FrankenHub is Created & A Very Minimal Power Supply

Okay, it was Halloween yesterday. In keeping with that I'll show you my replacement for the Pegasus Powerbox that died in Iowa. 

My Powerbox was a USB and Power Hub with the capability of controlling dew straps as well. My replacement would need to have those capabilities.

I already had a powered USB3 hub and a small Anderson Powerpole-based power bus from past tinkerings, so it was only a matter of replacing the dew control capability. A nice little Pegasus Dew Controller took care of that.  Yes, it's manually operated, but that's fine.

The USB Hub is so old it's no longer sold (not a surprise) and now is a 7-port, smaller device.

What was left was how to put it all together. And that's how it became Frankenhub.  I decided the perfect "suturing" was silicone bathtub caulk. Silicone caulk is waterproof, bonds to almost anything, remains pliable, and can be removed. As an adhesive it's more than adequately secure when joining two flat, clean surfaces.

Frankenhub is three layers, a dovetail to mate it to the ad-hoc finder shoe on the scope OTA, the USB hub, and then the dew controller on top:

Frankenhub

Definitely not as pretty as the Pegasus box, but it works just fine. What it lacks is software control of the dew controller, but I can live with that.

Wait, where is the power hub? Caulked to the side of my G11 saddle:

Power Bus

Think of this as one of those big bolts in the neck of Frankenstein's Monster. You know, these things:

OEM Heavy-Duty 300,000,000 volt / 30,000 amp Connectors

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Speaking of power, here's a minimalist approach to supplying power: A 50 Ah battery and cute 12 V distributer:

Power and Distribution

The battery has all the power I need for several nights of summertime imaging and the distribution box has three 12V automotive sockets, all I need to run my setup. Here's a closeup:

Power Distributor

Note that it includes a voltmeter that can be turned on/off, various sockets for charging your mobile devices, and a 3-level light that you could easily cover with red plastic.

The included 50 A Powerpole connector probably couldn't handle the 30,000 amps that jolted Frankenstein to life, but it's more than enough for my rig's peak draw of  5 amps.

 

50 amps of gray

Next time I'll have another Frankenstein that I hope isn't monstrous: the Veil Nebula mosaic, using parts (images) from a few sessions at Eagle Lake Observatory, The Iowa Star Party, and a friend's back yard near Stacy, Minnesota. A real Frankenimage. 

 

New Battery - LiFePO4!

New Battery and Power Box

I usually image from places that have AC power--but not always.  When there isn't AC I need to run from batteries. It's gotten to the point that I like batteries so much I use them even when AC is available. But -- the batteries I've been using are heavy and bulky; they are really not fun to lug around. And they're barely adequate for multiple-night star parties, too.

My goal has always been to eventually give up lead-acid batteries for lithium. Lithium's advantages over lead are well known, but it has always been the price that stopped me from jumping on their bandwagon. This year that changed; prices came down a lot!

I sprang for a 50Ah LiFePO4 battery--1/3 the weight of my 50Ah lead battery, and that's a world of difference. The upgrade motivated me to put it into a smaller case that I had sitting around, and to improve the connection ports over those on my old case. 

The new case outputs are three 12V automotive sockets and two Anderson Powerpole (APP) connections. A third APP allows me to connect to my solar panel or an AC charger. There's an on/off rocker switch to toggle between charging and power box use, and a panel meter for monitoring the state of the battery (more about that later).

There's one change inside the box, too. All the outlets pass through an APP distributer that allows each to be independently fused using common bayonet-style fuses.

The new power box

Testing the new battery

Of fundamental importance with any battery is knowing when to stop using it. Discharging a rechargeable battery too deeply will reduce its capacity and useful lifespan. But what exactly is "too deeply" and how will we know when the discharge is approaching that?

Here is a much-copied table that you can find all over the Internet: 

Table relating State of Charge to resting voltage and
suggesting what is "too deeply" 

Some terminology:

• State of Charge (SoC) is basically a measure of how much energy is left in a battery compared to what it has when fully charged. A full battery has SoC = 100%, an empty battery has SoC = 0%.
• Operating Voltage (Vo) is what a voltmeter reads across the battery poles while it is in use. 
• Rest Voltage (Vr) is what that voltmeter would read when the battery is not discharging and has not been discharging for at least an hour. The table's voltages are all rest voltages. Vr is impossible to measure while imaging--unless you're willing to shut down to let the battery rest.

