The Studeblogger

Thursday, October 23, 2008

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.

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