DIY Field Battery

Astrophotographers who regularly travel to remote observing sites require a reliable power source to last an overnight imaging session.

Deep-discharge lead-acid batteries typically used in portable power supplies are rated in terms of voltage (V) and current (Ah). Typical ratings are 12V 9Ah. The higher the capacity of the battery, the longer the battery life (e.g., a 12V 9Ah lasts longer than a 12V 7Ah battery). To provide continuous power that will last overnight, a battery must have enough capacity.

In my setup, I have used 4 identical 12 V 9 Ah deep-discharge lead acid batteries connected in parallel, which lasts for about 14 hours.

Connect the batteries in parallel. Attach the battery connector (cigarette lighter socket) observing correct polarity (the central contact point is connected to the positive terminal and the outer contact point is connected to the negative terminal). Insert a switch along the electrical path. Use a thick solid copper wire and then solder carefully all the connection points.

The DIY field battery must be charged with an appropriate charger. The charger should match the voltage (12 V) and should deliver enough current to the battery.

To charge the battery, connect the charger to the battery observing correct polarity: positive to positive (red), negative to negative (black). For chargers with an ammeter, upon connecting the charger to the battery, the meter indicates the amount of current being drawn by the battery at any particular instant. This is very helpful in determining the battery charging status.

I have been using this field battery since November 2011 and have not had problems powering my portable imaging setup. At my current load, the field battery lasts for approximately 14  hours, more than enough power for an overnight imaging session.

Night Sky in Focus
Astronomy and Amateur Radio
© Anthony Urbano (Manila, Philippines)

22 thoughts on “DIY Field Battery

  1. Interesting article. I am thinking of setting up myself, running my EQ6 mount, MacBook Air, QHY10 CCD. Your guide has helped clear up a lot of things! Thanks

  2. I just wanted to let you know that I followed your DIY field battery.  The instructions you gave were perfect. I also added some red lighting to help me out in the dark. I just wanted to say thank you. Wish I could post a picture.

    • Thanks Luis! I am glad I was able to help :)
      Let us know the capacity of your battery (AH), the equipment you are powering with it, and how long the field battery lasts :)

    • Great work! I see you still have space in your casing to accommodate perhaps an extra battery, and you’ve added a safety fuse :) I am happy I was able to help. Clear skies!

  3. Hi Anthony,

    I was wondering if you could help me out… I talked to some folks at another astronomy community and they’re saying that my suggested field battery wont be able to supply enough power to my laptop.

    I was planning to build a 12v/18ah battery to supply my laptop that’s rated at 19v/3.42ah. I know also for a fact that the laptop uses less than that because the battery is rated 10.8v/4800mah and this lasts me for a good 4 hours.

    I’d also appreciate if you could comment on this one. Thanks!

    • Hi Pao,
      Let me walk you through this:

      (1) Since your laptop is rated at 19V at 3.42 A, it means it consumes 19V X 3.42 A = 64.98 W of power. We will use this value in number 4.
      (2) We also know that your laptop has a battery pack rated 10.8V at 4.8 Ah, which means it can deliver 10.8V X 4.8Ah = 51.84 Watt-hours of power, and according to you, it can power your laptop for approximately 4 hours. It implies that during your usual imaging session, the laptop only consumes 51.84 Watt-hours / 4 hours = 12.96 Watts for every hour. We will use this value in item number 5.
      (3) The field battery you are planning to build has a total available power of 12 V X 18 Ah = 216 Watt-hours.
      (4) At maximum power consumption, this amount of available power will only last for about 216 Watt-hours / 64.98 W = 3.32 hours.
      (5) During your usual imaging session, the same amount of available power will last for about 216 Watt-hours / 12.96 W = 16.66 hours.

      This is also the same value that you will get if you try to do some ratio and proportion.

      (6) The laptop’s battery pack has a total of 51.84 Watt-hours of power, good for 4 hours (based on your reported usage; see item 2).
      (7) The battery you are planning to build has a total of 216 watt-hours (see item 3).
      (8) Through ratio and proportion: if 51.84 Watt-hours should last for 4 hours; 216 Watt-hours should last for a certain number of hours. Solving for the unknown will yield 16.66 hours. It also means that the field battery you are planning to build is roughly 4 times more powerful, and should last 4 times as long :)

      If you only intend to power your laptop, and nothing else (no cameras, no mount, no heaters, etc.), it should be able to last 3.32 hours (running at maximum consumption) up to 16.66 hours (during your usual imaging session). Hope this helps!

