It was mentioned not to let a Battery's Voltage get too Low.
Entry #5
According to 18650 Battery Studies...
It is best not to Charge a 3.7V Cell Battery, like an 18650, 18350, 14500, etc., all the way to 4.2 Volts, and especially not OVER 4.2V... Ever. A Charge to 4.22V won't make them explode, but Charging them to only about 4.18V will greatly extend the Battery Life Cycle - meaning the battery itslef will Last Longer. That's the Charge Voltage.
The Charge Rate will also effect the Life Cycle, but it will effect the Capacity, too - meaning How Long that charge will Last. For example (taken from a particular 2600 mAh Li-Ion 18650 Battery's Specs - See Attachment, page 4), charging an 18650 can be done Safely at several Charge Rates, like 500 mA up to 2C (2X the mAh Rating of the Battery), or 5200mA for this 2600 mAh Battery.
When charged at 520mA, Capacity = 100% (0.2 C charge rate)
When charged at 1300mA, Capacity = 95% (0.5 C charge rate)
When charged at 2600mA, Capacity = 90% (1.0 C charge rate)
When charged at 5200mA, Capacity = 80% (2.0 C charge rate)
What this means is that the Faster the Battery is Charged, the Less Time that charge will Last. So Charging your Batteries at 500mA is best for overall Battery Life, and it's best for the Charge Life.
According to the 18650 Battery Spec...
It is also important not to Trickle Charge, according to the 18650 Battery Spec. A Charger sould Terminate the Charge at Zero Volts. Most Chargers actullay do Trickle Charge, so you should monitor the Charger for its Full Charge indication, and remove the battery as soon as possible.
It's really quite difficult to find all of this in a Charger, so please try to find one with as much of this as possible. In general, the less you Pay, the fewer Features there are, and the farther it gets from a Proper Charge, much less one that will extend your Battery's Life Cycle.