Mark made a really nice battery monitor, he writes –
This mini-project came about from building a garden shed. I wanted to use an old UPS battery and some spare halogen downlighters for illumination. As I researched lead-acid technology, I decided that I needed more than a simple switch if I wanted the batteries to survive.
One of the biggest problems is deep discharge – in other words allowing the terminal voltage to drop too low. A Yuasa data sheet (for a NP10-6 battery) specifies the expected number of cycles verses the depth-of-discharge – for 100% DOD, you get 250 cycles, for 30% DOD, you get 1200. Unfortunately, the data sheet doesn’t expand on the full meaning of DOD, but it essentially translates to terminal voltage.