Al's Astrophotography Blog

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 for me I 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 I don’t draw quite as much for my dew heaters, and the TEC cooler is usually drawing at 60% or less of it’s full power draw.

So if I plan for 10 hours of use before recharge, I’d want at least a 60 Amp hour battery…  preferably much more since 6 Amps is a much higher draw than the current used used in establishing the rating (typically the amp hour rating is based on current value that will discharge the battery in 20 hours).   Let’s talk about that now.

How are batteries rated: Battery capacity is measured in Amp-Hours.  That is, how many amps for how many hours.  So, let’s say you needed 5 amps for your gear and wanted it to run for 8 hours, a 40 amp-hour battery is where you’d start.   It’s not actually that simple, but for our purpose it’s probably good enough.  For a more detailed discussion, check these links:

• Marine Battery Faq
• Understanding Amp Hours
• Batteries in Fact and Fiction
• Battery Storage Capacity Ratings

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 at least sort of close to 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:

Small Battey 40 AHrs

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