Battery parameter measurement

A method and system for evaluating lead-acid battery jars in a battery backup or standby system applies, in a stepwise manner, a current load across a group of adjacent jars. Voltage measurements are taken across each jar in the group at various times during the application of the current load. These measurements are utilized to calculate the ohmic resistance, the charge transfer resistance and the double layer capacitance of each jar.

BACKGROUND OF THE INVENTION 
This invention relates to battery backup and standby systems having from 
one to a large number of jars and, more particularly, to the measurement 
of jar parameters for use in determining the status of the jar(s). 
For lead-acid batteries, it is known in the art that some of the primary 
measures of battery jar health are the electrolyte resistance, the charge 
transfer resistance and the double layer capacitance. It is important that 
the electrolyte solution have the proper acid concentration, which is 
related to the amount of charge in the jar, and this can be determined by 
examining the resistance of the electrolyte, which should lie within a 
defined range. However, the actual electrolyte resistance cannot be 
measured by itself, since it is only one component (albeit the major one) 
of the overall ohmic resistance of the jar, which also includes the 
resistance of the jar grids, terminals, and interconnections or straps. 
But since the electrolyte resistance is the largest part of the overall 
ohmic resistance in a properly maintained corrosion-free installation, 
examining the overall ohmic resistance can provide an indication of the 
specific gravity of the electrolyte solution. The charge transfer 
resistance is the resistance between the battery plate and the electrolyte 
solution, and is related to the condition of the interface between the 
plate and the electrolyte. Ideally, the charge transfer resistance is 
zero, but it is considered acceptable if it is below some predetermined 
value. The plate/electrolyte interface can best be described as two rigid 
layers of ions which form what is referred to as a "Helmholtz double 
layer". With increasing plate sulphation and/or grid corrosion, there is 
less surface area for the inner layer of ions to adsorb to the plate and 
consequently fewer ions are present in the outer layer closest to the 
electrolyte. The resultant capacitance (also known as the "double layer 
capacitance") provides a measure of the plate surface area which is free 
of sulphation and/or corrosion, and measuring the decrease in capacitance 
over time provides an indication of the rate of the electrochemical 
deterioration of the jar. With increasing plate sulphation and/or 
corrosion, there is a corresponding increase in the charge transfer 
resistance as well. 
It would therefore be desirable to be able to measure the ohmic resistance, 
the charge transfer resistance and the double layer capacitance of a 
battery jar, and to be able to do so while the battery jars are on-line in 
the backup or standby system. 
SUMMARY OF THE INVENTION 
According to the present invention, a stepped current load is applied to a 
battery jar and voltage measurements across the jar are taken. The initial 
voltage change across the jar in response to the applied current load, 
when divided by the current, is equal to the ohmic resistance of the jar. 
The difference between the initial voltage change and the final 
(extrapolated) voltage of the jar, when divided by the applied current, is 
equal to the charge transfer resistance of the jar. By calculating the 
time constant of the voltage response of the jar to the stepped load and 
dividing the calculated time constant by the charge transfer resistance, 
the jar's double layer capacitance is obtained. 
In accordance with an aspect of this invention, the current load is applied 
in the form of a series of high frequency pulses and low frequency pulses. 
The short duration, high frequency, pulses are used for calculating the 
ohmic resistance. The long duration, low frequency, pulses are used for 
calculating the charge transfer resistance and the double layer 
capacitance of the jar. 
A system according to the present invention may be utilized for evaluating 
the condition of each of a plurality of rechargeable battery jars arranged 
in at least one parallel connected string of serially connected jars, with 
the jars of each string being divided into at least two groups. The system 
includes a switchable current load and a first controllable switch bank 
associated with each of the strings. The first switch bank is controllable 
for selectively coupling the current load across a selected group of 
adjacent jars in that string. The system also includes a bus system, 
voltage measurement modules coupled to the bus system, a current 
measurement system coupled to the bus system and a controller coupled to 
the bus system. There is a voltage measurement module associated with each 
group of adjacent jars in each string and each voltage measurement module 
includes a pair of voltage measurement terminals and a second controllable 
switch bank associated with the jars of that group. The second switch bank 
is controllable for selectively coupling one jar of the group at a time 
across the pair of voltage measurement terminals. The voltage measurement 
module is effective to collect data indicative of the voltage across the 
pair of voltage measurement terminals. The current measurement system is 
coupled to the switchable current load to collect data indicative of test 
current. The controller is effective to control the operation of the 
current load and the first controllable switch bank, and is further 
effective to transmit control signals over the bus system to actuate each 
voltage measurement module to control its second controllable switch bank 
and collect voltage data from the jars of the associated group of jars. 
