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As any astrophotographer who has travelled with their telescope will tell you the one thing you can't be without is a source of power. Being away from home means mains electricity is not available to you and so some form of a field battery is required. Field batteries can be purchased from a number of suppliers at a price but they generally have a maximum 17ah capacity which if you have a lot of kit to power means that your imaging session could be rather short.
There are a lot of plans on the Internet for field power supplies and so I first did some reading and then researched some suppliers of batteries. As my needs are rather modest, I'm only powering a HEQ5 mount, I settled on using 2 AGM type batteries used for mobility scooters having 12V and 12ah capacity. I wired these in parallel to obtain 12v and 24ah which for the mount I am driving gives me 12 hours of imaging time. I can't see me needing longer than this at the moment and there is some spare capacity for running heaters when required.
Parts list
15" Stanley toolbox
2 No 12v 12ah AGM batteries
2 No Cigarette lighter sockets
Voltmeter
Switch
Inline fuse holder
Crimp connectors
Heat-shrink tubing
Wire for connections
Total cost of parts was around £50. Considering the Skywatcher/Celestron power tank for the 7ah version costs around £55 I think I have a bargain here.
I'm no electrician but I do dabble as the needs of my hobby require and understand not to get the wires crossed but you follow my designs at your own risk. They have worked for me but may not necessarily be correct electrically.
So here is the circuit diagram of my setup. Not too difficult the batteries are wired in parallel so as to maintain 12v but this increases the capacity to 24ah which is what I wanted.
The fuse is an automotive blade type fuse rated at 5a.
The switch came from a redundant project
The volt meter is placed after the switch so also serves as a power on indicator. It is a red LED type which was purchased via Amazon along with the inline fuse holder and the Lighter sockets.
The batteries are Powerline PL12-12 mobility batteries obtained from Tayna Batteries in Abergele.
Heat shrink tubing was fitted to all crimp connectors except those for the lighter sockets which came insulated.
Wiring came from a redundant lamp and was rated for about 3 amps if I add extra circuits to this I will need to upgrade it but for now it works
If you haven't used crimp connectors before then they come in three sizes and for this circuit I used the blue ones but should have used the red ones which are smaller, for a better fit.
The finished job.
The inside layout, batteries are wired in parallel. Heatshrink tubing covers what would otherwise be bare metal on the crimp connectors
The lid and wiring of the various connectors and switches. The meter is in the middle.
I haven't given a blow by blow account of how I built this as it is simply a matter of drilling or cutting holes for the bits and pieces. All I would say is check for fit first before cutting the holes for the various connectors and other items as it was a close fit with the batteries, and the power connectors which had to fit between the batteries and the side of the box.
I've not shown it in the photos but I have packed the batteries with lumps of polystyrene to stop them moving around too much.
Improvements? Yes there are some, the most important would be to find a more robust box as this one is lightweight and flexes. I have overcome this by fitting zip ties through the lip of the lid and the body of the box which stops excessive flexing. I am considering an ammeter to monitor current draw as this will give me a better guide as to how much time I have left. A short test run of 5 hours saw the voltage drop from 12.8v to 12.5v in the first hour and then stayed steady at 12.5v. Any additional circuits will probably mean an upgrade to heavier wiring as well. Other improvements could result in my fan controller and heater controller being mounted in the same box.
The first night of use passed off without a hitch and seeing the reading on the voltmeter I consider I have enough power for two or three nights before a recharge becomes necessary. Well it has been five nights of use before the recharge so all in all the field battery has exceeded expectations.
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