Agilent Technologies Blood Glucose Meter AN 372 2 User Manual

Agilent AN 372-2  
Battery Testing  
Application Note  
An electronic load can be used to discharge batteries of various  
chemistries to determine actual capacity, capacity retention,  
and internal impedance.  
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Application Overview and Test Implementation  
1
Seven standard test procedures are  
Rated Capacity  
the specified minimum voltage called  
the “end of discharge voltage” (EODV).  
The EODV for nickel-cadmium batteries  
is typically 1.1 to 0.9 Volts.  
used to verify certain electrical char-  
acteristics of secondary batteries:  
The principal measurement of a  
battery’s performance is its rated  
capacity. Capacity ratings are attained  
in an accelerated test approximating  
the battery’s capacity in typical use.  
The capacity of a fully charged battery,  
at a fixed temperature, is defined as  
the product of the rated discharge  
current (in amperes) and the discharge  
time (in hours) to a specified mini-  
mum termination voltage (volts).  
See Figure 2. A battery is considered  
completely discharged when it attains  
1. Rated capacity  
2. Capacity retention  
3. Effective internal resistance  
4. Discharge rate effect on capacity  
at –20°C  
5. Discharge rate effect on capacity  
at 23°C  
The term C, or C-rate, is used to  
define the discharge current rate (in  
amperes), and is numerically equal  
to rated capacity, which is expressed  
in ampere-hours. The term 1C is  
defined as the rate of discharge that  
allows a battery to provide its rated  
current over a period of one hour.  
6. Life cycle performance  
7. Extended overcharge  
Other miscellaneous tests and proce-  
dures also involve discharging a  
battery such as: start-up voltage test,  
forced-discharge test, timed fast charge  
and dump-timed charge. Most battery  
tests typically require only about 1%  
accuracy unless otherwise specified.  
While battery tests do not require  
high accuracy, the tests must be very  
repeatable. Battery characteristics  
change with temperature so it is  
important to be able to control and  
monitor the temperature, usually to  
within ±2 degrees C. Other equipment  
requirements to consider are: a cur-  
rent source for charging secondary  
batteries, a voltage monitor, a current  
monitor, a load for discharge current,  
and a time keeping device. More  
information about test equipment  
is given in the “Test Equipment  
Requirements” section later in  
this application note.  
Figure 2. Typical Discharge Curve  
Note that a battery temperature rise  
of more than 5 degrees C above ambi-  
ent may require supplemental cooling  
to prevent battery performance degra-  
dation due to elevated temperatures.  
1. As specified in ANSI® C18.2-1984,  
American National Standards  
3
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Capacity varies with the rate of dis-  
charge as shown in Figure 3. Testing  
Average and maximum capacities are  
obtained by putting the battery through  
for how discharge rate affects capacity five successive charge/discharge sta-  
Capacity Retention  
This test characterizes how much of a  
fully charged battery’s capacity is  
retained over a long period of time  
under specific conditions. This time  
is sometimes referred to as the “shelf  
life” of the battery. This test is not to  
be confused with an attempt to char-  
acterize the self-discharge effect of  
the spontaneous internal chemical  
actions in batteries. Self-discharge  
occurs regardless of the battery’s  
connection to an external circuit.  
is discussed later in more detail.  
Generally, lower discharge rates over  
longer periods of time yield higher  
values of total capacity. It is impor-  
tant to realize that since discharge  
rate affects how the value of C is  
determined, battery manufacturers  
must decide on a standard time of  
discharge. Since different values for  
bilizing cycles. The batteries are given  
five stabilizing cycles where they are  
charged, discharged and rested at an  
ambient temperature of 23 degrees C.  
Batteries are charged at C/10 A for  
a period of from 20 to 24 hours and  
rested for a period of from 2 to 4 hours.  
The batteries are then discharged at  
a constant current of 1C amperes to  
capacity can be obtained for the same an EODV of 0.9 volts.  
battery, capacity is generally deter-  
mined over a “standard” period of  
time—from 5 to 20 hours at discharge  
rates from C/5 to C/20. A complete  
specification for capacity should  
The value of the capacity used in the  
The procedure to determine the effec-  
tive capacity retention of a battery is  
relatively simple. Immediately follow-  
ing the 5 cycles of capacity measure-  
ment, the battery is fully recharged. It  
is then stored open circuit for a peri-  
od of days at a specific temperature.  
