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    GEO instrumentation    

   LEAD ACID BATTERY: MEASURING AMP-HOUR CAPACITY

UNIVERSITY OF BERGEN
Department of Earth Science
 Allé gt. 41, N-5007 Bergen, Norway 
             
0.1 DRAFT - for comments - 4 February, 2019 O.M. - -
VER. STATUS CHANGE DATE BY CHECKED APPROVED
Yuasa rechargeable battery model RE7-12.

Yuasa rechargeable battery model RE7-12.

From the specifications:

  • 20-hr rate Capacity to 1.75VPC at 20°C: 7 Ah
  • 10-hr rate Capacity to 1.75VPC at 20°C: 6.2 Ah
  • "1.75VPC" means 1.75V per cell. There are 6 cells, so 1.75 V * 6 = 10.5 V measured at the terminals.
  • "20-hr rate" means that the battery should be discharged with a constant current over a period of 20 hours, i.e. current should be 7 Ah/ 20h = 0.35 A

1 INTRODUCTION


We purchased current/voltage sensor with I2C digital interface - Adafruit current/voltage monitor - with future Jan Mayen upgrades in mind: Several such sensors can be combined on the same serial (I2C) bus, making it possible to monitor current/voltage on many locations within any new prototype Jan Mayen power system test setup - by e.g. logging current from wind generator and solar panel, battery charge/discharge current and also voltages.

We tested the new sensor by obtaining discharge characteristics of a typical Seismo lab battery, manufactured by Yuasa, with model designation RE7-12 (12V/7Ah). Prior to testing, the battery was fully charged, using Nordic Power model SBC-8168 charger (see details below); during charging process the charger didn't indicate anything wrong (it has a "repair" LED indicator).

Afterwards a constant current circuit acted as a battery load; with current set by resistors and a LM317 regulator IC. The sensor was attached to Arduino board where a C-program collected sensor data, also halting further discharge of the battery when the terminal voltage reached the recommended lower limit (10.5V). The Arduino transmitted serial data to a laptop where a tiny Python scripts did time stamping and file storage.

2 TEST RESULT


Conclusion: Capacity was 2.1Ah - only 30% of the rated 7Ah.

The battery is old - from May 1999 - so perhaps we shouldn't expect anything else. So lead batteries requires some attention.

Click to enlarge.

Lead battery capacity test. Discharge curve shows both terminal voltage and accumulated charge [Ah]. Constant current load is 0.05C, where C is capacity, in this case 350 mA since it's a 7Ah battery. Click to enlarge.

Discharge characteristics for Yuasa NP-series lead acid battery. Probably quite similar to corresponding RE-series. Source: NP_7_12_DataSheet.pdf

Test should be repeated with controlled charging first - we are now dependent on battery charger - we have neither knowledge nor control over charging characteristics. What, for instance, determines when charging stops? We should consider acquiring a battery charger recommended by Yuasa, described here.


2.1 Charger used during May 2015 test: Nordic Power model SBC-8168


Before testing, the battery was fully charged using Nordic Power model SBC-8168 lead acid charger.

Nordic Power model SBC-8168 3-stage smart lead acid battery charger.

Features:

  1. Charger can be permanently attached to the battery (ref. User's Manual, p.1: "Features #4").
  2. Eventually, Float Mode is reached. In that case (User's Manual, p.4):
    "Float : PWM maintenance charging and cycle charging. In this mode the charger does not deliver current when battery voltage is above a set voltage. When battery voltage drops below the set voltage, charger will deliver small current until the battery voltage rises to about 13.5V. Then the maintenance charging cycle repeats. In the case of battery voltage drops more than 12.2V due to external load or otherwise , the charger will start a new charging cycle of Bulk, Absorption and Float."
  3. The charger returns to the last mode used, if 230 Vac is interrupted, after 2 minute delay.


3 TEST CONFIGURATION


Click to enlarge.

Battery discharge test setup. Click to enlarge.

Click to enlarge.

Battery discharge test setup. Constant current load, consisting of LM317 regulator and resistors, in upper left section. Click to enlarge.


