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".
What sulphation actually is
A healthy battery stores energy by shuffling lead sulphate between the plates and the acid. When you discharge it fully, soft sulphate crystals form on the plates. If you recharge it within a day or two, they break back down. Leave them sitting, and they harden into a crust that blocks the plates from holding charge — permanently. That's sulphation, and there's no proper fix once it's set in.
The realistic timeline
Hours 0-24: jump-start and a long drive will usually fully recover the battery. Days 2-7: capacity drops 10-30%; the battery will start the car but won't hold charge as long as it used to. Weeks 2-4: hard sulphation sets in; you'll typically lose half the cold-cranking amps. Month-plus: usually a write-off, especially in cold weather.
How to give it the best chance
Don't jump-start and drive 5 minutes home — the alternator can't fully recharge a flat battery on a short trip; you need at least 30 minutes of motorway-speed running. Better still, put it on a proper smart charger (£40 jobbie from Halfords or similar) for 12-24 hours. That gentle slow charge breaks down soft sulphate that a quick alternator charge won't touch.
When to just replace it
If it's gone flat twice in 6 months, or if it cranks slowly even after a full charge, the battery is done. Modern cars are very fussy about voltage — a weak battery causes phantom warning lights, electric window faults, even ABS errors. A new battery fitted is £100–£200 for most cars and worth it for the peace of mind alone.
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.