2009
11.27

Batteries

Going remote with your Astrophotography Rig? You’ll need power….  Quite a bit of it.  Most people won’t want to have a generator ruining the peaceful solitude of the night, so battery power is the usual solution.

So how much power do you need, what kind of batteries will do, how to connect to them, how to house them, how to charge them….  all are good questions.

How much power (capacity) do you need: This part isn’t hard to figure out, just be sure to count everything that needs power.  Here’s some rough guidlines:

  • Mount: less than 0.5 A while tracking (that’s what it’s doing most of the time).
  • Laptop:  I’ve measured a couple.  I got 2A for each of them.  You could measure your’s with a wattmeter (just divide the watts by 12, then again by 0.9 to account for inverter inefficiencies)
  • USB devices:  Unless I know otherwise, I use 0.5A for each device.
  • TEC coolers in cameras:  I use 1.5A for single stage units, 3A for two stage TECs.
  • Dew Heaters:  Check your manual.  I use my home made ones, and they are sized to use 0.5A each.

So, I conservatively  calculate 6A steady:  Mount, Laptop, 2 USB Cameras, one TEC cooled, 2 dew heaters.  In actuality, I think my steady draw is about 5A.  The difference is mainly that there isn’t quite as much draw for my dew heaters, and the TEC cooler is usually drawing at 40% or less of it’s full power draw. However, the cameras and laptop are powered through and inverter so there’s some loss there, maybe 15% more current used for those items.

So, now I know about how much current I’m using.  To determine how much battery is needed, I’ll need to now a bit about how batteries are rated..

How are batteries rated: Battery capacity is measured in Amp-Hours….  That is, how many amps for how many hours.  So you might say I need 6 amps for 10 hours, so I need a 60 amp-hour battery right?  It’s not actually that simple.  Battery amp-hour ratings are typically calculated by determining the current that will discharge the battery in 20 hours.  For example, a battery that becomes discharged in 20 hours with a current of 3 amps, would be a 60 Amp-hour battery.  The bad news is that batteries do  not behave linearly.  If for example, 6 amps were drawn from our 60 amp hour battery, it wouldn’t last 10 hours, it will become discharged much faster and might last maybe only 5 hours.  Likewise if less than 3 amps is drawn it will last quite a bit longer than would be calculated.  For a bit more detailed discussion, check these links:

You can see the rating systems are kinda mixed up and complicated.  However, there are a couple decent guidelines.  They are:

  • Suitable batteries for our overnight work will likely be labeled Deep Cycle.
  • Check the “Reserve Capacity” rating.  A rating up around 2 hours is good.
  • Use the Amp-Hour rating as a guide.  Check that the Amp-Hour rating divided by 20 is a number close to or bigger than your steady state draw.

These things together should get you a battery that will last though the night like you expect, AND be able to be cycled (discharged/charged) many times meaning you’ll get a battery that will last a long time.

Connections: I hate those cigarette lighter plugs.  I’ve had them get nudged and disconnect at the worst times.  I still use them when I have to because I have too many cords and too many configurations.  I like to use molex connectors (the white plastic connectors you find in computers).  I wire them with the ground to the two center pins, and the +12V to the two outer pins.  These are quite secure and cannot be plugged in incorrectly.  So my batteries have a mix of connectors:  Cigarette lighter, Molex, and perhaps a specific type to fit a particular mount.  There are also very good connectors available at hobby stores that are modular and indexing (can’t be reversed).

Where to buy batteries: Well, there are a lot of places.  I’ve found several online stores that sell scooter/wheel chair batteries.  These are nice in that you know they are expressly built to withstand many charge/discharge cycles.  Marine deep cycle batteries are good choices.  I like buying from places where you can get complete specs.  Sears is nice for that.  I have heard several bad reports about Everstart batteries (Walmart), but I also heard of several guys using them without incident.

Boxes: Well, suit yourself here.  The only advice I have here is to use a plastic box that is resistant to battery acids.   Here’s what Mine look like:

SmallBatt

Small Battey 40 AHrs

Big Battery

Big Battery with inverter for AC power

Chargers & Charging: I’ll keep this short.

  1. Don’t discharge your battery to below 10.5 volts.  It will shorten the battery’s life, perhaps drastically.  Additionally (fortunately in a way) your mount and photo equipment problably won’t be working well if your battery drops close to this level anyway.
  2. Recharge your battery as soon as possible after discharging.
  3. Get a good float charger (battery maintainer) to keep the battery at the proper state of charge.  The details of battery charging can get complicated.  Google the subject and you’ll get more info than you want to know.

Whew…  If you have corrections or additions please comment to this post.  Also, any recommendations for batteries that serve you well would be appreciated.

al

4 comments so far

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  1. Very good article, a very good read

  2. Well worth the read. A useful summary for planning power in the field.

  3. A Little Update:

    After some discussion on batteries in the Yahoo CGE forum, I got to wondering exactly how much power my rig was using…. So a test was in order.

    I had to shoot new darks anyway, so I plugged my laptop and ST-2000 camera through my “Save-a-Watt” meter so I could determine the power draw of the Laptop & Camera in the normal state: Camera cooled taking pics, laptop basically idle just controlling the camera with the monitor asleep. This would be the usual load as the laptop and camera work shooting a series of images through most of the night. I was surprised by the result.

    When first powering up, screen on, the laptop draws about 27 Watts, and if the laptop battery is still taking a little charge it gets up into the 30-35 watt range. While the camera was cooling down (the TEC at MAX power draw), the total load for the laptop & camera combined reached around 45 Watts.

    However, most of the night while taking images, the laptop battery is no longer charging, the moniter powers off, and the Camera TEC settles down to just holding the temperature at -15C. In this state, the laptop and camera dropped to just 20 +/-1 Watt. Even adding in inverter inefficiencies, that’s less than 2 amps at 12 volts.

    Wow…. that’s almost half of what I thought it would be!

    The full setup of course draws more juice since the laptop/camera will also have a guide camera and guiding program running, then add the mount and dew heaters. I’ve directly tested the mount while tracking and it’s less than 0.5 amps, and I built my own dew heaters and know they only draws 0.5A each (and I usually only need one powered up).

    So for my rigs, during the majority of the night while taking images, the load is typically:

    Laptop and SBIG Camera Less than 2.0 A (guider software running also)
    USB powered guide camera = 0.5 A
    Mount (tracking and guiding) = 1.0 A (probably less)
    Heaters (I actually shoot for 6W) = 0.5 A

    Total load for the majority of the night = 4 amps

    I haven’t accounted for inverter inefficiencies but still, these numbers would indicate that for my rigs, an 80 AH battery is fine. The 4 amp load is very close to the 20 hour rating current (80 AH/20H = 4A), so a fresh 80 AH battery should easily last a long night with ample reserve. This agrees with my experience…. I’ve only once had a battery die on me overnight, and that was due to battery condition (an aging battery, with the charge not well maintained).

    al

  4. This is great practical information, especially the field use example above. I’ve been mulling around what kind of power set up I needed for the field and your piece gave me a good idea what to shoot for. Despite what others have said about them, I’m going also with an inverter, a pure sine type in the 600W range for two reasons. One is keeping the plugs more uniform without having to deal with cigarette lighter connectors and I won’t have to get a DC/DC converter for the laptop. The second one is the the pure sine inverter delivers clean power to the laptop. I’m sure there are those folks who have a different opinion about this, thats OK, at least now I know I can move forward and get set up in a dark sky location without having to deal with heavy light pollution. Thanks for the help.