Then it is discharged at a constant  
current rate to an EODV of 0.9 V as  
before. The capacity obtained should  
not be less than 37% of the rated  
capacity for the battery. The number  
of days of shelf life are typically pro-  
vided for values of temperature from  
23 degrees C to 50 degrees C.  
following tests is the value obtained  
in the fifth stabilizing cycle. Also, the  
capacity obtained in the last three  
therefore have a C rate and the period cycles must not be less than that  
of time that was used to determine  
the capacity. For example, Capacity:  
450 mAh @ 5 hour rate.  
stated by the manufacturer as rated  
capacity (1C).  
Figure 3. Effect of Discharge Rate on Capacity  
4
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The battery is given five stabilizing  
cycles in accordance with the previ-  
ously outlined procedure.  
Impedance Test  
The battery must be fully charged as  
outlined above. An AC current source  
Effective Internal Impedance  
Battery impedance is dependent  
on temperature, its state of charge,  
and the load frequency. The effective  
internal impedance is lower for a fully  
charged battery than it is for a dis-  
charged one. Having a low internal  
resistance is very important when the  
battery must support a high current  
for a short time. Low temperature, use,  
and long storage periods all increase  
a battery’s internal resistance. Nickel-  
cadmium batteries also have a high  
effective capacitance. Their total effec-  
tive impedance is so low that, in  
applications where they are continu-  
ously being “trickel-charged” at rates  
from 0.01C to 0.1C, they make excel-  
lent ripple filters. Resistance and  
impedance tests are explained in the  
following paragraphs.  
( 1 kHz) is applied to the terminals of  
~
Life Cycles 1 through 48  
1. Charge 11 hours and  
20 minutes at C/10  
2. Discharge immediately at  
1C for 40 minutes  
the battery. The AC current through  
the battery and the voltage across it  
are measured. The impedance is simply  
calculated as V/I. An interesting  
alternative testing method that yields  
the same result is to place a varying  
3. No rest  
( 1 kHz) load across the fully charged  
~
Life cycles 49 and 50  
battery instead of the AC power source.  
1. Charge for 20 hours at C/10  
2. Rest 2 to 4 hours  
3. Discharge at 1C to 0.9 volts  
EODV  
Discharge Rate Effect on Capacity  
The rate of discharge has an effect on  
the total capacity of a battery. Heavy  
discharge rates decrease the total  
available capacity of a battery. The  
test is done at two temperatures:  
–20 degrees C and 23 degrees C.  
The battery is first fully charged at  
23 degrees C and then immediately  
stored for 24 hours at an ambient  
temperature of –20 degrees C. It is  
then discharged at an ambient tem-  
perature of –20 degrees C at a con-  
stant current rate of 1C to an EODV  
of 0.8 volts. Then the procedure is  
repeated at discharge rates of 5C and  
C/5. The whole test is then repeated  
at a temperature of 23 degrees C to  
an EODV of 0.9 volts.  
Repetition of Life Cycles  
Repeat cycles 1 to 50 as desired.  
The capacity at cycle 50, and multiples  
thereof, should be no lower than that  
stated for this procedure by the man-  
ufacturer.  
Resistance Test  
The battery must be fully charged as  
outlined above. Batteries rated 5 Ah or  
less are discharged at 10C for 2 min-  
utes and then switched to 1C. The  
battery voltage is recorded just prior  
to switching and again upon reaching  
its maximum value after switching.  
All voltage measurements are made at  
the terminals of the battery independ-  
ently of the contacts used to carry  
current. The effective internal resist-  
Extended Overcharge  
The ability of a battery to withstand  
overcharge is determined by charging  
the battery at a constant current of  
C/10, or at the maximum overcharge  
rate recommended by the manufac-  
turer, at an ambient temperature of  
23 degrees C for 6 months. The bat-  
tery should at no time show either  
electrolyte leakage or visual evidence  
of distortion beyond the standard  
maximum dimensions for that bat-  
tery. When discharged at a constant  
current of 1C to an EODV of 0.9V, the  
battery should have a capacity equal  
to or greater than the extended over-  
charge capacity specification.  
