Method and apparatus for detecting failed batteries

A technique for detecting failed batteries while the battery is attached to one or more electronic devices and is receiving a float charge is disclosed. The float voltage minimizes the normal voltage differences between battery cells. The technique employs a ratio comparative analysis of cell voltages of a battery provided across the terminals of the battery. Application of the ratio comparative analysis in assessing the condition of a battery assumes an equal voltage drop across each battery cell such that the cells are modeled as a series of resisters with respect to the float voltage. Such equal voltage drop enables a comparative ratio analysis of the voltage across each of the two portions of the battery's cell stack to the voltage across the entire battery. The comparative ratio analysis determines a voltage threshold that identifies whether a battery has a shorted or open cell.

TECHNICAL FIELD OF THE INVENTION 
This invention relates generally to a method and apparatus for detecting 
failed batteries, and more specifically, to a method and apparatus for 
determining whether a battery has shorted or open cells while the battery 
remains connected to other electrical devices and receives a float 
voltage. 
BACKGROUND OF THE INVENTION 
Detecting failed batteries has become particularly important in a number of 
key areas. One area, for instance, is to determine whether a battery has 
failed before a computer system attempts to rely upon it as a backup power 
supply following a failure of its primary power source. 
There are several conventional methods used to measure the condition of 
rechargeable batteries, including: electrolyte specific gravity, 
gas-gauging, cell impedance measurements, and open circuit cell voltage. 
Some methods specifically apply to a given technology. For example, the 
electrolyte specific gravity methods apply to the specific gravity of the 
electrolyte applicable to wet cell batteries, where the electrolyte is 
accessible for testing. Gas-gauging methods apply only to applications 
where the battery is periodically discharged then recharged (e.g., a 
cyclic application). Other conventional methods, such as cell impedance 
methods, tend to require a significant amount of precision circuitry (and 
expense). 
The open circuit method measures the condition of rechargeable batteries 
provided that the circuit voltage of an electrochemical system is at 
equilibrium. For a system to reach equilibrium, the battery, for 
twenty-four to one hundred and twenty hours, must neither receive a charge 
or current nor supply current to load. Accordingly, it is not practical to 
use the open circuit voltage method to measure the condition of batteries 
that power computer backup systems, since such batteries need to be 
continuously available. Therefore, it is unacceptable to use this method 
to test typical sealed lead-acid batteries used for computer backups and 
Uninterruptible Power Supply (UPS) applications. 
Since batteries tend to lose charge while not in use, many battery backup 
power applications continuously provide just enough energy that slightly 
reverse biases the battery to overcome the battery's internal 
self-discharge losses and to maintain the battery's charge. Such reverse 
biasing is generally provided by placing a "float voltage" across the 
terminals of the battery. 
Typical of conventional methods and apparatus that perform battery testing 
is the need to disconnect the battery from all connected electrical 
devices, especially lo the charging device before testing the battery. 
Further, because battery voltage tends to vary significantly when 
off-line, measuring off-line battery voltage generally does not 
conclusively establish whether the battery still has an acceptable charge. 
More particularly, Hawker Energy, a major manufacturer of typical sealed 
lead-acid batteries used in computer backup and UPS applications, 
indicates that the open is circuit voltage is an indicator of state of 
charge to an accuracy of .+-.20% for an off-line battery which has not 
been charged nor discharged for 24 hours. Hence, conventional testing 
methods that disconnect a battery for testing are unable to accurately 
determine battery voltage. 
Still further, conventional testing system's need to disconnect a battery 
from all electrical devices is particularly troublesome if the battery 
serves as a backup power source, whether used for a telecommunications or 
computer system, a UPS, or an emergency lighting system that depends upon 
the battery functioning when needed to supply power. For instance, if a 
battery backup is not connected to a computer when the computer's primary 
power source fails, since there is neither a primary nor secondary backup 
power supply, such a computer system is likely to fail and cause fatal 
system errors. Such errors may corrupt data and software. 
Since a computer may require a battery backup at a moment's notice, the 
battery should always be operational. Therefore, frequent battery tests 
are recommended to ensure that a battery is adequately charged when called 
upon to perform a power backup. However, because conventional battery 
testing systems generally need to have the battery disconnected from all 
electrical devices before testing the battery, frequent battery tests 
increase the likelihood that the battery backup will be unavailable when a 
primary power source fails. Without a charged battery backup, fatal system 
errors leading to lost and corrupted software and data could occur. 
