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Texas Instruments Incorporated
Power Management
Designing a multichemistry battery charger
By Keith Keller
Analog Field Applications/Power Management
The challenge
Designing a battery charger for nickel, lithium-ion (Li-ion)
and lead-acid cells requires special considerations for
proper charging and safety. A comprehensive charging
algorithm must be created that is dependent on the chem-
istry that needs to be supported. The easiest way to accom-
plish this type of design is to use an integrated, flexible
multichemistrybatterychargerinconjunctionwitha
microprocessor. The microprocessor is used to identify
thebatterychemistryandadjustforpropercharging
conditions, including termination criteria. It also monitors
operating conditions for safety.
Charging methods for different battery
chemistries
There are fundamental differences in how cells of different
chemistries are charged. Nickel-metal-hydride (NiMH) cells
require constant current throughout the charging cycle.
Li-ion cells require constant current followed by a constant
voltage based on their maximum-rated individual voltage.
Lead-acid cells also require a constant current followed by
a constant-voltage stage, but a final float-charge stage
must be added. Each of these will be discussed in turn.
NiMH cells
The accepted method of determining the full charge of a
NiMH cell while it is being charged with a constant current
is to monitor for either a voltage droop of approximately
8 to 16 mV or a rapid temperature increase of 10°C. In
either case, a charging rate of greater than 0.5C (preferably
1C) is necessary for these conditions to occur. (A “C rate”
is defined by the battery’s capacity. If a cell is rated at
1500 mAh, then a 1C charging rate will be 1.5 A). In stand-
alone chargers where the voltage-droop method is used,
care must be taken in the layout to minimize noise on the
battery sense line, which could cause a false full-charge
indication. To implement constant-current charging with a
multichemistry charger, the feedback resistors should be
set for a charge-regulation voltage higher than the cells
can reach.
Li-ion cells
The maximum-rated individual voltage for lithium-
manganese-oxide (LiMn 2 O 4 ) and lithium-cobalt-oxide
(LiCoO 2 ) cells is typically 4.2 V, and for the newer lithium-
iron-phosphate(LiFePO 4 ) cells, 3.7 V. When the maximum-
rated voltage is reached, it is held steady and the current
is allowed to decrease until the appropriate “taper” point
is reached, indicating a fully charged cell. The taper point
in stand-alone chargers is typically 1/10 the fast-charge
rate. However, with a flexible microcontroller-based archi-
tecture, the designer can choose to end charging at any
point in the charge cycle. If the design is charging a smart
battery with “bypass” cell-balancing circuitry, longer charge
times are required because the cell balancing happens only
during the end of charge when the voltages are relatively
constant and current is tapering.
Lead-acid cells
Like Li-ion cells, lead-acid cells require constant-current
charging followed by a constant-voltage stage, but they
also need a final float-charge stage. The speed at which
lead-acid cells charge is much slower than that of nickel or
Li-ion cells. Charging times can range from 12 to 36 hours,
depending on the battery capacity. Based on some impor-
tant trade-offs, the appropriate charging voltage of the
individual cells is between 2.3 V and 2.45 V. Charging the
cells to a lower voltage maximizes their service life but can
lead to sulfation of the negative plate. Charging them to a
higher voltage will shorten charging times but can cause
the cells to overheat at higher temperatures. For the final
float-charge stage, the voltage should be reduced to
approximately 2.25 V per cell. There are a few exceptions
to these specifications depending on the supplier, so cell
datasheets and specifications for proper charging and
safety conditions should be carefully studied.
The solution
Fortunately, all aspects of the charging algorithm for any
battery chemistry can be implemented with a flexible
multichemistry battery charger and a companion micro-
processor. For portable applications where a Li-ion or
nickel pack could be used interchangeably, other design
aspects to consider are proper chemistry identification
(ID), detection of pack insertion/removal, charging tem-
perature range, charge thresholds, system monitoring and
fault reporting, and loss of input power.
If there is a need to determine pack chemistry, an easy
solution is to use an ID resistor on an additional pin that
the microcontroller can read via its analog-to-digital con-
verter’s input. This setup not only monitors for the ID
resistor but can also determine when the battery pack is
inserted into the charging cradle. Different-size batteries,
such as a double-capacity pack, can also be distinguished.
Cells are typically charged within a temperature window
of 0 to 50°C, but this range can be extended if the cells are
charged at a slower rate and a lower voltage. In extremely
high ambient temperatures, active discharge of Li-ion cells
to anywhere from 3.7 to 3.9 V per cell can also be consid-
ered. In all cases, high temperatures shorten battery life.
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4Q 2009
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Power Management
Texas Instruments Incorporated
After determining the chemistry ID and an acceptable
temperature range, the charger must determine if the cells
are undervoltage (indicating a deeply depleted state) and
therefore need to be charged slowly, typically at 1/10 the
fast-charge rate. For nickel cells, the undervoltage safety
threshold is considered to be 0.9 V or less per cell; for
standard lithium cells, less than 3.0 V per cell; and for the
newerLiFePO 4 cells, as low as 1.5 V. During trickle charge,
if the cell’s voltage does not increase above these safety
thresholds within 30 minutes, the cell is deemed to be
damaged, and charging stops.
After all these safety checks are complete, the batteries
are deemed good, and the fast-charge mode begins.
Throughout the charging process, the temperature, charge
current, and battery voltage must be continuously moni-
tored. Three arrays of values should be stored for each of
these measurements. Each data point can be taken approx-
imately every 100 ms, and an average of the values can be
used for calculations. A watchdog safety timer is a good
idea to protect against any unforeseen coding errors that
could cause the microprocessor to enter an unknown state.
Specific fault conditions can be stored as unique bits in a
fault register that the system can read through interrogat-
ing the microprocessor or that can be indicated to the
user through LEDs or a display. Finally, the charging algo-
rithm should be designed so that if there is a loss of input
power or the battery pack is removed and inserted, the
whole identification and charging process will start again.
Texas Instruments (TI) offers several multichemistry
battery ICs to meet different requirements for charger
design. For example, the bq24703 multichemistry charger,
along with an MSP430F2012 ultralow-power microcon-
troller, is used in the PMP3914 evaluation module to detect
and charge either a NiMH or a Li-ion battery pack. Also
included in the design is a 75-W off-line converter, with an
input voltage of 108 to 132 V and an output voltage of 25 V,
thatusesTI’sUCC28600green-modequasiresonantflyback
PWM controller.
The bq24703 is a nonsynchronous charger with a high-
side pFET control, which makes it ideal for charging a
high-voltage (21-V) 5s2p Li-ion battery pack or a 25-V
15s1p NiMH battery pack. The letter “s” indicates how
many cells are connected in a series string to achieve the
desired pack voltage. The letter “p” indicates how many
strings of cells are connected in parallel to achieve the
desired pack capacity. With a nonsynchronous charger, the
charge current is limited to around 3 A. Several other
multichemistry chargers from TI, such as the bq24704 or
bq24750A, are synchronous buck converters that can sup-
port 10 A or more of continuous-charge current.
Conclusion
Creating a multichemistry battery charger requires knowl-
edge of individual cell characteristics and overall safety
considerations. This article discussed differences between
charging Li-ion, nickel, and lead-acid batteries and outlined
how a charging algorithm can be implemented for a multi-
chemistry charger and a microcontroller. Safety consider-
ations were also discussed, including system monitoring
for under- and overvoltage conditions, overcharging, and
extreme temperatures.
Related Web sites
PMP3914 Evaluation Module User's Guide:
www.ti.com/sc/device/ partnumber
Replace partnumber with bq24703, bq24704 , bq24750A ,
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SLYT357
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