My mid-2009 15″ MacBook Pro (A1286) has developed a nasty habit in the last couple of days. It suddenly discharges its entire battery without warning, and won't start until it's had about 10 minutes charging afterwards. This happens even when plugged into the mains and on 100% charge. So far it's only happened after a long day's use (plugged in all day), but it's happened three times now. The cycle count is 768, according to Apple support the maximum cycle count on my model is 1000, so it shouldn't be end days yet. It's also displaying the condition as "normal".
I've ordered another battery which should be arriving tomorrow, but I'd really appreciate knowing why this is happening, and how I can avoid it happening again. I've lost some important work due to these shutdowns, and I'm worried about where the charge is going - if the battery fried the logic board I'd be up the creek.