Method and device for enhancing smart battery performance

The subject invention pertains to a method and device for implementing a dynamic End of Discharge Voltage (EODV) for rechargeable batteries and battery packs (batteries/packs). This dynamic EODV can be adjusted based on, for example, environment, operating conditions, temperature, residual capacity, cell chemistry, cell size, form factor, discharge rate, and/or the number of charge/discharge cycles the battery has undergone. The subject invention can provide for enhanced run time from each discharge cycle of the battery/pack. In addition, the subject invention can provide for more accurate save-to-disk alarms, while ensuring adequate energy for the actual save to disk operation. Furthermore, the subject invention can also extend the useful life of the battery/pack.

BACKGROUND OF THE INVENTION 
This invention relates to rechargeable batteries and battery packs and, in 
a particular embodiment, to smart batteries and battery packs 
(batteries/packs). The subject invention pertains to improving the 
accuracy of battery/pack capacity, remaining capacity, and remaining run 
time determinations. Advantageously, this invention can utilize a dynamic 
end of charge voltage to enhance the performance of rechargeable 
batteries/packs throughout the battery/pack life and over a wide variety 
of operating conditions and environments. This invention can enable 
battery/pack users to get more useable capacity out of each charge of the 
battery/pack and to more accurately track the battery/pack capacity. 
Rechargeable batteries/packs are often used in applications, for example 
laptop computers, where battery/pack supplied energy is necessary to 
maintain certain information related to the user's application. In these 
situations, when the battery no longer can supply the ongoing energy 
requirement, some or all of this information can be lost. Accordingly, 
device designers and manufactures have incorporated techniques to alert a 
user and/or signal a host device as to the remaining capacity in the 
battery/pack, for example to initiate save-to-disk routines for laptop 
computers to avoid loss of information. By alerting a user when the 
battery/pack is approaching a point where the battery/pack has only enough 
capacity remaining to perform the save routine, a user can perform the 
save routine in time to avoid loss of valuable information. It is the goal 
to initiate these save routines at a point in the discharge cycle where 
enough capacity remains to perform the save routine with adequate margins 
for error, while minimizing excess unused capacity in the battery/pack. 
Additionally, other events may be triggered when the battery/pack reaches 
certain output voltage levels, wherein the remaining charge is presumed to 
be correlated to these certain output voltage levels. For example, fuel 
gauge operations, shut down, and other remaining capacity communications 
with a host device or user can be triggered based on the output voltage 
level of the battery/pack. 
Typically, a fixed End of Discharge Voltage (EODV) is used in battery/pack 
operating and charging algorithms, where the EODV is a voltage output of 
the battery/pack which, when reached, indicates that essentially all of 
the usable capacity of the battery/pack, over and above the capacity 
needed for any necessary terminal function, has been removed. Accordingly, 
for example in the context of a smart battery/pack, once this fixed EODV 
is reached an indication would be given to the host device or user to 
shutdown and recharge the battery/pack. Additionally, this fixed EODV 
often functions as the endpoint for determining the capacity of a 
battery/pack such that the capacity is defined to be essentially zero, or 
fixed finite amount, at the EODV. Accordingly, the accuracy of this EODV 
is very important to the performance of the battery/pack. 
Due to internal resistance (IR) drops, for example due to the cell and 
interconnects, a fixed EODV is often inaccurate for these purposes. These 
IR drops from contact resistance, cell can resistance, wire resistance, 
trace resistance, electrode resistance, electrode/electrolyte interface 
resistance, electrolyte resistance, and protection circuitry can lead to 
an incorrect determination of the actual potential of the electrode of the 
battery/pack. In particular, at high discharge rates these IR drops can 
alter a battery/pack's discharge profile as a function of discharge rate 
and, due to the use of a fixed EODV, may lead to the determination that 
the battery/pack has delivered all the available capacity even when more 
capacity may still be available from the battery/pack. Furthermore, these 
IR drops can reduce the accuracy, or repeatability, of the fuel gauge at 
various discharge rates, for example when a fixed EODV is used to define 
zero, or near zero, capacity. 
