Another Imagining Year Ends; Solar Panel Recharging of Batteries.

Bye-bye, 2019

Winter has fallen onto the region with a loud thud. Snow depth is now seven inches and the temperatures are near normal (for the next week, anyway). For a warm weather person like myself this essentially means outdoor astronomy is in hibernation until spring.

It remains possible that I may use the imaging platform at Cherry Grove, but I can't say I'm in the mood for that at the moment. So let's see what else there is to do.

I have some mirrors that are in polishing/figuring stage, so I could get back into those.

There's always the task of learning image processing software, but if things continue on like they have the last two years with awful weather there's no rush.

I could do some programming and try to quantify the meteor reflection data I collected a couple of years ago. But instead, let's have...

More Power Fun!

One thing that intrigues me is solar power for recharging batteries at remote sites. A few years ago when I priced this out solar panels were too expensive compared to buying batteries. But with the continuing decline in panel prices it's time to reassess.

The first step is to determine my power needs. This is the product of my imaging setup's power requirement and the number of hours spent imaging in a typical evening. The first is something I've measured; my current setup (mount, laptop already charged, dew heaters set higher than usual at 50%, CCD with cooling at 70%, autoguide camera) setup draws about 3A. That means I can image with about 40W of power.

Next we need to know how many hours this power will be needed in an evening of imaging. Let's consider the cases of an equinox and summer solstice at 45N latitude, and for each imaging through either nautical, astronomical twilight, or full darkness.

Summer solstice: full dark 3:20, astronomical twilight and darker, 5:31; nautical twilight and darker, 7:10; civil twilight and darker, 8:25; sunlight, 15:35.

Equinox: full dark, 8:31; astronomical twilight and darker, 9:42; nautical twilight and darker, 10:57; civil twilight and darker, 11:50; sunlight, 12:11.

Let's say you image during astronomical darkness and start up about a half an hour before that for polar aligning, target acquisition, and letting things settle. Depending on the time during the summer you will be using power for about 6 hours (solstice) or 10 hours (equinox).

Multiply the above hours by 3 amps and you get 18Ah (solstice) or 30Ah (equinox). These are what you need to put back into the battery after an all night imaging session. (In terms of energy in watt hours, these are about 216 and 360 watt hours.


[Digression: My primary battery is 50Ah. Draining that by 18Ah is only 36%; a 30Ah draining is 60%. By August 1 this has changed to a drain of 22Ah or a 44% drain.]

At this point it's tempting to say a 100W solar panel can bring a battery back up to full charge in only a few hours. Can it? There are a few wrinkles to consider that can reduce that 100W:

• Clouds: Cirrus are no problem, but typical fair weather cumulus can drip power by anywhere from 8 to 20% [ref]. Total overcast drops power by 50 to 75%.
• Heat: Higher temperatures are doubly bad. Panels become less efficient as the temperature rises, and AGM batteries are better off being charged at lower voltage. For typical NE afternoon temperatures (37C or 95F) this leads to a panel efficiency drop of about 2.5% and an increase of about 6% in battery charging time.
• Resistance losses can be minimized by using wires that are heavy enough for their lengths. In my possible system this means using 12AGW throughout and keeping the runs reasonably short, and should not be a factor.
• Charge controller efficiency. MPPT controllers generally have an efficiency in the area of 95%.
• Panel orientation isn't a big factor so long as you can keep the panel face reasonably perpendicular to the sun. This means turning it hourly and using some kind of adjustable altitude brace.

In the worst case it's a hot, overcast day (which are rather mutually exclusive) and we multiply 100W times 0.5 for overcast x (0.975x0.94) for a 95F day x 1.0 for resistance loss (none) x 0.95 for controller loss x 0.86 for being 30 degrees off perpendicular all day. In other words, that 100W becomes 37W. 37W times total daylight time minus two hours is about 500 watt hours (solstice) or 370 (equinox).

Conclusion: Even in an unlikely worst-case situation a 100W panel + MPPT controller should be able to do nightly recharges adequate for imaging using my setup.