TECHNICAL NOTE TN-09002
Understanding the Value of Monitoring
Intercells & Intertiers in a Battery String
Scope:
This Technical Note (TN) describes the value to monitoring Intertiers and Intercells in a Battery String.
Revision:
Published by: Fran Losey
Director of Technical Services
11 May 2010
Rev. A
UPS Battery Strings are joined together by plates or cables to create a contiguous electrical series
connection. This is usually accomplished with copper stranded cable with lugs crimped at each end,
which fasten to the battery terminal via a bolt.
As with any electrical circuit, all items in the path contain a particular resistance. This resistance is
published for a particular conductor.
Alber definitions:
• INTERTIER (It): the conductor connecting one group of batteries to another, typically when
transitioning tiers.
• INTERCELL (Ic): the conductor connecting one battery to another
INTERTIER
INTERCELL
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TECHNICAL NOTE TN-09002
Understanding the Value of Monitoring
Intercells & Intertiers in a Battery String
For the sake of this paper, we will establish a baseline of 1 string, having (240) 2 volt jars (1x240x2).
Within this string, there are 232 intercells, consisting of a plate (or plates) bolted to the battery posts;
typically these plates are ~6µΩ in resistance.
For the intertiers, a typical configuration is to use multiple copper cables in parallel to carry current from
one shelf, tier, or stand to another. In this string we will use a common configuration of 8 groups of 30
jars, connected via 7 Intertiers. Typically, each of these intertiers are ~300µΩ in resistance.
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TECHNICAL NOTE TN-09002
Understanding the Value of Monitoring
Intercells & Intertiers in a Battery String
Let’s view a schematic illustration, representing a portion of this particular string. If this string were to
experience a discharge, we can calculate the voltage and power loss (heat) during this expected state:
Cell 1
Cell 2
Cell 3
Cell 29
Cell 30
+
IC 1
IC 2
6µΩ
6µΩ
Cell 31
Cell 32
IC 29
6µΩ
IC 28
6µΩ
Cell 33
Cell 239
IT 1
300µΩ
Cell 240
IC 31
6µΩ
IC 32
6µΩ
IC 238
6µΩ
Cell 30
Discharge I
= 1000A
Cell 31
IT 1
300µΩ
IC 239
6µΩ
Cell 32
IC 31
6µΩ
IT 1Voltage = 1000A * .0003Ω
IT 1 = .3V
IC 31Voltage = 1000A * .000006Ω
IC 31= .006V
IT 1 Power = I * E
IT 1 = 1000A * .3V
IT 1 = 300 Watts
IC 31 Power = I * E
IC 31 = 1000A * .006V
IC 31 = 6 Watts
In a properly working system, with a discharge current of 1000Amps as shown above, there will be a
total voltage loss and a total power loss of:
•
•
Vloss of 232 IC and 7 IT
Vloss = (232 * .006V) + (7 * .3V)
Vloss = 1.392V + 2.1V = 3.492V (.73% of 480V)
Ploss = 232 IC and 7 IT
Ploss = (232 * 1000A * .006V) + (7 * 1000A * .3V)
Ploss = 1392W + 2100W = 3492 Watts
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TECHNICAL NOTE TN-09002
Understanding the Value of Monitoring
Intercells & Intertiers in a Battery String
Loose connections are typically introduced during construction and maintenance, as batteries are
installed or replaced, or when battery trays are removed and reinstalled. Poor connections can also
develop from a buildup of corrosion, as well as a change in contact pressure between the posts and their
connections. The result will be increased resistance. Let’s now run that same analysis on a connection
for an intertier that is not properly torqued.
Loose Connection at IT
Cell 30
Discharge I
= 1000A
Cell 31
IT 1
10,000µΩ
Cell 32
IC 31
6µΩ
IT 1Voltage = 1000A * .01Ω
IT 1= 10.0V
IT 1 Power = I * E
IT 1 = 1000A * 10V
IT 1 = 10,000 Watts
In the string above, with a discharge current of 1000Amps, there will now be a total voltage loss and a
total power loss of:
•
•
Vloss of 232 IC and 7 IT
Vloss = (232 * .006V) + (6 * .3V) + (1 * 10.0V)
Vloss = 1.392V + 1.8V + 10.0V = 13.192V (2.69% of 480V)
Ploss = 232 IC and 7 IT
Ploss = (232 * 1000A * .006V) + (6 * 1000A * .3V) + (1 * 1000A * 10.0V)
Ploss = 1392W + 1800W + 10,000W
Ploss = 13,198W
Clearly these numbers indicate significant concern, as the excessive voltage loss will result in the UPS reaching
V-cutoff much faster than it should, potentially dropping the load prematurely. Additionally, the power
dissipation in this example will generate significant heat, possibly leading to melt down at the connection point,
and potentially hazardous thermal runaway conditions.
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TECHNICAL NOTE TN-09002
Understanding the Value of Monitoring
Intercells & Intertiers in a Battery String
Without monitoring such key parameters, reliability and safety may be jeopardized. A monitoring system must be
able to identify and notify of such conditions to enable proactive intervention before potential damage occurs.
Two Alber test conditions are capable of providing identification of issues related to the complete series
conduction path within a battery string:
RESISTANCE TESTING
Alber’s patented resistance techniques provide accurate measurement of a cell’s internal resistance along with the
interconnecting hardware in the conduction path (intercells and intertiers). This is a critical, often overlooked
feature which can assist in identifying when issues such as loose connections jeopardize the reliability of the
backup power system.
DISCHARGE RECORDING
As an additional feature, Alber monitors also take the intertier and or intercell into account when discharges
occur. A combined reading system will subtract the voltage drop of the Intertier from the adjacent cell, so that a
true cell voltage is recorded during a discharge. A discrete reading system provides an added benefit of
subtracting all interconnecting strap voltages (intercell and intertier), so that all cells display only the true cell
voltage. When a recorded discharge is replayed, the Engineer can be assured the voltage displayed accounts for
such.
With Alber’s unique ability to view a discharge in real time, Engineers have a significant safety tool available to
quickly identify critical conditions, such as cell dropout and cell reversal, and if conditions warrant abort before
damage occurs.
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TECHNICAL NOTE TN-09002
Understanding the Value of Monitoring
Intercells & Intertiers in a Battery String
In summary, Alber systems excel in detecting issues such as torque, either following maintenance, or
before system testing, such as a discharge that is about to be performed. Our Service team always
recommends a resistance test be manually performed following any maintenance on the Battery String,
to assure system conduction path integrity. The consequences are too dire not to perform this test and
utilize these very valuable features.
END OF TECH NOTE
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