4 BATTERY TEST EQUIPMENT


4.1 Hioki


We consider acquiring battery test equipment that Hioki makes: Battery HiTester 3554. THIS MODEL IS NOT PRODUCED ANY LONGER.

Battery test equipment recommended by Yuasa.

From Hioki model 3554 manual:

4.2 Bosch


Bosch model BAT 110 Battery Tester

4.3 Actmeters


ACT CHROME 12V SLA Intelligent Battery Tester -- we have purchased this unit

ACT 612 6V/12V Lead Acid Intelligent Battery Tester

'

5 LEAD BATTERY FLOAT CHARGE RECOMMENDATIONS, DATA SHEET


Below is datasheet for another Yuasa battery series where details about how to charge in order to obtain rated lifetime is stated. The life time is clearly reduced if the so called float voltage is a bit outside specifications. See separate figure Relationship between Float Charge Voltage and Battery Life at 20°C; at that temperature it should be 2.275V/cell ±1%, i.e. 13.514V < VFLOAT < 13.787V; temperature compensation is also important.

Attach:Yuasa-battery-model-series-NPL-plNPL_data_de-en.pdf

6 TERMINAL VOLTAGE AS FUNCTION OF SOC (STATE OF CHARGE), LEAD-ACID BATTERIES


This table shows relation between terminal voltage (open-circuit) and SOC (state of charge) for lead-acid batteries. [Capacity of battery is something else.] Assuming 20 deg C.

SOC = State of charge 12 V battery Individual cell

7 LINKS


8 BATTERY DISCHARGE TEST SOFTWARE


./projects/battery-capacity-test/Arduino-read-current-and-voltage-sensor/

8.1 Arduino software - interface to current / voltage sensors


#include <Wire.h>
#include <Adafruit_INA219.h>

Adafruit_INA219 ina219;

void setup(void) 
{

  // Initialize digital pin 13 as an output.
  pinMode(2, OUTPUT);
  digitalWrite(2, HIGH);   // turn the LED on (HIGH is the voltage level)

  uint32_t currentFrequency;

  Serial.begin(115200);
  //Serial.begin(9600);
  //Serial.println("Hello!");  
  //Serial.println("Measuring voltage and current with INA219 ...");
  ina219.begin();
}

void loop(void) 
{
  float shuntvoltage = 0;
  float busvoltage = 0;
  float current_mA = 0;
  float loadvoltage = 0;
  float power = 0;
  long sec_since_epoch = 0;

  shuntvoltage = ina219.getShuntVoltage_mV();
  busvoltage = ina219.getBusVoltage_V();
  current_mA = ina219.getCurrent_mA();
  loadvoltage = busvoltage + (shuntvoltage / 1000);
  power = loadvoltage * current_mA;
  //sec_since_epoch = now();


  //Serial.print("Bus Voltage:   "); Serial.print(busvoltage); Serial.println(" V");
  //Serial.print("Shunt Voltage: "); Serial.print(shuntvoltage); Serial.println(" mV");
  //Serial.print("Load Voltage:  "); Serial.print(loadvoltage); Serial.println(" V");
  //Serial.print("Current:       "); Serial.print(current_mA); Serial.println(" mA");

  Serial.print("$PUIBBM,");   // $PUIBBM = Proprietary, UIB, Battery Measurement
  Serial.print(current_mA);   Serial.print(",mA");  Serial.print(",");
  Serial.print(loadvoltage);  Serial.print(",V");   Serial.print(",");
  Serial.print(power);        Serial.print(",mW");  Serial.print(",");
  Serial.print(shuntvoltage); Serial.print(",mV");  Serial.print(",");
  Serial.print(busvoltage);   Serial.print(",V");   Serial.print(",");
  //Serial.print(sec_since_epoch);
  Serial.println("*00");

