State-of-Charge (SoC) Temperature Compensation Table4.4. Measure State-of-Charge (SoC)
State-of-Charge (SoC) Temperature Compensation Table
While working with lead-acid batteries, please help to prevent blindness and wear glasses in the unlikely event of an explosion. Below are eight simple steps in testing a Car or Deep Cycle battery. If you have a non-sealed wet battery (with filler caps), it is highly recommended that you use a good quality temperature compensating hydrometer, like an E-Z Red SP101, which can be purchased at an auto parts or battery store for less than $10.
How Do I Use a Hydrometer? A hydrometer is an inexpensive a float-type device used to measure the concentration of sulfuric acid (Specific Gravity) of battery electrolyte ("battery acid"). From this reading you can easily and accurately determine a non-sealed battery's State-of-Charge. A hydrometer is a glass barrel or plastic container with a rubber nozzle or hose on one end and a soft rubber bulb on the other. Inside the barrel or container, there is a float and calibrated graduations used for the Specific Gravity measurement. The following is a list of instructions on how to correctly use a battery hydrometer: BATTERY HYDROMETERS
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If you have a sealed battery or need to troubleshoot a charging or electrical system, you will need a digital voltmeter with 0.5% (or better) DC accuracy, such as a Fluke 175. A digital voltmeter can be purchased at an electronics store for less than $200. Analog voltmeters are not accurate enough to measure the millivolt differences of a battery's State-of-Charge or output of the charging system. Do not use a 12-volt test light to troubleshoot vehicle electrical circuits, except for testing the parasitic load at the battery, because you might damage the emissions computer or other sensitive electronic devices. A good source of information on measuring voltage and for maximum voltage drops can be found at Exide's Caring For Your Battery. A battery load tester is optional. Another way of testing the CCA (Cold Cranking Amp) or capacity of lead-acid car batteries is by using a conductance tester, such as a Midtronics, costing $100 to $600. The most accurate way of testing a starting or Deep Cycle battery is a full capacity load test.
Visually inspect for obvious problems such as a low electrolyte levels, dirty or wet battery top, corroded or swollen cables, corroded terminals or battery posts, loose hold-down clamps, loose cable terminals, or a leaking or damaged battery case. If the electrolyte levels are below the tops of the plates, add enough distilled, deionized or demineralized water to cover the plates and recharge the battery. The plates need to be covered at all times to prevent an internal battery explosion or sulfation. Please see Section 3.2 for electrolyte fill levels.
Charge the battery to 100% State-of-Charge. If non-sealed battery has a .030 (sometimes expressed as 30 "points") or more difference in specific gravity reading between the lowest and highest cell or if a cell is .010 or 10 "points" below the reading for a fully charged cell, then you should equalize the battery using the battery manufacturer's procedures. (Please see Section 9.)
Surface charge is the uneven mixture of sulfuric acid and water along the surface of the plates as a result of charging or discharging. It will make a weak battery appear good or a good battery appear bad. You need to eliminate the surface charge by one of the following methods after recharging a lead-acid car battery:
4.3.1. Allow the battery to sit (or rest) without discharge or recharge for between six to twelve hours in a warm room, if possible, to allow for the surface charge to dissipate. (Recommended method.)
4.3.2. Turn the headlights on high beam for five minutes, turn them off, and wait five to ten minutes.
4.3.3. With a battery load tester, apply a load at one-half the battery's CCA rating for 15 seconds and then wait five to ten minutes.
4.3.4. Disable the ignition, turn the engine over for 15 seconds with the starter motor, and wait five to ten minutes.
4.3.5. Apply a load that is 33% of the ampere-hour capacity for five minutes and wait five to ten minutes.
4.3.6. With a battery load tester, apply a load is one third the battery's amp-hour rating for five minutes and wait five to ten minutes.
