Box your Battery

Tired of lugging a heavy battery around by its carrying strap? Inconvenienced by connecting to it using alligator clamps? Messed up by dew getting the battery all wet? Make a battery box!

It's simpler than you might think and you can make it as fancy as you want. Here's my rather minimalist box:

A Simple Battery Box

At the right side under the lid protuberance designed to allow dew to run off are two 12V sockets. I may add a couple more on the far side. Each is individually fused so that if one device blows the rest keep working.
 

The voltmeter springs into action

The only other thing I added was an LED voltmeter that's actuated by a momentary-on push button. This helps me monitor the battery's state of charge.

Under the hood

The 35Ah battery (group U1) is dwarfed by the box, which is designed to handle batteries in groups 24-31. I've used rigid foam to hold it in place. I haven't done a deep recharge of the battery yet, so I don't know how warm it will get. I suspect it won't get warm at all--My charger is only 1.1A. With an accessory the charger can charge the battery though one of the sockets

What you'll need:

• The box: The one I used comes with a useful divider and can be used with a  battery that provides 75Ah and still have room for the sockets
• Sockets are found on eBay for about $6 each. I used waterproof ones that come with their own cables and fuses. I bought the kind that attach with a locking ring; they've been solid in use.
• The voltmeter (about $4) is also from eBay, waterproof, and also attaches with a ring.
• Connecting wire. If you run your own wire to the sockets, be sure to use 14AWG or heavier wire, and check that the fuses are appropriate for the wire and devices you plan to connect. Wire to the voltmeter can be much lighter as it will carry only a tiny current.
• Tools: The only helpful tool you may not have is a variable drill bit that will help make the socket and meter holes. You don't need one as large as the holes--it can be used the start the hole and ream it to proper size.

What you don't need:

• Battery terminals for jumper cables. This should be a deep-cycle battery, and it doesn't like jump starting. 
• A carrying strap. The box comes with handholds at either end. A 35Ah battery and box weighs under 25 pounds, so it's easy to carry.

As you can see from the above picture a U1-sized battery leaves a lot of room for accessorizing. An amp meter could be useful to install if you want to measure battery drain. If you have the need for other voltages, add some step-down DC converters. They're small and very inexpensive. You should probably isolate them from any charger, perhaps with a two-way rocker switch. Go wild and add lights and a radio and you have one of the overpriced commercial "power tanks."

I'll be field testing this battery over the summer at a number of star parties, and may modify it as a result.

My battery box delivers twice the watt hours of a commercial "power tank," and costs less (about $125 total,  $100 of which is the battery and charger).

Trying My New Battery; Color Balance Issues

The weather has been cooperating a little better this last week, and I've been able to try out my battery-based imaging setup. I haven't done much more than image an hour here, a couple of hours there, but the results are about as expected.

Here are two images from my inner red zone back yard,

M42:

As you can see I was unable to completely eradicate the sky brightness gradient.

The Rosette:

Both are taken without filters only because I don't have one. A decent filter would have improved these greatly.

Here are a couple from some distance north of the Twin Cities in a yellow zone,

75m of the M81/82 area:

I have this delusion that I'll be able to image the integrated flux nebula.

The southern portion of Auriga (including some dark lines from tree branches):

The first two images came out horribly red/magenta heavy, and it was a lot of work getting the color right. I passed that off as a consequence of the camera mod and light pollution. The second two appeared just as red on the display screen while working on getting the exposure right, but they came out fairly neutral because I chose the ImagesPlus RAW conversion that ignores the camera's white balance; I used "Bayer No White Balance" instead of "At Capture White Balanced Color."

Color balance isn't an issue when CCD imaging, particularly if your filters are balanced by the manufacturer to give equal white signals in each channel.

The question is now which is better, a custom white balance or using the "no white balance" processing option. There's only one way to find out, and that will require a sunny day, a few sheets of white printer paper, and a clear night. That's my next task.

Oh, and the battery worked perfectly. It's nice to have the power right there in such a light package. 35Ah, less than 23 pounds. My little battery case now has two power sockets, but the volt meter has yet to arrive.

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.

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.

Batteries

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.