  //Serial.println("");

  if (loadvoltage < 10.5)
    digitalWrite(2, LOW);    // turn the LED off by making the voltage LOW

  if (loadvoltage >= 10.7)
     digitalWrite(2, HIGH);   // turn the LED on (HIGH is the voltage level)


  delay(991);
}

8.2 Example data file


./projects/battery-capacity-test/Arduino-read-current-and-voltage-sensor/2015-05-03_163305.txt

Example content of 2015-05-03_163305-a.txt:

$PUIBBM,350.10,mA,12.02,V,4207.86,mW,35.02,mV,11.98,V,*00,0.0
$PUIBBM,351.00,mA,11.99,V,4208.87,mW,35.07,mV,11.96,V,*00,1.0
$PUIBBM,350.90,mA,11.98,V,4204.86,mW,35.08,mV,11.95,V,*00,2.0
$PUIBBM,351.00,mA,11.98,V,4204.66,mW,35.10,mV,11.94,V,*00,3.0
$PUIBBM,351.00,mA,11.97,V,4201.85,mW,35.09,mV,11.94,V,*00,4.0
$PUIBBM,351.10,mA,11.97,V,4201.65,mW,35.10,mV,11.93,V,*00,5.0
$PUIBBM,351.20,mA,11.96,V,4201.44,mW,35.10,mV,11.93,V,*00,6.0
$PUIBBM,350.70,mA,11.96,V,4195.45,mW,35.08,mV,11.93,V,*00,7.0
$PUIBBM,350.70,mA,11.96,V,4194.05,mW,35.09,mV,11.92,V,*00,8.0
$PUIBBM,350.70,mA,11.96,V,4194.04,mW,35.06,mV,11.92,V,*00,9.0
$PUIBBM,350.80,mA,11.96,V,4193.84,mW,35.08,mV,11.92,V,*00,10.0
$PUIBBM,350.90,mA,11.95,V,4193.65,mW,35.12,mV,11.92,V,*00,11.0
$PUIBBM,351.10,mA,11.95,V,4196.02,mW,35.08,mV,11.92,V,*00,12.0
$PUIBBM,350.70,mA,11.95,V,4189.83,mW,35.06,mV,11.91,V,*00,13.0

8.3 Gnuplot commands


#GNUPLOT commands related to battery tests May/June 2009

set terminal png size 900,600 font "Helvetica,12"
set output 'output.png'

set title "NNSN - SEISMO LAB - Battery capacity test\nBattery: Yuasa mod. RE7-12 (12 V, 7.0 Ah)\nDischarge current: 0.05 CA (350 mA).\nMeasured 4 May 2015, by O.M."

set grid

set logscale x
set xrange [0.016667:8]
unset xtics
set xtics format " "
set xtics add ("1m"  0.016667)
set xtics add ("2m"  0.03333)
set xtics add ("4m"  0.06667)
set xtics add ("6m"  0.1)
set xtics add ("10m" 0.16667)
set xtics add ("20m" 0.33333)
set xtics add ("40m" 0.66667)
set xtics add ("1" 1, "2" 2, "3" 3, "4" 4, "5" 5, "6" 6, "8" 8)

set xlabel 'Discharge time [Hours]'

set yrange [10:13]
set ylabel 'Terminal voltage [Volt]'
#set ytics add ("Initial value 12.08V" 12.08)
#set ytics add ("Discharged 10.5V" 10.5)

set y2tics 0.5
set y2range [0:3]
set y2tics border
set y2label 'Charge [A*Hour]'

set datafile separator ","

plot '2015-05-03_163305-a.txt' using ($13/3600):4 title 'Voltage' with lines,\
     '2015-05-03_163305-a.txt' using ($13/3600):(($2/1000)*($13/3600)) title 'Charge' with lines axes x1y2 

This script yields the following plot:

Click to enlarge.

Lead battery capacity test. Discharge curve shows both terminal voltage and accumulated charge [Ah]. Constant current load is 0.05C, where C is capacity, in this case 350 mA since it's a 7Ah battery. Click to enlarge.

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Page last modified on February 04, 2019, at 05:25 PM
Electronics workshop
Department of Earth Science - University of Bergen
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