4.4. Measure the State-of-Charge (SoC)
If the battery's electrolyte is above 125° F (51.5° C), allow it to cool. To determine the battery's SoC with the battery's electrolyte temperature at 80° F (26.7° C), please use the following table. The following table has a baseline that assumes that a 1.265 specific gravity and 12.65 Open Circuit Voltage (OCV) reading is a fully charged (100%), wet, Low Maintenance (Sb/Ca) lead-acid battery at rest and with an open circuit (no external current running through it and the negative battery cable disconnected). For electrolyte temperatures other than 80° F (26.7° C), please use the Temperature Compensation table below to adjust the Open Circuit Voltage (OCV) or Specific Gravity readings. The Specific Gravity or OCV readings for a battery at 100% SoC will vary by plate chemistry, so check the battery manufacturer's specifications for their State-of-Charge definitions for your battery. If you do not know the baseline for your battery at 100% SoC, please see Section 9.5. How Do I Know When My Battery Is Fully Charged? A fully charged wet battery at 80° F (26.7° C), can range from 1.215 to 1.300 Specific Gravity (12.22 to 13.00 VDC) and a sealed Valve Regulated Lead-Acid (VRLA) Gel Cell or AGM battery from 1.285 to 1.310 (12.85 to 13.1 VDC).
STATE-OF-CHARGE (SoC)
|
Digital Voltmeter Open Circuit Voltage at Rest |
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 |
|
12.45 |
75% |
1.225 |
-35°F |
|
12.24 |
50% |
1.190 |
-10°F |
|
12.06 |
25% |
1.155 |
15°F |
|
11.89 or less |
DISCHARGED |
1.120 or less |
20°F |
[Source: BCI]
UPDATED STATE-OF-CHARGE
TEMPERATURE COMPENSATION
|
Electrolyte Temperature Degrees Fahrenheit |
Electrolyte Temperature Degrees Celsius |
Add or Subtract to Hydrometer's SG Reading |
Add or Subtract to Digital Voltmeter's Reading |
|
120° |
48.9° |
+.016 |
-.0129 |
|
110° |
43.3° |
+.012 |
-.0109 |
|
100° |
37.8° |
+.008 |
-.0084 |
|
90° |
32.2° |
+.004 |
-.0049 |
|
80° |
26.7° |
0 |
0 |
|
70° |
21.1° |
-.004 |
+.0070 |
|
60° |
15.6° |
-.008 |
+.0158 |
|
50° |
10° |
-.012 |
+.0279 |
|
40° |
4.4° |
-.016 |
+.0435 |
|
30° |
-1.1° |
-.020 |
+.0621 |
|
20° |
-6.7° |
-.024 |
+.0843 |
|
10° |
-12.2° |
-.028 |
+.1083 |
|
0° |
-17.8° |
-.032 |
+.1335 |
Electrolyte temperature compensation, depending on the battery manufacturer's definition of 100% State-of-Charge, will vary. If you are using a digital DC voltmeter or a non-temperature compensated HYDROMETER, make the adjustments indicated in the table above. Please note that some battery manufacturers express their SoC definitions at 77° F (25° C), and some slight temperature compensation should occur to normalize the definitions at 80° F (26.7° C) to use the tables above. For example, if the electrolyte is at 80° F (26.7° C), and the specific gravity reading is 1.265 for a 100% SoC, when the electrolyte is at 20° F (-6.7° C), the actual specific gravity reading would be 1.289 for a 100% State-of-Charge because the liquid is more dense. However, when you subtract .024 from 1.289, the corrected reading would be 1.265 or 100% State-of-Charge. At 100° F (37.8° C), the actual specific gravity reading would be 1.257 for 100% SoC, but the compensated reading, after .008 is added, would be 1.265 for 100% State-of-Charge. This is why using a temperature compensated hydrometer is highly recommended and more accurate.
If you are using an accurate (.5% or better) DIGITAL DC VOLTMETER, make the adjustments indicated in the table above. For example, if the electrolyte is at 80° F (26.7° C), and the voltage reading is 12.65 for a 100% State-of-Charge, when the electrolyte is at 20° F (-6.7° C), the actual voltage reading would be 12.566 for a 100% State-of-Charge. Before you correct the reading by adding .0843 volts (84.3 millivolts). At 100° F (37.8° C), the actual voltage reading would be 12.658 for 100% SoC.
