Anker Astro3
2 USB @ 5V 2A and DC @ 9V/12V 2A
10000mAh@5V
37Wh = 3.1Ah@12V
I have been using this battery pack for the Nexstar SLT mount with C6 OTA for about 3 months. It has 12V or 9V @2A (selectable one at a time) output unlike most if not all USB battery pack which only comes with 5V output. It is larger than a regular 5V USB battery pack but way more portable than acid lead battery.
The rated energy at 12V (37Wh) seems to be lower than 5V (50Wh). Assuming the battery is 5V, there could be energy loss when converting from 5V to 12V i.e. 75% efficiency. However, I don't know the actual battery rating as I haven't cracked open the battery pack yet.
It has 4 blue LED indicators to show the battery capacity. There are rather nice with your desktop but a bit distracting to the eye in the field at night. I need to shield the LEDs behind the mount.
The instruction manual for the mount states that it requires 12VDC@750mA. I guess that is the maximum current draw when slewing at maximum speed in both Alt/Az axes. When tracking, it should use less current as it is moving at slower speed. Even at maximum current of 750mA, the battery pack should last for about 4 hours.
During an observation session, I typically only observe 2 or 3 objects in the sky. So, that is not a lot of slewing at maximum speed. Throughout the session, the mount is always in tracking mode for both axes. When viewing at very high magnification, I need to correct the tracking by slow slewing back and forth in order to bring the object into the centre of the eyepiece or camera. The battery usage is less than 50% after about 3 hours. Sometimes, during short session, I only used out one LED. So, there is plenty of juice left. 5 or 6 hours of tracking should be possible in a single charge. May be I should measure the current consumption during tracking and slewing for better estimation.
I've retrofitted a diy arduino motor focuser to the C6 OTA and later will add a nichrome dew heater as well. We'll see how the Anker battery pack performs with the two extra gadgets.
Update 25/11/12 : Nexstar SLT and Arduino Motor Focuser Current Measurements
Power(W) = Current(A) * Voltage(V)
Wh = Ah * V
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