For each of the six discharge cycles,  
the manufacturer supplies the value of  
capacity to be expected as a percent of  
C1. Charging and discharging at tem-  
peratures below the specification sheet  
recommendation should be avoided.  
ance (R ) is then calculated as follows:  
e
R
V  
I = H– L  
V
I
V
I
=
L– H  
e
=
I
I
V
H, H = the current and voltage,  
recorded just prior  
Life Cycle Performance  
to switching  
Life cycle testing is a measure of  
expected battery performance in actual  
service. Life cycle performance is  
characterized by dynamically loading  
the battery in a simulated “real-life”  
situation for 50 or more charge and  
discharge cycles as follows:  
V
L, L = the current and maximum  
voltage, recorded  
after switching  
5
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Many cells can be quick-charged at a  
One final test, called the “forced dis-  
rate up to C/3 in as little as 3 to 5 hours charge test,” determines the safety of  
instead of the standard 12 to 15 hours a battery under certain abusive con-  
Miscellaneous Tests  
In addition to the tests already men-  
tioned, there are also other miscella-  
neous tests performed on nickel-  
cadmium batteries. These tests usually  
involve high rate charge and/or  
discharge.  
at the C/10 rate. High rate charging  
should be done under controlled con-  
ditions where temperature, voltage,  
pressure, or some combination of  
ditions. This test is very dangerous  
because, during the test, the battery  
is very likely to explode. The test  
must be done under extremely well  
these parameters can be monitored to controlled conditions in an explosion  
assure they are within specifications.  
proof safety chamber to prevent per-  
sonal injury. The test involves con-  
necting a current source in series  
with the battery. The polarity is in  
High rate discharge and charge of  
nickel-cadmium batteries is possible  
with today’s new and better designed  
cells having advanced plate and cell  
construction. The low internal resist-  
ance of nickel-cadmium batteries  
yields high discharge currents. If they  
are discharged continuously under  
short circuit conditions, however,  
self-heating may do irreparable dam-  
age. Continuous discharge at rates  
greater than 1C should be prevented  
to avoid potentially hazardous condi-  
tions due to high internal gas pres-  
sure build-up.  
One fast-charge method involves  
charging the battery at a rate exceed-  
ing the specified maximum charge rate the same direction as normal or short  
for a finite period of time, after which circuit current flow. See Figure 4. The  
the charge rate is reduced to currents current source is set to a value such  
below C 10. This method, called “timed that the resultant current flow is  
greater than the short circuit current  
“boost” charge to a partially discharged flow. This test simulates what may  
fast charge,” can indeed give a quick  
battery, but unfortunately has the  
potential of permanently destroying  
the battery. The destruction occurs  
due to overcharging the battery be-  
happen if a battery were improperly  
installed in a circuit where it may not  
be the only source in the application.  
Ideally the battery should withstand  
cause its unused capacity is unknown the stress, with some degree of margin,  
prior to charging.  
when the test currents are similar to  
actual conditions.  
Very high currents (>2C) can be with-  
drawn in low duty cycle pulses pro-  
viding that internal temperatures and  
pressures are maintained. Output  
capacity in any type of pulse discharge  
application is difficult to predict  
because of the infinite number of  
possible combinations of discharge  
time, rest time, and EODV. Simulation  
of actual events, as in the Life-Cycle  
test, is the best way to quantify a  
battery exposed to such conditions.  
A safer variation of the timed fast  
charge method is called “dump timed-  
charge” where the battery is first  
fully discharged (“dumped”) to its  
EODV before recharging via the “timed  
fast charge” method. The “dump  
timed-charge” method has the advan-  
tage of knowing just how much energy  
must be pumped back into the battery  
to bring it to full capacity; the risk of  
overcharging is therefore eliminated.  
Figure 4. Forced Discharge  
6
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Test Equipment Requirements  
From the various tests described  
so far, we can see some common  
requirements for test equipment. All  
the tests require a discharge cycle  
using a constant current. A constant  
discharge current cannot be attained  
with a simple resistor because the  
battery voltage changes as current is  
drawn from it. An active device is  
required, such as an electronic load  
with a constant current mode of  
operation. Also note that, because  
many levels of constant current are  
used from test to test, you should be  
able to control the electronic load  
dynamically as the test demands.  
The second alternative (Figure 7) shows  
that using a power supply may be more  
desirable because timed fast charge,  
dump-timed charge, and forced dis-  
charge tests all require a DC power  
source anyway. Additionally, a con-  
stant current power supply could then  
be used to test ampere-hour efficiency  
of secondary batteries. This rating is  
simply the ratio of the ampere-hours  
delivered during discharge to the  
Figure 5. Single Battery Test Configuration  
The first alternative (Figure 6) requires  
a method of scanning the voltage of  
each battery in the stack so that when  
any one battery reaches its EODV,  
either the test can be halted or the  
battery switched out of the circuit  
and replaced by a short circuit. Even  
as each battery is switched out of the  
circuit, the discharge current will  
remain the same if the load has a  
constant current mode of operation.  
ampere-hours required to restore the  
initial state of charge to the battery.  
