Checking specific gravity on a sealed battery

While I was sussing out my battery troubles, I came across a formula to calculate the specific gravity of a sealed battery (an indication of the water-to-acid ratio in the cells, which indicates its state of charge. You used to be able to perform this test with a hydrometer on open-cell batteries, but those days are gone. So, just for reference, here is the process:

1. Disconnect the battery cables.
2. Measure the voltage between the posts.
3. Divide the voltage reading by 6 and then subtract 0.85 from the product.
   

So, say you measured 12.6 volts at the posts. Dividing by 6 equals 2.1. Subtract 0.85 and the result is 1.25, is the specific gravity of the battery.

The Studebaker Shop Manual helpfully lists what the specific gravity test reveals:

• 1.260 = Fully charged.
• 1.220 = 75% charged.
• 1.170 = 50% charged.
• 1.120 = 25% charged.
• 1.070 = Discharged.

The manual also suggests that any battery that measures below 1.220 specific gravity should be recharged.