I am currently watching a thread where an owner of a water damaged MacBook Pro needs to recover the data from the soldered onboard SSD. The battery has gone flat and can't be charged, but the laptop was otherwise working OK on battery power. I would think that the cheapest option would be to recharge the battery out of the laptop, recover the data, and then junk the motherboard (I wouldn't trust it).
Are these batteries too "smart" to be externally charged? Does anyone have any suggestions other than borrowing a similar laptop to charge the battery? Is there a cheap and easy solution for an end user who is not electronically savvy?