How to retain power-off phone batteries?

Sometimes, you may get an important call when your phone battery is using up and it's unavailable to recharge. How to retain limited power of the battery? Actually, it is very simple. Try to make the battery warm so that it release the remaining power for working. First, you can withdraw the battery from the mobile phone, and then rub them back and forth with hands. If it's an iPhone, you can shut it down, warm it up under your arms, and then turn it on again. It may regain the power in a short time. However, you have to finish the call as soon as possible for such regained power is very limited.