Conventional battery testing systems do not remedy the problems associated 
with testing a battery's charge while the battery is connected to a 
charging device. A reason for such failings is due to conventional 
systems' inability to distinguish between the voltage placed across the 
terminals of a battery by a battery charger and the battery's chemically 
induced voltage. 
The purpose for placing a float voltage across the terminals of a battery 
is to prevent the natural degradation of the battery's charge, thereby 
prolonging the power and, thus, "life" of the battery. In order to charge 
a battery, the float voltage placed across the battery terminals must be 
greater than the voltage naturally generated by the battery's cells. 
Although helpful with prolonging the life of a battery, the float voltage 
tends to impede a conventional battery testing systems' ability to measure 
a battery's charge. Instead of measuring a battery's charge, conventional 
methods measure the greater of the two voltages, which is generally the 
float voltage, and are thus unable to determine when a battery fails. 
Consequently, conventional battery testing methods only determine whether 
the device providing the float voltage is working, but do not identify 
when a battery receiving the float voltage fails or is failing. 
Also, typical of conventional battery testing systems is an inability to 
distinguish between the natural variability of voltage of each battery 
cell and a shorted-out battery cell. In a battery consisting of "n" cells 
in series, for example, a 48 volt battery consisting of 24 lead-acid 
cells, each with a potential of 2.0 volts, any one cell provides only 1/n 
of the total voltage. The battery terminal voltage under the range of 
charge and discharge conditions will typically vary from 40 to 56 volts in 
the above example (.+-.16.7% of the nominal 48 V), while the voltage of 
one cell represents only slightly more than 4% of the battery voltage. 
Hence, the effect of one bad cell is difficult to distinguish from the 
normal variance of battery terminal voltage in the conditions of use. 
Because conventional battery testing methods are unable to distinguish 
between a battery having cells with a voltage charge less than nominal and 
a battery having a shorted-out cell, the testing method assumes the worst 
case, i.e., shorted cells, and therefore classifies the battery as 
"failed." Consequently, such a battery is prematurely discarded and 
replaced regardless of whether the battery is operational and meets 
acceptable voltage and current requirements pursuant to specifications. 
Further, cell imbalances during charging and discharging are normal 
occurrences. Individual battery cell manufacturing variances will cause a 
series string of battery cells to have slightly different voltages at any 
given time in the charge or discharge cycle. In order to overcome 
differences between cell voltages, the float-charge maintains a small 
overcharge current in order to ensure that the weakest cells in the string 
receive sufficient charge. Typical conventional battery testing methods do 
not consider the minimizing effects to cell imbalances caused by a float 
voltage placed across the terminals of a battery. 
As a result, there has been a need for a battery testing method and 
apparatus that measures battery voltage while on-line to determine whether 
the battery has failed due to either shorted or open cells, avoids the 
need to take the battery off-line, avoids the errors caused by measuring 
float voltage, distinguishes between shorted cells, open cells and less 
than nominal average cell voltages, does not require periodic 
discharge/recharge cycles and is inexpensive and does not include 
complicated test circuitry. 
SUMMARY OF THE INVENTION 
The present invention substantially improves on the prior art's method and 
apparatus for testing batteries to determine whether a battery has failed 
due to a shorted or open cell. The method and apparatus enables 
measurement of battery voltage while the battery remains connected to one 
or more electrical devices and receives a float voltage. For example, the 
present invention may test a battery while the battery backup power supply 
is connected to a computer and is receiving a float voltage. The float 
voltage provides slow rate battery recharging and in doing so minimizes 
voltage imbalances between cells. Further, there are normally small 
voltage differences between cells in a battery. These differences tend to 
be magnified during discharge and while capacity is returned to the cells 
during high rate recharging. Applying the float charge over a time period 
minimizes the normal voltage difference between cells, so that this method 
as applied distinguishes between normal cell voltage differences and cells 
that are open and shorted. 
In particular, the method and apparatus of the present invention detects 
failed batteries by employing a ratio comparative analysis of voltage 
drops across the cells of a battery irrespective of the float voltage 
concurrently provided across the terminals of the battery. Application of 
ratio comparative analysis in assessing the condition of a battery assumes 
an equal voltage drop across each good battery cell. In this analysis, the 
cells of a battery are modeled as a series of resisters with respect to 
the float voltage placed across the terminals of the battery. Hence, it is 
possible to assume that each properly operating cell provides an 
equivalent voltage drop across each cell. Such equal voltage drops enable 
a comparative ratio analysis of the difference between the voltage drops 
across two half stacks of the battery's cells to the voltage across the 
entire battery. 
