Our trusty diesel Yaris is parked on our drive for the duration being as Her Ladyship is working from home.
I've rigged up a 13.5V charger that's connected to a weatherproofed external mains point via a 24-hour 'segmented' timeswitch. The segments are set for 'half an hour on' four times a day and this keeps the battery (and hence alarm system) in working order.
The charger is a heavy-duty 13.5V/27V switchable output unit that I built (as part of the "Restoration Project That Dare Not Speak Its Name"
) back in 1992 and was designed to float (or boost-charge if needed) a twin set of 12V commercial vehicle lead-acid batteries.
Guy