The Lust for Power, Part 1

Okay, maybe not so much lust as desire.

For the last five years or so my dark sky imaging has relied on two deep cycle batteries. One of the batteries had been allowed to discharge to nearly dead but with regular recharging seems to have recovered, although there's no doubt it lost some of its life.

These are group 27 unsealed lead acid batteries that weigh about 55 pounds each. With that weight you might expect them to have good amp hour (Ah) ratings. What are their capacities in Ah? They're labeled with two RC values: 200 for a non-standard 23A drain rate, and 175 for the standard 25A rate. The higher drain rate translates to a capacity of 73Ah and as expected the slightly lower drain of 23A gives a capacity of 76.7Ah. My expected drain rate of 6.5 amps is much lower and should suggest a larger yet AH value. Another practice is to take the Ah to be half the RC; for my batteries this would be 87.5Ah.

The upshot of all that uncertainty--and battery capacity is notoriously difficult to quantify--is that I'll assume the batteries started their lives with an AH value around 80. What it is now I can't say, other than it's less.

Even that conclusion has to be questioned, for some of those amp hours are coming when the voltage is well below 12V. Will everything keep working at 11 volts? SBIG says my CCD will work even at 10V. Kendrick controllers basically turn off when the voltage drops below 11.6V. (They're quite adamant about this and have refused pleas to disable the low voltage cut-off.) The CGEM's ability to handle low voltage is questionable, though; there are reports that it will begin to fail when the voltage goes below 12V.  So even if my batteries are able to produce 80Ah, they're not all usable.

Time for some "ground truth." How have the batteries performed in the past? Probably their biggest single star party workout came at the 2014 Nebraska Star Party where I imaged for seven and one half hours at an hourly drain of about 6A (see below). This probably says more about the lack of clear sky time than it does about the batteries.

Hauling batteries like this on long road trips is a bit of work, and does present a small risk that the batteries could leak acid. So far I've never tipped them over, but an unpleasant accident almost seems inevitable. So it may be time to replace them, and what follows is my exploration of the options.

Power Requirements

Some dark sky star parties are three nights, others are four, and all of them that I attend are during the summer or early fall. A typical summer night is completely dark for only about five hours; by the equinox this stretches to nine hours. Rather than estimate a nightly power need, an hourly power consumption is probably more sensible to use. ADDED: I was able to actually measure some of the values, and those are added in [red].

• CGEM Mount: During fast slews it can require 1.5A [1.4A], but when tracking it's more like half of that. Let's assume a 0.75A [0.35A]demand while imaging.
• SBIG ST-8300M CCD Camera: The spec sheet says the camera draws 3A at 100% cooling. A more typical cooling load is 60% of this, so I'll assume a steady 2A draw.
• DSLR instead of CCD? probably more like half an amp. [With the display off, my Canon T2i, draws 0.13A while idle,  0.19A while imaging. The 12VDC-to-7.4VDC converter is 0.03A of those values.] 
• Laptop: My old Gateway's AC power adapters says it runs at a maximum output of 3.4A @ 19V, so at 12V that's more like 5.4A. This agrees with my 12DC adapter's spec sticker that says it permits up to 5.6A. That's the load when it's running and charging the battery. A more realistic load is closer to half that, so I'll say 3A to be on the high side and include losses in the 12VDC to 19VDC adapter. [While charging it draws 5.6A, and 2.1A when fully charged. These values don't take into account computational demand of autoguiding.  Included in these values is 0.12A for the 12VDC-to-19VDC converter. Plugging in the Orion StarShoot Autoguider adds about 0.5A. Dimming the display to its minimum cuts half an amp from the draw.]
• Dew Prevention: I use Kendrick dew prevention, and the power need varies greatly with the telescope objective diameter. At 100% power the strip I use for the guide scope draws 0.3A, for the 4" scope 0.9A, and for the 9.25" scope, 2A.  So my range is 1.2 to 2.3A. Because I almost always use a power setting half this, I'll take the dew demand as .5 to 1A [The low setting actually ranges from 0.17A (6" strap) to 0.97A (28" strap)]

How does this add up?

