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The following chart can be used to determine the approximate level of charge for a wet cell battery. To use this chart you will need a good digital multi-meter or a reasonably good hydrometer. The hydrometer is more accurate, but a digital meter is probably good enough for most RV use. To determine battery condition, readings should be taken after a battery has been charged and then allowed to "rest' with no charge or discharge for about two hours. If you use a hydrometer, all cells of a battery should measure within 0.01 of each other.

This data is from the Yuasa "Battery Handbook and Technical Guide."

	Yuasa-Exide
P.O. Box 14145
Reading, PA 19612-4145

Voltage vs percent charge

Note that the voltages calculated are for an unloaded battery. This means that you shouldn't be charging or discharging while you take these readings. Always wait several minutes after disconnecting a load or at least one hour after charging before measuring the voltage. For specific gravity you do not need to lift the leads to the battery in order to be accurate enough for most RV use.

Temperature: 27 degrees Fahrenheit

 Percent  Hydrometer  Unloaded
  charge     reading   voltage
     100       1.248     12.53
      75        1.193     12.20
      50        1.143     11.90
      25        1.103     11.66
       0         1.083     11.54


Temperature: 37 degrees Fahrenheit

 Percent  Hydrometer  Unloaded
  charge     reading   voltage
     100       1.252     12.55
      75        1.197     12.22
      50        1.147     11.92
      25        1.107     11.68
       0         1.087     11.56


Temperature: 47 degrees Fahrenheit

 Percent  Hydrometer  Unloaded
  charge     reading   voltage
     100       1.255     12.57
      75        1.200     12.24
      50        1.150     11.94
      25        1.110     11.70
       0         1.090     11.58


Temperature: 57 degrees Fahrenheit

 Percent  Hydrometer  Unloaded
  charge     reading   voltage
     100       1.258     12.59
      75        1.203     12.26
      50        1.153     11.96
      25        1.113     11.72
       0         1.093     11.60


Temperature: 67 degrees Fahrenheit

 Percent  Hydrometer  Unloaded
  charge     reading   voltage
     100       1.262     12.61
      75        1.207     12.28
      50        1.157     11.98
      25        1.117     11.74
       0         1.097     11.62


Temperature: 77 degrees Fahrenheit

 Percent  Hydrometer  Unloaded
  charge     reading   voltage
     100       1.265     12.63
      75        1.210     12.30
      50        1.160     12.00
      25        1.120     11.76
       0         1.100     11.64


Temperature: 87 degrees Fahrenheit

 Percent  Hydrometer  Unloaded
  charge     reading   voltage
     100       1.268     12.65
      75        1.213     12.32
      50        1.163     12.02
      25        1.123     11.78
       0         1.103     11.66


Temperature: 97 degrees Fahrenheit

 Percent  Hydrometer  Unloaded
  charge     reading   voltage
     100       1.272     12.67
      75        1.217     12.34
      50        1.167     12.04
      25        1.127     11.80
       0         1.107     11.68

 

Freezing points vs specific gravity

The specific gravity in this chart is measured at 77 degrees Fahrenheit, at the point where the electrolyte actually freezes, it will be lower..

The  % charge is interpolated these numbers from the 77 degree chart above. The reason that 1.15 come out to 50% instead of a lower number is rounding.

           Freezing,
Percent Specific Degrees
Charge Gravity Fahrenheit
100 1.265 -75
76 1.225 -35
74 1.200 -17
50 1.150 5
0 1.100 18




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