MegaSack DRAW - This year's winner is user - rgwb
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I'm spending my evenings making a copy of one of [url= http://www.lovehulten.com/rkaidr.html ]these[/url] at the moment. I've now got all the electronics sorted and a nice prototype banged up in MDF.
My current version needs mains power to run. But when I start on my proper version I want to use a 12v battery system instead that is charged by an external DC power supply. It has to be 12V to drive the display.
Any battery experts got any recommendations as to what to use? I've been looking at sealed lead acid units designed for alarms at the moment but am struggling to find something nice and low profile so that my end product is not a huge box.
Model boats/aircraft?
The display requires 12V? What's the range of input voltage (ie how low and how high)? The trouble with batteries is that they decrease in voltage as they discharge, and unless you can get one which will power the display through the whole voltage range of the discharge then you'll need a regulator - at which point you need a higher voltage battery (well that or a lower voltage and convert up, but it will be more trouble to find a regulator).
Depending on what you do need, 3 cell LiPo batteries used for RC models are nominally 11.1V and go from 12.6V down to 9V during discharge.
The display will run at a lower voltage, problem is that it comes with no tech sheet or manual at all. I think it will run at lower than 9v, I need to give it a test.
If you haven't already, have a look at Absorbed Glass Mat (AGM) and Gel batteries. Their construction allows them to be placed in different orientations that might give you a lower profile, the difficulty is that the cases are commonly to standard dimensions
You'd definitely be better off with a LiPo battery it'll be lighter, less bulky and last longer than any Lead based tech. Additionally it won't be significantly damaged by being left flat.
Try and find something to repurpose from RC cars/planes, car jump starters, phone or laptop chargers. Charging will possibly be more of a issue. You'll either need an external smart battery charger (no reason you couldn't connect that to a DC jack/plug) OR possibly you can find a battery which has it's own charging system built in - more likely with car jump starters and phone chargers.
HTH.