item, because it runs day and night. Dometic publishes a valuable figure: the average consumption in amp-hours per hour, at a stated outside and inside temperature. For its CFX3 35, a 36-litre compressor cool box, that is 0.98 Ah per hour at 12 V, with 32 °C outside and 4 °C inside [2]. Over 24 hours, that comes to about 24 Ah, or roughly 280 Wh. In cooler weather, the compressor runs less often.

In winter, the heater takes over. A 2 kW diesel air heater such as the Webasto Air Top 2000 STC draws 14 to 29 W for its fans and electronics [3], or up to 348 Wh over a 12-hour night. A Truma Combi 4 gas heater quotes an average current draw of about 1.2 A [4], or about 14 amp-hours over a 12-hour night. Our guide to campervans in winter looks at these figures in detail.

Everything else is worked out the same way. Four 3 W LED spotlights on for 4 hours: 48 Wh. A laptop whose charger delivers 45 W, plugged in for 3 hours a day: 135 Wh at most. Two phones, each with a battery storing about 15 Wh: 30 Wh. These are only examples: the power of your own appliances is shown on their label or on their charger.

Lead or lithium: the capacity you can really use

A battery is not meant to be run right down to empty. For AGM lead-acid, the most economical depth of discharge is generally set at 50%; for lithium iron phosphate, Victron works on 80% [5]. Dometic goes further and considers lithium iron phosphate usable almost to 100% [6]. Sticking to 80% leaves a margin.

The reason fits on one line of a data sheet. A Victron AGM battery is rated for 600 cycles at 50% discharge, but only 400 at 80% [7]: draining it fully wears it out faster. The same manufacturer’s SuperPack lithium batteries claim 2,500 cycles at 80% discharge, at 25 °C [8].

The figure on the label also assumes 25 °C and a slow discharge, over 20 hours [7]. Colder, older or worked harder, a battery gives back less: Victron’s AGM data sheet shows capacity dropping sharply at low temperatures [7], and in the blog’s example, a high current alone takes away about 16% of the capacity, before an ageing factor of 0.8 [1].

Enlarge diagram

Two days without charging: a worked example

Let’s do the maths with the budget above and two days without charging, in bad weather or while parked up. In summer: 495 Wh a day, or 990 Wh for two days. With lithium, 80% usable, you need 1,238 Wh of capacity, which is 103 Ah at 12 V. With AGM lead-acid, at 50%, you need 1,980 Wh, or 165 Ah.

In winter, with the heater at its maximum consumption: 891 Wh a day, 1,782 Wh for two days. You then need 186 Ah of lithium or 297 Ah of lead-acid. In this example, a 100 Ah lithium battery just about lasts two days in summer; in winter, you would need two of them.

The same reasoning answers the most common question: how long will the cool box run on its own? Dometic advises dividing the battery’s usable capacity by the appliance’s hourly consumption [6]. With a 100 Ah lithium battery used to 80%, the CFX3 35 runs for about 82 hours at 32 °C, almost three and a half days; with 100 Ah of lead-acid used to 50%, about 51 hours.

The inverter and the other loads people forget

A 12 V to 230 V inverter draws power even when nothing is plugged into it. Among Victron’s 12 V models, no-load consumption ranges from 4.2 W for the smallest to 10 W for the 1,200 VA model, with a maximum efficiency of 87 to 91% across that range [9]. Left on day and night, an inverter that draws 6 W with no load costs 144 Wh a day: more than the lighting and the phones of our example put together. Its ECO mode, which puts it to sleep when there is no load, reduces this loss [9].

Efficiency matters too: a laptop charged through an inverter running at 90% uses 150 Wh for 135 Wh of useful energy. Small appliances on standby, the water pump and the roof fan add up in the same way, a few watt-hours each. An honest budget lists them all, then adds a margin.

That leaves the number of days without charging. I would recommend allowing two days for a van that is often on the move, and three for a winter stay parked up: it is the setting to adjust to the way you travel, and the one that changes the capacity most.

Charging, the factor that changes everything

The number of days without charging weighs as much as the budget. A van that is driven every day charges its leisure battery from the alternator; a van that stays put relies on solar power or a mains hook-up. In winter, short days and a low sun cut a panel’s output just when the heater and the lights use the most: it is the season that sets the capacity. To run the numbers with your own battery, panels, fridge and region, Best Batteries’ off-grid calculator estimates how many days you can last unplugged, under cloud as well as sun.

One last winter trap, specific to lithium: some batteries refuse to charge below 5 °C, such as Victron’s Smart lithium batteries, whose management system cuts off charging below that temperature [10]. Installed inside the heated space rather than in an outside locker, the battery avoids this lock-out. Beyond that, installation and wiring are a job for a professional: this guide stops at sizing.

A leisure battery is chosen by multiplying the watt-hours you use by the days without charging, then dividing by the share you can use: the rest is just the label talking.

Worth watching

A walk-through of what amp-hours actually measure and how to turn a day’s power use into the battery capacity a campervan needs, the same calculation this guide sets out.

The video is hosted by YouTube (Google): nothing loads until you play it, and Google may then set cookies.

Julien de Jurquet

Campervans, motorhomes, cycling and calculators

Julien de Jurquet writes for Outfarer about campervans, motorhomes, cycling and calculators. He reads instruction manuals to the very end and never lets a figure through without its unit, so that your choices rest on sums rather than promises.

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