My brother was getting Mum’s unused car ready for disposal but the battery was dead. I carry a lithium-ion jump-starter but hadn’t topped it up for a few months, so it was down to 91% charge. Hooked it up to the sick battery, pressed the boost button to equalise the surface charge of the two batteries and it started OK, initially turning over a few times before roaring into life. The booster battery’s capacity dropped to 90% after this job.