## Portable Power

I recently attended a four day star party in Illinois (Illinois Dark Sky Star Party). Electronics has come to dominate observational astronomy. Power demands span the range from flashlights to high end laptops (plural) and telescope controllers. In my case, I have a netbook, an equatorial telescope mount and several smaller battery operated devices. All need to be recharged after a nights use. Electric power is not readily available at the park. We all use batteries of one type or another.

A quick survey of power demands shows that I draw approximately three amps of power during normal operation. Since an observing session can last up to six hours, this means I will use about 18 amp hours (or 216 watts) of power each session. During the day, I will use about two amps but the period of use is dependant upon how much data needs to be processed. This can take as long as four hours. This is an additional eight amp hours (or 96 watts). So the total daily power requirement is now approximately 26 amp hours (312 watts).

With three full days of observing, I would need a power source capable of supplying at least 78 amp hours (936 watts). A standard deep discharge battery rated at 96 amp hours would be marginally sufficient but it is too large and too expensive. I say marginally because not only is it not good to completely discharge a battery, but you can't really expect to get the full capacity over the lifetime of the battery. A typical correction value of 80% is used for these calculations. That reduces the expected 96ah to about 77ah or  924watts. I do have a 72 amp hour (864 watt, 696 watts adjusted) marine battery but it would not supply the worst case power requirements.

An Internet search found a small 45watt 12volt Photovoltaic panel for \$139.00 (just about \$3.10 per watt). Under full sun, this panel will supply approximately 3.5 amps of charge current. Considering a nominal efficiency of 80% for the six hour period from 9AM to 3PM, this panel, when driven to track the sun, should provide approximately 2.8 amp hours of charge current during this period. This works out to 16.8 ah (201 watts) returned to the battery per sunny day. While this results in a daily deficit of 9.2 amp hours (110 watts), the energy recovered is more than sufficient to allow operation for a bit over six days at the above rate of charge & discharge.

As an aside: Using the above numbers with totally clear skies, a 36AH battery would be sufficient to supply power for three days. That certainly would be much smaller, less expensive and less massive than the 96ah battery.

In order to maximize panel output, the panel is mounted to the equatorial telescope mount (the telescope is not used during daylight hours). The mount will track the sun during the daytime so maximal power is provided for battery charging.

Click on the image at right for time lapse images of solar tracking. Caution: This is a nearly 20meg file so it may take a moment or two to load (It might look better if it was viewed locally after downloading).

The panel can be quickly setup to catch whatever light might be available. In this case a very cloudy day provided about 300 milliamps - enough for a trickle charge - and better than nothing.

This project was the stimulus for a spotlight article in the ShowMeSolar newsletter.

### Bonus:

I used some of that power to drive a camera to capture a time lapse image of a night at IDSSP. This is four hours starting at about 9PM. This is a small, 2meg animated gif so it should look fine in a browser.

## Jones-Bird telescope goes green

St Charles County Missouri holds the distinction of having the only park in the state dedicated
to Astronomy - Broemmelsiek. The park has two telescopes permanently installed. One of the scopes, a 16", is battery  powered.

Normally, the battery is removed from the scope and placed on a trickle charger in another building. This necessitates the opening of that building in order to retrieve the battery when the 16" is opened up for use. There are several inconveniences involved (not the least of which is the fact that after a weekend of heavy usage, the trickle charger would not fully recharge the battery by the next  weekend).

Without going into the kind of detail as above, a small, \$20.00 1.5 watt solar panel was found that would easily recharge the battery during the week following a weekends' heavy use of the telescope. The panel was mounted onto the front access door of the 16" enclosure and the scope rewired to accept the charger.

Now it's a simple matter of just opening the scope and rolling back the shed. No longer do we need to go through the effort to open and close both buildings when only one scope is in operation nor connect and disconnect batteries and chargers.

Only once, has the battery not been brought back to a full charge. This followed a four week cloudy period during which the telescope was heavily used on five of the only clear nights during that period. The spare battery  in the other building, was used as intended.

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