Advanced Battery Management (ABM) and Battery Testing in Eaton UPS
The battery system in a UPS forms the heart of the power protection benefit to the user. It is a key
element which performs two functions: 1) delivers energy during a power outage, and (2) stores energy
efficiently for extended periods of time. That stored energy is instantaneously available when needed to
support the critical load on the UPS. In order to perform the above functions reliably, the charge level on
the battery must be maintained. At the same time, it is desired that the battery charge be controlled to
maximize system efficiency and, more importantly, to maximize the service life of the battery system. The
unfortunate fact is that Valve Regulated Lead Acid (VRLA) batteries are often marketed as “10 year
design life”, yet real world data shows that in UPS applications, the battery is replaced every 4-5 years on
average. Because the battery can often be 30% of the cost of the UPS system, users frequently request
that the UPS vendor ensure that the service life of the battery be extended as much as possible. They
require tangible evidence that “battery monitoring” and “battery management” systems actually perform
as advertised. The benefit of longer service life is lower life cycle cost and capital expense for the user.
Two types of battery charging schemes have traditionally been used for UPS battery systems. The older
and more commonly known is the “float” charge scheme. This involves applying a constant voltage
charge to the battery continuously for purposes of maintaining full charge during day-to-day operation of
the UPS. This works quite well in many conventional battery applications. However, it can result in
reduced battery life, due to overcharging, for batteries that are used very occasionally as in standby
applications like UPS. In a UPS, the battery system may sit in float mode for many months, without ever
experiencing a discharge. Float charging for long periods of time means that “trickle charge” energy is
constantly forced into a battery which is effectively “already full”. This results in very gradual degradation
of the lead plates (positive grid corrosion), and significantly reduces service life. Standby applications are
better suited for “cyclic” charging schemes. The system Eaton utilizes is called Advanced Battery
Management (ABM) charging system. ABM is a set of charger controls, and automated battery tests. It is
implemented in Eaton 3 phase UPS systems from 10 kVA to 3.3 MVA. Cyclic charging schemes like ABM
allow for periods of time where the battery is being fully charged, and periods of time when the charger is
disabled. This reduces the time that the battery is being overcharged when compared to a traditional float
charger. This reduction in overcharging yields a measurable increase in battery life for UPS applications.
ABM Operational Summary
Advanced Battery Management consists of three operating modes:
1) Charge mode
2) Rest mode
3) Test mode
These modes are shown graphically in figure 1.
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Figure 1: Depiction of ABM Modes
Charge Mode
The UPS enters Charge mode under any of the following conditions:
Whenever the UPS is commanded to turn on
After any Utility power outage, lasting longer than 15 seconds
Whenever the battery is replaced (or the battery breaker is opened and re-closed)
In Charge mode, bulk charging of the batteries is used to recharge a discharged battery after a power
outage, or whenever the ABM process is restarted. Charge voltage target is set to 2.30 V/cell, and charge
current will be greater than 0.1 C A. Bulk charging lasts only as long as it takes to bring the battery
system up to a predetermined float level (there is a 100-hour maximum time limit). Once this level is
reached, the UPS battery charger enters a constant voltage mode, maintaining a 2.3 V/cell level. Current
is at trickle charge levels during this time, and a 24-hour clock is started. At the end of 24 hours of float
charging, the UPS automatically performs a battery test (see fig. 1) at two different load levels to verify
that the battery is performing, and to collect data for comparison to previous and subsequent automatic
battery tests. If the test should fail, an alarm is annunciated on the UPS and also through the remote
monitoring system that may be connected to the UPS. At the end of the test, the charger resumes
constant voltage mode, and remains in that state for an additional 24 hours.
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Rest Mode
Rest mode begins at the end of Charge mode; that is, after 48 hours of float charging, and after a
successful battery test. In Rest mode, the battery charger is completely turned off. The battery system
receives no charge current during this mode, which lasts about 28 days. Then, the Charge mode is
repeated as described above. It is clear, then, that the battery spends most of its time in Rest mode. As a
result, the following benefits are realized:
1) Battery is not subjected to forced charge current; therefore, overcharging is not possible.
2) Thermal runaway is not a concern with the charger off.
3) The battery system can not be damaged by ripple currents, since the charger is off.
4) The likelihood of positive grid corrosion is greatly reduced, allowing extended service life.
