a low state of charge (SOC) point. When you choose a controller, be sure to
consider the effects of temperature and gassing.
State of Charge
A battery’s state of charge (SOC) is the available portion of the battery capacity.
An SOC of 90 percent means that the battery is charged to 90 percent of its
capacity. This requires 10 percent more to be full at 100 percent SOC. It can be
extremely difficult to measure this state of charge in PV systems due to its irreg-
ular operation. Use the two methods below to determine the state of charge:
For unsealed batteries (with fluid electrolyte) —You can measure the
acid concentration using a sensor or a hydrometer. You can also check the
battery voltage after a full charge and a voltage-settling period.
For sealed batteries (with immobilized electrolyte) —You cannot
measure the acid concentration. The charge status is determined by
measuring the voltage.
Voltage and SOC from the original full battery capacity will drop with time
and usage.
Aging Effects
The largest drawback to lead-acid grid-plate batteries is their life span. They func-
tion for three to 20 years with 100 to 4,000 full cycles. This provides relatively
broad ranges of life span that are the result of various reversible or irreversible
ageing processes. Some of these influence each other.
Sulfation (irreversible)—Sulfate crystals can grow if the battery is not
recharged to a high enough voltage or after discharge. This causes the active
mass to decrease. The crystals that form are unusable over time, thereby
allowing the chargeable/dischargeable capacity to drop.
Drying out (irreversible)—If a battery does not have enough distilled water,
it can dry out and quit working. Plates that are exposed and damaged will
not hold a proper charge.
Acid stratification (reversible)—The heavy acid drops settle on the bottom
while the lighter water migrates to the top. This creates a high-acid
concentration. This is reversible during maintenance by adding a gassing
equalization charge to mix up the acid.
Corrosion (irreversible)—Corrosion on the lead grid of the positive pole
leads to higher grid resistance. This occurs at cell voltages under 2V or over 2.4V.
Corrosion occurs when there is electrochemical activity. Electrochemical
activity results when two dissimilar metals are immersed in an electrolyte and
affected by a current in a charge or discharge mode.
CHAPTER 7 Introduction to Energy Storage Systems 119

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