In addition, a fixed EODV can lead to inaccurate capacity determination as 
the number of times the battery/pack has been charged and discharged 
increases. For rechargeable batteries/packs, the voltage profile as it 
relates to capacity changes as a function of cycle life. Specifically, for 
certain batteries/packs, as the number of charging cycles increases the 
fixed EODV is reached at a point where more capacity is actually available 
than when the same fixed EODV is reached when the battery/pack had 
undergone fewer charging cycles. 
Furthermore, other factors such as temperature, cell chemistry, form 
factor, residual capacity, and operating conditions can affect the 
accuracy of the capacity determinations. Accordingly, there is a need in 
the art for a method and apparatus to more accurately gauge remaining 
capacity and more accurately determine the end of discharge voltage for 
rechargeable batteries/packs. 
BRIEF SUMMARY OF THE INVENTION 
The subject invention pertains to a method and device for improving the 
accuracy of battery/pack capacity, remaining capacity, and remaining run 
time determinations. A specific embodiment of the subject invention 
relates to implementing a dynamic End of Discharge Voltage (EODV) for 
rechargeable batteries and battery packs (batteries/packs). This dynamic 
EODV can be adjusted based on, for example, environment, operating 
conditions, temperature, residual capacity, cell chemistry, cell size, 
form factor, discharge rate, and/or the number of charge/discharge cycles 
the battery has undergone. The subject invention can provide for enhanced 
run time from each discharge cycle of the battery/pack. In addition, the 
subject invention can provide for more accurate timing of save-to-disk 
alarms, while ensuring adequate energy for the actual save to disk 
operation. Furthermore, the subject invention can also extend the useful 
life of the battery/pack. 
In a specific embodiment of the subject invention, a dynamic, as opposed to 
a fixed, EODV which depends on the number of charge/discharge cycles is 
utilized. A number of methods for dynamically changing the EODV can be 
utilized in accordance with the subject invention. In another specific 
embodiment of the subject invention, a dynamic EODV which depends on the 
discharge rate is utilized. Further embodiments can utilize a dynamic EODV 
which depends on temperature or some combination of cycle count, discharge 
rate, and/or temperature.

DETAILED DISCLOSURE OF THE INVENTION 
The subject invention pertains to a method and device for improving the 
accuracy of battery/pack capacity, remaining capacity, and remaining run 
time determinations. A specific embodiment of the subject invention 
relates to implementing a dynamic End of Discharge Voltage (EODV) for 
rechargeable batteries and battery packs (batteries/packs). This dynamic 
EODV can be adjusted based on, for example, environment, operating 
conditions, temperature, residual capacity, cell chemistry, cell size, 
form factor, discharge rate, and/or the number of charge/discharge cycles 
the battery has undergone. The subject invention can provide for enhanced 
run time from each discharge cycle of the battery/pace In addition, the 
subject invention can provide for more accurate timing of save-to-disk 
alarms, while ensuring adequate energy for the actual save to disk 
operation. Furthermore, the subject invention can also extend the useful 
life of the battery/pack. 
In a specific embodiment of the subject invention, a dynamic, as opposed to 
a fixed, EODV which depends on the number of charge/discharge cycles is 
utilized. A number of methods for dynamically changing the EODV can be 
utilized in accordance with the subject invention. Referring to FIG. 1, 
typical discharge profiles for a single Li ion cell is shown for 2, 200, 
300, and 400 charge/discharge cycles. It is shown that as the battery/pack 
undergoes additional charge/discharge cycles, the discharge profile 
changes. Although FIG. 1 illustrates typical discharge profiles for a 
Li-ion cell, the discharge profiles for cells of other cell chemistries 
can also change form as a function of the number of charge/discharge 
cycles. For example, the subject invention is applicable to Li-ion, NiCd, 
NiMH, Li-polymer, Li-ion family, as well as other present and future 
rechargeable cell chemistries. 
