12 volt Batteries
Standard car starter batteries are an ideal way of getting 12v power.
You can pick up a second hand car battery for £10 at a scrap yard - if it looks well kept it'll probably be in good condition. You can usually see a 'manufactured' date. Anything less than 3 years old will be good and less than 5 will probably be OK.
If you have one, take along a voltmeter. If you get a reading of 12.8 volts you're onto a winner (don't let 'em see you smiling though or the price will double). Anything above 12.6 is worth a go. Don't touch anything that reads less than 12.0 under any circumstances.
You can trickle charge your battery with a plug-in adapter, but it'll need to be more like 14v to push enough charge into your battery, and less than an amp to be sure you're not going to fry it.
Ideally you'll find a cheap charger at a boot sale or on e-bay. If you get the chance to take your voltmeter to it before buying, you'll want to see about 14.4v. There always needs to be a 'gradient' of voltage - the charger needs to be higher than the charged.
A battery connected to a charger, when full will have a voltage of about 14.4v. As soon as its taken off charge the voltage will fall, over a 6-hour period, to 12.8v. A good battery will then stay at 12.8v for a long time, losing maybe 0.05v a week.
A cheap solar panel (less than 5 watts) can be fixed directly to your battery without the need for a charge-controller. If it starts to bubble then its full up and you can disconnect the panel for a while.
When batteries are being charged they give off hydrogen as a by-product of the process. Keep your battery in a ventilated area, not a closed room. Hydrogen explodes and its no good for breathing either.
Whatever you find, even a half-capacity battery, that'll hold 30 amp hours of charge will power, for example, a 'Summer Lights' kit for 600 hours non-stop (nearly a month).
Make sure you check the electrolyte level in each of the battery's cells and, if necessary, fill them up to the bar inside the cell (which should be just covered) with distilled water. Don't skimp on the water, impurities in tap water will not help your battery last. And don't be fooled by a battery that calls itself 'no' or 'low' maintenance - if you can unscrew the 6 cell covers then they should be checked at least every 6 months and filled up as necessary.
A last note on safety; car and leisure batteries are capable of delivering all their stored power pretty-much instantaneously. If you
a fat battery such as these with a wire, the wire will glow red, then white, smell badly of burnt plastic and burn through - a bit like a giant fuse wire. You'll have done some lasting damage to your battery (let's hope it was a cheap scrap one) and you may have burnt yourself and anything else that got in the way.
And our disclaimer - none of the above advice comes with any sort of guarantee or promise of effectiveness. However, it all comes from real experience and is meant as a guide for those wanting to learn.