After some discussion on batteries in the Yahoo CGE forum, I got to wondering exactly how much power my rig was using…. So a test was in order.

I had to shoot new darks anyway, so I plugged my laptop and ST-2000 camera through my “Save-a-Watt” meter so I could determine the power draw of the Laptop & Camera in the normal state: Camera cooled taking pics, laptop basically idle just controlling the camera with the monitor asleep. This would be the usual load as the laptop and camera work shooting a series of images through most of the night. I was surprised by the result.

When first powering up, screen on, the laptop draws about 27 Watts, and if the laptop battery is still taking a little charge it gets up into the 30-35 watt range. While the camera was cooling down (the TEC at MAX power draw), the total load for the laptop & camera combined reached around 45 Watts.

However, most of the night while taking images, the laptop battery is no longer charging, the moniter powers off, and the Camera TEC settles down to just holding the temperature at -15C. In this state, the laptop and camera dropped to just 20 +/-1 Watt. Even adding in inverter inefficiencies, that’s less than 2 amps at 12 volts.

Wow…. that’s almost half of what I thought it would be!

The full setup of course draws more juice since the laptop/camera will also have a guide camera and guiding program running, then add the mount and dew heaters. I’ve directly tested the mount while tracking and it’s less than 0.5 amps, and I built my own dew heaters and know they only draws 0.5A each (and I usually only need one powered up).

So for my rigs, during the majority of the night while taking images, the load is typically:

Laptop and SBIG Camera Less than 2.0 A (guider software running also)
USB powered guide camera = 0.5 A
Mount (tracking and guiding) = 1.0 A (probably less)
Heaters (I actually shoot for 6W) = 0.5 A

Total load for the majority of the night = 4 amps

I haven’t accounted for inverter inefficiencies but still, these numbers would indicate that for my rigs, an 80 AH battery is fine. The 4 amp load is very close to the 20 hour rating current (80 AH/20H = 4A), so a fresh 80 AH battery should easily last a long night with ample reserve. This agrees with my experience…. I’ve only once had a battery die on me overnight, and that was due to battery condition (an aging battery, with the charge not well maintained).

al