Does anyone still measure the specific gravity of electrolyte in a wet lead acid battery to determine the battery's state of charge? I doubt it, but it's still interesting to know and a tool you can use to help determine the condition of a wet battery.
Specific gravity is the ratio of the density of a liquid and the density of the same volume of pure water at 80F. I've also heard this explained as the ratio of the weight of a liquid and the weight of pure water at 80F, to make it easier to understand. For example, the specific gravity of pure water is 1.0 because both liquids are pure water and have the same density. The specific gravity of sulfuric acid is 1.835 at 80F, and has a higher density than pure water. That means sulfuric acid weighs more than the same volume of pure water at 80F. The electrolyte used in a wet battery is 64 percent water and 36 percent sulfuric acid, and has a specific gravity of 1.260 to 1.280 at 80F.
When a battery discharges, the sulfates in the electrolyte solution react with the lead plates in the battery turning them into lead sulfate. The sulfate leaves the electrolyte solution and reacts with the lead plates when a battery is discharging. This chemical reaction changes the percentage of both the sulfuric acid and water in the electrolyte and changes the specific gravity. When a battery is charged, the sulfate in the lead plates go back into solution and changes the percentage of water and sulfuric acid in the electrolyte and it's specific gravity. This is why measuring the specific gravity of the electrolyte can tell you the battery's state of charge.
The tool used to measure specific gravity is called a hydrometer. It's basically a glass tube with some type of float inside the tube, and a rubber collapsible ball at one end to suck and push electrolyte in and out of the glass tube. How high the float inside the tube floats when electrolyte is in the glass tube determines the electrolytes specific gravity. This instrument works because the specific gravity of a liquid can affect how an object floats in the liquid. A good example of this is salt water compared to fresh water. If a boat is loaded with cargo it will float differently in the ocean than it does in fresh water. If the boats journey starts in the ocean but transitions to fresh water during the trip, it could sink when it reaches fresh water if the specific gravity of salt water and fresh water is not taken into account when loading the boat.
You can see what a hydrometer looks like and read about it more using this link. This is a paid Amazon link:
https://amzn.to/3EAcmZwRelationship between specific gravity, battery voltage and state of charge:
| Specific Gravity | Battery Voltage | State of Charge |
| 1.265 | 12.6 or higher | 100% Charged |
| 1.225 | 12.4v | 75% Charged |
| 1.190 | 12.2v | 50% Charged |
| 1.155 | 12.0v | 25% Charged |
| Lower than 1.120 | 11.9v or lower | Discharged |
When using a hydrometer the reading must be corrected if the temperature is above or below 80F by adding the number shown below:
160F / +.032
150F / +.028
140F / +.024
130F / +.020
120F / +.016
110F / +.012
100F / +.008
90F / +.004
80F / 0
70F / -.004
60F / -.008
50F / -.012
40F / -.016
30F / -.020
20F / -.024
10F / -.028
0F / -.032
For example, the hydrometer reading is 1.236 and the temperature is 60F. You want to add -.008 to the reading. Adding a negative number to a positive number is the same as subtracting .008 from your reading. The hydrometer reading should be 1.236 - .008 = 1.228. Another example, you have the same hydrometer reading of 1.236 but the temperature is 90F. You need to add .004 to your reading, which will give you 1.236 + .004 = 1.240
