A flat lead-acid battery starts permanent sulphation damage within 24-48 hours of going dead. Leave it flat for a week and you'll lose maybe 30% of capacity; a month and it's usually scrap. Charge it within 24 hours of going flat and most batteries recover fully.
Honest answer to a question I get every Monday morning after a weekend of "the car won't start".
Why did my battery go flat — I haven't left anything on? — Modern cars draw a small parasitic current 24/7 (alarm, ECU memory, keyless entry). A healthy battery handles it for a month or two of being parked. If yours goes flat in a week or two, either the battery is on its way out or there's an excessive drain — a glovebox light, dashcam, or a sticky relay.
Will jump-starting it damage anything? — On most cars no, provided you connect the leads correctly (red to dead positive, then donor positive; black to donor negative, then earth on the dead car — not the battery). On some hybrids and stop-start cars there are restrictions; check the handbook.
Can a fully sulphated battery ever be saved? — There are 'desulphator' chargers that pulse high voltage and sometimes recover a bit of capacity, but in 32 years I've rarely seen one bring a properly-dead battery back to genuinely useful condition. Cheaper and more reliable to just fit a new one.
Bob's Mechanical Repairs — independent family-run garage in Birnam, Dunkeld, Perthshire. Call 01350 727 276 or email [email protected].