Battery charging of an eBook reader is a basic requirement for portable use. These days all batteries used in portable readers and tablets are Li-Ion batteries.
A Li-Ion battery has a nominal voltage of 3.6 to 3.7 Volts and it generally charged at 4.2 Volts. The battery circuit needs to limit the charging current once the charge voltage is reached. The cell must not be overcharged and has protection circuits to prevent overcharging. Battery cell life can be extended by not fully charging the battery, but, of course, it will run down quicker if it is not full charged. Some believe that the charger should be removed after charging to improve battery cell life. There are also considerations for storage of a battery powered device for long periods.
Many eBook Readers come with only a USB cable that can use a computer to perform both connection for eBook download and charging. This is limited to 500mA. Most people will use a Wall Charger.
 Wall Chargers
The standard charger for nearly all portable devices is a 5V USB charger. The charging circuit inside the device drops the output and regulates it to 4.2 Volts. Chargers are available in several amperages from 1/2 A and up and will usually support any input voltage from 100V to 250V AC, 50 or 60 Hz.
It is important to use a charger that provides adequate amperage. While most devices can use 500 mA they will often charge faster if more current is available. Chargers are usually rated by wattage. This can be calculated by multiplying the 5V by the amperage. Thus a 500 mA charger would produce 2.5W while a 10Watt charger will supply 2.0 Amps. A Web Tablet or small computer 2-in-1 will generally need a 10Watt charger. If the charger gets hot when used you need a larger amperage. Having a charger with more amperage available will not harm a device. It will use only what it needs. There are also wall chargers that can support multiple devices simultaneously. Do not exceed the total current available.
As charging requirements increase the need for current exceeds the capability of current connectors so the USB Type-C connector is now available for this task.
 Car charging
Many new cars now have USB connections available that can be used for charging. There is also at least one 12V accessory outlet. 5V chargers are available that will plug into this accessory outlet. These will have one or possibly two USB charging connections.
 Portable charging
Portable chargers include auxiliary batteries and/or solar cells. In each case the portable charger begins with a variable input voltage and produces an output voltage of 5V to recharge the attached device. This means that the charging device includes electronics to boost the output to 5V or reduce the output to 5V.
The most annoying feature of the auxiliary battery device is that it has to be charged separately and may have lost some of its charge before it is needed. However it is still handy insurance to get a few more hours out of the device.
The solar cell approach may or may not have internal batteries itself. Internal batteries mean that it can be used even if the sun is not currently present assuming it was recharged earlier. Otherwise the device must be in the sun for the recharge period.
 Limiting chargers
- Nitecore charger UM20 is charger that will shut itself off when the battery is fully charged. This charger uses 5V input and converts it to 4.2V output. It can charge at 1A or 500mA rate using a 5V source of at least 1.5A (better with 2A to 3A). It will work directly on the portable device using a USB cable. It has an LCD display to show status.
- Apple device chargers are limited in that the current supply tapers off as the battery approaches full charge. This is somewhat true of all chargers but Apple is more aggressive in ensuring this behavior.
 For more information
- charging lithium ion batteries - from Battery University.
- PRST1 Compatible wall chargers
- http://www.usb.org/kcompliance/view/CertifiedUSBPowerBricks.pdf Approved Certified powerBricks by the USB organization (This is new: late 2016 or 2017 devices)
- USB#USB Power