UPS battery overview
UPS battery overview
It’s well known that the battery is the most vulnerable part of a UPS. In fact, battery failure
is a leading cause of load loss. Yet understanding how to properly maintain and manage UPS
batteries can not only extend battery service life, but can also help prevent costly downtime.
Positive Flag
Extruded Intercell
Welded Connection,
Low Resistance
Current Path
Strap Joining
Plates in
Pasted Plate
Lead Alloy
VRLA batteries are frequently used in UPS
or other high-rate applications.
Internal and external components of a valve-regulated lead
acid (VRLA) battery.
eaton corporation Power Quality
The most common type of battery used in
UPSs is valve-regulated lead acid (VRLA)
batteries, also known as sealed or
maintenance free. VRLA batteries are
sealed, usually within polypropylene plastic,
which offers the advantage of not containing
any sloshing liquid that might leak or drip.
Because water cannot be added to VRLA
batteries, recombination of water is critical
to their life and health, and any factor that
increases the rate of evaporation or water
loss — such as temperature or heat from
the charging current — reduces the life of
the battery.
Frequently asked
questions: batteries
1. What is the “end of useful life?”
The IEEE defines “end of useful life” for a
UPS battery as being the point when it can
no longer supply 80 percent of its rated
capacity in ampere-hours. When your
battery reaches 80 percent of its rated
capacity, the aging process accelerates and
the battery should be replaced.
2. Is there any difference between the
batteries used by smaller UPSs, from 250
VA to 3 kVA, and the ones used by larger
While basic battery technology, and the risks
to battery life, remains the same regardless
of UPS size, there are some inherent
differences between large and small
applications. First, smaller UPSs typically
have only one VRLA battery that supports
the load and needs maintenance. As
systems get larger, increasing battery
capacity to support the load gets more
complicated. Larger systems may require
multiple strings of batteries, introducing
complexity to battery maintenance and
support. Individual batteries must be
monitored to prevent a single bad battery
from taking down an entire string, and
putting the load at risk. Also, as systems get
larger, wet-cell batteries become much
more common.
3. My UPS has been in storage for over a
year. Are the batteries still good?
As batteries sit unused, with no charging
regimen, their battery life will decrease. Due
to the self-discharge characteristics of lead-
acid batteries, it is imperative that they be
charged after every six to 10 months of
storage. Otherwise, permanent loss of
capacity will occur between 18 and 30
months. To prolong shelf life without
charging, store batteries at 10°C (50°F) or
6. If I add more batteries to a UPS can I
add more load?
Adding more batteries to a UPS can
increase the battery runtime to support the
load. However, adding more batteries to the
UPS does not increase the UPS capacity. Be
sure your UPS is adequately sized for your
load, then add batteries to fit your runtime
4. What is the difference between hot-swappable and user-replaceable
9. Why are batteries disconnected on
small, single-phase UPSs when they are
holdover time available will be less than it
would have been if the batteries were fully
This is done to ensure that they are in
compliance with Department of
Transportation regulations.
13. What are the risks associated with a
lack of battery maintenance?
10. Does the UPS need to have a load on
it to charge its batteries?
The primary risks of improperly maintained
batteries are load loss, fire, property damage
and personal injury.
The UPS should have a minimum of 10%
load to charge its batteries. Once connected
to a standard supply of electricity (via input
plug or hardwiring), your UPS should charge
its batteries regardless of how much load, if
any, is attached to it
Hot-swappable batteries allow the batteries
to be changed out while the UPS is running.
User-replaceable batteries are usually found
in smaller UPSs and require no special tools
or training to replace. Batteries can be both
hot-swappable and user-replaceable.
Adding extended battery modules increases
runtime but does not increase the power rating
or capacity of the UPS
7. If my UPS is in storage how often
should I charge the batteries?
The batteries should be charged every three
or four months to prevent loss of capacity.
UPS models like the Eaton 9130 feature hot-swappable
batteries for maximum uptime
8. What is the average lifespan of UPS
The standard lifespan for VRLA batteries is
three to five years. However, expected life
can vary greatly due to environmental
conditions, number of discharge cycles, and
adequate maintenance. Have a regular
schedule of battery maintenance and
monitoring to ensure you know when your
batteries are reaching their end-of-life. The
typical life of an Eaton UPS with ABM
technology is 50% longer than with standard
5. How is battery runtime affected if I
reduce the load on the UPS?
The battery runtime will increase if the load
is reduced. As a general rule, if you reduce
the load by half, you triple the runtime.
11. How can you be sure UPS batteries
are in good condition and ensure they
have maximum holdover in the event of
a power failure? What preventive
maintenance procedures should be done
and how often?
14. What is thermal runaway?
Thermal runaway occurs when the heat
generated in a lead-acid cell exceeds its
ability to dissipate that heat, which can lead
to an explosion, especially in sealed cells.
The heat generated in the cell may occur
without any warning signs and may be
caused by overcharging, excessive charging,
internal physical damage, internal short
circuit or a hot environment.
15. Why do batteries fail?
The batteries used in the UPS and
associated battery modules and cabinets are
sealed, lead- acid batteries often referred to
as maintenance-free batteries. While this
type of battery is sealed and you do not
need to check the fluid level in the battery,
they do require some attention to assure
proper operation. You should inspect the
UPS a minimum of once per year by
initiating a self-test of the UPS.
Batteries can fail for a multitude of reasons,
but common reasons are:
12. How long does it take for the UPS
batteries to recharge?
16. How is battery performance generally
On average, it takes 10 times the discharge
time for the UPS batteries to recover. (A
30-minute battery discharge requires about
300 minutes to recharge.) After each power
outage, the recharge process begins
immediately. It is important to note that the
load is fully protected while the batteries are
recharging. However, if the batteries are
needed during the recharge time, the
Batteries are generally rated for 100+
discharges and recharges, but many
batteries show a marked decline in charging
capacity after as few as 10 discharges. The
lower the charge the battery can accept, the
less runtime it can deliver. Look for batteries
with a high-rate design that sustains stable
performance for a long service term.
• High or uneven temperatures
• Inaccurate float charge voltage
• Loose inter-cell links or connections
• Loss of electrolyte due to drying out or damaged case
• Lack of maintenance, aging 1.800.356.5794
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