Lead-Acid Storage BatteriesDOE-HDBK-1084-95LEAD-ACID BATTERY TYPESRev. 0Page 11BatteriesLEAD-ACID BATTERY TYPESGenerally there are two types of lead-acid storage batteries, based on their method ofconstruction. These batteries are either called flooded (or vented) or sealed. Flooded andsealed batteries also differ in their operation. All lead-acid batteries produce hydrogen andoxygen gas (gassing) at the electrodes during charging through a process called electrolysis. These gases are allowed to escape a flooded cell, however the sealed cell is constructed sothat the gases are contained and recombined. It should be noted that hydrogen gas isexplosive in air at only 4% by volume. Flooded and sealed lead-acid batteries are discussedin the following paragraphs.FloodedLead-AcidBatteriesFlooded cells are those where the electrodes/plates are immersed in electrolyte. Since gasescreated during charging are vented to the atmosphere, distilled water must be addedoccasionally to bring the electrolyte back to its required level. The most familiar example ofa flooded lead-acid cell is the 12-V automobile battery.SealedLead-AcidBatteriesThese types of batteries confine the electrolyte, but have a vent or valve to allow gases toescape if internal pressure exceeds a certain threshold. During charging, a lead-acid batterygenerates oxygen gas at the positive electrode.Sealed lead-acid batteries are designed so that the oxygen generated during charging iscaptured and recombined in the battery. This is called an oxygen recombination cycle andworks well as long as the charge rate is not too high. Too high of a rate of charge may resultin case rupture, thermal runaway, or internal mechanical damage.The valve-regulated battery is the most common type of sealed battery. It was developed forstationary and telecommunication battery applications. These types of sealed batteries have aspring-controlled valve that vents gases at a predetermined pressure. Typical pressurethresholds are from 2 to 5 psig, depending on the battery design. Although the term "valve-regulated" is often used synonymously to describe sealed lead-acid batteries, not all sealedbatteries are valve-regulated. Some battery designs employ replaceable vent plugs or othermechanisms to relieve excess pressure. Sealed batteries were developed to reduce themaintenance required for batteries in active service. Since electrolyte levels are preserved bytrapping and recombining off-gasses, there should not be any need to add distilled water overthe life of the battery. These batteries are often misnamed "maintenance free." In fact, allmaintenance practices applicable to unsealed type batteries are applicable to sealed typebatteries. The only exception is that electrolyte levels cannot, and should not need to be,maintained.
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