The comparative ratio analysis determines the voltage threshold that 
categorizes a battery as either good or bad. 
As described herein, the present invention is a battery testing system for 
identifying a failed battery, the system comprising: a battery having two 
portions connected in series, each portion including a plurality of 
serially connected cells; an electric circuit including the battery 
connected within the circuit; a battery charger connected to the battery 
and providing a float voltage across the plurality of serially connected 
cells; a voltage measuring device for measuring voltages across the 
battery, wherein the voltage measuring device separately measures voltages 
across the first portion, the second portion, and a combination of the two 
portions; and a battery tester determiner for receiving signals 
representing the measured voltages and determining whether a battery has 
failed based upon the voltage measured across the first portion, the 
voltage measured across the second portion, and the voltage measured 
across a combination of the two portions. 
As further described herein, the present invention is a method of 
identifying a failed battery, wherein the battery includes two portions 
connected in series, each portion including a plurality of serially 
connected cells, the method comprising the steps of: receiving a first 
signal representing a voltage measured across the first portion while the 
battery receives a float voltage while connected to an electric charging 
circuit; receiving a second signal representing a voltage measured across 
the second portion while the battery receives the float voltage while 
connected to the electric charging circuit; receiving a third signal 
representing a voltage measured across a combination of the two portions 
while the battery receives the float voltage while connected to the 
electric charging circuit; determining whether a battery has failed based 
upon the voltage measured across the first portion, the voltage measured 
across the second portion, and the voltage measured across a combination 
of the two portions; and alerting an operator if the battery is bad. 
It will be appreciated from the foregoing that a significant aspect of the 
present invention is the ability to determine the condition of the battery 
without disconnecting the battery from electrical charging devices. 
Further, the present invention determines the condition of the battery 
after minimizing the normal voltage differences between each of the 
battery's cells. Further, based upon a voltage ratio derived from measured 
voltages across a battery, the present invention is able to identify 
whether the cause of a battery failure is due to either a shorted or open 
cell. 
More particularly, because the voltage across each serially connected 
portion of the battery is measured separately to calculate a voltage 
ratio, the testing method determines whether the battery has failed 
irrespective of the float voltage. This permits computer operators or 
automated computer programs to test a battery without disconnecting the 
battery from a device providing a battery charge. Thus, when the battery 
testing system of the present invention tests a computer system's battery 
backup supply, the battery may remain connected to the computer, 
accordingly providing a reliable power supply. 
The invention may be better appreciated from the following Figures, taken 
together with the accompanying Detailed Description of the Invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The following description is of the best presently contemplated mode of 
carrying out the invention. The description is made for the purpose of 
illustrating the general principles of the invention and is not to be 
taken in a limiting sense. 
Referring first to FIG. 1A, a diagrammatic view of a battery receiving a 
float voltage of the present invention may be generally appreciated. As 
shown, the battery 100 is center-tapped, and includes one half of the 
cells between terminals 106 and 108, and the other half between terminal 
108 and 110. A power supply (Vps) 112 provides a float voltage across each 
of the terminals, 106 and 110, to slightly reverse bias the battery 100 so 
as to maintain an appropriate charge. 
Referring next to FIG. 1B, a diagrammatic view of a battery testing system 
including the battery 100 and power supply of FIG. 1A may be generally 
appreciated. In particular, a preferred embodiment of a battery testing 
system including a battery 100, power supply 112, voltage measuring device 
114 and battery testing determiner 116 is shown. In a preferred 
embodiment, the battery 100 is a lead/acid battery. However, alternative 
embodiments of the present invention may include other battery types. As 
shown, the lead/acid battery is a 24 cell center-tapped battery, including 
cells comprising a first 1/2 stack of 12 cells 102 and a second half stack 
of 12 cells 104. 
The Vps 112 provides a float voltage across each cell of the first and 
second cell stacks (102 & 104) of the battery 100. In a preferred 
embodiment, each cell is identical and operates properly, so that the Vps 
voltage drop is divided evenly across the first and second 1/2 cell stacks 
(102 & 104) of the battery. In an exemplary but not limiting embodiment, 
the first 1/2 stack of cells 102 has a voltage "V1" of 25.8 volts, and the 
second 1/2 stack of cells 104 has a voltage, "V2" of 25.8 volts. 