Also shown in FIG. 1, is the impact of using a fixed voltage, 3.0 volts in 
FIG. 1, for triggering a suspend to disk operation. As this Li-ion cell 
preferably should not be discharged below the minimum critical operating 
voltage of 2.7 volts, the remaining available capacity is the difference 
in capacity, as read on the horizontal axis, from where the profile 
reaches 3.0 volts until the profile reaches 2.7 volts. In this example, 
after 2 cycles, this difference corresponds to approximately 40 mAh of 
capacity. After 400 cycles, this difference corresponds to approximately 
180 mAh of capacity. If approximately40 mAh are needed to perform the 
save-to-disk operation and the save-to- disk operation is triggered at 3.0 
volts, there is essentially no unused capacity for the battery/pack after 
just 2 cycles but approximately 140 mAh of unused capacity for the battery 
pack after 400 cycles. 
Alternatively, if the save-to-disk operation is triggered at 2.8 volts for 
a cell which has undergone 400 cycles, there is only approximately 40 mAh 
of remaining capacity left. Accordingly, for a battery/pack having 
undergone 400 cycle/discharge cycles, less unused capacity is left when 
the save-to-disk operation is triggered at a voltage of approximately 2.8 
volts rather than 3.0 volts. Thus, triggering save to disk at 2.8 volts, 
rather than 3.0 volts, allows a user a longer run time. 
In a specific embodiment of the subject invention, the Li-ion battery/pack 
of FIG. 1 is utilized with the EODV being determined by a straight line 
approximation, based on the number charge/discharge cycles. In this 
embodiment the EODV is determined for a Li-ion battery/pack, for which the 
discharge profile is shown in FIG. 1, by a straight-line approximation. 
EQU EODV=EODV.sub.0 -(constant) (number of cycles) (1) 
##EQU1## 
In this embodiment, equation (2) is used to determine EODV until 400 
cycles is reached. After 400 cycles, EODV is maintained at 2.8 volts to 
avoid reaching the minimum critical operating voltage of 2.7 volts for 
this particular cell chemistry. This can also prevent the EODV from 
reaching a point where insufficient capacity is available at the EODV for 
example, for the save-to-disk operation. 
The slope and initial EODV, EODV.sub.0, with respect to the straight line 
approximation can be determined experimentally, and/or theoretically, for 
each battery/pack, and can vary for different cell chemistries, 
temperatures, discharge rates, residual capacity, and physical 
constructions. For example, measurements can be conducted for various 
combinations of factors and the resulting discharge profiles incorporated 
into the EODV determinations. 
Alternative embodiments can employ, for example, second-order, third-order, 
or logarithmic approximations. In a preferred embodiment, a best-fit curve 
can be utilized with respect to the point on the discharge profile which 
leaves enough remaining capacity to perform the desired shutdown or 
save-to-disk operation, for each cycle count. One skilled in the art will 
appreciate from FIG. 1 that a selected number of discharge curves for any 
number of cycles can be generated for a particular application. A point on 
each discharge curve is then determined such that the remaining available 
capacity (for example, the difference between the capacity at the EODV and 
the capacity at the minimum critical operating voltage) of the battery at 
each point corresponds to a preselected capacity, such as the capacity 
necessary to perform a save-to-disk operation. These data points in turn 
correspond to a function that can be used to adjust the EODV based on the 
number of cycles. The function can be approximated using curve fitting 
methods know in the art such as polynomial equations or any other 
equations that best fit the data. Further embodiments, can employ a step 
wise EODV function or other functions which enhance the battery/pack 
performance and meet other system constraints, for example limited memory. 
