Secondhand batteries are worth considering, but unless virtualy free, should be carefully tested.
The cheaper types of lead acid battery only have a reliable life of about 3 years, they may last up to 6 years, but with declining reliability.
Vehicle SLI (Starting, Lighting, Ignition) batteries are not suitable.
Sources of secondhand batteries include companies who service large UPS systems or fork lift trucks.
The batteries used in such applications are often replaced as a precaution when some useful life remains.
How big a battery are you considering ?
And at what voltage ?
Bigger is better , within reason.
12 volts is far preferable for small systems, but higher voltages are better for larger systems.
Remember that all batteries in a battery bank should be same type and age.
Large numbers of small batteries tend to be less reliable than smaller numbers of larger batteries.
Batteries are a relatively safe technology, but by no means risk free, and I would recomend a study of this thread
http://www.powerswitch.org.uk/forum/vie ... php?t=5455"Installers and owners of emergency diesels must assume that they will have to run for a week or more"