Therefore, the combined voltage "V3" (or Vps) 110 is 51.16 volts. Having 
an equal voltage drop across each cell (2.15 volts), when a cell is good, 
and having an equal number of cells in each 1/2 stack of cells, enables 
the invention to detect shorted or open cells. 
A voltage measuring device 114, comprising voltmeters M.sub.1, M.sub.2 and 
M.sub.3, is electrically coupled to the battery 100. As shown, the voltage 
measuring device 114 coupled to the battery 100 measures a voltage across 
the first cell group 102, "V1," the voltage across the second cell group, 
"V2," and the voltage "V3" across all cells of the battery 100. Due to 
this time-dependent cell balancing which occurs during the float voltage, 
it is important that the battery test method allows sufficient time for 
cell imbalances to equalize before deciding that the battery pack is 
"failed." In a preferred embodiment, the decision as to whether the 
battery pack is "bad" is determined after a float voltage is applied to 
the battery after 24 hours. However, alternative embodiments before 
reading the voltages may apply a float voltage to minimize normal cell 
differences for more or less than 24 hours. 
A battery tester determiner 116 receives and processes signals representing 
each of the voltage measurements, V1, V2 and V3 to determine whether the 
battery 100 has an open or shorted cell. In a preferred embodiment, the 
battery tester determiner 116 may include a software program running on a 
computer. Alternative embodiments of the invention may include a battery 
tester determiner 116 that is solely hardware, software, firmware, etc. In 
consideration of the various embodiments of the present invention provided 
herein, it is apparent that these examples are provided solely for 
illustrative purposes and should not be construed to limit the invention. 
Referring next to FIG. 2, the process of determining whether the battery 
100 has an open or shorted cell in accordance with the present invention 
may be better appreciated. In step 200, the battery tester determiner 116 
receives a voltage V1 measured across the first 1/2 stack of battery cells 
102. In step 202, the battery tester determiner 116 receives a voltage V1 
measured across the second 1/2 stack of battery cells 102. In step 204, 
the battery tester determiner 116 receives a voltage measured across a 
combination of the first and second 1/2 stacks of the battery cells. In 
step 206, the battery tester determiner performs a voltage analysis to 
determine whether the battery has failed due to a shorted or open cell by 
using the measured voltage data of V1, V1 and V3. 
Referring next to FIG. 3, the voltage analysis performed by the battery 
tester determiner 116 to determine whether the battery 100 has an open or 
shorted cell may be understood in greater detail. In step 300, the battery 
tester determiner 116 determines the voltage difference between voltage V1 
of the first portion of the battery 100 and voltage V1 of the second 
portion of the battery 100. After determining a voltage difference, in 
step 302, the battery tester determiner 116 determines a voltage ratio by 
dividing the voltage difference by the voltage across the combination of 
battery cells V3. In step 304, the battery tester determiner 116 compares 
the voltage ratio to a threshold value. The threshold value is a 
predetermined value set as the maximum percentage difference from nominal 
voltage that a battery with an adequate charge may possess. If, in step 
306, the battery tester determiner 116 finds the voltage ratio less than 
the threshold value, the battery testing system passes the battery as 
possessing a satisfactory charge. In contrast, if the voltage ratio is 
greater than the threshold value then the battery testing system fails the 
battery as possessing an unsatisfactory charge. If the battery fails the 
test, it may be necessary to replace it with an alternative battery. In 
this case the computer may be programmed to so advise an operator based 
upon the output of the battery tester determiner. Further, based upon the 
voltage ratio, the operator will be able to identify whether the battery 
failure is due to either an open or shorted cell. 
Referring next to FIG. 4, the setting of a threshold value may be 
understood in greater detail. In step 400, a battery's voltage ratio is 
determined if one portion of the battery has a shorted cell. In step 402, 
a battery's voltage ratio is determined if one portion of the battery 100 
has an open cell. After determining the theoretical voltage ratio of each 
test case, in step 404, the threshold value is set with a value proximally 
less then the lesser value of the voltage ratio when the battery 100 has a 
shorted cell and when the battery 100 has an open cell. 
