Lithium-ion Battery Charging Basics |
 Lithium-ion charger catalog page |
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Lithium Ion Charging Basics 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 lithuim 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. 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 page here some quantitative data on the relative capacity of lithium-ion batteries that are charged below 4.2 volts. 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. Safety Every lithium ion battery pack should have (must have?) 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:
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