By Ed Spears

When it comes to power management, regular monitoring and maintenance of uninterruptible power systems (UPSs) is critical to ensuring continuous uptime of electrical systems. This is a particularly crucial consideration for UPS batteries, which serve as the backbone of these systems and are also the most vulnerable component.

Battery failure is a leading cause of load loss, so knowing how to mitigate risks will help to prolong battery life while also saving you time and troubles down the road. Administrators typically approach this matter based on design life, or knowing how long a battery can be expected to perform under ideal conditions. Estimating actual battery life, however, depends on the four factors that can affect it:

1. Ambient temperature: The rated capacity of a battery is based on an ambient temperature of 25°C (77°F). It’s important to realize that any variation from this operating temperature can alter the battery’s performance and shorten its expected life. To help determine battery life in relation to temperature, remember that for every 8.3°C (15°F) average annual temperature above 25°C (77°F), the life of the battery is reduced by 50%.

2. Battery chemistry: UPS batteries are electrochemical devices whose ability to store and deliver power slowly decreases over time. Even if you follow all the guidelines for proper storage, usage, and maintenance, batteries still require replacement after a certain period of time.

3. Cycling: During a utility power failure, a UPS operates on battery power. Once utility power is restored, or a switch to generator power is complete, the battery is recharged for future use. This is called a discharge cycle. At installation, the battery is verified to be at 100% of rated capacity. Each discharge and subsequent recharge reduces its relative capacity by a small percentage. The length and depth of the discharge cycle determines the reduction in battery capacity. Lead-acid chemistry, like others used in rechargeable batteries, can only undergo a maximum number of discharge/recharge cycles before the chemistry is depleted. And once the chemistry’s depleted, the cells fail and the battery must be replaced.

 4. Maintenance: Battery service and maintenance are critical to UPS reliability. A gradual decrease in battery life can be monitored and evaluated through voltage checks, load testing, or monitoring. Periodic preventive maintenance extends battery string life by preventing loose connections, removing corrosion, and identifying bad batteries before they can affect the rest of the string. Even though sealed batteries are sometimes referred to as “maintenance-free,” they still require scheduled maintenance and service. Maintenance-free simply refers to the fact that they don’t require adding fluid.

Without regular maintenance, your UPS battery may experience heat-generating resistance at the terminals, improper loading, reduced protection, and premature failure. With proper maintenance, however, the end of battery life can be accurately estimated and replacements can be scheduled without unexpected downtime or loss of backup power.

Battery Maintenance for Extended Life

Unfortunately, quantifying the combined effect of the four factors that affect battery life is difficult. Instead, you need a way to determine when a battery is near the end of its useful life so you can replace it while it still works—before the critical load is left unprotected. Performing a battery rundown test is your best option. With this method,the module is taken offline, connected to a load bank, and operated at rated power until the specified runtime elapses or the unit shuts down due to low battery voltage. If battery capacity is less than 80% of its rated capacity, the battery should be replaced.

Thermal scanning of battery connections during the battery rundown test identifies loose connections. What’s good about this test is that it gives you the chance to see the battery during an extended, high-current discharge. Remember: Scanning should take place during discharge and recharge cycles.

An effective UPS battery maintenance program must include regular inspections, adjustments, and testing, with thorough record-keeping of all readings. Trained technicians should:

  • Inspect batteries and racks/cabinets for signs of corrosion or leakage
  • Measure and record the float voltage and current of the entire bank
  • Record the voltage, impedance, and electrolyte density of selected battery cells
  • Check the electrolyte level in each cell, if accessible
  • Log the ambient temperature


Lead-acid batteries have benefited from one of the most successful recycling campaigns in the world—with more than 96% of them being recycled from 1997 to 2001, according to Battery Council International statistics. Many states require lead-acid batteries to be recycled, and several options exist to dispose of used batteries, including:

  • If you’re in the process of making a battery upgrade or replacement, the supplier may take your old batteries and recycle them for you. If you participate in Eaton’s UPSgrade program, for instance, we take the old UPS and recycle it.
  • Check your local listings or search for a local recycler at
  • Visit an automotive store; some accept batteries for recycling.
  • Check to see if your municipality has a dump or recycling location that will accept batteries for recycling—many do. When disposing of batteries in this manner, be sure to get a dated receipt clearly detailing what batteries were dropped off, including the quantities, with the recycler’s full name, address, and phone number. Such information will come in handy in the unlikely event that you get audited.

Spot Replacement of Batteries

A series of batteries is similar to a string of holiday lights: When one unit fails, the entire string no longer works. When a battery or group of batteries connected in a series ceases to work, not only is the battery string no longer functional, but it can also be difficult to determine which battery has failed. The most effective way to combat this potential problem is to “spot” replace bad batteries that are less than 3 years old. While the four factors referenced above play a large role in determining when a battery is vulnerable to failure, there’s no surefire way to predict it.

The only way to identify bad batteries early enough for spot replacement is through continuous battery monitoring and scheduled maintenance. Spot replace bad batteries that are less than 3 years old and replace the whole string between the fourth and fifth year.

Handling and Storage

Store batteries in cool, dry, well-ventilated areas with impervious surfaces and adequate containment in the event of spills. Batteries should also be stored under a roof for protection against adverse weather and separated from incompatible materials. And be sure to store and handle them only in areas with adequate water supply and spill control, and avoid damage to containers. Finally, keep batteries away from fire, sparks, and heat.

State and local governments may have regulations concerning how and where your UPS batteries are installed—usually depending on the amount of electrolytes in the batteries. Flooded-cell batteries, for instance, require special ventilation because of their liquid electrolyte and how much hydrogen they emit. Consequently, they’re usually stored away from the load and other equipment. Valve-regulated lead-acid batteries, on the other hand, are much less hazardous due to their immobilized electrolyte, which means they’re often exempt from stringent regulations.

Determining battery life can be tricky, but understanding your UPS battery can extend its lifecycle, prevent costly downtime, and save time and money. By taking into account the four factors that affect battery life, in addition to implementing a comprehensive maintenance plan, administrators can rest assured that electrical components will stay up-and-running in the most critical conditions.


Ed Spears is a product marketing manager in Eaton’s Critical Power Solutions Division in Raleigh, NC.