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J.R. Buchanan

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Lead/Acid Batteries

Intro

This data is from the Yuasa "Battery Handbook and Technical Guide", which I got for free from Yuasa at:

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

I don't know if they are still giving them away, but if they are, it's worthwhile sending for one.

There was a lot of data in the book referring to Yuasa specific chemistries, as well as conventional techniques (also used in some Yuasa batteries).

Voltage vs percent charge

The Yuasa book has a chart of specific gravity vs. % charge at 77 degrees Fahrenheit. Then they specify that to find the cell voltage (this is the voltage of a single cell, not the entire battery!), one should add .84 to the specific gravity. To find the voltage for a 6 cell (nominal 12V) battery, multiply by 6. It is stated that the specific gravity changes by .001 for every 3 degrees Fahrenheit. I used this data to build these charts from the one supplied for 77 degrees. Since the calculated values for the voltage were off by a small amount at 77 degrees, I used the calculated values in my chart so that it could be compared to the charts calculated for other temperatures.

Note that the voltages calculated are for an unloaded battery. This means that you shouldn't be charging while you take these readings either! I'd suggest waiting several minutes after disconnecting a load or charger before measuring the voltage.

It is also mentioned that Yuasa's "Sulphate Stop" will make for both higher specific gravity readings and voltages. One presumes that other anti-sulphation products may have similar results.

Temperature: 7 degrees Fahrenheit

 Percent  Hydrometer  Unloaded
  charge     reading   voltage
     100       1.242     12.49
      75       1.187     12.16
      50       1.137     11.86
      25       1.097     11.62
       0       1.077     11.50


Temperature: 17 degrees Fahrenheit

 Percent  Hydrometer  Unloaded
  charge     reading   voltage
     100       1.245     12.51
      75       1.190     12.18
      50       1.140     11.88
      25       1.100     11.64
       0       1.080     11.52


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


Temperature: 107 degrees Fahrenheit

 Percent  Hydrometer  Unloaded
  charge     reading   voltage
     100       1.275     12.69
      75       1.220     12.36
      50       1.170     12.06
      25       1.130     11.82
       0       1.110     11.70

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.

I'm presuming the last entry (way less than 0% charge) is a battery that's been sitting a long time.

The book didn't have % charge, I was curious and 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
     <0   1.050          27

Self discharge

According to the Yuasa book, conventional lead-acid batteries discharge at a rate of 1% per day at 77 degrees Fahrenheit. That means that in a bit more than three months, your battery is dead. Faster if your bike/car draws some current when not running, which many modern vehicles do.

An increase of 18 degrees doubles the discharge rate. Although not stated, I presume that a decrease of 18 degrees halves it.

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