Knowing what causes these backup power sources
to expire early will help you prevent it from happening
The lead-acid battery is the primary source of backup power for today's mission critical systems. It's used to protect everything from switchgear to telecommunications equipment. And it will continue to do so for the foreseeable future. But despite their important role, batteries fail. The challenge is to keep those failures from happening prematurely.
Batteries have a life expectancy, just as people do. They won't last forever, but you can address the factors that diminish their health and cause premature failure. You can reduce failures dramatically if you understand — and correct for — their causes: excessive cycling, improper charging, poor temperature control, installation errors, manufacturing deficiencies, and operational errors.
Excessive cycling. Every time a battery cycles (a discharge followed by a recharge), the electrochemical generator goes to work. Its job is to convert acid and paste into electricity (the paste on the positive grid changes from PbO2 to PbSO4). The deeper the discharge, the more acute — and stressful on the paste — this change must be. Thus, deeper discharges shorten battery life more than shallower discharges do. Deeper cycling also means increased rates of corrosion of the grid structure. Life-shortening grid structure corrosion is especially pronounced in lead-calcium batteries, which are the most popular batteries in use today.
The cycling capability of the lead-acid battery depends on the depth of discharge. For example, a lead-calcium battery is capable of only 50 deep cycles, or those that remove more than 80% of energy. But it can deliver 300 cycles that are 25% depth of discharge. So a UPS battery that normally delivers about 25% of its stored energy during its 15-minute rated reserve time can do that about 300 times.
Improper charging. Battery manufacturers specify charging voltage ranges for various cell designs. If your float voltage isn't within specifications, the battery will fail prematurely in one of two ways, depending on which direction the voltage skews. Low float voltage (undercharging) causes sulfate crystals to form on the plate surfaces. Sulfate crystals harden within days and won't go back in solution even if you restore proper charging voltage. The result is a permanent loss of capacity. Extended undercharging will also cause the negative plates to lose active material. High float voltage (overcharging) causes excessive gassing of hydrogen and oxygen. Flooded cells lose water, and valve-regulated lead acid (VRLA) cells dry out (Sidebar below). Overcharging also causes higher float current, which accelerates corrosion and shedding of active material from positive plates. In VRLA cells, the recombination of gases to form water generates heat, which causes higher float currents. Excessive gassing in VRLA cells can lead to thermal runaway.
Poor temperature control. Do you have batteries stacked in a rack four high? The problem with this is each row will be at a progressively higher temperature because it's heated by the row underneath it. Measure the temperature on the bottom row and then on the top row. If these cells have the same charge voltage, you're probably in trouble because the correct float voltage is temperature dependant. So you can't charge both rows at the same voltage because it will be wrong for one of them. If you read any manufacturer's float voltage specification, you'll notice a temperature specification that goes with it.
At 77°F, the highest float voltage at which a cell can still recombine all the gases driven off the plates is about 2.32V. If the cell temperature is 90°F at this voltage, it will dry out and possibly go into thermal runaway. Thermal runaway can cause the jar to melt, but worse, it sets the stage for fire and explosion.
To set the proper charging level, you must either account for the interaction between voltage and temperature or control the battery temperature. It's better to control the temperature. Try to maintain the operating temperature where the manufacturer specifies — typically 77°F. Tip: Do everything possible (provide proper ventilation, place the batteries in a location that doesn't restrict air flow) to control temperature passively. You may not be able to run all cooling equipment from battery power during an outage.
Temperature is especially critical for VRLA cells. The recombination of gases within them can occur only at a certain rate. Exceeding this rate will cause gas pressure to build up beyond the safety valve level. When that happens, gases and water will be vented out and permanently lost.
Don't allow temperature to drop too low or climb too high. Low temperatures diminish battery capacity. For example, at 62°F, capacity is about 90% of what it is at 77°F. At lower temperatures, you need a higher float voltage to maintain full charge. On the other hand, high temperatures kill batteries. Every 15°F rise in operating temperature cuts battery life in half. High temperature increases float current and, subsequently, corrosion. It also causes gassing, loss of water in flooded cells, and dryout/thermal runaway in VRLA cells.
Installation errors. Many battery problems stem from improper installation. Eliminate the following three common installation errors:
Loose intercell connections — These can lead to abrupt failures, fire, or explosion.
Damaged post seals — Improper cell handling or unsupported cables can damage post seals. Damaged seals allow acid to migrate up the post and corrode the post-to-intercell connection.
Shipping caps not replaced with vent caps — In flooded batteries, a lack of vent caps allows internal gas pressures to force gases to escape past the post seals — and thus cause post corrosion.
Manufacturing deficiencies. Few battery problems stem from manufacturing deficiencies. But, these deficiencies do occur. You should be able to spot these deficiencies so you can work with the manufacturer to resolve them. At the annual BattCon conference for battery users, manufacturers repeatedly make it clear they want to know about — and resolve — manufacturing-related defects. Three areas of concern:
Faulty post seal design — A leaky post seal allows acid to migrate up to — and damage — the post-intercell connection. Sometimes a new design appears to work well, but then fails after six to eight years in the field.
Internal connection problems — In multicell jars, such as 6V or 12V modules, the intercell connection between adjacent cells may fail due to a poor lead burn.
Paste inadequacies — Paste formula errors or improper paste curing can reduce battery capacity. Find paste problems by testing the capacity of new batteries.
Operational errors. Avoid the following operational errors for longer battery life:
Discharge without recharge — When a cell is nearly or fully discharged, you must recharge it within 24 to 48 hours or you'll damage it. As a battery discharges, the electrolyte starts changing from an acid solution to almost pure water. Lead dissolves in water, and some of the plate material mixes with water to form lead hydrate, which forms a short circuit between the plates, irreversibly damaging the battery.
Over-discharge — This causes abnormal expansion of the active material in the plates and leads to recharge problems and permanent damage. It can happen in a lightly loaded UPS system that experiences an extended outage.
Excessive discharge cycling — Some users have local requirements to test their critical backup systems weekly or monthly. If these tests include cycling, don't expect the batteries to last long.
With your knowledge of battery failure causes, you can spend less on replacement batteries, increase safety, and improve reliability. A small investment in a systematic program to eliminate these causes of failure can easily pay for itself over the long haul. You want to ensure that when the power fails, your batteries don't.
Rafter, P.E., is the president of Tier IV Consulting Group, Lee's Summit, Mo.
Lead acid batteries are available in two types: VLA and VRLA. They share some failure causes, but each also has its own weaknesses.
In VLA batteries, positive grid corrosion is the normal sign of impending failure. As the grid corrodes, the effective cross section of the conduction path narrows — and the internal cell resistance increases. At the same time, the grid structure swells and deforms, causing it to lose contact with the paste (active material). Because the resistance between paste and grid increases, internal cell resistance increases. If you ignore the increased resistance and fail to remove the battery from service in a timely manner, the positive grids will lose their mechanical strength and start to break apart.
The VRLA has a lifespan of only seven to 10 years. Consequently, VRLA cells don't live long enough to “die of natural causes” (e.g., grid corrosion). They most often die from loss of water in the electrolyte. Current research indicates that secondary reactions from internal recombination of hydrogen and oxygen gases adversely affect the polarization voltage of the negative plates and accelerate positive grid corrosion. Both problems cause capacity loss.