      Eteny

  4. Hello and thank you for your great DIY! I have constructed the field battery and have used it on several occassions using the same batteries and components. I checked the math and it all works out. I am using the Celestron AVX, Macbook (Black 2008), and a Dew Heater. I should be able to power my setup for hours, however, my Macbook will charge for an hour and then stop. Battery charger indicates that it is connected but not charging.

    I am not sure if maybe the power flow drops (~12V) with everything connected to the same battery and thus cannot consistently power the laptop and thus no charge. Have you experienced anything like this?

    Any response would help.

    • Victor I have heard of a laptop with this problem but it was caused by an aftermarket charger that had a missing center pin that sends voltage or some data to the laptop to let it know when to stop. In your situation since u fear a low voltage what u need to do to test your theory is first try charging in your car and see if it stops after one hour. If it does the charger is at fault. If not u need to buy a 12volt splitter. Connect a 12volt voltmeter (automotive type from amazon.com $15) and notice the voltage when u start charging and when u stop. It could even be as simple as u needing to test using a better 12 charger or a USB port that provides more amps thatn what u r using now. So my best guess is u r not using a adequate 12 charger even tho it works fine on ac power. Or u could be using an inverter. Try one specific to 12 volt power etc. Test several chargers.

      -bruce

    • Hi Victor,

      Can you provide the specs of your DIY battery and charger (voltage/current, etc)? I suspect that not enough power reaches the laptop. For the laptop to indicate a ‘charging’ status, it requires a certain amount of power, which I believe is adequately supplied within the first hour of use, and then drops eventually. It is possible that there is a problem with either (1) the battery itself, not supplying enough power, (2) the converter/inverter, also not supplying enough power or probably supplying too low voltage or current. We need to determine first where the problem lies. Maybe you can measure the actual output of the laptop charger when connected to AC, and then compare it with the output when using the field battery.

      Connect the charger/inverter/converter to a car’s cigarette plug as Bruce suggested. If still the same thing happens (power dropping after an hour), then probably, (1) a charger/inverter/converter problem, or (2) car plug also not supplying enough power. If no problem occurs, then it is most likely an issue with the battery. Alternatively, you can test several chargers as well, to determine if you have a faulty charger. Let us know how it goes :)

  5. Hi Anthony,

    I’m a new reader in your site. Having done some simple astro-photography way back in the mid 80’s while I was living and working in the USA. All that I had with me was a used Minolta SRT 101 SLR camera (with mirror lock up feature) some Kodak 3200 ASA Film fresh from the refrigerator, a cable release, a sturdy Bogen Tripod, my watch. I timed and bracketed my exposures and jotted all these down in an old notebook. No laptops yet then!

    Viewing the magnificent and inspiring astro-photos of yours got me interested to re-kindle this hobby again but now, I’m using a Nikon D3100 DSLR camera and I just got myself a Celestron C70 spotting scope, T-Adapter and just waiting for the Camera Adapter to arrive in stock.

    I live in San Juan and I’m hoping to be able to catch a glimpse of a clear night sky and do some astro-photography once more on the roof deck of my condo unit.

    Thanks for sharing your tips on the DIY section.

    Facinating and intelligently written articles.

    Richard

  6. Built this battery about 4 years ago. Used it only a few times and it worked great. Tried to use it last night but it only worked for a few minutes. I thought it was the charge so i put it on the charger. It only charges up to 75%. Are my battries dying?

    • It is normal for batteries to experience reduced charging capacity as they age. This is true even for old batteries that have never been used. After 2 years, you’ll start to notice this degradation, which is most likely the case with your battery (or any other battery for that matter).

      • It should still power my laptop for a while, right? I went to use it the other night and it ran my laptop for only a few minutes. It had worked for hours in thr past. I was wordering if ot might be because of the charge or if there’s a problem with my converter?

  7. I’d like to try this. Have an Orion Eqg mounta and use laptop and dslr. I have a marine long cycle battery but it never makes it through the night. My concern is connecting and soldering.

    I’m not a diyer

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