The third column in the table tells us SoC shouldn't slip past 20% if the battery is to stay nice and healthy. My battery is advertised to endure 4000 recharge cycles before needing replacement -- if the recharge comes before the SoC slips to 50%. Given my current age and how often I image, the battery will never see 4000 recharges; in fact I'd be surprised if it sees more than 200, and the majority of those will be from a SoC above 50%. But it's nice to know that if circumstances require it I can take it deeper. 

To be conservative about this, I'll preselect SoC = 20% as a safe stopping point. I'll probably never reach that because I can always recharge the next day using AC or my solar panel. 

If I can't recharge the next day and want to image anyway, how do I track the SoC so that I can prevent the battery from going too low? First, I'll need to know its energy content when the SoC is 100%. If I can determine how much energy it has provided since the last charge I can estimate SoC by

    SoC = 100% * (1 - (energy discharged / energy content at SoC 100%))

Given the battery's rated voltage and amp-hour capacity I can estimate the battery's full capacity:

   Energy content at SoC 100% = (rated V) x (rated amp-hour capacity) 

        = 12.8V x 50Ah 

        = 640 watt hours (Wh)

Why do I call this an estimate? Every battery is a little different even when new, and over time some of that capacity will be lost. Generally new batteries have a slightly larger capacity than their rating, but I'll go with 640Wh to be conservative.

I added a panel meter to the new box that gives me the amount of energy discharged; this makes  estimating SoC easy.

Panel meter showing a battery at rest after discharging 416Wh

If my aim is to not let the battery fall below a SoC of 20%, all I need do is make sure that number never goes over (1 - 20%) * 640Wh = 512Wh. That way, happy battery and happy imager!

Now you're probably wondering about that solar panel and asking if that isn't heavy and bulky, too. Oh, it is! But combined with the very light lithium battery it still beats the multiple lead batteries required for that rarest of events, a five clear night remote star party!

One more thing, a tiny hack. The panel meter has a button that lets you zero the energy discharge counter after a charge. It's also what turns on and off the panel backlighting.  The button is countersunk and can only be depressed by something relatively pointy. My solution is a 3mm glass bead held in the button hole by stretchy friction tape. You can see it just to the right of the panel display.

That's more than enough battery talk. Next time some even more mundane chatter about upgrading my Pegasus FocusCube to Version 3, and putting my old Version 2 on an AT-65.

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.

A Tale of Three Batteries

I recently complete my piggyback power and USB distributor that should allow me to image using only two cables running to the mount. (I'll have more about that after it's been field tested.) I'm standardizing power connections using Anderson Powerpole connectors, and the plan is to use all my batteries connected in parallel. This means adding Powerpole connectors to all of the battery boxes, which got me thinking about the batteries themselves.

The old batteries are 6 and 7 years old, and there was little doubt they were starting to show their age. Both are the wet lead-acid type, which is a little messy and a tad bit risky to haul around. They're also heavy, each being rather large (Form 27) and weighing about 55 pounds.  I could justify replacing them with nice new AGM sealed batteries if they proved to be in poor shape, which raised the question of how to test a battery's capacity.

Both batteries have reserve capacity (RC) values of 175 and were advertised as being deep-cycle. I can't attest to their deep-cycle ability, but they served me through several Iowa Star Parties and four Nebraska Star parties.

RC is the number of minutes a battery at 80°F can provide 25A before it is fully discharged (it's open circuit voltage falls to 10.5V). RC is not the same as amp hours, and certainly not the same as usable amp hours.

To see why, let's pretend we can convert RC to Ah directly:

Amp hour capacity = amps provided • elapsed hours =  25A • (RC  / 60) =  0.417 • RC

An RC of 175 gives a capacity of 72.9 Ah. Seem easy, doesn't it? The catch is that you almost never want to drain a battery that far-it will gradually decrease the battery's capacity. If you only discharge it about 80% you probably won't notice any loss of capacity until you're well past 200 discharges. 200 discharges is a lot of nights out imaging! So that 72.9Ah is really more like 58Ah. Sorry!

(Much of the literature about deep cycle batteries is written for solar power users who discharge their batteries nightly. In their case a maximum discharge of only 50% or less is needed if the batteries are to be economical.)

But wait, it gets more complicated! Astrophotography seldom requires anything like 25A. (For example, my setup for imaging with a DSLR requires only about 3 to 4A; it jumps to about 6A when using my CCD) Batteries are more efficient at providing power when the current is lower, meaning that the "real" Ah capacity in that case is higher. My batteries also have a stated alternate RC value of 200 based on 23A draw, which gives a full drain capacity of 23A • (200 / 60) =  76.7Ah. Two amps less and you get 3.8 more Ah. Great!

