Thursday, March 3, 2016

The Lust for Power, Part 3: Generators

Last time I found two imaging configurations that called for the use of big, heavy, and expensive deep cycle batteries. The common alternative is to buy a generator; it will supply 110V AC, and then your AC adapters will feed your equipment.

Generators as the primary power source

Generators come in a variety of sizes. The factors to consider, roughly in order of their importance, include:
  • Amperage rating (AC). While some generators have DC outputs, you'll probably be using the AC side.
  • Running and peak wattage (W). Wattage is easily computed by taking the product of amps times volts. [Energy is watt-hours (Wh); A battery's available Wh is just the product of its Ah and voltage. For example, a 100Ah 12V battery has a capacity of 1200Wh, of which about 60% is available before recharging.] 
  • Sound level. If you're at a remote site, chances are you'll have people camped nearby. Running a loud generator could get you booted out.
  • AC regulation. Is the ouput voltage well regulated, and is it in the form of a relatively noise-free sine wave? This is difficult information to come by.
  • Available low-energy modes. Does it reduce fuel consumption and sound level if the demand is small?
  • Subjective things like ease of use, noise level, fuel consumption, reliability, etc. Generally the more you pay the more you get in terms of these.
What kind of power will we need? Configuration 1 (CCD, guided large telescope) required 6A. At 12V this is only about 72W. Peak amps is about twice that and demands 150W. Configuration 2 has an even smaller demand (130W peak).

The smallest generators have about 800W peak power and a price point around $300. The PortaSource IG800W ($313) has marginal amps; the Generac ix800 ($287) gets mediocre reviews and a lot of 1-star votes on Amazon.

Medium-capacity generators have around 1600 to 2000W running power and twice the amperage that I require. Prices range from $400 to $600, although some brands can demand about $1000. It's not clear that the added cost is worth it. These generally weigh 50 pounds or more.

In this class I think the Wen 56200i ($429) fits my needs best .

Generators with greater capacity are not needed for imaging, but can also serve as emergency generators. I'm not going to consider them. 

Generators to recharge batteries

It may make some sense to get a small generator and then use it to recharge a battery. because the recharge takes place during daytime noise is relatively unimportant. What matters is the amperage of the charger, since that will be "restocking" the Ah lost overnight. Because some chargers operate at lower amps than your imaging use, you may be able to get by with a generator that doesn't meet specs as the primary power supply.

The required charging time will be the number of hours you imaged  times the amps used while imaging divided by the charger's charging amps.

Example: Imaging at 6A for four hours and using a 1.1A charger. The recharge time is 4h x 6A / 1.1A, or almost 22h. The most you would probably recharge for would be 12h, so you could recover only 13Ah of the 24 you used the night before. Not practical.

Now imagine imaging at 2.9A for four hours and the 1.1A charger. Recharge time is now 4h x 2.9A / 1.1A, or  10.5h. That's actually doable.

I've used a relatively slow charger in this example. A 3.5A charger would cut these times by 2/3 , to 7 and 3.5 hours. A 50Ah battery and 800W generator could make a nice tandem.

Generators to Augment Batteries

Okay, why not power things from both a battery and small generator, thereby easing the power demands on both, then using daytime to top off the battery? Let's see some examples.

Let's consider Configuration 2, small telescope, CCD, guiding, dew prevention: 5.2A. the two big power consumers are the laptop and CCD. Let's let the generator power the laptop and mount (3A), the battery handle the CCD and dew prevention (2.2A). 3A is easily within reach of the small generators, and the nightly Ah drawn from the battery is 8.8Ah. Recharge time is 8 hours. Suppose it's fall and you can image for 6 hours; you'll pull 13.2 Ah out of the battery and need 12 hours to put it back. The generator will run a lot--18 hours a day.

The downside is that nighttime running of the generator brings noise back into consideration. And you'll be running the generator quite a bit--both during imaging and the daytime. It's an interesting approach to powering your gear, but I think either battery or generator is better than a hybrid solution.

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