Leaving batteries on low charge for any length of time is a death sentence. Once the cells have fallen below a certain level it is almost impossible to recharge them and requires specialised equipment to even make the attempt. Manufacturers recommend to store batteries for long periods with not more than 80% charge. This is why whenever you buy electrical equipment with 'lipo' batteries, you have to charge them first before you use them. This ensures that if the equipment has been stored for a long time before being sold that a possible low state of remaining charge doesn't take it below the threshold of the charge limit of the cell.