During Rest mode, the open circuit battery voltage is monitored constantly, and battery charging is
resumed if any of the following occur:
A power outage lasting longer than 15 seconds occurs
The OCV (open circuit voltage) of the battery drops below 2.1 V/cell after 10 days of rest mode
o If OCV drops below 2.1V during the first 10 days, an alarm is triggered
28-day timer expires (end of Rest Mode)
The battery is replaced, or the breaker is opened and re-closed
There are two other battery tests that are performed as a part of the ABM cycle. The first of these is
meant to detect battery conditions which could lead to thermal runaway. The bulk charging period is
timed and if the float voltage is not reached in a predetermined time an alarm is signaled, and the charger
shut down. The second test is performed after the charge cycle is completed (i.e., at the beginning of
Rest mode). The battery is discharged for 25% of the expected discharge time. Upon reaching this point
the battery voltage is measured. If the voltage is below a specified threshold, dependent on the load, then
an alarm is signaled indicating the battery is nearing the end of its service life and should be replaced.
Other Modes
Disabled mode: note that ABM may be disabled by the user or an Eaton field service engineer at any
time, if desired. In this case, the UPS battery charger operates as a conventional float charger only. This
is recommended when a wet cell or flooded electrolyte battery is used with the UPS. ABM is intended for
use with VRLA batteries, wet batteries will not benefit from ABM controls.
Many observers express concern regarding the remaining capacity of the battery if called upon to support
the UPS near the end of its Rest mode. In other words, how much battery capacity is available on day 27
of a 28-day rest mode? In general, and using a 15 minute battery as an example, under this condition, the
battery would provide all but about 30 seconds of its 15 minute backup time. This is proportionally true for
other battery sizes.
ABM Performance
The ABM process above describes benefits of using a “cyclic” charging scheme. Those benefits,
specifically extended service life, are in fact substantiated by data and empirical testing performed by
Eaton, and also by other independent sources. Some of this testing is recent, and some was performed
as much as 20 years ago. Eaton has 18 years of experience using ABM in its UPS products. It is not a
“new battery management” feature. It has proven itself beneficial in the field for almost two decades.
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Battery Life Data
Back up time min.
10 11 12 13 14 15 16 17
Tim e in test, m onths
Figure 2: Accelerated Life Testing of ABM at 40 degrees C
In figure 2 above, the testing was performed at a very high ambient temperature to provide meaningful
data in a shorter period of time. The service life enhancements become evident after about only 7 months
of this accelerated test. This test was done with conventional UPSs and VRLA batteries.
Battery Life Test
Back up time min.
Time in test, months
Figure 3: Long Term Cyclic Charge vs. Float Test
In figure 3 above, the effect of a cyclic charging regime over several years is demonstrated. This testing
was done almost 20 years ago, by a battery manufacturer not associated with Eaton, or any UPS vendor.
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Figure 4: Calculated Service Life Extension for ABM Charge Algorithm
Note that in figure 4 above, the curve identified as “23/23 days” represents a float charger, and ABM (as
implemented today) is best represented by the curve labeled “12/23 days”. At an ideal 25 degrees C,
there is a theoretical increase in service life of 6 years reflected in this analysis.
The above information shows a clear benefit of cyclic charging in UPS applications, both in simulated and
in actual performance tests. These results would not be expected with non-VRLA batteries, or in
applications like motive power chargers, where the battery is discharged/recharged daily, and therefore
not deployed in a standby application.
ABM is unique in the UPS industry, but similar cyclic designs are utilized by battery manufacturers and
battery charger designers worldwide. The criticality and cost of the battery subsystem of any UPS
dictates that special consideration be given to battery longevity. Additionally, with environmental concerns
relating to battery removal and disposal becoming more prevalent, it is desirable to reduce the frequency
of battery replacements during the life of the UPS electronics. ABM offers a significant benefit over
conventional “battery monitors” which don’t provide charging control, and “multi-stage chargers” which
protect the battery, but do not provide useful extension of battery service life.
Over the past 18 years, ABM has proven itself in large and small UPS systems, from the desktop to the
data center, to the medical lab, and the factory floor. Anywhere a UPS is installed, a battery system is
depended upon to provide backup power protection for critical business processes and even for
personnel safety. The battery is all too often ignored as a “maintenance-free” product, not requiring
attention or inspection. This neglect, though common, can be costly and possibly disastrous. The ABM
system, by its nature, helps to protect the battery from unnecessary failures like electrolyte dry out and
thermal runaway, while functioning to extend the useful life of this key component of Power Quality.
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Questions and comments regarding this white paper can be directed to Ed Spears, Eaton Product
July 2009