Monitoring circuitry, for example within a smart battery pack, can be 
utilized to monitor the terminal voltage of the battery/pack and to 
trigger, for example, a save-to-disk routine when the EODV is reached. For 
example, a status bit can be set when the terminal voltage is less than or 
equal to the EODV to allow the circuitry to initiate the save-to-disk 
routine. Monitoring circuitry can also be used to monitor the number of 
charge/discharge cycles for input into equation (1). 
In another specific embodiment of the subject invention, a dynamic EODV 
which depends on the discharge rate is utilized. This dynamic EODV can 
provide a more accurate indication of the remaining capacity in the 
battery/pack, as compared with using a fixed EODV. With respect to the 
battery/pack of FIG. 2A, the EODV is lower for higher drain rates. In this 
embodiment, the EODV is a function of the instantaneous drain rate, due to 
the internal resistance components involved. If a fixed EODV is selected 
based on a C-rate discharge and the battery/pack is operated at 3 C, more 
capacity will be left in the battery/pack at EODV because the battery/pack 
voltage is depressed at higher discharge rates, Conversely, if the fixed 
EODV is selected such as to leave a desired capacity in the battery when 
discharged at a high rate, say 3 C, and the battery/pack is discharged at 
C-rate, less capacity will be left in the battery/pack and the terminating 
functions may not get completed. Using a dynamic EODV based on the actual 
discharge current gives a more accurate estimate of the residual capacity. 
The usual practice is to be conservative and select a fixed EODV 
corresponding to a low discharge rate, for example 1 C or below, resulting 
in unused capacity in the battery/pack when discharging to the EODV at a 
higher discharge rate. FIG. 2B shows the relationship between discharge 
profiles for a typical NiMH cell for different discharge rates, namely 0.2 
C, 1 C, 2 C, and 3 C in FIG. 2B. The shift in the discharge profiles for 
different discharge rates can, at least in part, be attributed to the 
resistance components such as cell contacts and interconnects. Again, one 
skilled in the art will appreciate from FIG. 2B that a selected number of 
discharge curves for any number of discharge rates can be generated for a 
particular application. A point on each discharge curve can then be 
determined such that the remaining available capacity (for example, the 
difference between the capacity at the EODV and the capacity at the 
minimum critical operating voltage) of the battery at each point 
corresponds to a preselected capacity, such as that capacity necessary to 
perform a save-to-disk operation. These data points in turn correspond to 
a function that can be used to adjust the EODV based on the discharge 
rate. The function can be approximated using curve-fitting methods known 
in the art such as polynomial equations or any other equations that best 
fit the data. 
If a fuel gauge measurement is to be performed by draining the battery/pack 
to the EODV and then charging the battery/pack, this fuel gauge 
measurement can be more accurate if the appropriate EODV for the drain 
rate is used. For example if a battery/pack is drained to a fixed EODV and 
then charged, assuming the remaining capacity to be zero at the EODV, the 
integration of the charging current over time can be used as a new learned 
capacity for the battery/pack. Note that when discharging the battery/pack 
to the fixed EODV to start the learned capacity measurement, the amount of 
residual capacity in the battery/pack at the EODV is related to the 
discharge rate. This will obviously affect the amount of additional charge 
the battery/pack can hold, thus impacting the learned capacity. Next, as 
the battery/pack is discharged by a user at a different rate than the 
discharge rate for the learned capacity measurement, the fixed EODV will 
be reached at a point on the discharge profile with a different remaining 
capacity than the remaining capacity at the beginning of the learned 
capacity measurement. 
Accordingly, the shift in the discharge profile with respect different 
discharge rates and the corresponding difference in remaining capacity in 
the battery/pack at a fixed EODV for different discharge rates can lead to 
inaccuracies in remaining capacity predictions, unused remaining capacity 
upon reaching the fixed EODV, and/or reaching a fixed EODV without 
sufficient remaining capacity to perform any terminal functions which may 
be necessary. 
