Lead-acid battery: Voltage, State of Charge, and S.G.

As I think about Lead-acid batteries and the voltage and s.g. as measures of the state of charge, I am getting confused.
If I charge a battery fully, both the s.g. and the voltage will be reasonable indicators of the state of charge, right?
Now what happens if I realize that the electrolyte level is low and add water and let the electrolyte stabilize? The s.g. will be reduced, I assume, but what happens to the voltage? Does that drop because of the dilution of the electrolyte? If not, the voltage still indicates full charge, but the s.g. indicates less than full charge.
Answers, please -- simple, if possible.
Perce
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For single cells of lead-acid batteries: Specific Gravity = Cell Open Circuit Voltage - 0.845
I think this is for 78F/25C temperature, for cells that have rested (no charge/discharge) for at least an hour.
Bill Kaszeta Photovoltaic Resources Int'l Tempe Arizona USA snipped-for-privacy@pvri-removethis.biz
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In your example, the electrolyte will read high until the water is added, since adding water is replacing the water displaced during the charging process.
On Sun, 21 Aug 2005 19:46:52 -0400, "Percival P. Cassidy"

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Percival P. Cassidy wrote:

Here's, a very simplified description, that nonetheless tells you what is happening. During discharge, heavy "stuff" that is in solution is deposited on the plates of the battery, thereby making the liquid lighter, so the SG decreases. The opposite happens when charging - the heavy "stuff" that is on the plates is driven back into the liquid solution, making the SG increase.
When the electrolyte level is low, it is because the water, not the heavy "stuff" has evaporated. Adding water raises the level of the electrolyte and dilutes it, just as you said. But the heavy "stuff" is still in the battery, deposited on the plates. Charging the battery forces that stuff back into solution, and the SG returns to where it should be.
SG is, as you said, an indicator of the state of charge of a battery. In the scenario you asked about, where some of the water evaporated and then the level was brought back up by adding water, both the SG and the voltage would indicate that the battery was not fully charged. Charging it would raise both the voltage and the SG to their fully charged levels.
Ed
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The SG for a fully charged battery *assumes* normal electrolyte level.
For large cells, there is usually a 'level correction' that is applied to the SG reading, much like the temperature correction. So for every xx below normal, you subtract .001 from the SG reading to get the corrected reading.
It's best to add water just before charging. But it is important to not over-water as the charging will create bubbles that can cause the electrolyte to overflow. But the charging will help to mix the electrolyte. If you add water to a battery and then leave it open circuit, the water (being lighter than the acid) will not mix evenly and this can be bad for a cell. It can also screw up tomorrow's SG reading if it hasn't mixed in yet.
daestrom
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