Heres a short, true story.
My next door neighbours son left to work as a doctor in Australia. He parked his 3 or 4 year old Toyota Auris (not a hybrid) in his parents garage and left for the promised land.
18 months later, my next door neighbour came round and asked for my help starting said car as her son was visiting on holiday and wanted to sell the car.
So armed with multimeter in hand, opened the bonnet and measured the battery at 0.01 volts. Zero point zero one volts ....mmm, pretty knackered I remarked, but not unexpected as no-one had thought to disconnect the battery. Probably will need a new battery I said, but I'll try recharging it but highly unlikely to recover it...
Removed battery to my garage, connected my charger (which is a variable lab power supply) set volts to 14.8 volts, current flowing showing as between 0 and 1 mA, hmmm not good. Left it to have a cup of tea, came back thinking of handing it back and the current had risen to 7 to 8 mA. So I thought I'll leave it connected to see what happens. Half an hour its risen almost 100mA, rising a few mA every minute. 3 hours later its at 900mA and rising. Left overnight, and its charging at max (2.2A limit on the power supply) and the voltage is now due to current limiting now at 14.1v or so, but slowly rising.
Neighbour asks me how its going, I said we'll pop it back on and see, but I doubt it'll turn the engine over. Put it back in the Toyota, turn the ignition on, it starts first turn! Wow, not even a struggle, Toyota engine immediately purring smoothly after being stood for 18 months, unbelievable!
Next task, try to move car with rear parking brake discs left on for 18 months and stuck on, lots of revs and eventually bang, and away we go. Checked lots of restarts to check it wasn't a freak start.
Son came over, drove it around for two weeks, then sold it to first buyer to view.
The battery was a Yuasa EFB battery, small about 40Ah IIRC. Made in UK. Impressed.