The dynamic EODV of the subject invention which varies as a function of 
discharge rate can enhance the accuracy of fuel gauge operations and 
consistently allow a user to utilized more of the battery/pack's capacity 
during each discharge cycle. To illustrate, assume a battery is discharged 
at a 1 C rate and then charged, wherein the amount of charge is measured 
to provide a new learned capacity of the battery/pack. If the battery/pack 
has an accompanying fuel gauge, the fuel gauge will read full. As the 
battery/pack is discharged, the fuel gauge will indicate less than full, 
for example by integrating the discharge current over time to determine 
what percent of the now learned capacity is left. Note that the fuel gauge 
could instead indicate, for example, time remaining, capacity remaining, 
or any other desired format. 
If the discharge rate is above 1 C and a fixed EODV is used, the fixed EODV 
would be reached before the fuel gauge indicates zero remaining capacity 
and, depending on the protocol, the user could be instructed, or forced, 
to shut down and charge the battery. This would leave unused remaining 
capacity in the battery/pack, set an incorrect zero remaining capacity for 
the next learned capacity, and give the user an incorrect indication of 
remaining capacity while the battery/pack was in use. Alternatively, if a 
dynamic EODV in accordance with the subject invention is utilized, the 
EODV can be lowered for the higher discharge rate allowing the use of more 
of the remaining capacity, making the fuel gauge indication more accurate, 
and set a more accurate zero remaining capacity for the next learned 
capacity. 
If the battery/pack is discharged at a lower discharge rate than 1 C, for 
example 0.2 C, and a fixed EODV is utilized, then the fuel gauge will 
indicate the battery/pack is discharged before reaching the fixed EODV. 
Depending on the protocol used, the user may be instructed, or forced, to 
shutdown. The fuel gauge is showing no remaining capacity because the new 
learned capacity was zeroed when there was actual remaining capacity, thus 
causing the fuel gauge to underestimate the actual capacity of the 
battery/pack. Again, if the user shuts down when the fuel gauge indicates 
no remaining capacity, unused remaining capacity will be left in the 
battery. Alternatively, if a dynamic EODV in accordance with the subject 
invention is utilized, the EODV at the originally 1 C rate discharge which 
began the new learned capacity would have been lower and the new learned 
capacity would have been higher. Accordingly, the fuel gauge would have 
provided a more accurate indication of remaining capacity, allowing the 
user to use more of the battery/packs, remaining capacity. 
In a preferred embodiment, as shown in FIG. 2A, the determination of the 
EODV as a function of discharge rate can be made by use of a straight line 
approximation. FIG. 2A is for illustration purposes and shows how the EODV 
can depend on discharge rate. The slope and intercept for the straight 
line approximation can be derived by experiment, and/or by theory, and can 
vary for different cell chemistries, temperatures, residual capacity, 
cycle counts, and physical configuration. Alternative embodiments can 
employ, for example, second-order, third-order, or logarithmic 
approximations. In a further specific embodiment, a best-fit curve can be 
utilized. 
Additional embodiments of the subject invention can adjust the EODV as a 
function of, for example, one or more of the following factors: 
charge/discharge cycle count, discharge rate, temperature, residual 
capacity, cell chemistry, form factor, and physical configuration. 
Measurements of discharge profiles can be taken for various combinations 
of factors and corresponding discharge profiles generated. These profiles 
can be incorporated into the algorithm for adjusting the EODV of the 
battery/pack. As mentioned, these algorithms can be implemented with 
equations that incorporate the desired factors. In addition, look up 
tables can be utilized. In a specific embodiment, a multi-dimensional look 
up table can be utilized with each dimension correlating to one of the 
factors considered. 
It should be understood that the examples and embodiments described herein 
are for illustrative purposes only and that various modifications or 
changes in light thereof will be suggested to persons skilled in the art 
and are to be included within the spirit and purview of this application 
and the scope of the appended claims.