Lithium Ion Battery Charging
These remarks apply equally to lithium ion and lithium polymer batteries. The chemistry is basically the same for the two types of batteries, so charging methods for lithium polymer batteries can be used for lithium-ion batteries.
Charging lithium iron phosphate 3.2 volt cells is identical, but the constant voltage phase is limited to 3.65 volts.
The lithium ion battery is easy to charge. Charging safely is a more difficult. The basic algorithm is to charge at constant current (0.2 C to 0.7 C depending on manufacturer) until the battery reaches 4.2 Vpc (volts per cell), and hold the voltage at 4.2 volts until the charge current has dropped to 10% of the initial charge rate. The termination condition is the drop in charge current to 10%. The top charging voltage and the termination current varies slightly with the manufacturer.
However, a charge timer should be included for safety.
The charge cannot be terminated on a voltage. The capacity reached at 4.2 Volts per cell is only 40 to 70% of full capacity unless charged very slowly. For this reason you need to continue to charge until the current drops, and to terminate on the low current.
It is important to note that trickle charging is not acceptable for lithium batteries. The Li-ion chemistry cannot accept an overcharge without causing damage to the cell, possibly plating out lithium metal and becoming hazardous.
Float charging, however, is a useful option. The safety issue with keeping the battery on constant charge is that if the charger should somehow go haywire and apply a higher voltage there could be problems. And, so the logic goes, the shorter the charger is turned on the less likely the charge will go haywire while connected to the battery. However, there is another safety method, the battery protection board, which should be included either on the battery or in other circuitry between the battery and the charger. The BPB (also known as PCB for "protection circuit board") or other battery management circuit will stop the charge if the voltage gets too high.
The question occasionally comes up "What is the effect of charging with less than 4.2 volts?" Unlike other battery chemistries the battery will charge, but it will never reach full charge, it will only be partly charged. The reason for this is that stuffing the ions into the anode or cathode crystals requires more voltage than the simple electrochemical cell voltage. The higher the voltage the more ions can be inserted. The page linked here contains our research and some quantitative data on the relative capacity of lithium-ion batteries that are charged below 4.2 volts. An advantage to charging at lower voltages is that the cycle life goes up dramatically.
Charging Lithium ion batteries at slow rates
When the charge rate during the constant current phase is low, the charger process will spend less time during the constant voltage tail. If you charge below about 0.18 C, the cell is virtually full when the 4.2 volts is reached. This can be used as an alternative charge algorithm. Just charge below 0.18C constant current and terminate the charge when the voltage reaches 4.2 volts per cell.
Every lithium ion battery pack should have a method of keeping the cell balanced and preventing them from being over-discharged. This is usually done with a safety board which monitors the charge and discharge of the pack, and prevents dangerous things from happening. The specifications of these safety boards are dictated by the cell manufacture, and may include the following:
Reverse polarity protection
Charge temperature--must not be charged when temperature is lower than 0° C or above 45° C.
Charge current must not be too high, typically below 0.7 C.
Discharge current protection to prevent damage due to short circuits.
Charge voltage--a permanent fuse opens if too much voltage is applied to the battery terminals
Overcharge protection--stops charge when voltage per cell rises above 4.30 volts.
Overdischarge protection--stops discharge when battery voltage falls below 2.3 volts per cell (varies with manufacturer).
A fuse opens if the battery is ever exposed to temperatures above 100° C.