Keeping Lead-Acid Batteries Healthy and Effective

Keeping Lead-Acid Batteries Healthy and Effective

Jim Baker, Ventev Senior Electrical Engineer

July 2021

Lead-acid batteries are the most frequently used batteries in UPS systems. They are economical
components that have proven to be effective over many years in use. Lead-acid batteries do age
and their capacity slowly deteriorates until they need replacement.

Why do they deteriorate?

The most common problem is the sulfation of the battery plates. Sulfates are created and consumed as
part of the normal charging and discharging battery process. However, a residual amount of sulfate
can be left on the battery plates each cycle. This sulfate coating builds up over time and reduces the
useful area of the plates, which also reduces battery capacity. Additionally, the sulfation process is
accelerated if the batteries are left in a discharged state.

What else impacts the life of a battery?

A variety of other factors, like high temperatures, will cause a decline in battery capacity.
Battery temperature can rise significantly during high rates of discharge or high charge rates and
overcharging. In Sealed Lead Acid batteries this can cause the battery to swell, subsequently
warping the thin battery plates, and cause venting of hydrogen from the cells.


If the battery left is unused for an extended period, the electrolyte can experience stratification. In
this situation, the acid and water components in the battery electrolyte will separate with the acid
settling to the bottom of the cell leaving the water at the top. The higher acid concentration at the
bottom of the cell will corrode away plate material, reducing battery capacity, eventually leading to
battery failure. This situation can be avoided when the battery is in routine use because in the
discharge and charge processes small gas bubbles form that promotes mixing of the electrolyte.
Most of these conditions are not observable by monitoring the battery terminal voltage while in
standby mode. That results in an AC Line Down situation, where batteries are expected to back up
critical applications. These conditions will result in shorter runtimes, or in the worst case, an immediate
system shutdown.

How do people typically test batteries in the field now?

The two most popular tests are Run and Ohmic Tests. In a Run Test, the battery is either partially
discharged or fully discharged. From the runtime data collected, capacity can be determined directly.
An Ohmic Test is used to determine the battery’s internal impedance. In most cases, there is a
relationship between loss of capacity and increased battery impedance. This measured value can
be compared to the impedance value, as specified by the manufacturer, to arrive at an estimate of
the battery’s health.

Are there more expensive battery testing devices that can be used?

Ohmic Tests need to be capable of measuring the sub-milliohm variations in battery impedance with
high precision. This requirement tends to make them quite expensive. Additionally, not all capacity
issues can be resolved by this method. For example, an Ohmic Test on fully charged batteries may not
be able to differentiate between a battery that is 100 percent good and one that is reduced to 80
percent. This applies to batteries that may have relatively few cycles with plates still in good
condition but have been partially vented due to higher temperatures or overcharging.

What is the best way to truly test a battery’s health?

The most accurate evaluation of a battery’s health is to allow the batteries to power its respective
load and time the test to low voltage cut-off. This measurement is then compared to the original
design specification. However, this full discharge Run Test not only adds cycles to the battery, which
can decrease its capacity, running the battery down to a low state of charge also carries a
considerable risk of the system being unable to perform if a backup situation occurs.

How does the BTRM evaluate battery health? How does it know when to test the battery
and when not to? Why is this method the most effective?

The BTRM utilizes the Run Test method but limits the Run Test to a partial discharge window, about
20 percent or less, and from the collected data estimates the full runtime capability. This minimal
discharge limits any impact on battery health due to cycling and minimizes the risk of the batteries
not being ready for a backup situation. Additionally, the Run Test’s discharge and charge cycle help
offset the stratification issue caused by long-term inactivity.


When an alert is sent, does that mean my battery bank will fail? Or will it not meet the
defined battery backup time? Do I need to replace the batteries immediately? How much
time do I have to replace the batteries?

One of the primary advantages of having the BTRM monitor the capacity on a scheduled basis is
that the BTRM can better assess the battery’s health over time. For example, the BTRM allows you
to differentiate between a battery that continues to consistently perform, even at some reduced
capacity, but still beyond the standby requirement, or a battery that is showing a continual or
rapid decline.
The battery industry recommends replacing batteries that have reached 85 percent of their capacity.
However, having the information the BTRM provides allows you to potentially extend the lifetime
of batteries that have declined in capacity but remain stable performers.

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