The controller also collects test current data over the bus system from 
the current measurement system. A computer is coupled to the controller 
and is effective to cause the controller to control the current load, the 
first controllable switch bank, each voltage measurement module and the 
current measurement system so that the current load is applied in a 
stepwise manner to each group of adjacent jars one at a time, the voltage 
measurement module associated with that group collects voltage data from 
each jar during the stepwise application of current load to that group, 
and the current measurement system collects test current measurement data. 
The computer is further effective to collect all of the voltage and 
current measurement data and to utilize the collected data to calculate at 
least one of the ohmic resistance, the charge transfer resistance and the 
double layer capacitance for each of the cells. 
BRIEF DESCRIPTION OF THE DRAWINGS 
The foregoing will be more readily apparent upon reading the following 
description in conjunction with the drawings in which like elements in 
different figures thereof are identified by the same reference numeral and 
wherein: 
FIG. 1 is an overall block diagram of a prior art battery backup system in 
which the present invention finds utility; 
FIG. 2 illustrates a simplified equivalent circuit for a model of a battery 
jar, which is useful for understanding this invention; 
FIG. 3 illustrates the voltage response of a battery jar to a stepped pulse 
of load current; 
FIG. 4 illustrates a load current control signal pulse stream according to 
the present invention; 
FIG. 5 is an overall block diagram of a system constructed in accordance 
with the present invention for evaluating the status of battery jars in 
the system of FIG. 1; 
FIG. 6 is a block diagram of the data acquisition portion of the system 
shown in FIG. 5, showing a portion of a voltage measurement module; 
FIG. 7 is a block diagram showing the remaining portion of the voltage 
measurement module; and 
FIG. 8 is a block diagram showing the current measurement system.

DETAILED DESCRIPTION 
Referring now to the drawings, FIG. 1 illustrates a typical prior art 
battery backup system coupled to the commercial power grid 10. The backup 
system is adapted to provide power to the load 12 upon detection of 
failure of the power grid 10. What is not shown in FIG. 1 is the direct 
connection of the load 12 to the power grid 10 and the arrangement which 
disconnects the load 12 from the power grid 10 upon detection of the power 
grid failure, such arrangement being conventional and well known in the 
art. 
The battery backup system shown in FIG. 1 includes a bank of batteries 14 
connected to a charger 16 and an inverter 18. The battery bank 14 
typically comprises at least one string of serially connected rechargeable 
battery jars. The charger 16 maintains the battery jars within the battery 
bank 14 at their full level of charge when the power grid 10 is 
operational, as is well known. Upon detection of a failure of the power 
grid 10, the inverter 18 becomes operative to transform energy from the 
battery bank 14 into the same form normally provided by the power grid 10 
and supply such transformed energy to the load 12, so that the load 12 
does not see any interruption of power. Typically, the power grid 10 
provides alternating current so that the inverter 18 functions to convert 
the direct current provided by the battery bank 14 into alternating 
current. The foregoing is conventional and well known in the art and will 
not be described in any further detail. 
In the following discussion, reference will be made to battery jars. It is 
well understood in the art that rechargeable lead-acid batteries, with 
which the present invention is particularly adapted for use, are provided 
in the form of one or more cells enclosed within a "Jar", at which 
positive and negative voltage terminals are accessible. 
FIG. 2 shows a simplified equivalent circuit diagram for a model lead-acid 
battery jar. As shown, the jar can be considered to include the ohmic 
resistance R.sub.OHMIC in series with the parallel combination of the 
charge transfer resistance R.sub.CT and the double layer capacitance 
C.sub.DL. The voltage response of the battery jar to a stepwise applied 
current load has the general form: 
EQU V(t)=V.sub.OHMIC (t)+V.sub.CT (1-exp(-t/R.sub.CT C.sub.DL)). 
FIG. 3 illustrates the actual voltage across a battery jar in response to a 
stepped pulse of load current. The load is applied at the time t.sub.0. 
Prior to the time t.sub.0, the voltage across the battery jar is 
substantially constant, having broadband noise superimposed thereon. 
