The geek squad was smart in doing a boost charge to the battery. What most people are unaware of is that most lithium ion batteries in phones and other devices actually have a protective circuit enclosed in the battery. This helps prevent damage to the battery due to overcharging, discharging, extreme temperatures, and other factors. If a battery discharges below a certain voltage it can trigger the protective circuit. When it is in this state the battery will read at 0volts with a multimeter. You can wake the battery by doing a boost charge. It's kind of like jumping a car battery. You are supplying the battery with a higher voltage for a short period of time. By doing this it wakens the battery from this sleep state and then you can use the original charger to finish charging the battery. A lot of people throw away perfectly good lithium ion batteries because they think they are dead when in reality they actually are not and just need boosted to get them to charge and function again. This is especially true for batteries that have sat for awhile unused. To help prolong the life of your batteries try to connect your phone to a charger when it's between 15-20% and let it charge to at least 90% if possible. I know this is not always possible but it will help extend the batteries lifetime by preventing from overcharging and discharging to often, which can affect the health of a battery. Extreme hot and cold temperatures can also affect the life of your battery.