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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.