For non-sealed batteries, please check the specific gravity in each cell with a hydrometer and average cells readings. For sealed batteries, measure the Open Circuit Voltage (OCV) across the battery terminals with an accurate (.5% or better) digital DC voltmeter. This is the only way you can determine the battery's SoC. Some batteries have a built-in hydrometer, Magic Eye, which only measures the State-of-Charge in ONE of its six cells.
Magic Eye Built-in Hydrometer
[Source: Popular Mechanics]
If the State-of-Charge is BELOW 75% using either the Specific Gravity, voltage test or the built-in hydrometer does not indicate "good" (green or blue), then the battery has a low charge and needs to be recharged before proceeding. If the battery is sealed, the battery could have low electrolyte, especially in a hot climate. You should replace the battery, if one of the following conditions occur:
4.4.1. If there is a .050 (sometimes expressed as 50 "points") or more difference in the specific gravity reading between the highest and lowest cell, you have a weak or dead cell(s). Applying an EQUALIZING charge per the battery manufacturer's procedures may correct this condition. (Please see Section 9.)
4.4.2. If the battery will not recharge to a 75% or more State-of-Charge level or if the built-in hydrometer still does not indicate "good" (green or blue), which indicates a 65% SoC or better).
4.4.3. If a moderate load is applied and if there is no or very little current flowing there is an probably an open cell or a completely sulfated battery. Without a load, a voltmeter reading may or may not indicate an open.
4.4.4. If the digital voltmeter indicates 10.45 to 10.65 volts, there probably is a shorted cell. A shorted cell is caused by plates touching, sediment ("mud") build-up or "treeing" between the plates.
If the battery's State-of-Charge is at 75% or higher or has a "good" built-in hydrometer indication, then you can load test the Car or Deep Cycle battery by one of the following methods.
4.5.1. Car Batteries:
4.5.1.1. With a battery load tester, apply a load equal to one half of the CCA rating of the battery for 15 seconds. (Recommended method).
4.5.1.2. With a battery load tester, apply a load equal to one half the OEM cold cranking amp specification for 15 seconds.
4.5.1.3. Disable the ignition and turn the engine over for 15 seconds with the starter motor.
DURING the load test, the voltage on a good Car battery will NOT drop below the following table's indicated voltage for the electrolyte at the temperatures shown:
Capacity Load Test
|
Electrolyte Temperature Fahrenheit |
Electrolyte Temperature Celsius |
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 |
|
0° |
-17.8° |
8.5 |
[Source: BCI]
4.5.2. Deep Cycle Batteries:
If the battery is fully charged or has a "good" built-in hydrometer indication, then you can test the capacity of a deep cycle battery by applying a known load and measuring the time it take to discharge the battery until measures 50% SoC as defined by the battery manufacturer. Normally a discharge rate that will discharge a battery in 20 hours can be used. For example, if you have an 80 ampere-hour rated battery, then an average load of four amps would discharge the battery to 50% SoC in approximately 10 hours. For an estimate of the battery's capacity, double the amount of time it took to discharge to 50% SoC and multiply by the average discharge rate in amps. Some new batteries can take up to 30 charge/discharge "preconditioning" cycles before they reach their rated capacity. Depending on your application, fully charged batteries with 80% or less of their original rated capacity available are considered to be bad. If the Deep Cycle battery passed the Capacity Load Test, then skip the next test, Section 4.6 Bounce Back Test and go to Section 4.7. Recharge below.
If the Car battery has passed the load test, please go to Section 4.7. Recharge below. If not, remove the load, wait ten minutes, and measure the State-of-Charge. If the battery bounces back to less than 75% SoC then recharge the battery (please see Section 9.) and load test again. If the Car battery fails the load test a second time or bounces back to less than 75% SoC, then replace the battery because it lacks the necessary CCA capacity.
You should recharge your battery to 100% SoC as soon as possible to prevent lead sulfation and to restore it to peak performance.
When the non-sealed wet battery (with filler caps) has cooled to room temperature, recheck the electrolyte levels and, if necessary, fill to the correct level. Please see Section 3.2 for electrolyte fill levels.
