Lithium-ion Battery Charging Basics

 

Today beside the lead acid batteries lithium ion batteries are also being regular and famous and the use of the devices are also increasing.
So to get the right backup from the battery and making the battery life longer, it is necessary to know about the charging procedure of the same and to implement the better charging.
The charging procedure for the lithium ion battery is in two phase i.e.
1. Constant current (CC)
2. Voltage source (CV)
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 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 also you need to take care of overcharging of the battery as this may damage the plates. 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.
The lithium ion cell offer more comfortable charging at lower temperature so the charging should be performed within 5°C to 45°C.

Safety
Every charger must have below mentioned safety points
· 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.