Ad Astra Observatory

After the powertank debacle of the past weekend, I’m beginning to think that maybe I need to maintain a battery setup in the observatory, so that when I go on the road, everything is ready to roll.

I’m picturing a deep cycle battery (need to determine how many amp hours I need), with a charging circuit, inverter, and outlets (for AC and DC) all built into one box.

I have 4 object that need power:
– laptop (5.5A ouch)
– G-11 ( < 1A normal, ~1.5A slewing )
– ST-4 (1A )
– camera ( ? call it 1A )

So, if I leave the laptop out (you have to run on your own battery, Mr. Laptop), then I’m looking at about 2A normal, with spikes up to about 4A. Figure the mount needs to run for 8h each night (10p-6a), and that’s 16Ah per night. To make it through a 3 day weekend, I would need 60Ah total (that’s 16A * 3 nights = 48Ah, but you don’t want to run below 20% on the battery, so 60Ah). A deep cycle battery of this size is something like $150.

Now, if I want to keep running on a regular basis, I could buy a solar panel to charge during the day. If I’m drawing 16Ah in a night, that’s 16Ah * 12V = 192 Watt hours. If you figure 6 hours of peak sun in CA during the summer, then I need to produce 32 Watts each hour in order to totally refill the battery. A 40-watt solar panel at Wind and Sun is $250.

There’s something to be said for tuning the whole rig for a smaller panel that can’t put a full charge on the battery in one day, requiring a slightly larger battery which drains to 50% on night one, then 40% on night 2, then 30% on night 3, …

I would need to buy or build a charging circuit. I haven’t really looked into this, but I know that the circuit has to play around with voltages as the battery gets charged up, in 3 stages. PWM should help, and this seems like a place the Arduino could help, in particular, it could hopefully drive an LCD or other display in between charging pulses to the battery, and keeping track of the inbound voltages and outbound charge level.

My long term plan might be to run the observatory off of batteries all the time, maybe install a panel on the roof (or charge off of mains power). That would keep the batteries “exercised” and would make moving out to a remote site more straightforward, since I’d have a power source that I was very familiar with.