1. The maximum is imaging with the C9.25 on a dewy night with autoguiding: 6.75A [6.0A]
2. Small scope on a dewy night with autoguiding, 6.25A [5.2A]
3. On a dewless night both drop to about 5.75A [5.0A].
4. DSLR + lens, no guiding? 4.3A [0.7A].
5. There are more combinations, but let's stop here.

The reality is that I seldom image more than a few hours a night. If we cap the maximum number of hours at four per night, the nightly power need for Case 2 above is about 26AH [21Ah], so a four-night party would need 104AH [83Ah] if it was clear every night.

At the other extreme is using a DSLR and using the laptop only for focusing. This would require only 21AH [11Ah]!


Next time in Part 2, can I use batteries to meet my imaging needs?

A Truss Mirror Grinding Stand

Years ago I built a mirror grinding stand from 2x4s and plywood designed to be so solid that it could be rigid when bolted together--no glue was necessary. It was very, very heavy, which was great for stability but a bother to keep around the basement when not in use.

The club has been talking about a mirror-making workshop, and although it doesn't seem like something that's going to take place any time soon I was motivated to build something better for my mirror work. After looking around the Internet for designs I settled on something like the Stellafane design. It's a simple sort of truss barrel.

What I didn't like about the Stellafane design was 24" diameter of the top--it's too big for my purposes--and the four-corner truss. I decided to try a three-corner truss instead. The four-corner truss is a standard for big Dobs and is used by Starmaster, Teeter's, Discovery, and Obsession. The thee-corner truss has proven itself on Orion and Meade Dobs, so perhaps it would work well for ATM work. It's a little lighter and less work to assemble, too.

Because I was using a three-corner truss it seemed logical to make the top and base hexagonal instead of the circles used by Stellafane. My hexagons are inscribed in a 20" diameter circle, which will be of adequate size to handle any mirror I can foresee working on up to 12" diameter.

Here's what it looks like after first assembly:

The stand is 37" from floor to top, just right for me.

What you can't see is that there are three rubber feet under the base to help it sit flat on the floor without rocking. At this point it's held together by wood screws. It seems rigid enough, but I won't know how really solid it is until it's in use. There may very well be some glue in its future.

Still to be done is the addition of adjustable cleats for standard mirror sizes: 6", 8", 10", and 12" should do it. The entire assembly will get sealed with polyurethane, with the top getting a nice sanding and a triple coat of poly to resist the water it's going to see.

Correcting the Too-short Celestron Hand Control Cable

One of the perennial, if minor, "what were they thinking" topics in astronomy hardware is the short coiled cable used to connect Celestron hand controls to their mounts. The cable, stiffly coiled like an old phone handset line, is simply too short.

This wasn't so bad on my old CG5 ASGT where the mount head isn't all that large, but on my CGEM it couldn't be ignored. It was possible to have the handset pulled right out of its tripod leg cradle as the scope turned in RA. That's not something you want to see when you're imaging, since it means the coiled cord was torquing the mount and then letting the handset become a free-swinging weight in whatever breeze there might be.

One solution I tried for a while was a coiled extension cable. The added cable was so heavy and droopy that it was awkward to handle and tended to get snagged on the mount. Then I came across a video that shows how to replace the stock cable with one that's more user-friendly.

All you need is

• A piece of flat 6-conductor telephone cable, preferably one with at least one end having an RJ12 connector attached--if not, you'll have to do that yourself. These can be found many places; on Amazon they're typically around $5 to $6.
• A soldering gun and solder (A good, fast-heating gun is much preferable to the old pencil type)
• A wire cutter
• A craft knife for stripping very thin wires
• Heat-shrink wire tubing (I found this on eBay)

Extremely helpful to have is a soldering jig to hold the ends of the wires together as you solder them.

The most important consideration is getting the wire connections correct. In my case the wire colors and order in the cable exactly matched that used by Celestron, so it was easy to get things right. If you get the connections wrong its quite possible you'll ruin your handset.

The filter from the old cable will be reused on your new cable. Aside from that, you can discard the old cable. Or toss it into your pile of stuff you probably should trash but are keeping "just in case" it might be useful someday.

The job takes an hour or so, when you're done you'll have the kind of cable Celestron should have provided in the first place!