We're not done yet, though. What you're powering also will come into play. As the battery is drained its voltage falls, and as that happens voltage converters and inverters may have to work harder to provide regulated power. Some devices (Kendrick dew controllers, for example) may shut off. So draining a battery by 80% may not work for you.

Possibly the "best" way to determine how a battery will perform is to test it yourself. I recently found a nice way of doing this that employs a simple AC electric clock, AC lamp, and an inverter with a low voltage alarm. An inline DC power meter can be useful, too, but it's not necessary.

I used a 40W bulb to better match my typical current demand. (The calculation is easiest if we use amps = power / volts, so in this case 40W / 12V = 3.33A.) The inverter and clock draw power, too, so  My inline meter suggested that inverter, bulb and clock used about 43 watts and would draw about 3.6A initially. My inverter is programmed to sound an alarm when the loaded voltage drops to 11V; in reality it appeared to cut off at 11.2V

As a reality check I tested my relatively new and well cared for 35Ah AGM battery. I was unable to find an RC value for it that I could trust, so I took values from its spec sheet and (with the help of a spreadsheet) interpolated a capacity of 31.65Ah at a drain of 3.6A.

The battery powered the inverter for 399 minutes, and the meter reported 28.5Ah were provided, but a closer look at current and voltage measurements suggests this should be adjusted by a factor of about 0.93--so 28.5 becomes 26.5. given the uncertainties, I'll use that as the output of the battery. 26.5 is 83% of the estimated 31.65Ah capacity.

Even though all of these numbers are fairly approximate, I think it's safe to conclude that the relatively healthy 35Ah battery was able to provide something like 80% of its capacity before the voltage fell low enough to shut down the inverter. Given that, the test can be repeated with the other batteries; if they are  in perfect shape they could be expected to each deliver about 80% of their 73Ah, which is 58Ah. This is very likely an underestimate because we drain at a much lower amperage than 25A.

Battery One, which is seven years old and was once allowed to go dead, provided power for only 213 minutes. That means it provided only about 3.6A • 213 minutes • (1 hour / 60 minutes) = 13Ah. Awful!

Battery Two, which is six years old and was better maintained, provided power for much longer and ended up delivering 26.5Ah, which is still less than half of what a new battery its size could be expected to provide. It should be replaced, too.

The options are 50Ah batteries, $100 ($2/Ah), 30 pounds, or 75Ah, $140, ($1.90/Ah). I think I'll go with the 50Ah ones! Time to place an order!

A Battery Box Addition

The Battery Box got one refinement over the last couple of weeks: A built-in 7.5VDC power supply.

I had intended to use an external supply, but the little Drok unit gets good reviews and is much smaller than the adjustable supply I had planned to use. It was perfect for mounting within the box. All it needed was an output plug. For that I went with the same sort of plug used on many mounts, a lockable panel-mount socket and plug. (Both were purchased on eBay.)

Drok 12VDC to 7.5VDC step-down converter

5.5x2.1mm socket and plug

A built-in converter presents two problems if it's left connected to a battery: A continual power drain (0.1 to 0.12W) and possible interaction with a smart charger. The drain doesn't sound like much, but it equals about 7Ah per month; that's a lot for a 35Ah battery. To prevent these problems I isolated the converter using a rocker SPST switch. As a reminder to turn off the converter when it's not needed I added a tiny LED that draws a minuscule 3mA. Here's the switch and LED:

Switch and indicator light

As I said, it's a tiny LED! It's just bright enough to let you know that the converter is active.

12V sockets (above) and new 7.5V socket (below).

I made an extension patch cord that fits the plug and the power cord from the camera's dummy battery; this will permit me to use other power supplies.

One other add-on is intended to make this easier to use: An accessory cable for my charger that will let it charge through one of the sockets. Because this is a small battery it can be charged at a relatively low amperage of 1.1A. While I haven't yet recharged it from a state of deep discharge, I'm hoping that it will remain cool during charging even when left in the box. It's my hope to never need to remove the cover.

NEXT UP: It's the Messier Marathon, April 8 or 9! I plan on imaging some Messier galaxies as a part of the Arp galaxy imaging project. There are six Messier/Arp galaxies I haven't imaged yet: 49, 60, 65, 66, 77, 87, and 90. Only 77 is badly placed; it will set too soon for imaging. Because it's a marathon night, I'll shoot only luminance with my ST-8300M, and probably only an hour apiece at most using my TV102. Last year the MM had only so-so skies. Here in Minnesota we're overdue for some good Messier luck.

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?