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#Post#: 39--------------------------------------------------
Charging Lithium Ion Batteries
DIR By: FLF
Date: October 22, 2012, 11:46 pm
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Charging lithium-ion batteries
There is only one way to charge lithium-based batteries. The
so-called 'miracle chargers', which claim to restore and prolong
batteries, do not exist for lithium chemistries. Neither does
super-fast charging apply. Manufacturers of lithium-ion cells
have very strict guidelines in charge procedures and the pack
should be charged as per the manufacturers "typical" charge
technique.
Lithium-ion is a very clean system and does not need priming as
nickel-based batteries do. The 1st charge is no different to the
5th or the 50th charge. Stickers instructing to charge the
battery for 8 hours or more for the first time may be a leftover
from the nickel battery days.
Most cells are charged to 4.20 volts with a tolerance of
+/-0.05V/cell. Charging only to 4.10V reduced the capacity by
10% but provides a longer service life. Newer cell are capable
of delivering a good cycle count with a charge to 4.20 volts per
cell. Figure 1 shows the voltage and current signature as the
lithium-ion cell passes through the charge stages.
Figure 1: Charge stages of a lithium-ion battery. Increasing the
charge current on a lithium-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.
HTML http://imageshack.us/a/img803/5682/lionbattschart.png
The charge time of most chargers is about 3 hours. Smaller
batteries used for cell phones can be charged at 1C; the larger
18650 cell used for laptops should be charged at 0.8C or less.
The charge efficiency is 99.9% and the battery remains cool
during charge. Full charge is attained after the voltage
threshold has been reached and the current has dropped to 3% of
the rated current or has leveled off.
Increasing the charge current does not shorten the charge time
by much. Although the voltage peak is reached quicker with
higher charge current, the topping charge will take longer.
Some chargers claim to fast-charge a lithium-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%. The
topping charge typically takes twice as long as the initial
charge.
No trickle charge is applied because lithium-ion is unable to
absorb overcharge. A continuous trickle charge above 4.05V/cell
would causes plating of metallic lithium that could lead to
instabilities and compromise safety. Instead, a brief topping
charge is provided to compensate for the small self-discharge
the battery and its protective circuit consume. Depending on the
battery, a topping charge may be repeated once every 20 days.
Typically, the charge kicks in when the open terminal voltage
drops to 4.05V/cell and turns off at a high 4.20V/cell.
What happens if a battery is inadvertently overcharged?
lithium-ion is designed to operate safely within their normal
operating voltage but become unstable if charged to higher
voltages. When charging above 4.30V, the cell causes plating of
metallic lithium on the anode; the cathode material becomes an
oxidizing agent, loses stability and releases oxygen.
Overcharging causes the cell to heat up. If left unattended, the
cell could vent with flame. Do not leave charging batteries
unattended. Check the temperature of the charging batteries. If
excessively hot, remove from charger.
Much attention is focused to avoid over-charging and
over-discharging. Commercial lithium ion packs contain a
protection circuits that limit the charge voltage to 4.30V/cell,
0.10 volts higher than the voltage threshold of the charger.
Temperature sensing disconnects the charge if the cell
temperature approaches 90°C (194°F), and a mechanical pressure
switch on many cells permanently interrupt the current path if a
safe pressure threshold is exceeded. Exceptions are made on some
spinel (manganese) packs containing one or two small cells.
Extreme low voltage must also be prevented. The safety circuit
is designed to cut off the current path if the battery is
inadvertently discharged below 2.50V/cell. At this voltage, most
circuits render the battery unserviceable and a recharge on a
regular charger is not possible.
There are several safeguards to prevent excessive discharge. The
equipment protects the battery by cutting off when the cell
reaches 2.7 to 3.0V/cell. Battery manufacturers ship the
batteries with a 40% charge to allow some self-discharge during
storage. Advanced batteries contain a wake-up feature in which
the protection circuit only starts to draw current after the
battery has been activated with a brief charge. This allows
prolonged storage.
In spite of these preventive measures, over-discharge does
occur. Advanced battery analyzers (Cadex C7000 series) feature a
'boost' function that provides a gentle charge current to
activate the safety circuit and re-energize the cells if
discharged too deeply. A full charge and analysis follows.
If the cells have dwelled at 1.5V/cell and lower for a few days,
however, a recharge should be avoided. Copper shunts may have
formed inside the cells, leading a partial or total electrical
short. The cell becomes unstable. Charging such a battery would
cause excessive heat and safety could not be assured.
Battery experts agree that charging lithium-ion batteries is
simpler and more straightforward than the nickel-based cousins.
Besides meeting the tight voltage tolerances, the charge circuit
can be designed with fewer variables to consider. Full-charge
detection by applying voltage limits and observing the current
saturations on full charge is simpler than analyzing many
complex signatures, which nickel-metal-hydride produces. Charge
currents are less critical and can vary. A low current still
permits proper full charge detection. The battery simply takes
longer to charge. The absence of topping and trickle charge also
help in simplifying the charger. Best of all, there is no memory
but aging issues are the drawback.
The charge process of a lithium-ion-polymer is similar to
lithium-ion. These batteries use a gelled electrolyte to improve
conductivity. In most cases, lithium-ion and lithium-ion-polymer
share the same charger.
Preparing new lithium-ion for use
Unlike nickel and lead-based batteries, a new lithium-ion pack
does not need cycling through charging and discharging. Priming
will make little difference because the maximum capacity of
lithium-ion is available right from the beginning. Neither does
a full discharge improve the capacity of a faded pack. However,
a full discharge/charge will reset the digital circuit of a
'smart' battery to improve the state-of-charge estimation
State-of-charge reading based on terminal voltage
The open circuit voltage can be used to estimate the battery
state-of-charge of lithium, alkaline and lead-based batteries.
Unfortunately, this method cannot be used for nickel-based
packs.
On a lithium-ion cell, 3.8V/cell indicates a state-of-charge of
about 50%. It must be noted that utilizing voltage as a fuel
gauge function is inaccurate because cells made by different
manufacturers produce a slightly different voltage profile. This
is due to the electrochemistry of the electrodes and
electrolyte. Temperature also affects the voltage. The higher
the temperature, the lower the voltage will be.
Hints to long battery life
• Limit the time at which the battery stays at 4.20/cell.
Prolonged high voltage promotes corrosion, especially at
elevated temperatures. (Spinel is less sensitive to high voltage
than cobalt-based systems).
• 3.92V/cell is the best upper voltage threshold for
cobalt-based lithium-ion. Charging batteries to this voltage
level has been shown to double cycle life. Lithium-ion systems
for defense applications make use of the lower voltage
threshold. The negative is reduced capacity.
• The charge current of Li-ion should be moderate (0.5C for
cobalt-based lithium-ion).
The lower charge current reduces the time in which the cell
resides at 4.20V. It should be noted that a 0.5C charge only
adds marginally to the charge time over 1C because the topping
charge will be shorter. A high current charge tends to push the
voltage up and forces it into the voltage limit prematurely.
Note: In respect to fast-charging and topping charge, the charge
behavior of lithium-ion is similar to lead acid. Here, the
voltage threshold of 2.35V/cell during regular charge needs to
be lowered to 2.27V/cell when the VRLA is on standby. Keeping
the voltage at the high threshold would contribute to corrosion.
A similar effect occurs with lithium-ion.
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