RigTonightPowering an astrophotography rig in the field: how long will your battery last?
Nothing ends a clear night faster than a flat battery at 2 am. The arithmetic to avoid it is simple once you have a power budget.
Step 1: what does everything draw?
Use average draw for runtime and peak draw to check that the supply can keep up:
| Part | Typical average | Notes |
|---|---|---|
| Cooled camera | 8–15 W | Much more on warm nights; the cooler works harder |
| Mount (tracking) | 3–12 W | Slews can briefly reach 2–4× that |
| Controller (Raspberry Pi, mini PC) | 4–15 W | A laptop powered from the battery can draw 30 W or more |
| Dew heater strip | half its rating | Heaters usually run at about 50% power |
| Filter wheel, guide camera | 1–2 W |
Step 2: usable capacity
Battery capacity is in watt-hours (Wh), but you can't use all of it:
- Portable power stations lose some energy converting to 12 V, so count about 85%
- LiFePO4 batteries can be run deep, so count about 90%
- Lead-acid and AGM batteries shouldn't go below half charge, so count 50%
Step 3: divide
runtime (hours) = usable Wh ÷ average W
Example: a 500 Wh power station at 85% is 425 Wh usable. A rig averaging 30 W runs for about 14 hours, enough for a long winter night.
Check peak draw too. If a cooler starting up during a slew draws more than the 12 V outlet allows (often 120 W on power stations), the supply can cut out.
Ways to stretch a battery
- Set the camera cooler 5–10 °C warmer. It saves a lot, and modern sensors have little dark current anyway.
- Turn dew heaters down on dry nights. The builder has a switch to leave them out of the budget. How to stop dew explains how much heat you really need.
- Power the laptop from its own battery, or use a low-power controller.
- In winter, keep the battery off the frozen ground; cold batteries deliver less.
Set your location on the Tonight tab and the builder compares your runtime with tonight's sunset-to-sunrise length.
Stay safe
Fuse every 12 V connection at the battery, keep cables and connectors off wet grass and dew-soaked ground, and follow the battery maker's charging and storage instructions. A shorted lithium battery can start a fire.