Lithium Ion Battery Charging
Basics
These remarks apply equally to lithiumion and lithium polymer batteries. The chemistry is basically the same for thetwo types of batteries, so charging methods for lithium polymer batteries canbe used for lithium-ion batteries.Charging lithium iron phosphate 3.2 voltcells is identical, but the constant voltage phase is limited to 3.65volts.
The lithium ion battery is easy to charge. Charging safely is amore difficult. The basic algorithm is to charge at constant current (0.2 C to0.7 C depending on manufacturer) until the battery reaches 4.2 Vpc (volts percell), and hold the voltage at 4.2 volts until the charge current has droppedto 10% of the initial charge rate. The termination condition is the drop incharge current to 10%. The top charging voltage and the termination currentvaries slightly with the manufacturer.
However, a charge timer should beincluded for safety.
The charge cannot be terminated on a voltage. Thecapacity reached at 4.2 Volts per cell is only 40 to 70% of full capacityunless charged very slowly. For this reason you need to continue to chargeuntil the current drops, and to terminate on the low current.
It isimportant to note that trickle charging is not acceptable for lithiumbatteries. "Trickle charging" means that a certain current is forced into thebattery even when full. Trickle charging is used in lead acid, NiCad, and NiMHbatteries to keep them topped off. The Li-ion chemistry cannot accept even atrickle overcharge without causing damage to the cell, possibly plating outlithium metal and becoming hazardous.
Float charging, however, is auseful option. "Float charging" means that a specific voltage is applied to thebattery, the battery is free to accept current or not, depending on its stateof charge. The safety issue with keeping the battery on constant charge is thatif the charger should somehow go haywire and apply a higher voltage there couldbe problems. And, so the logic goes, the shorter the charger is turned on theless likely the charge will go haywire while connected to the battery. However,there is another safety method, the battery protection board, which should beincluded either on the battery or in other circuitry between the battery andthe charger. The BPB (also known as PCB for "protection circuit board") orother battery management circuit will stop the charge if the voltage gets toohigh.
Charging Lithium ion batteries at slow rates
Whenthe charge rate during the constant current phase is low, the charger processwill spend less time during the constant voltage tail. If you charge belowabout 0.18 C, the cell is virtually full when the 4.2 volts is reached. Thiscan be used as an alternative charge algorithm. Just charge below 0.18Cconstant current and terminate the charge when the voltage reaches 4.2 voltsper cell.
Safety
Every lithium ion battery pack should have a methodof keeping the cell balanced and preventing them from being over-discharged.This is usually done with a safety board which monitors the charge anddischarge of the pack, and prevents dangerous things from happening. Thespecifications of these safety boards are dictated by the cell manufacture, andmay 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.
- Over-discharge 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.
