These things are dangerous. You absolutely should get a high quality charger designed explicitly for lithiums and learn about the hazards. Lithium batteries can catch fire, explode, and cause fires, particularly if abused. Abuse can be either overcharging or discharging below a safe level. Most people should use only protected cells.
A solid rule is that no lithium cell should ever be discharged below 3.0 volts. 3.5 volts is a recommendable point to take a battery out of service and recharge it. Batteries should not be stored fully charged. It is recommended to charge a battery to 3.8 volts and store them in that state.
People say that at 3.3 volts, from 92 to 98 percent of the battery capacity has been used. It is also said that at 3.6 volts (at rest) is pretty much empty. Under load, a battery may drop as low as 2.5 volts when discharged.
Keep this table in mind:
4.2 volts 100% 4.1 about 90% 4.0 about 80% 3.9 about 60% 3.8 about 40% 3.7 about 20% 3.6 empty for practical purposes <3.5 = over-dischargedStoring batteries not in use in a refrigerator at about 40 percent charge level is probably a great idea. High temperatures are definitely not good.
I think of these two types of batteries as the same thing, except that lithium polymer batteries have a soft package. The is almost but not quite true. There is some advertising hype that lithium polymers offer higher energy density. This is true, but not enough to really care about. On top of that, the lithium polymers cost signficantly more. So unless you need an unusual thin battery shape, don't worry about it. Also certain lithium polymers can produce extremely large currents, which may be of interest to the RC model community, but not to me. My interest is flashlights and low power electronics.
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