Micrel, Inc.

MIC79110

September 23, 2013

12

Revision 2.1

Lithium-ion Batteries

Lithium-ion batteries are charged in two stages to reach

full capacity. The first stage charges the battery with

maximum charge current until 90% of the battery cell’s

voltage limit is reached. The second stage tops off the

charge with constant voltage charge as the charge

current slowly decreases. End of charge is reached when

the current is less than 3% of the rated current. A third

stage will occasionally top off with charge with constant

voltage charge if the battery voltage drops below a

certain threshold.

Figure 2. Typical Two-Stage Lithium Charge Profile

All lithium-ion batteries take approximately 3 hours to

charge with the second stage taking twice as long as the

first stage. Some chargers claim to be fast chargers by

skipping the second stage and just charges the battery

until the cell voltage is reached. This only charges the

battery to 70% capacity.

An increase in the charge current during stage 1 does not

shorten the total charge time. It will only shorten the time

for stage 1 to complete and lengthen the time in stage 2.

The lithium-ion loses charge due to aging whether it is

used or not. Do not store the batteries at full charge and

high heat because it will accelerate the aging process.

Try and store with 40% charge and in a cool

environment.

Lithium-ion Safety Precautions

Every lithium-ion battery pack should have a safety circuit

which monitors the charge and discharge of the pack and

prevents dangerous occurrences.

The specifications of these safety circuits are dictated by

the cell manufacturer and may 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.

•

Protection circuitry for over voltage applied to the

battery terminals.

•

Overcharge protection circuitry to stop charge when

the voltage per cell rises above 4.3V.

•

Over discharge protection circuitry to stop discharge

when the battery voltage falls below 2.3V (varies with

manufacturer).

•

Thermal shutdown protection for the battery if the

ambient temperature is above 100°C.

Auto Top-Off Charger Application Circuit

Figure 3. Auto Top-Off Charger − Application Circuit

Lithium-ion batteries will begin to lose their charge over

time. The MIC79110 can be configured to automatically

recharge the battery when the voltage drops below the

minimum battery voltage. This minimum voltage is set by

a resistor divider at the battery and is connected to the

voltage falls to 3.72V, SD pin gets divided down by R1

and R2 to 0.93V and starts the normal charging process.

While charging the DEOC indictor is turned on, pulling

the SD pin to GND, keeping the MIC79110 on. When the

end of charge is reached, the DEOC pin opens. The

divided down BAT voltage is now 1.05V at the SD pin,

ending the charging process.