Battery Automatic Load Tester

Introduction

Every season, FIRST includes a 12V battery in the Kit of Parts (KOP). Year after year, we have cumulate quite a few batteries. Unfortunately, batteries do have a limited life. Some last longer than others. There are many factors affecting the life of batteries.

When batteries go bad or about to go bad, they can appear to be fully charged. But when they are used in competition, the charge drained very quickly. Therefore, it is very difficult to tell a bad battery by looking at the voltage of a fully charged battery. It is beneficial to have a tool that can tell us if a battery can hold its charge or it is about to die. Measuring the terminal voltage of the battery without any load (Open Circuit Voltage) is not indicative of whether the battery is good or bad. The proper way is to test them under load for a set period of time. At the end of the period, the terminal voltage must not drop below a certain threshold. In order to test a battery under load, we must use a load tester that draws a constant current from the battery for a set period of time while monitoring its terminal voltage. This document will explain the procedures of using a load tester to test the batteries. It also explain the design of such a load tester.

Theory of Load Testing

In automotive battery testing, the recommended load testing should draw one half Cold Crank Amps (CCA) of a fully charged battery for 15 seconds. For 12V batteries, the following table defines the state-of-charge of the battery.

Open Circuit Voltage

Approximate State-of-Charge at 80°F (26.7°C)

Hydrometer Average Cell Specific Gravity

Electrolyte Freeze Point

12.65

100%

1.265

-77°F(-67°C)

12.45

75%

1.225

-35°F(-37°C)

12.24

50%

1.190

-10°F(-23°C)

12.06

25%

1.155

15°F(-9°C)

11.89 or less

DISCHARGED

1.120 or less

20°F(-7°C)

Load testing should only be done if the state-of-charge of the battery is above 75% (12.45V). If the Open Circuit Voltage is below 75%, the battery must be recharged before load testing or the result will not be accurate. At the end of load testing, the terminal voltage under load should not drop below 9.6V (assuming temperature at 70°F). The following table shows the minimum terminal voltage under load for various temperatures.

Electrolyte Temperature °F

Electrolyte Temperature °C

Minimum Voltage Under LOAD

100°

37.8°

9.9

90°

32.2°

9.8

80°

26.7°

9.7

70°

21.1°

9.6

60°

15.6°

9.5

50°

10.0°

9.4

40°

4.4°

9.3

30°

-1.1°

9.1

20°

-6.7°

8.9

10°

-12.2°

8.7

-17.8°

8.5

Load Testing Procedures

  1. Make sure the battery is at room temperature (70-80°F). If the battery has been sitting out cold, it must be warmed up to room temperature for 4 to 12 hours before proceeding.
  2. Measure the state-of-charge of the battery (Open Circuit Voltage). If state-of-charge is below 75% (12.45V), the battery must be recharged before proceeding.
  3. Connect the battery to the voltmeter and the load for 15 seconds while monitoring the terminal voltage on the voltmeter.
  4. At the end of the 15 seconds, if the terminal voltage is below 9.6V (assuming at 70°F), remove the load, wait 10 minutes and measure the state-of-charge. If the state of charge is below 75% (12.45V), recharge the battery and do the test again (repeat from step 3). If the test failed again, the battery is bad and must be replaced.
  5. If the battery passes the test, you should recharge the battery as soon as possible to prevent sulfation and to restore it to peak performance.

Testing Procedures With an Automatic Load Tester

The testing procedures outlined above, although simple, are still tedious to execute. One must connect and disconnect the battery from the load and the voltmeter, watch the time for the load test duration and read the voltmeter at critical moment to determine the test result etc. To make our life easier, we have designed a battery automatic load tester that will simplify the above procedures to the following.

  1. Make sure the battery is at room temperature (70-80°F). If the battery has been sitting out cold, it must be warmed up to room temperature for 4 to 12 hours before proceeding.
  2. Connect the battery to the automatic load tester. Turn the power of the load tester ON.
  3. Read the built-in voltmeter on the load tester to make sure the battery voltage is at or above 12.45V. If not, disconnect the battery from the load tester and recharge it. Then repeat the procedures from step 2.
  4. Make sure the GREEN LED on the load tester is ON. If not, press the RESET button to reset it to GREEN.
  5. Press and release the TEST button. This will automatically connect the load to the battery for 15 seconds. At the end of the 15 seconds, the load will be disconnect automatically.
  6. If the battery passed the load test, the GREEN LED will remain ON. If it failed, the RED LED will be ON instead.
  7. If the battery failed, wait 10 minutes and check the voltmeter reading. If the voltage is below 12.45V, recharge the battery and try the test again (repeat from step 2). If the test failed again, replace the battery.
  8. If the battery passes the test, you should recharge the battery as soon as possible to prevent sulfation and to restore it to peak performance.

