The question on every hunting and truck-camper board in October is the same: will a 100 amp-hour lithium battery run a diesel heater all night, or do I need a power station, or a bigger battery, or solar? The answers on the boards range from "easily" to "mine died at 3 am", and both are true, because the answer depends on three things the question leaves out.
What a diesel heater actually draws
A 12 volt diesel air heater is a fan, a fuel pump, a controller and a glow plug. Only one of those uses real power, and only for a minute.
| Phase | Draw | How long |
|---|---|---|
| Glow-plug start | 8 to 12 amps, about 100 to 140 watts | 30 to 60 seconds, then again at shutdown for the purge |
| Running, medium fan | 1 to 3 amps, 12 to 36 watts | As long as it runs |
| Running, lowest setting | 0.5 to 1 amp | As long as it runs |
Those are owner-measured figures, not manufacturer ones; the makers of these heaters do not publish electrical draw, including the one we sell. They are consistent across the van and solar boards to within an amp, so they are safe to plan with.
The consequence is that a diesel heater is a fuel appliance, not an electrical one. Ten hours on low is roughly 10 to 20 amp-hours. Ten hours on medium, cycling, is more like 20 to 30. On the fuel side the 8 kW heater we stock burns 0.16 to 0.62 litres an hour and runs up to 16 hours on its 10 litre tank, which is the number that actually limits a night.
The three things that decide whether the battery lasts
1. Cold. The lithium bank you are drawing from is also cold. Lithium delivers fine below freezing but most batteries will not accept charge below about 0°C, and their usable capacity at minus 15 is noticeably less than the label. A 100 amp-hour battery at minus 15 in a truck bed is not a 100 amp-hour battery.
2. What else is on. The heater is rarely alone. A 12 volt fridge in a cold truck bed cycles less than in summer but still draws 1 to 2 amps when it runs. Lights, a phone, a CPAP if you use one. The heater's 20 amp-hours becomes 40 or 50 for the night once those are added, and that is the overnight number people actually see.
3. How the heater restarts. A heater on a thermostat cycles off and on through the night, and every restart is another glow-plug cycle at 10 amps. A heater that cycles ten times uses more than one that runs steadily on low. Set it low and let it run.
So what battery
A 100 amp-hour lithium battery runs a diesel heater and a fridge overnight with margin in ordinary cold, and gets tight over two nights without charging. That matches what owners report: "easily" is one night; "died at 3 am" is night two with the fridge and no charge in between.
- One night out, then drive: 100 amp-hours is enough. The Victron 12.8 volt 100 amp-hour Smart has cell-level Bluetooth monitoring and needs an external BMS; for a simpler drop-in, the Eco-Worthy 280 amp-hour has its own BMS with low-temperature charge protection built in, which is exactly the protection a truck bed in November needs.
- Two or three nights without driving: 200 amp-hours, or 100 plus solar that actually sees sun, which in a November hunting camp is not a given.
- A power station instead: it works, and it is portable, but you pay a premium per amp-hour, you cannot charge it from the truck at a useful rate without its own adapter, and it has to be carried in and out. For a camper that stays on the truck, a fixed battery with DC-DC charging is cheaper per night and never gets left at home.
Charging it: the drive is the charger
The reason a fixed battery wins is the drive in. An hour of driving through a DC-DC charger at 50 amps replaces a full night's use. The Orion XS takes its power from the truck's starter battery and alternator, delivers a proper lithium charge profile, and never draws more than the current you set, so the alternator is protected. On a truck with a smart alternator it detects engine running from voltage rather than needing an ignition wire, though running one is still the cleanest install. Our Orion XS versus Orion-Tr Smart guide covers the choice.
Size the charger to the alternator. A 50 amp charger on a small alternator running heated seats and lights on a cold morning is asking a lot. A rule of thumb is a third of the alternator's rating; on most modern half-ton trucks that comfortably allows 50 amps.
Solar is the second source, not the first, in the season this matters. A 100 to 200 watt panel through a SmartSolar MPPT covers the fridge on a clear day and keeps the battery from sitting flat, which lithium tolerates but does not enjoy. In a canyon in November it is a supplement.
Wall tents and cargo-trailer camps
The same arithmetic applies to a heater ducted into a wall tent, with one change: there is usually no truck alternator nearby, so the battery has to last the trip. Owners quoting "120 watts at start, 30 to 45 running" for a tent heater are describing exactly the figures above. A 100 amp-hour battery is a night and a half of heat alone; take a second battery, or a panel, for a longer camp. And mount the heater outside the tent with only the ducting inside, which is the safety point every tent-heater thread rightly insists on.
Know your number instead of guessing
A SmartShunt between the battery and everything else counts amp-hours in and out and shows you, on the first night, exactly what your heater, fridge and lights use together. After that you are not sizing from a forum thread; you are sizing from your own camp. It shares data with the Orion XS in the same app, so you also see what the drive in put back.
Our guides to diesel heater mounting and undersized heat and diesel versus propane versus electric heat cover the installation and the choice of heater. Everything ships from Upland, California.