Referring next to FIG. 5, determining the voltage ratio when one portion of 
the battery has a shorted or an open cell may be understood in greater 
detail. More particularly, the following sequence of steps may be applied 
to determine the voltage ratio when either portion of cells of battery 100 
has a shorted or an open cell. In step 500, the battery tester determiner 
116 measures voltage V1 across the first portion of battery cells 102, and 
in step 502, measures the voltage V2 across the second portion of battery 
cells 104. In step 504, the battery tester determiner 116 measures the 
voltage V3 across a combination of the first and second portions of the 
battery cells. In step 508, the battery tester determiner 116 derives a 
voltage ratio by dividing the voltage difference V1-V2 by the voltage V3 
across the combination of battery cell portions. 
Tables 1 to 3 below show an example of FIGS. 4 and 5, where a battery 
having 24 cells are grouped into a first and second portion of 12 cells, 
where in table 1 all of the cells are good, in table 2 one cell of the 
first portion of cells is bad, and in table 3, one cell of the first 
portion of cells is open. 
TABLE 1 
__________________________________________________________________________ 
All Good Cells 
__________________________________________________________________________ 
Battery = 24 cells = (1.sup.st Cell portion of 12 cells + 2.sup.nd Cell 
portion of 12 cells) 
Voltage drop per Cell = 2.15 volts 
1.sup.st Cell Portion = 12 good cells 
2.sup.nd Cell Portion = 12 good cells 
V3 = float voltage = 56.25 volts 
Voltage Ratio Equation: 
VR = (V1 - V2)/V3 
##STR1## 
= 28.125 - 28.125/56.25 
VR = 0 
__________________________________________________________________________ 
TABLE 2 
__________________________________________________________________________ 
One Cell is Shorted 
__________________________________________________________________________ 
Battery = 24 cells = (1.sup.st Cell portion of 12 cells + 2.sup.nd Cell 
portion of 12 cells) 
Voltage drop per Cell = 2.15 volts 
1.sup.st Cell Portion = 12 good cells 
2.sup.nd Cell Portion = 11 good cells & 1 shorted cell 
V3 - float voltage = 56.25 V 
Voltage Ratio Equation: 
VR = (V1 - V2)/V3 
##STR2## 
= (29.35 - 26.90)/56.25 
= 4.35% 
__________________________________________________________________________ 
TABLE 3 
______________________________________ 
One Cell is Open 
______________________________________ 
Battery = 24 cells = (1.sup.st Cell portion of 12 cells + 2.sup.nd Cell 
portion of 
12 cells) 
1.sup.st Cell Portion = 12 good cells 
2.sup.nd Cell Portion = 11 good cells & 1 open cell 
VD per cell = 2.15 volts 
V3 = Vps = F.V. = 56.25 V 
Voltage Ratio Equation: 
V1 = (12 * 2.15) 
V2 = V3 - V1 
= Vps - (12 * 2.15) 
V3 = Float voltage = 56.25 V 
VR = (V1 - V2)/V3 
= (12 * VD) - (V3 - 12 * VD)/V3 
= (12VD - V3 + 12VD)/V3 
= (24VD - V3)/V3 
VR = 24VD/V3 - 1 
= 24VD/V3 - V3/V3 = (24VD - V3)/V3 
= (2(12 * 2.15) - 56.25)/56.25 
= 8.26% 
______________________________________ 
Once the theoretical voltage ratios for the cases where all cells of the 
battery are good, one cell of the battery is open, and one cell of the 
battery is shorted, a recommended threshold is set at a level proximally 
lower than the lower of the voltage ratio values of the case when the 
battery has a shorted cell, and the case when the battery has an open 
cell. Hence, the voltage threshold ratio level may preferably be set to 
approximately 4 percent, a percentage proximally lower than the 4.35% 
voltage ratio derived for the battery having a shorted cell. The 4.0% 
value was chosen based upon the natural variability of battery voltage. 
It can therefore be appreciated that a new and novel method and apparatus 
for determining whether a battery has shorted or open cells while the 
battery remains connected to an electrical charging device has been 
described. It will be appreciated by those skilled in the art that, given 
the teaching herein, numerous alternatives and equivalents will be seen to 
exist which incorporate the invention disclosed herein. For example, the 
present invention need not use lead acid battery; other types of batteries 
may be used. Battery need not be center tapped, but could have unequal 
numbers of cells on either side of tap. As a result, the invention is not 
to be limited by the foregoing exemplary embodiments, but only by the 
following claims and equivalents thereof.