At a dark site there is no outlet. The mount, cooled camera, dew heaters and computer all run from one 12 volt battery until dawn. Here is how to size it, when to regulate the voltage, how to fuse it, and how to charge lithium iron phosphate (LiFePO4) safely on a cold night.
List every load first
Every device has an input rating on its label, manual or power brick. Write down the amps at 12 volts. If the label gives watts, divide by 12.
- GoTo mount. It tracks all night and draws more when it slews.
- Cooled camera. The cooler is often the biggest single load. ZWO, for example, recommends a 12 V, 3 A supply for the ASI2600's cooler and says a lithium battery at 11 to 15 V also suits it. The camera itself draws at most 4.7 W over USB.
- Dew heater straps. One per optic. A controller that pulses them saves a lot.
- Laptop or mini-PC. Run it from 12 V directly if you can. An inverter adds conversion loss and its own idle draw.
- Small loads. Guide camera, focuser, filter wheel and USB hub.
A worked amp-hour budget
The draws below are examples only. The camera line uses ZWO's supply rating, which is a ceiling rather than an average. Replace every line with the figures from your own labels.
| Load | Example draw (replace with yours) | Hours | Amp-hours |
|---|---|---|---|
| Mount, tracking | 2.0 A | 10 | 20 |
| Cooled camera (ZWO supply rating) | 3.0 A | 10 | 30 |
| Two dew heater straps | 2.0 A total | 10 | 20 |
| Mini-PC | 1.5 A | 10 | 15 |
| Guide camera, focuser, hub | 0.5 A | 10 | 5 |
| Total | 9.0 A | 90 Ah |
Add 20 percent for slewing, warm-up and the first cooldown, and this example rig needs about 108 amp-hours for a 10-hour winter night.
Now allow for the cold. Victron rates its 100 Ah Lithium SuperPack at 80 Ah at 32 °F (0 °C), and the 200 Ah model at 160 Ah. So a 100 Ah battery falls short for this rig on a freezing night, and a 200 Ah battery covers it with room to spare. For a two-night star party without solar, double the number. That is where the Eco-Worthy 280 Ah fits; Eco-Worthy rates it at 3,584 watt-hours.
Better still, measure. A SmartShunt counts every amp-hour in and out. One night gives you real numbers, and it tells you at midnight whether you will make it to dawn.
Why LiFePO4 suits an imaging rig
- Flat voltage. A LiFePO4 battery holds around 13.2 to 13.4 volts for most of its range. The mount and camera see nearly the same voltage at 3 a.m. as at dusk. Lead-acid sags all night.
- Usable capacity. You can use most of a LiFePO4 battery's rating. A lead-acid battery lasts longest if you use only about half.
Lithium voltage barely moves as it empties, which is the other reason for the shunt.
12 V straight from the battery, or a regulated 13.8 V?
Most astronomy gear says "12 V", but the manual usually gives a range. ZWO's cooler accepts 11 to 15 V. A LiFePO4 battery stays inside that all night, so for many rigs the battery can feed the gear directly through fuses.
Regulate when:
- A device's range is narrow, or its manual asks for a steady 12 V or 13.8 V.
- You charge while imaging. The gear then sees the charging voltage. Victron's SuperPack absorbs at 14.2 to 14.4 V, so check each device's maximum.
- Long, thin cables let the mount's voltage sag when it slews.
A DC-DC converter in power supply mode holds a fixed output whatever the battery and load do. The Orion-Tr Smart 12/12-30 non-isolated can be used as a power supply, with the output set from 10 to 15 V over Bluetooth and 30 A continuous. Victron notes that setting it above its 12.2 V nominal raises the minimum input for a stable output by the same percentage. At 13.8 V that is about 9 V, far below where a LiFePO4 battery sits. The larger Orion XS has a power supply mode with a 10 to 17 V output and a current limit you set from 1 to 50 A.
Victron rates the Orion-Tr Smart at 87 percent efficiency, so run only the loads that need regulation through it. Dew heaters are simple resistive loads and work fine on battery voltage.
Fused distribution with a small busbar
Build it in this order:
- Main fuse at the battery. Put a MIDI fuse in a holder within a few inches of the battery positive. Size it to protect the cable to the busbar, not to the load. Our cable and fuse sizing chart has the table.
- A positive and a negative busbar. Every negative returns to the negative busbar, then through the SmartShunt to the battery negative.
- A branch fuse for each device. Rate it above the device's draw and below what its lead can carry. We don't stock small blade fuse blocks, but any good ATC or ATO block does this job.
- Connectors. Many imagers use Anderson Powerpole for the device leads. We don't stock Powerpole connectors. We do stock a 50 amp SB50-style Anderson plug, which suits the heavy link between the battery box and a charging lead or vehicle. It does not mate with Powerpoles.
Charging at home
Use a charger with a lithium profile. The Blue Smart IP22 comes in 15, 20 and 30 amp 12 V versions and has a Li-ion preset. The 30 amp model puts 90 amp-hours back in about three hours, plus a short absorption stage. The sealed Blue Smart IP67 suits a damp garage.
Its power supply mode can also run the rig with or without a battery connected, which is handy for a backyard test before a trip.
Charging in the field with solar
A portable panel and a SmartSolar MPPT at the battery can refill it between nights. Victron rates the 75/15 at 15 A and 220 W of panel at 12 V, and the 100/20 at 20 A and 290 W. The panel voltage must be at least 5 V above the battery before the controller starts. As an example, 15 A for five hours of good sun is up to 75 amp-hours back. Winter sun gives less.
Cold nights: never charge lithium below freezing
LiFePO4 cells can be damaged if charged below 32 °F (0 °C), and Victron's IP22 manual says not to. The charger can also stop charging in the cold, typically below 5 °C, when a compatible battery monitor shares the battery's temperature with it.
The batteries protect themselves too:
- Victron SuperPack (serial HQ2040 and newer): charging is blocked below 0 °C, give or take 3 °C, and resumes above 3 °C. Discharge is blocked below -20 °C.
- Eco-Worthy 280 Ah: low-temperature charge protection stops charging in the cold. We could not confirm its exact cut-off on Eco-Worthy's own page, so check the manual, and treat it as a backstop, not permission.
Running the rig on a frosty night is fine within those limits, with less capacity. The risk is solar charging a battery that is still below freezing at sunrise. Charge at home, or let it warm first.
Observatory trailers: check the bearings
Trailer observatories take a beating on washboard roads. Cloudy Nights builders suggest dual axles, shocks, and spare bearings with the tools to fit them. Repack before the season. Our 10-inch Timken bearing kit suits many light trailers; check the fit with our bearing kit guide by axle rating and follow the Timken repack guide. If the trailer carries the lithium bank, our guide to charging a lithium trailer from a truck's 7-pin covers charging it on the drive.
Send us your list of loads and we will help size the battery and fusing. Everything ships from Upland, California, on the I-10 toward the desert dark sites, and the counter is open for pickup.