How can I measure my battery's “State of Charge”?
If the battery's temperature, or electrolyte in flooded types, is above 110° F (43.3° C), allow it to cool. To determine the battery's state of charge with the battery's temperature at 80° F (26.7° C), use the following tables. Table 1 assumes that a 12.65 voltage reading or a 1.265 specific gravity reading represents a fully charged battery. For other battery or electrolyte temperatures, use the Temperature Compensation tables below to adjust the Open Circuit Voltage for VRLA DRY CELL batteries or Specific Gravity readings for flooded types.
Table 6 - Electrolyte Freezing Point
|
Digital Voltmeter |
State of Charge |
Hydrometer Reading |
Electrolyte |
|
Open Circuit Volts |
Approximate |
Specific Gravity |
Freezing Point |
|
12.65 |
100% |
1.265 |
-75° F (-59.4° C) |
|
12.45 |
75% |
1.225 |
-55° F (-48.3° C) |
|
12.24 |
50% |
1.190 |
-34° F (-36.7° C) |
|
12.06 |
25% |
1.155 |
-16° F (-26.7° C) |
|
11.89 |
Discharged |
1.120 |
-10° F (-23.3° C) |
Table 7 - Hydrometer Temperature Compensation
|
Temperature |
Add or Subtract to Hydrometer Reading |
Add or Subtract to Voltmeter Reading |
|
0° / -17.8° |
-.032 |
-.192 |
|
10° / -12.2° |
-.028 |
-.168 |
|
20° / -6.7° |
-.024 |
-.144 |
|
30° / -1.1° |
-.020 |
-.120 |
|
40° / 4.4° |
-.016 |
-.096 |
|
50° / 10° |
-.012 |
-.072 |
|
60° / 15.6° |
-.008 |
-.048 |
|
70° / 21.1° |
-.004 |
-.024 |
|
80° / 26.7° |
0 |
0 |
|
90° / 32.2° |
+.004 |
+.024 |
|
100° / 37.8° |
+.008 |
+.048 |
|
110° / 43.3° |
+.012 |
+.072 |
|
120° / 48.9° |
+.016 |
+.096 |
|
130° / 54.4° |
+.020 |
+.120 |
|
140° / 60.0° |
+.024 |
+.144 |
|
150° / 65.6° |
+.028 |
+.168 |
|
160° / 71.1° |
+.032 |
+.192 |
Table 8 - Voltmeter Temperature Compensation
|
Temperature |
Add or Subtract to Hydrometers Reading |
Add or Subtract to Voltmeter Reading |
|
0° / -17.8° |
-.032 |
-.192 |
|
10° / -12.2° |
-.028 |
-.168 |
|
20° / -6.7° |
-.024 |
-.144 |
|
30° / -1.1° |
-.020 |
-.120 |
|
40° / 4.4° |
-.016 |
-.096 |
|
50° / 10° |
-.012 |
-.072 |
|
60° / 15.6° |
-.008 |
-.048 |
|
70° / 21.1° |
-.004 |
-.024 |
|
80° / 26.7° |
0 |
0 |
|
90° / 32.2° |
+.004 |
+.024 |
|
100° / 37.8° |
+.008 |
+.048 |
|
110° / 43.3° |
+.012 |
+.072 |
|
120° / 48.9° |
+.016 |
+.096 |
|
130° / 54.4° |
+.020 |
+.120 |
|
140° / 60.0° |
+.024 |
+.144 |
|
150° / 65.6° |
+.028 |
+.168 |
|
160° / 71.1° |
+.032 |
+.192 |