Please note: this text concerns Lithium-Ion batteries only.
 
HOW TO CHARGE
If a charger (power supply) comes with your product, use the original charger.
If you have a device with USB charging socket use chargers with capacity above the minimum listed in the product specifications. Charging using a USB charger with higher capacity is not a problem.
It is wise to charge only where you can observe the device, so you can stop charging if overheating should occur.
 
BATTERY LIFE
All batteries, over time, will slowly wear down. A good battery will keep more than 70% of its original capacity even after 500 charging cycles. Capacity will decrease gradually, until almost no capacity is left, and the battery will need replacing, if this is possible and economical. Life expectancy is optimal when the battery is being used between 10 and 50 degrees Celsius.
 
WHAT NOT TO DO
There is no memory effect in Li-ion batteries, so do not treat them like old Nickel-Cadmium types (which you had to run dry regularly).
This means that you should never run Li-ion batteries completely dry. Actually, this will damage the battery. Whenever you can, keep them between 20 and 80 per cent capacity, where they perform best. However, it is better to charge the battery to 100% capacity than to run it dry.
Never charge if temperature is at freezing point or lower (0 degrees Celsius, 32 F), since this will almost certainly damage the battery. 
Never place the device that is being charged on a sofa, rug or clothing when charging. This can cause heat to build up, which can in turn cause overheating. Always place it on a flat, hard surface, where heat can dissipate quickly.
 
 
GENERAL BATTERY INFORMATION
Most rechargeable devices use Lithium-Ion (Li-ion) polymer batteries these days.
This type has high energy density, very low memory effect and low self-discharge.
You can find them in electric cars, golf carts, power tools and other applications, besides portable headphones, telephones and loudspeakers.
Various subtypes exist that have different strengths and weaknesses, depending on the chemicals and metals used.
Batteries gradually self-discharge, even if they are not connected to anything. Usually, you can expect a self-discharge of about 2% per month when the battery is new, and this may increase somewhat with battery age and temperature.