How many times have you heard the expression, "The battery won't take a charge" or "The battery won't hold a charge"? More often than not, the culprit is hardened sulfate on the battery plates. This tech bulletin discusses, sulfation, what causes it are, and some potential solutions to prevent the sulfate from permanently damaging your battery.

Look inside a battery cell. Basically, there are positive plates, negative plates, separators to keep the plates apart, and electrolyte (sulfuric acid and water).

Sulfation is part of normal battery chemistry and occurs during every cycle. When a battery is discharged the lead active material on the plates (lead peroxide on the positive plates and sponge lead on the negative) reacts with the sulfate from the sulfuric acid in the electrolyte forming lead sulfate on the plates. Now, sulfate is being removed from the electrolyte increasing the ratio of water to sulfate in the solution and since sulfuric acid is heavier than water, as the battery is discharged, the Specific Gravity goes down and when the process is reversed it goes up. When all of the lead active material has been converted to lead sulfate and the electrolyte to water the battery is completely discharged. When a battery reaches this state, it must be recharged. During recharge, the lead sulfate is re-converted into lead active material and the sulfate returned to the electrolyte as sulfuric acid.

If a battery is left standing in a discharged condition the lead sulfate will become increasingly harder and have a high electrical resistance. Now we have a sulfated battery. The lead sulfate may become so hard that normal recharging will not break it down. Most charging sources, engine alternators and battery chargers, are voltage regulated. Their charging current is controlled by the battery's state of charge. During charging, battery voltage rises until it meets the charger's regulated voltage, lowering the current output along the way.

When hard sulfate is present, the battery shows a false voltage, higher than its true voltage, fooling the voltage regulator into thinking that the battery is fully charged. This causes the charger to prematurely lower its current output, leaving the battery discharged. Charging at a higher than normal voltage and low current may be necessary to break down the hardened sulfate.

Hardened sulfate also forms in a battery that is constantly being cycled in the middle of its capacity range (somewhere between 80% charged and 80% discharged), and is never recharged to 100%. Over time, a portion of the plate's active materials turns into hard sulfate. If the battery is continually cycled in this manner, it will lose more and more of its capacity until it no longer has enough capacity to perform the task for which it was intended. An equalizing charge, applied routinely every three to four weeks, should prevent the sulfate from hardening.

In both cases, the fact that the battery "won't take a charge" is a result of improper charging procedures which allowed the sulfate to harden. In most instances, it is possible to salvage a battery with hardened sulfate. The battery should be charged at 2.6 to 2.7 - volts per cell and a low current rate (approximately 5 Amps for small batteries and 10 Amps for larger ones) until the specific gravity of the electrolyte starts to rise. (This indicates that the sulfate is breaking down.) Be careful not to let the internal temperature of the battery to rise above 125 F. If it does, turn the charger off and let the battery cool. Then, continue charging until each cell in the battery is brought up to full charge (nominal 1.265 specific gravity or higher). The time needed to complete this recharge depends on how long the battery has been discharged and how hard the sulfate has become. Most commonly available battery chargers are not sufficiently adjustable to meet these requirements, see our battery charging page for chargers that will not only correct this problem but will prevent it from occurring in the first place or the future.

The next time your batteries don't seem to be taking or holding a charge, check the specific gravity with a hydrometer. If all cells are low even after a long time on charge, chances are you've got some hardened sulfate that has accumulated on the plates. By following the instructions outlined above, the problem may be corrected.

With a sealed battery the same problem can exist. Unfortunately hydrometer readings cannot be taken to determine the problem. If you subject a sealed battery to overcharging you may lose the electrolyte and ruin the battery.