Specifications

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Figure 2.12.2-2: Simple charging circuit. Permission granted by Colin Mitchell of
http://talkingelectronics.com/ .
This is sufficient for a slow overcharge rate. This may be practical for a user with
a consistent schedule. However, as a matter of practicality, a faster charge time
is needed. Faster charging can be accomplished by charging it at C/3.33. Due to
the obvious increased risk of overcharging, a timer is also necessary in this case.
However, it is a simple timer, with no additional logic. As such, the battery would
have to be discharged fully in order for the timer-based accelerated charging
circuit to work well. This is not practical as lithium ion batteries should not be
regularly completely discharged then recharged. In addition, they were chosen
primarily because they could withstand recharging from multiple states of
discharge. It would not be practical to choose a circuit that does not also take this
consideration into account.
2.12.3 Intelligent Charging Circuit
The last way of charging is at the fastest rate of charging, at 1*C. At this point, it
becomes mandatory to directly monitor the battery for an end-of-charge
condition. The method of achieving this is to monitor both voltage and current
conditions of the battery. For a lithium ion battery, the voltage must be monitored
until it reaches the 4.1Vdc – 4.2 Vdc level. At this point, current must be
monitored for a drop in current.
There is no additional “conditioning” or special charging treatment required for
lithium ion. As such, the charging behavior will not need to be adjusted
depending on how many cycles have been through the battery. However, there is
consideration with the voltage and current levels used to charge the battery.
Lithium ions require a very narrow range of voltage and current. Voltage should
be no lower than 4.1Vdc, and no higher than 4.2 Vdc. In addition, current should
be constant. Ramping up the current does not significantly shorten charge time; it
serves only to reduce the life of the battery.
The charging voltage can be set at 4.2 Vdc. However, setting it to 4.1Vdc does
reduce the capacity of the battery; at the same time, it extends the usable life of