Immediately upon application of the load current, the jar voltage drops by 
an amount equal to V.sub.1. This is the drop across the ohmic resistance 
of the battery jar. As discussed above, the major component of the ohmic 
resistance is the electrolyte resistance of the battery jar. Therefore, by 
measuring this voltage drop and dividing by the applied current, the ohmic 
resistance of the battery jar is determined. The remaining exponential 
change in voltage is due to the parallel combination of the charge 
transfer resistance and the double layer capacitance, with the time 
constant of that exponential change being the product of the charge 
transfer resistance and the double layer capacitance. The voltage drop 
V.sub.2 between the initial voltage drop V.sub.1 and the final 
(extrapolated) steady state voltage across the jar is equal to the charge 
transfer resistance times the applied current load. Therefore, by 
determining the final steady state voltage across the jar and subtracting 
from it the measured voltage drop V.sub.1, this is divided by the applied 
current load to determine the charge transfer resistance. By calculating 
the time constant of the exponential portion of the response and dividing 
the calculated time constant by the determined charge transfer resistance, 
the double layer capacitance is obtained. When the current load is removed 
at the time t.sub.R, the voltage response of the battery jar is the 
inverse of its response to the application of the current load. 
According to the present invention, a stepped current load is applied to a 
battery jar and the voltage across the jar is measured at various times. 
These voltage measurements are utilized to calculate the ohmic resistance, 
the charge transfer resistance and the double layer capacitance of the 
jar. Thus, the voltage across the jar is measured at least once prior to 
the time t.sub.0 to obtain a baseline unloaded voltage. If such 
measurements are taken several times and averaged, the effects of 
broadband noise are reduced. The voltage across the jar is then measured 
immediately after the time t.sub.0 to obtain the voltage drop V.sub.1. Two 
or more measurements are then taken prior to the time t.sub.R and curve 
fitting techniques are utilized to obtain an exponential curve from which 
the voltage drop V.sub.2 and the exponential time constant are determined. 
It is preferred that a series of current pulses be applied to the jar and 
measurements taken during each of those pulses to cancel out the effects 
of broadband noise. Although it is possible to use a relatively long load 
pulse to take voltage measurements and obtain all the desired information, 
there are reasons why this would be disadvantageous. Thus, in order to 
cancel out broadband noise to obtain a reliable measurement of the initial 
voltage drop V.sub.1, a large number of pulses are required. If long 
pulses are used, this results in an undesired draining of charge from the 
battery jar. It is therefore preferred to use several short duration 
pulses to obtain the voltage drop V.sub.1 and fewer longer duration pulses 
to determine the voltage drop V.sub.2 and the exponential time constant. 
Thus, a pulse train of the type illustrated in FIG. 4 is preferred. 
Illustratively, the pulse train comprises twenty short duration (high 
frequency) pulses and three long duration (low frequency) pulses. 
Preferably, the short high frequency pulses will have a time duration 
between about 0.5 ms to about 10 ms and the long low frequency pulses will 
have a time duration between about 1 ms to about 50 ms, with about one 
second between pulses. This will keep the test time for each jar to less 
than one minute, which is desirable since a large battery backup system 
may comprise upwards of 256 jars. 
FIG. 5 shows in block diagram form a system for practicing the present 
invention. Before describing the system shown in FIG. 5, a discussion of 
the battery bank 14 is in order. The battery bank 14 comprises a number of 
parallel strings of serially connected battery jars. Illustratively, each 
string includes thirty two serially connected jars and there are eight 
such strings connected in parallel, for a total of two hundred fifty six 
(256) battery jars. It is understood that this number is for illustrative 
purposes only, and any particular battery backup or standby system may 
have more or fewer strings each with more or fewer jars. 
As shown in FIG. 5, the battery evaluation system includes a plurality of 
voltage measurement modules 20, a current measurement system 22, and a 
system/load controller and interface 24, all interconnected via a bus 
system 26, which is preferably an asynchronous serial communications bus. 
A remote personal computer 28 is coupled to the system/load controller and 
interface 24 via a communications link 30, which may be a hard-wired 
connection, a modem, or any other appropriate link. According to the 
present invention, each string within the battery bank 14 is divided into 
quarters, and there is a voltage measurement module 20 dedicated to each 
such quarter. 
FIG. 6 illustrates the data acquisition portion of the system shown in FIG. 
5, coupled to one string of the battery bank 14. As is conventional, 
within the battery bank 14 the battery strings are connected in parallel 
between a first rail 32 and a second rail 34. Only one string 36 is 
illustrated herein and, as shown, the string 36 is divided into four 
serially connected quarters 38, with each quarter being made up of eight 
serially connected battery jars. It is understood that the string need not 
be divided into equal quarters in order to practice the present invention. 
