Use and Care of Sealed Lead Acid Batteries


The SLA (sealed lead acid) battery is the best choice for many uses; it is rugged, will not leak, easily rechargeable, and offers a good amount of energy in a convenient form. The primary drawback is the weight of the cell. To get the most out of SLA batteries takes just a few precautions in their use and care.

SLA batteries can be charged with a current limited, regulated voltage source. A 12V battery should be charged to a around 14.5 to 14.7V if a quick charge is desired. After the battery is fully charged this voltage should be reduced to about 13.5V to maintain the charge or the cell should be removed from the charger. The lower maintaining voltage is often referred to as "float" charge. A good battery charger will do this automatically, making charging relatively foolproof and will not damage the battery if left connected.

Most rechargeable batteries (Including NiCd, NiMH and SLA, but not Lithium Ion) suffer from self discharge, a small leakage current in the battery that will discharge the battery when not in use. After several weeks a significant portion of the battery's stored energy will be gone. To combat this problem, simply charge the battery before use, or utilize a float charger to keep the battery charged until use.

When checking a battery for full charge you need simply measure the battery voltage. This is done in the "open" state, with nothing connected to the battery. Use a voltmeter to simply measure the battery voltage and look for a reading around 12.5V (give or take a couple tenths of a volt depending upon the temperature of the battery). If the battery was just removed from a charger the reading will be high and will take a few minutes to relax to the open cell voltage. A discharged battery will read lower than this, but not a great deal lower, usually around or just below 12V with no load on it. When an SLA battery fails, it usually has one or more bad cells inside. Each cell contributes 2V to the battery voltage with six cells making up a 12V battery. Thus if the reading is two volts low, even after charging, the battery has a bad internal cell and should be replaced.

Heat is the enemy of SLA's, and indeed most battery types. Heat degrades the electrolytes and increases the rate of self discharge, causing a battery to slowly discharge when not in use. Rechargeable batteries should be stored in a cool place when not in use. A few days in a hot vehicle will probably not significantly damage a battery, but more than that certainly will.

Overdisharge of an SLA battery is a quick way to kill it. A battery should not be allowed to go below 1/2 its rated voltage, or 6V for a 12V battery. Properly designed devices meant to use this type of battery should contain circuitry to automatically shut off when the voltage gets too low (this is true of our IR lamps). This is also the reason that SLA batteries should be stored charged, as self discharge will eventually completely discharge the battery and damage it. Thus these batteries should always be charged after use and charged every few months if stored unused for extended periods.

Lead acid batteries can supply a great deal of current, enough current to create a hazard if the battery output is shorted. A short can result in melted wiring, badly damaged equipment, and even the danger of fire. The battery circuit should ALWAYS have an appropriately rated fuse in it.

SLA batteries contain a large amount of lead, a toxic heavy metal, and should never be thrown away, but should be recycled in a battery recycling program. Indeed, unsafe disposal of a lead acid battery is against both federal and many local laws. Many city and county waste programs have a lead-acid battery collection point or pickup for recycling. Many battery retailers also accept batteries for recycling. Sealed lead acid batteries are chemically identical to automotive batteries and can be recycled by the same programs.

Although there appear to be many problems with SLA batteries, problems are easily avoided by remembering the following points...