The Li-ion charger is a voltage-limiting
device similar to the lead acid battery charger. The difference
lies in a higher voltage per cell, tighter voltage tolerance
and the absence of trickle or float charge when full charge
While the lead acid battery offers some flexibility
in terms of voltage cut-off, manufacturers of Li-ion
cells are very strict on setting the correct voltage. When
the Li- ion was first introduced, the graphite system
demanded a charge voltage limit of 4.10V/cell. Although higher
voltages deliver increased energy densities, cell oxidation
severely limited the service life in the early graphite cells
that were charged above the 4.10V/cell threshold. This effect
has been solved with chemical additives. Most commercial Li-ion
cells can now be charged to 4.20V. The tolerance on all Li-ion
batteries is a tight +/-0.05V/cell.
Industrial and military Li-ion batteries
designed for maximum cycle life use an end-of-charge voltage
threshold of about 3.90V/cell. These batteries are rated lower
on the watt-hour-per-kilogram scale, but longevity takes precedence
over high energy density and small size.
The charge time of all Li-ion batteries,
when charged at a 1C initial current, is about 3 hours.
The battery remains cool during charge. Full charge is attained
after the voltage has reached the upper voltage threshold
and the current has dropped and leveled off at about 3 percent
of the nominal charge current.
Increasing the charge current on a Li-ion
charger does not shorten the charge time by much. Although
the voltage peak is reached quicker with higher current, the
topping charge will take longer. Figure 4-5 shows
the voltage and current signature of a charger as the Li-ion
cell passes through stage one and two.
Some chargers claim to fast-charge a Li-ion
battery in one hour or less. Such a charger eliminates
stage 2 and goes directly to ready once the
voltage threshold is reached at the end of stage 1. The charge
level at this point is about 70 percent. The topping
charge typically takes twice as long as the initial charge.
No trickle charge is applied because the Li-ion
is unable to absorb overcharge. Trickle charge could cause
plating of metallic lithium, a condition that renders the
cell unstable. Instead, a brief topping charge is applied
to compensate for the small amount of self-discharge the battery
and its protective circuit consume.
Depending on the charger and the self-discharge
of the battery, a topping charge may be implemented once every
500 hours or 20 days. Typically, the charge kicks
in when the open terminal voltage drops to 4.05V/cell and
turns off when it reaches 4.20V/cell again.
Charge stages of a Li-ion battery.
Increasing the charge current on
a Li-ion charger does not shorten the charge time by much.
Although the voltage peak is reached quicker with higher current,
the topping charge will take longer.
What if a battery is inadvertently overcharged?
Li-ion batteries are designed to operate safely within
their normal operating voltage but become increasingly unstable
if charged to higher voltages. On a charge voltage above 4.30V,
the cell causes lithium metal plating on the anode. In addition,
the cathode material becomes an oxidizing agent, loses stability
and releases oxygen. Overcharging causes the cell to heat up.
Much attention has been placed on the safety
of the Li-ion battery. Commercial Li-ion battery
packs contain a protection circuit that prevents the cell
voltage from going too high while charging. The typical safety
threshold is set to 4.30V/cell. In addition, temperature sensing disconnects
the charge if the internal temperature approaches 90°C (194°F).
Most cells feature a mechanical pressure switch that permanently
interrupts the current path if a safe pressure threshold is
exceeded. Internal voltage control circuits cut off the battery
at low and high voltage points.
Exceptions are made on some spinel (manganese)
packs containing one or two small cells. On overcharge, this
chemistry produces minimal lithium plating on the anode because
most metallic lithium has been removed from the cathode during
normal charging. The cathode material remains stable and does
not generate oxygen unless the cell gets extremely hot.
case of rupture, leaking electrolyte or any other cause of
exposure to the electrolyte, flush with water immediately.
If eye exposure occurs, flush with water for 15 minutes
and consult a physician immediately.