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Lithium-ion Battery Charging Basics

Lithium-ion Battery Charging Basics

Lithium Ion Charging
These remarks apply equally to lithium ion and lithium polymerbatteries. The chemistry is basically the same for the two types ofbatteries, so charging methods for lithium polymer batteries can beused for lithium-ion batteries.Charging lithuim iron phosphate 3.2volt cells is identical, but the constant voltage phase is limitedto 3.65 volts.The lithium ion battery is easy to charge. Chargingsafely is a more difficult. The basic algorithm is to charge atconstant current (0.2 C to 0.7 C depending on manufacturer) untilthe battery reaches 4.2 Vpc (volts per cell), and hold the voltageat 4.2 volts until the charge current has dropped to 10% of theinitial charge rate. The termination condition is the drop incharge current to 10%. The top charging voltage and the terminationcurrent varies slightly with the manufacturer.However, a chargetimer should be included for safety.The charge cannot be terminatedon a voltage. The capacity reached at 4.2 Volts per cell is only 40to 70% of full capacity unless charged very slowly. For this reasonyou need to continue to charge until the current drops, and toterminate on the low current.It is important to note that tricklecharging is not acceptable for lithium batteries. The Li-ionchemistry cannot accept an overcharge without causing damage to thecell, possibly plating out lithium metal and becoming hazardous.Thequestion occasionally comes up "What is the effect of charging withless than 4.2 volts?" Unlike other battery chemistries the batterywill only be partly charged. The reason for this is that stuffingthe ions into the anode or cathode crystals requires more voltagethan the simple electrochemical cell voltage. The higher thevoltage the more ions can be inserted. The page linked page heresome quantitative data on the relative capacity of lithium-ionbatteries that are charged below 4.2 volts.Charging Lithium ionbatteries at slow ratesWhen the charge rate during the constantcurrent phase is low, the charger process will spend less timeduring 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 canbe used as an alternative charge algorithm. Just charge below 0.18Cconstant current and terminate the charge when the voltage reaches4.2 volts per cell.SafetyEvery lithium ion battery pack should have(must have?) a safety board which monitors the charge and dischargeof the pack, and prevents dangerous things from happening. Thespecifications of these safety boards are dictated by the cellmanufacture, and may include the following:
Reverse polarity protection
Charge temperature--must not be charged when temperature is lowerthan 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 shortcircuits.
Charge voltage--a permanent fuse opens if too much voltage isapplied to the battery terminals
Overcharge protection--stops charge when voltage per cell risesabove 4.30 volts.
Overdischarge protection--stops discharge when battery voltagefalls below 2.3 volts per cell (varies with manufacturer).
A fuse opens if the battery is ever exposed to temperatures above100° C.

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