The ability to control the load with a  
computer is important because dis-  
charge is typically over a long period  
of time and, if the test were not auto-  
mated, constant attendance would be  
an unproductive use of an operator’s  
time. Long term tests also bring about  
another requirement: reliability. The  
electronic load must be very reliable  
because, if it should fail, the test  
Figure 7. Using an Offset DC Power Supply  
would take a long time to repeat.  
Voltage and current must be moni-  
tored throughout all the tests because  
actual battery voltage varies with the  
battery chemistry as well as the dis-  
charge rate involved. Therefore, a  
voltmeter and ammeter are required.  
They should be computer controlled  
so that the various tests can be halted  
when the EODV is reached. If an  
In battery or single cell testing the  
electronic load only has to function  
down to the EODV, not zero volts. See  
Figure 5. If the minimum load operat-  
ing voltage is above the EODV for the  
battery being tested, two alternatives  
are available: stack more than one  
battery in series until the required  
voltage is reached (Figure 6) or place  
a DC power supply (of sufficient volt-  
age and current) in series with the  
battery (Figure 7). A power supply  
applied in this way is sometimes  
called an “offset supply.”  
Figure 6. Batteries in Series  
ammeter is unavailable, a current  
shunt can be used in conjunction with  
either a second voltmeter or a scanner.  
7
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Battery Testing with Agilent Electronic Loads  
By internet, phone, or fax, get assistance  
with all your test and measurement needs.  
Agilent Technologies Electronic  
Loads are ideally suited for battery  
test applications. Their many features  
make the test system easy to config-  
ure and provide safe, reliable, and  
repeatable operation.  
bus so that time consuming discharge  
tests can be attended automatically.  
Agilent’s electronic loads truly provide  
a “One Box” solution.  
Online Assistance  
Phone or Fax  
United States:  
(tel) 1 800 452 4844  
Testing cells down to an EODV of  
0.9 volts is easily done with the Agilent  
6060A, 6063A, 60501A, 60502A,  
The Agilent 6060A Electronic Load  
Canada:  
(tel) 1 877 894 4414  
(fax) (905) 206 4120  
and 6050A Electronic Load mainframe 60503A, or 60504A Electronic Loads.  
have the required constant-current  
modes as well as constant-resistance  
and constant-voltage modes. Built-in  
voltmeters and ammeters eliminate  
the need for external meters and pro-  
vide measurement accuracy which, in  
most cases, greatly exceeds the 0.5 to  
1% that is typically required.  
While the operating characteristics  
of these loads are guaranteed to meet  
all specifications above 3 volts, the  
DC operating characteristics extend  
below 3 volts (see Figure 8). This figure  
shows that at 0.9 volts the Agilent  
6060A Electronic Load is capable of  
reliably drawing up to 27 amperes.  
That means an 80 Ah battery could  
be discharged to an EODV of 0.9 volts  
at a discharge rate of C/3. For appli-  
cations requiring V/I characteristics  
below the operating curve of Figure 8,  
Agilent offers a full family of DC power  
Europe:  
(tel) (31 20) 547 2323  
(fax) (31 20) 547 2390  
Japan:  
(tel) (81) 426 56 7832  
(fax) (81) 426 56 7840  
Latin America:  
(tel) (305) 269 7500  
(fax) (305) 269 7599  
These electronic loads can be con-  
trolled from their front panel, from  
a computer via GPIB, or by a 0 to  
10 volt analog signal. By varying the  
analog control input (up to 10 kHz),  
Australia:  
(tel) 1 800 629 485  
(fax) (61 3) 9210 5947  
New Zealand:  
(tel) 0 800 738 378  
(fax) (64 4) 495 8950  
a battery’s effective internal impedance supplies to be used as an offset supply.  
can be easily measured. The electronic  
load’s built-in GPIB interface makes it Agilent’s full featured Electronic Load  
Asia Pacific:  
(tel) (852) 3197 7777  
(fax) (852) 2506 9284  
simple to connect any computer that  
supports GPIB. Agilent’s electronic  
loads are not limited to just being  
controlled over the bus. Measured  
current, voltage, power and complete  
status can also be read back over the  
Family offers quality and reliability  
backed with a three year warranty.  
Refer to the 1990/91 DC Power Supply  
Catalog with Electronic Loads (Part  
Number 5952-4203) for more informa-  
tion about Electronic Loads.  
Product specifications and descriptions in this  
document subject to change without notice.  
Copyright © 1988, 1991, 2000 Agilent Technologies  
Printed in U.S.A. 9/00  
5952-4191  
Figure 8. Operating Characteristics of an Agilent Electronic Load  
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