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LABC1 
• 5  
solution. As more energy is drained from the battery, the ratio of sulfuric acid to 
water decreases and the created lead sulfate byproduct begins forming on the 
electrode plates. A low hydrometer reading means the chemical makeup that 
generates the free electrons is diminished so not as much energy is stored for 
use.  
The term ‘specific gravity’ is often used to benchmark a lead acid battery’s 
SOC. The specific gravity of a substance is a comparison of its density to that 
of water (1.000). Imagine a one gallon bottle filled with water and a second 
filled with feathers. There are equal volumes of material present in both but the 
bottle with the feathers will weigh less than that containing the water. The 
resultant specific gravity value of the bottle of feathers would be less than that 
of the bottle of water. With lead acid batteries, the sulfur atoms break down and 
leach out of the electrolyte solution as it discharges. The breakdown of the 
electrolyte reduces its overall ‘weight’ as the sulfur is removed from the solution 
thus reducing the specific gravity measurement. Take a look at Table 1
Great care should be taken to avoid discharging a battery beyond the 75% 
SOC point. Once the specific gravity drops below the 1.210 level, excessive 
sulfate deposits form on the electrode plates. This process is called ‘sulfation’ 
and leads to the hardening of the electrode plates. If the battery is kept in a low 
charge state for long a period of time, the sulfation process will eventually 
reduce the ability of the battery to generate ion charge carries to the point that 
it no longer provides the needed power. This point is otherwise known as a 
DEAD BATTERY! 
When you recharge the battery, the process is reversed and the sulfur returns 
to the electrolyte solution. Proper cycling of the battery will ensure a long and 
functional life. If the battery is abused by allowing sulfation of the electrode 
State Of 
Charge 
Specific 
Gravity 
 
Open-
Circuit 
Voltage 
(approximate) 
100% 
1.265 
12.63 
75% 
1.210 
12.30 
50% 
1.160 
12.00 
25% 
1.120 
11.76 
0% 
1.100 
11.64 
State of Charge as related to Specific 
Gravity and Open-Circuit Voltage 
 
Table 1