According to the updated testing procedure above, the load tester must be able to automatically connect the load when the TEST button is pressed and time the 15 second testing period. During the test period, it will continuously compare the battery voltage to the minimum voltage under load (9.6V). If the voltage dropped below the threshold, it will light up the RED LED. At the end of the test period, it will automatically disconnect the load from the battery.

Design of the Battery Automatic Load Tester

The design of the Automatic Load Tester can be divided into three sub-components (Automatic Load Tester Design PDF and Battery Load Tester Parts List).

Test Period Generator

The simplest way to generate a 15-second test period is to use the NE555 timer configured as a monostable circuit generating a 15-second square pulse when the trigger button is pressed (pulling the trigger pin low). The period of the generated pulse is controlled by the RC network (VR1 and C1) using the following equation:

Period = 1.1*VR1*C1

In our case, Period is 15 seconds, VR1 is a variable resistor that will be tuned to approx. 500kΩ. Therefore, Period = 1.1*0.5*27 ≈ 15s. The reason why the design uses a variable resistor to tune the period is that component values may not be accurate. For example, capacitors have an error tolerance of 20%. So we need to compensate this error by fine tuning the resistance.

The output of the NE555 timer will generate a 15-second positive pulse. The low of the pulse should be close to zero volt and the high should be close to Vs which is 15V in our case. According to the data sheet, NE555 can drive fairly high current (200mA) and since the coil of the relay consumes less than 80mA, we can sufficiently drive the relay with the NE555 output directly. To prevent back EMF from the inductive coil of the relay, we added the diodes for extra protection.

Test Load

To test whether the battery is good or bad, we need to draw one half of the Cold Crank Apms (CCA) for 15 seconds. Unfortunately, I was unable to find the CCA of the battery in any data sheet so I will take the advice from Team Skunk Work, who has also designed a battery load tester, that they are drawing 20A as the test current. The following equation is used to calculate the test load.

R = V/I

where V is 12V and I is 20A. Therefore, R is approx. 0.6Ω.

Resistance of the test load is only half of the equation, we also need to calculate power dissipation by using the following equation.

P = V*I

where V is 12V and I is 20A. Therefore, P is 240W.

When designing load resistors, it customary to give it some margin by at least doubling P. Therefore, the load resistor should be 0.6Ω 500W. Unfortunately, it cost about $100 for such a high power resistor. However, 1.5Ω 50W resistors are a lot cheaper. They are about $4 each. By using 10 of these connecting in series and in parallel, we can achieve an overall resistance of 0.6Ω and the combined power dissipation of 500W.

Because 20A is a very large current, a relay is used to connect and disconnect the test load from the battery. To prevent sparking when the relay is breaking contact, a 6800uF capacitor is connected across the contact. This will give approx. 4 ms of ramp down time when the contact is breaking.

Battery Voltage Monitor

The battery voltage monitor consists of an OpAmp, a NE555 timer configured as a bistable circuit (flip flop), two status LEDs and a digital voltmeter. The OpAmp is constantly comparing the battery voltage to the reference voltage of 9.6V. If the battery voltage is higher than the reference voltage, the output of the OpAmp remains low that will keep the NE555 timer in reset state. This in turn keeps the NE555 output low and thus lights up the GREEN LED. However, if the battery voltage dropped below the reference voltage, the output of the OpAmp will become high that will trigger the NE555 timer to flip its output to high. This will in turn light up the RED LED and hold it ON until the reset button is pressed in which case the GREEN LED will light up again. The reference voltage is established by the voltage divider resistors VR2 and R4. VR2 is a variable resistor that will be tuned to approx. 5.625kΩ calculated from the following equation.

   15V*(10kΩ/(10kΩ + VR2)) = 9.6V
=> 10k + VR2 = 150k/9.6
=> VR2 = 150k/9.6 - 10k = 5.625kΩ

The R6 resistor established a positive feedback for the OpAmp making it a comparator with Hysteresis. This eliminates possible noises that may make the output of the OpAmp flipping back and forth. However, this is not really required in our application because even if the OpAmp output is flipping back and forth, the bistable circuit will ignore further trigger anyway.

When the battery is connected to the load tester, the digital voltmeter will continuously monitor its terminal voltage. The digital voltmeter has no other purpose in the circuit than providing the voltage reading of the battery.

References

http://autorepair.about.com/library/weekly/aa101604a.htm

http://www.batterystuff.com/tutorial_battery.html#6

http://www.kpsec.freeuk.com/555timer.htm#monostable

http://www.onsemi.com/pub_link/Collateral/LM339-D.PDF

BattTester (last edited 2012-10-04 17:25:45 by MikeTsang)