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This data is from the Yuasa "Battery Handbook and Technical Guide"
Yuasa-Exide
P.O. Box 14145
Reading, PA 19612-4145

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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 3 hours 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.

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Temperature: 7 degrees Fahrenheit

Percent Charge	Hydrometer reading	Unloaded 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 Charge	Hydrometer reading	Unloaded 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 Charge	Hydrometer reading	Unloaded 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 Charge	Hydrometer reading	Unloaded 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 Charge	Hydrometer reading	Unloaded 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 Charge	Hydrometer reading	Unloaded 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 Charge	Hydrometer reading	Unloaded 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 Charge	Hydrometer reading	Unloaded Voltage
100	1.295	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 Charge	Hydrometer reading	Unloaded 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 Charge	Hydrometer reading	Unloaded 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 Charge	Hydrometer reading	Unloaded 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

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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 Charge	Specific Gravity	Degrees 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

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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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