Chris, as with most things in Engineering, mechanical and electrical, you could spend a lifetime studying batteries.
In brief, there are two major parameters of interest to motorists where car batteries are concerned, CCA (Cold ranking Amperage) and Capacity. The former (
http://batteryuniversity.com/learn/article/how_to_measure_cca_cold_cranking_amp) determines how well suited the battery is for starting the car and the higher the CCA figure, the better it is at turning over the engine, especially diesel.
The latter, Capacity (
http://batteryuniversity.com/learn/article/how_to_measure_capacity), measured in Ampere-hours, indicates how long a battery will supply current for into a given load. The unit is a product of load (amps) and time (hours).
My battery is rated at 560A CCA and 60A-h Capacity. The CCA figure is more than adequate to start my 2.6 petrol and the Capacity figure would be fine if it were true! However, my own tests (discharging into a 12 amp load) shwed that it lowered its terminal voltage to 7 volts after just over 1 hour, giving a true figure for an aging battery of approx, 15 A-h.
In this case, given that the Omega has a quiescent current drain of 85mA (0.085A), it would be discharged in 15/0.085 hours, i.e. 176 hours or 7.35 days. I consider this to be inadequate and will be looking to replace it.....
Ron