August 20, 2001

Questions and Answers on Battery Life in S&C Distribution Automation Products.

S&C has been working closely with customers, battery manufacturers, and consultants in the interest of increasing battery life in distribution automation products. This article includes information gleaned from those discussions, and provides the answers to questions that customers have frequently asked with respect to battery life.

What’s the difference between float and cyclic applications?

A float application is one in which the battery undergoes extended periods of time on charge and is only rarely discharged, for example, when a switch performs an open/close operation or when system voltage is lost.

A cyclic application is one in which the battery is discharged more frequently (as little as once a month), which, in turn, means that it is recharged more frequently. Cyclic loading is very hard on batteries. In fact, depending on the extent of the discharge and the ability of the charger to completely replenish battery capacity before the next discharge, cyclic loading can cut battery life in half.

Are float applications more reliable than cyclic applications?

Battery life is certainly more predictable in float applications. The life expectancy of batteries in the cyclic mode is too complex to predict and is heavily dependent on reliable replenishment of capacity between discharges. The more frequently a battery is discharged and the deeper the depth of discharge, the higher the risk that it will not be adequately recharged before the next discharge occurs. Discharging the battery before it is fully replenished reduces its life.

Are charging requirements different for cyclic and float applications?

For float applications, the primary consideration is to maintain the float voltage properly. The recommended settings for the battery charger are well supported by empirical data from a number of different researchers.

For cyclic applications, the primary consideration is ensuring that the batteries are fully recharged before the next discharge. To accomplish this, the ampere-hours of cyclic loading should be replenished with an overcharge of 105 to 110%. Unless the charge cycle is carefully coordinated with the discharge cycle, there is a risk of overcharging or undercharging.

Does the ampere-hour capacity of a battery have any bearing on battery life?

No. As long as the ampere-hour capacity is sufficient to drive the load during loss of ac control power for the prescribed time, the ampere-hour capacity does not affect battery life. Higher ampere-hour capacity increases the ride-through time when ac power is lost but, as a consequence, the time to fully recharge the batteries also increases.

Does testing affect battery life?

It can. Some RTUs feature frequent automatic battery testing. One RTU manufacturer tests the batteries for thirty minutes, every 24 hours. If this function is enabled, the application is most likely cyclic because it frequently deep-discharges the batteries in order to test them. S&C recommends that customers either disable this function or contact the RTU manufacturer to determine how to decrease the frequency of the tests from daily to monthly cycles.

But EnergyLine tests the batteries in their switch controls daily. Why isn’t this a cyclic application?

Unlike other RTU manufacturers, EnergyLine conducts an intelligent battery test that is microprocessor-controlled. The frequency and duration of the test depends on the condition of the battery, the elapsed time since the most recent ac power loss, the pre-programmed maximum load, and numerous other factors. EnergyLine also measures battery impedance, which is impacted by the condition of connectors and battery terminals. And the discharge level is low enough that the battery capacity is fully replenished between test cycles. Thus, in EnergyLine switch controls, battery testing is considered to be a float application.

What tests can a customer perform to determine if the battery is good?

The best way to determine battery capability is to measure the ampere-hour capacity by applying a load to the battery while monitoring the current and time. A fully charged, 12-Vdc, 5-ampere-hour battery will provide 5 amperes of current for about an hour before the battery voltage drops to less than 11 volts. If the battery capacity is measured to be less than 4 ampere-hours (80% of rated capacity), the battery should be considered to be at the end of its life. For best results, the load current should be kept constant over time. The batteries under test should be stabilized at a room temperature of about 75°F. Note that new batteries may need to undergo several charge-discharge cycles before coming up to full capacity.

How does temperature affect battery life?

As a rule of thumb, for every 8°C to 10°C rise above 25°C, battery life is cut in half. This generality applies both to batteries in the field and in storage.

How does temperature affect battery charging?

As temperature increases, the voltage needed to recharge the battery decreases. Battery charger output voltage must therefore be reduced to avoid overcharging the battery.

Does low temperature affect battery life too?

No, battery life is not affected by cold temperature. But cold temperature can affect battery capacity. A 5-ampere-hour battery has more than sufficient capacity to drive its load for the prescribed time, down to -40°C.

Would other batteries provide longer life?

No. Both S&C and EnergyLine have selected Hawker sealed-lead batteries for back-up power sources. These batteries are widely believed to be the best on the market. But increased battery life depends on accurate and efficient charging and eliminating cyclic loading conditions.

How should batteries be stored?

Battery manufacturers state that batteries may be stored at 25°C for up to two years and then recharged without affecting their reliability or capacity. Most battery manufacturers recommend an open-circuit voltage test every six months and recharging when the reading approaches 2.00 volts per cell. If the batteries are stored at temperature above 25°C, testing should be performed more frequently. The figure below shows how storage temperature and time affect capacity. Proper storage tips are included in the applicable S&C instruction sheets.

What happens if the battery is stored for longer than the recommended time?

Batteries stored for longer than the recommended time without recharging may be damaged beyond recovery. No amount of recharging will bring them back.

What is the specified battery life?

Most starved-electrolyte battery manufacturers claim battery life in float applications is eight to ten years at 25°C, under ideal charging conditions.

What is “real world” battery life?

Unfortunately, under real-world conditions, battery life is considerably less than the ideal. Depending on the type of loading, charging method, and temperature, realistic battery life is likely to be in the 2- to 6-year range.

S&C’s customer survey revealed that battery life varies greatly according to average ambient temperature. There is no definitive answer, except that cooler is definitely better.

When has a battery reached its end of life?

The battery supplier defines end of life as the time when a battery reaches 80% of its rated capacity or when the battery’s internal resistance increases to the extent that it limits the ability to deliver power to the load.

How does charging voltage affect battery life?

Both overcharging and undercharging can have a negative impact on battery life. Undercharging causes unrecoverable loss of capacity while overcharging dries out the electrolyte.