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Lifetime Effects of Voltage and Voltage Imbalance on VRLA Batteries in Cable TV Network Power Lifetime Effects of Voltage and Voltage Imbalance on VRLA
Batteries in Cable TV Network Power



Brian Kuhn
René Spée
Philip T. Krein
SmartSpark Energy Systems
Alpha Technologies
University of Illinois
Champaign, Illinois, USA
Bellingham, WA, USA
Urbana, Illinois, USA


Abstract

Charge balance is known to be an essential consideration in series valve-regulated lead-acid (VRLA) battery
applications. Conventional applications use passive equalization to maintain balance: periodic overcharge
sequences. Active equalization uses external circuits to enforce charge balance without the need for overcharge.
There is little published data to show the potential value of active equalization in float applications such as those
common in telecommunications. Measured voltage balance data for a population of 2538 batteries in a set of
846 series strings are reported. Batteries showed a clear pattern of degraded life for charge voltages that differ
by more than about ±0.1 V from an optimum charge voltage chosen to minimize positive grid corrosion.
Passive equalization was able to keep about 63% of the batteries in the population within this range. The
remaining 37% showed life degradation in a direct relationship to the charge voltage error. It is estimated that
active equalizers capable of balance at the level of about 15 mV/cell would have maintained all the batteries
near enough to the optimum range to avoid life degradation. An economic analysis suggests that active
equalization would reduce tangible battery and associated maintenance costs by 31-36%. There would be
additional economic benefits that are more difficult to quantify.


1 Introduction

State-of-charge (SOC) balance is important to
maintain long-term life in series strings of all types of
rechargeable batteries. The issues are well known for
valve regulated lead-acid (VRLA) batteries. In the
literature, it has been established that voltage balance
is a suitable surrogate for SOC balance [1,2]. It has
also been established that voltage imbalance gradients
need to be less than about 15 mV/cell to avoid life
degradation [2]. A major issue is that although a
baseline threshold for the voltage gradient has been
established, there are little or no data demonstrating
the value of low gradients. Some questions that need
to be answered include: How much is life degraded as
imbalance increases? How does a 15 mV/cell target,
obtained in a laboratory, reflect on actual usage of
batteries? Do low gradients yield long life?

In addition to charge balance, the absolute charge
voltage is a critical parameter. When voltage can be
held close to the optimum float value for a given
temperature, corrosion effects are minimized and
lifetime is maximized. The potential benefits of
equalization can be compromised by incorrect charge
voltage or inaccurate temperature compensation.

A wide range of low-loss methods for equalization
has been proposed [1,3-8]. All of these techniques
are designed to move charge from one battery cell to
another to perform equalization. The techniques vary
in their complexity, with some requiring transformers
[3, 6], microprocessors [6, 8], and semiconductors
with a full battery system voltage rating (instead of a
cell or monoblock rating) [1, 8]. Modular methods
were introduced in [4], which applies buck-boost
converters between adjacent cells or monoblocks, and
[5], which applies switched capacitor methods. The
modular approach in [10] is related to a forward
converter topology as discussed in [3]. All of the
techniques except [5] require measurement of cell
voltages, and many only work while the battery is
charging [1, 7, 8].

The purpose of this paper is to determine what
equalization might be able to accomplish in terms of
battery life extension in float applications. There
have been very few studies that confirm the benefits
of any equalization approach, or that establish the
value of equalization in field use. A 20-cycle test in
[2] showed that water loss was cut nearly in half for
flooded lead-acid batteries with equalizers compared
to those without. Actual street tests of a fleet of
electric vehicles showed modest cycle life extension
from equalization, in the range of 15-30% for those
devices that worked [9]. Tests in [10] show much
more significant improvement a factor of about 3
cycle life extension in an aggressive hybrid electric vehicle test sequence. Other long terms tests [11]
have been less conclusive.

The above studies address cycling applications.

During cycling, impedance mismatches and other
small differences among cells would be expected to
produce differential charging. It is reassuring that
cycling tests in [10] show that equalization extends
the cycle life of a series string to that of the individual
elements. It is expected that effective equalization
will provide this level of performance when designed
into a cycling application. The benefits in float
applications such as telecom power systems are less
obvious. Imbalance in float applications probably
comes about because of mismatches in self-discharge
characteristics or temperature gradients across a
battery pack. It is important to study how
equalization is likely to impact float situations.

2
Field Test Data

Needs for and possible benefits of equalization are
explored here through field testing of a large
population of VRLA batteries in a
telecommunications float application. These field
data quantify the lifetime effects of voltage imbalance
or error. A sample of 846 battery strings in a cable
TV system in a U.S. upper Midwest location, each
constructed from three matched 12 V monoblocks
[12] in series, was monitored after a service life of
approximately 2 years to establish operating life
performance. These batteries are maintained on float
with a proprietary CATV power supply [13]. The
target charge voltage is selected based on previous
results to minimize positive grid corrosion. The
voltage is temperature compensated in accordance
with well-established practice. A typical power
supply system for an outdoor CATV plant is
illustrated in Figure 1.

Fig. 1. Open enclosure showing batteries and CATV
power supply with integral battery charger.
Installations of these types must meet increasing
levels of performance. As voice-over-internet-
protocol (VOIP) is being introduced in many cable
systems, battery life and performance should tailored
to achieve the lifeline power backup performance
expected today from conventional wire telephony.
For example, in a long-term outage in the plant, the
CATV provider expects a certain run time, which in
general is predicted based on equalized batteries. The
unit in Fig. 1 protects its monoblocks with a low
voltage disconnect, set to 10.5 V on the lowest
battery. For mismatched strings, the discharge time
can be reduced substantially and significant run time
can be lost.

In these installations, a short-term overvoltage
equalization charge is provided at regular intervals.
This represents a passive equalization process as
defined in [2]. The net effect is that these data
represent a best case field test of 36 V battery
strings in float applications. They use the best
available conventional charging process, and do not
apply any external balancing process.

It is noted in [14] that poor charging is the primary
cause for short battery life. Lead acid batteries obtain
their maximum lifetime if they are charged at the
proper voltage, which needs to be temperature
compensated. Excessive overcharging leads to
positive grid corrosion and active material shedding
leading to a shorter cell life. Overcharging also leads
to excessive gassing which can dry out the cell and
cause other problems if the battery is not maintained
properly. On the other side, overdischarging a cell is
bad also, since it can reduce the electrolyte
concentration low enough to damage the pore
structure of the battery. Additionally, if the battery is
stored in a discharged state, sulfation can occur on the
negative plate, leading to decreased capacity and life
of the battery.

It is apparent that it is important to maintain the
battery at a proper voltage to obtain maximum life.
With active equalization, it is possible to control the
voltage of individual monoblocks or cells and prevent
them from drifting apart. Without equalization, with
a charge process based on the full stack voltage, it is
possible that one of the cells will be overcharged (and
damaged) during the charge process, while the other
cells do not obtain full charge. Similarly during
discharge, one cell may be overdischarged and
damaged while the others have not yet been fully
drained. Systems which monitor individual
monoblock voltages during discharge will not deliver
full capac