Friday, February 26, 2016

The Lust for Power, Part 2

In Part 1 I looked at the power requirement of my gear with the purpose of seeing how I might replace my aging deep-cycle batteries. The required amps for several configurations can now be given. The Ah requirement for hour hours a night for four nights is given in parentheses.
  1. C 9.25 on guided CGEM, CCD, dew prevention: 6.0A
  2. AT65 on guided CGEM, CCD, dew prevention: 5.2A
  3. AT65, guided GGEM, DSLR, dew prevention: 3.4A
  4. Camera lens on DSLR, unguided CGEM, dew prevention:  0.9A
Don't worry if the numbers don't exactly map with the empirical values given in Part 1--I'm usually rounding up here. What are our power supply options for these configurations?

Commercial Portable power packs

Most of commercially produced power packs are based on 17Ah batteries. Examples are the Celestron PowerTank 17 ($122) and Orion Dynamo Pro ($145). When brand new, these may be capable of delivering 80% of that 17Ah. That's 13.6Ah. As time goes on you'll see that decrease depending on the number of times you cycle the battery and how well you maintain it. A battery pack like this is sufficient for 14 hours of Configuration 4 and marginal for one night of Configuration 3. It doesn't meet the 16-hour requirement for either case without one or more recharges.

Generally speaking power units like these are wildly overpriced--unless you put a high premium on bells and whistles like radios, spotlights, and DC outlets at other voltages. You're get much better economy if you buy a larger battery and charger. For example, a 35Ah sealed AGM battery and charger cost around $100.

Some power supplies (Duracell, Black and Decker, etc.) are more focused on cranking power and include inverters so you can run your gear as if you had a AC outlet at hand. An inverter sounds nice, but it will eat up a small portion of whatever power you need to supply; a battery build for starting cars is quite different from your need (prolonged low current for many hours).

Recommendation: Don't buy any power supply that includes car starting in its list of features--unless the low Ah rating it provides is all you need. Even in that case, you're better off to simply invest in a battery and charger.


For lowest cost you can use flooded (also known as wet) lead-acid batteries. These have caps on top for adding water and venting gas during charging. While less expensive than other battery types they have several downsides. The acid can spill or leak out and damage equipment or even cause personal injury. For this reason wet batteries have to be kept upright at all times.

While charging hydrogen gas can accumulate and cause an explosion.

Fortunately there are sealed lead-acid batteries that are spill proof and can be used in any orientation. Those that employ Absorbed Glass Mat (AGM) technology and its variations also have better deep-cycle characteristics than flooded batteries. Other advantages of sealed batteries are that they can be shipped without worries about acid spills and the need for the user to initially add the acid, and that they're maintenance-free (aside from recharging).

(Important note: You can't use a flooded battery charger on an AGM battery unless that charger specifically has an AGM capability.)

The battery size you need will be determined by your gear and the type of battery. I think the only practical type of battery to consider is AGM; other technologies (mainly lithium ion) tend to be more expensive. If you take care of your battery (keep it charged, avoid thermal extremes and physical abuse) and use it infrequently (a dozen times a year, maybe?) it will provide years of 
reliable ability to deliver between 50 and 80 percent of its Ah rating. I'll apply the 60% rate in what follows in order to be conservative.

Configuration 1 (large scope and CCD): 6A x 6h is 96Ah. This is 70% of a 160Ah battery. A single battery with that capacity weighs over 100 pounds and costs $300 or more. This doesn't fit my definition of portable power.

Two 80AH batteries would be a somewhat better solution because although being higher in cost they're a bit more portable--each is about 50 pounds. I've imaged this way, but I don't enjoy lugging the batteries around, and consider it a marginal solution in this case.

Configuration 2 (small scope and CCD)

This needs a battery with about 140Ah capacity. This is also met by a single heavy, expensive battery. The same two-battery solution works here as in Configuration 1, so we're again stuck with the non-optimal use of very heavy batteries.

Configuration 3 (short lens or scope, guiding, dew and DSLR)

This needs a 90Ah battery. Two 50Ah batteries would provide more than enough power and cost about $180. Total weight would be around 70 pounds.

Configuration 4 (short lens and DSLR)

This is clearly a case where a battery is the best solution, requiring only a 24Ah battery. It can't be much easier.

The last two configurations clearly can use batteries to meet the requirements. But what about the first two? You can either lug big batteries around or find an alternative: A generator. That's for Part 3.

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