In any event, substantially centrally of each string may be a central 
disconnect switch 40, which forms no part of the present invention, but is 
utilized to remove the string from the battery bank 14. 
The system/load controller and interface 24 includes a load control circuit 
42 which has a controllable switch bank 44 associated with each of the 
strings. The switch bank 44 is controllable for selectively coupling the 
load control circuit 42 across a selected quarter string 38. The current 
load used for battery jar measurements is generated within the current 
path 46, there being a controllable switch 48 by means of which the long 
and short duration current pulses are generated. A sensing resistor 50 is 
provided in the current path 46, across which is connected the current 
measurement system 22. 
Each voltage measurement module 20 includes a switch bank 52 coupled 
between the individual jars within the associated quarter string 38 and a 
pair of voltage measurement terminals 54, 56. The switch bank 52 is 
controllable by the microprocessor 58 (FIG. 7) within the voltage 
measurement module 20 to selectively couple the jars within the quarter 
string 38 one at a time across the pair of voltage measurement terminals 
54, 56. The voltage measurement terminal 56 is connected directly to the 
analog to digital converter 60. The other voltage measurement terminal 54 
is connected through the switches 62 to the low pass filters 64, then 
through the switches 66 to the analog to digital converter 60. When the 
voltage measurement module 20 is activated and one of the battery jars 
within the quarter string 38 is connected to the voltage measurement 
terminals 54, 56, the switches 62 are all closed and the switches 66 are 
all open. During a current load pulse, the switches 62 are opened one at a 
time so that a time-related sequence of voltage measurements are held in 
the low pass filters 64. The timing of the opening of the switches 62 
corresponds to the times when measurements are taken. The switches 66 are 
then sequentially closed to transfer these voltage measurements to the 
analog to digital converter. Alternatively, a single low pass filter with 
a very long time constant and one set of switches 62, 66 can be utilized. 
However, the double layer capacitance cannot be computed in this case. 
As shown in FIG. 8, the current measurement system 22 is similar to the 
voltage measurement module 20 in that the leads 68, 70 which are connected 
across the sensing resistor 50 are connected one directly to the analog to 
digital converter 72 and one through the switches 74, the low pass filters 
76 and the switches 78. The current measurement system 22 also includes a 
microprocessor 80 which, among other things, controls the switches 74, 78. 
Each of the voltage measurement modules 20 and the current measurement 
system 22 includes an input/output circuit 82 coupled to the bus system 26 
for receiving instructions from the system/load controller and interface 
24 and for transmitting collected voltage measurement data to the 
system/load controller and interface 24. The system/load controller and 
interface 24 does preliminary processing of the data and transmits the 
data to the remote personal computer 28 for final processing and 
evaluation of battery condition. Thus, the personal computer 28 utilizes 
the voltage and current measurement data to calculate and/or display the 
ohmic resistance, the charge transfer resistance and the double layer 
capacitance for each of the jars, in the manner previously described. 
Thus, typically, in a large battery backup or standby system the remote 
personal computer 28 initiates the collection of voltage measurement data 
once a week during an off hour. The system/load controller and interface 
24 sends signals over the bus system 26 to cause a string of long and 
short duration current pulses to be applied to each quarter string and to 
have the voltage measurement module 20 associated with that quarter string 
take voltage measurements from each jar within that quarter string. During 
the data collection from each jar, a stream of both long and short 
duration current pulses are applied to the quarter string. All of the 
voltage measurement data is collected by the system/load controller and 
interface 24 and transmitted to the remote personal computer 28 for 
processing. 
The system/load controller includes an internal real time clock and may 
also be programmed to autonomously take ohmic resistance, charge transfer 
resistance, and double layer capacitance measurements for each jar at 
predetermined specific time intervals at predetermined specific times. The 
acquired data in this case is temporarily stored in the voltage 
measurement modules as well as the system/load controller for future 
downloading to the remote personal computer. 
Accordingly, there has been disclosed an improved method and system for 
evaluating the status of battery jars in a battery backup or standby 
system. While an exemplary embodiment of the present invention has been 
disclosed herein, it will be appreciated by those skilled in the art that 
various modifications and adaptations to the disclosed embodiment may be 
made and it is intended that this invention be limited only by the scope 
of the appended claims.