Originally Posted by Nick Hearne
I have no problem leaving batteries for over 4 months & still being in tip top condition!
Not saying you don't Nick, but most of the evidence points to the contrary:
Self discharge of batteries
Function of temperature
Self-discharge is the electrical capacity that is lost when the cell simply sits on the shelf. Self-discharge is caused by electrochemical processes within the cell and is equivalent to the application of a small external load.
Lead-acid and nickel-cadmium batteries lose their charge very quickly. For example, a lead-acid battery stored at 30oC would lose half its initial charge in about 3 or 4 months while, for nickel-cadmium, this would only take about 6 weeks. In normal use, this might present no real problem, as these types of battery can be recharged, but such batteries are clearly unsuitable for "fitting and forgetting".
Note that I snipped stuff about other technologies, as they don't really fit the conversation.
One other site I glanced at put the self-discharge rate of lead-acid batteries at between 1 and 60% per month (nice that they're so generous.) I filed this site under "useless".
Apparently, self-discharge rate is pretty quirky, and is affected by manufacturing tolerances, number of cycles the plates have seen, ambient temperature, etc. But, it does occur.
I've got a second truck which sits in the driveway for months at a time. It usually requires a charger session before it will crank. Once started and run for an hour, it's good for a couple more months.
Hope that your system is more robust than mine.