title_body: How to restore LiPo-powered aircraft that have not been flown for years?
upvoted_answer: You may need new batteries.
  
  When a LiPo battery fully discharges, thin whiskers of lithium metal form between the plates of the battery.  Attempting to recharge the battery will cause these whiskers to overheat, which may cause the battery to catch fire or explode.
  
  Commercial batteries from reputable manufacturers have self-monitoring circuitry that will permanently disable the battery if the charge level drops too low.  If the batteries are of this sort, it's safe to test them by recharging -- if the battery has been damaged by over-discharge, it simply won't take a charge.
  
  Cheap third-party batteries and homemade battery packs don't have this protection.  A battery that's only been sitting for a year may still be good -- I'd try recharging it in the middle of a concrete driveway or some other well-ventilated, fireproof location.  With a battery that's been sitting for three years, I wouldn't risk it.