In practice, the TI algorithms use
coulomb counting when the system is
on and open-circuit voltage measurement
when the system is off or in
sleep mode to adjust remaining stateof-
charge (RSOC) as appropriate. This
provides extremely realistic predictions
of remaining battery life.
One key side benefit of impedancetracking's
real-time updating of actual
state of charge is that it allows the battery
gas gauge to reside on the system
board, rather than on the battery
pack. That means one gas-gauge chip
per system, rather than one per battery
pack, can be used to account for
end-user battery swaps.
According to TI, if the battery pack
remains in system, an impedancetracking
gas-gauge chip uses the created
cell profile as the basis for its fuel
gauging. When removing/re-inserting
or replacing the system's battery, the
chip arbitration algorithm compares
the measured characteristics of the
inserted battery pack with the default
profiles and the previously created cell
profiles and chooses the profile that
matches the characteristics of the battery
pack best.
Charging
Battery-charger chips
range from autonomous drop-in ICs to
highly programmable devices. With so
many contemporary chargers hiding
their algorithms on-chip, it's hard to
find resources for explaining what
actually goes on in battery charging.
Happily, a Maxim Integrated Products
applications note (www.maxim-ic.com/appnotes.cfm/an_pk/680) from
2000 explains basic battery charging
using the state diagram in Figure 4.
Here's an edited and condensed version
of what Maxim had to say:
The state machine starts even
before the battery is connected, with
the charger initializing itself and performing
a self-test, including checking
whether a battery is present at its output.
The point of the test is to catch
events in which the charging process
has been interrupted, perhaps by a
user unplugging the charger before
charging is finished.
Actual charging begins with cell qualification, a state in
which the charger detects the installation of the battery and
determines whether it can be charged. The charger may look
for a voltage on its charging terminals or look for the external
jumper or thermistor that's present in some battery packs. If
an authentication scheme is used, the charger will determine
whether the battery is an approved type. The next state, qualification,
determines whether the cell is functional. The charger
checks for opens, shorts, and (sometimes) temperature.
After qualification, the next state for some batteries, notably
nickel-cadmiums (NiCds), is preconditioning, essentially discharging
the battery output down to a level of 1 V over the
course of several hours. This is done to help prevent the NiCd's
notorious (and misnamed) "memory effect." It's not necessary
for more modern battery chemistries.
What happens in the fast-charge state depends on battery
chemistry. Broadly speaking, for fast-charging NiCd and nickelmetal-
hydride (NiMH) batteries, the charger applies a constant
current while monitoring battery voltage and other variables to
determine when to terminate the charge. The most common
fast-charge rate is C/2, so a totally discharged battery with a 2-Ah
rating would be fast-charged at 2 A for approximately two hours.
Of course, without preconditioning, a battery may be connected
to a charger at any state of charge. Therefore, the charger
must be able to avoid overcharging. When a constant charging
current is applied in NiCd and NiMH batteries, the cell voltage rises
slowly and eventually peaks. NiMH charging should stop when
dV/dt hits zero. NiCd charging should stop when dV/dt inflects
downward. Chargers that use faster charging rates than C/2
monitor temperature as well as voltage and terminate fast charging
based on the rate of increase in cell temperature.
Li-ion battery chargers need more precise control of charging
voltage than nickel chemistry, and their maximum charging
rate is set by current limiting. They also add a top-off charging
state after reaching the nominal float-voltage.
Different battery chemistries possess different self-discharge
rates, which affect the trickle-charge state. Li-ion batteries
self-discharge very slowly, and chargers designed for
them rarely include trickle-charging. The Maxim note observes
that "NiCds, however, can usually accept a C/16 trickle charge
indefinitely. For NiMH cells, a safe continuous current is usually
around C/50, but trickle charging for NiMH cells is not universally
recommended."
As for those autonomous chargers (a market in which Maxim
is a fierce competitor), designers may find their task considerably
simplified from what it was when the app note was written.
To play fair, I'll pick an example from Linear Technology's line
card, the LTC4075 Li-ion charger, introduced last spring.
Figures 5a and 5b show a block diagram of the charger and
the charging curves it produces. With this charger, programming
is achieved via three package pins. The pins labeled IUSB
and IDC use resistors to program the currents for USB and wall
adapter voltage sources. A resistor on ITERM programs termination
current threshold. The other pins provide status indications
or accept input power.
This is not to say that all chargers must have this degree of
autonomy. For a contrast, take a look at Summit Microelectronics'
SMB137 (see "Where Are We?" p. 50).