Every winter the ski boards ask whether a battery-powered heater can keep a van warm overnight. It cannot, and three lines of arithmetic show why. What works is a diesel heater, a lithium battery kept warm enough to charge, and a charger that refills it on the drive up.
First, plan where you sleep. Mammoth Mountain's parking page says overnight parking is prohibited in all of its parking areas during the winter season, and violators are towed or ticketed for blocking snow removal. The Town of Mammoth Lakes has also said there is no street parking from November 1 through April 30. Big Bear has its own rules. Check the current ones and book a legal spot before you leave.
Why a battery heater cannot heat a van
| What | Energy |
|---|---|
| Our Eco-Worthy 280 Ah battery, full | 3,584 Wh |
| A plug-in space heater on high, for one hour | about 1,500 Wh |
| A 2 kW diesel heater on its lowest setting, for 10 hours | about 9,000 Wh of heat |
| One liter of diesel | about 10,600 Wh |
Portable electric space heaters are rated at up to 1,500 watts. Through an inverter, that empties the Eco-Worthy 280 amp-hour battery in under two and a half hours, before inverter losses.
A 2 kW diesel heater such as the Webasto Air Top 2000 STC turns down to 0.9 kW. Ten hours at that lowest setting is 9,000 watt-hours of heat, two and a half times the whole battery. The heater gets it from about 1.2 liters of fuel, because it burns 0.12 liters an hour on low and a liter of diesel holds about 10.6 kilowatt-hours.
The battery's job is to run the heater's fan, fuel pump and glow plug, which is a small job.
What a diesel heater actually draws
Two established makers publish their figures:
| Heater, 12 V | Running draw | Start-up |
|---|---|---|
| Webasto Air Top 2000 STC | 15 to 30 W across its 0.9 to 2.0 kW range | Not given on the datasheet |
| Eberspaecher Airtronic D2 | 4 to 34 W, depending on the heat setting | 100 W or less |
Eberspaecher's manual also describes the start. The glow plug comes on with the fan at low speed, fuel starts after about 65 seconds, and the glow plug switches off 60 seconds after the flame is detected. At shutdown the fan runs on for about four minutes and the glow plug comes back on for about 40 seconds to burn off residue. Even at a full 100 watts for all of that, one start and stop uses under 5 watt-hours.
Over a 10-hour night, the Webasto's 15 to 30 watts is 150 to 300 watt-hours, or about 12 to 23 amp-hours from a 12.8 volt battery. That is under a tenth of the Eco-Worthy.
The 8 kW diesel air heater we stock burns 0.16 to 0.62 liters an hour and runs up to 16 hours on its 10 liter tank. Its maker does not publish electrical draw, so use the figures above as a guide and measure yours with a SmartShunt.
The start still matters for one reason: voltage. The Airtronic shuts down if its supply stays below about 10.5 volts for 20 seconds. A nearly flat battery on a long, thin cable can sag that far during the glow phase. Our truck-camper battery sizing guide adds the fridge and lights to the overnight number.
Lithium in the cold: it discharges, but it must not charge
Lithium iron phosphate delivers power below freezing but must not be charged there. Victron's Lithium Battery Smart is rated to discharge from minus 20°C to 50°C (minus 4°F to 122°F) but to charge only from 5°C (41°F) up. Capacity drops too: Victron rates its 100 amp-hour SuperPack at 80 amp-hours at 0°C.
Batteries handle this in one of two ways:
- A heated battery has a built-in heater that warms the cells before they accept charge. We do not stock a heated lithium battery.
- Low-temperature charge protection means the battery's BMS stops charging when the cells are too cold. It does not warm them. The Eco-Worthy 280 Ah we sell has this protection. After a night well below freezing, it will refuse charge from the alternator or the sun until the cells warm up.
The practical answer is to keep the battery inside the heated space, not in a truck-bed box, under the chassis or in an unheated rear locker. Then the heater that keeps you warm also keeps the battery warm enough to charge.
Charge it on the drive up
The drive up US 395 is the best charger you have. An Orion XS delivers up to 50 amps from the alternator. Its input current limit is adjustable from 1 to 50 amps, so you can keep it within what your alternator can spare. It detects a running engine from the voltage, so on most vehicles it needs no ignition wire. Four hours at 50 amps is 200 amp-hours on paper, most of a 280 amp-hour battery.
Two conditions apply: the battery must be above its charge temperature, and the alternator must have current to spare with the lights and wipers running. The Orion XS also has a low-temperature shutdown you can set in the app. It works only when a BMV-712 or a SmartShunt fitted with Victron's optional temperature sensor shares the battery temperature with it. We do not currently stock that temperature sensor, so ask us before you plan on this feature. Our Orion XS versus Orion-Tr Smart guide covers sizing.
Solar in winter is a bonus
Short days, a low sun and snow on the panel all cut winter harvest, and a panel under snow makes next to nothing. A SmartSolar MPPT 100/30 takes up to 440 watts of panel on a 12 volt battery, but it charges the same battery, so the same rule applies: a cold battery with low-temperature protection will not take solar charge either. Brush the panel off after a storm, and plan the trip around the drive and the battery.
Condensation and ventilation
Everyone breathes out moisture all night, and in a cold van it ends up on the windows and walls. Unvented propane heaters add to it, because burning propane produces water vapor and those heaters release their combustion gases into the cabin. A diesel heater burns in a sealed chamber and sends its exhaust outside, so what it blows in is dry heat. The Webasto can run in fresh-air or recirculation mode, and fresh air helps dry the van.
Still, crack a vent. A roof vent fan on low, with a window cracked at the other end, moves damp air out without dumping the heat. Wipe the windows each morning and keep sleeping bags off cold walls.
Carbon monoxide
A diesel heater is sealed combustion, but a cracked exhaust, a loose joint or an outlet buried in snow can still put carbon monoxide into the van.
- Route the exhaust away from doors, windows and the heater's own intake. Check the joints each season.
- After a storm, dig out the heater's exhaust and intake before you start it.
- Do not idle the engine for heat while parked in snow. The CDC warns that with a blocked tailpipe, carbon monoxide can build up quickly inside the vehicle.
- Fit a battery-powered CO alarm at about breathing height in the sleeping area. Ours records the last CO event, so you can tell if it went off while you were on the hill.
Our guide to CO detector requirements in RVs covers placement and the rules.
A checklist for the trip
- A legal place to sleep, booked before you leave.
- A diesel heater with a clear exhaust and enough fuel for every night planned.
- A lithium battery mounted inside the heated space.
- A DC-DC charger to refill it on the drive.
- A CO alarm and a cracked vent.
For more, read diesel heat versus propane and electric and diesel heater mounting and cold spots. Everything ships from Upland, California, and you can collect at the counter before you head up the mountain.