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Use of Ni-Fe Batteries in Solar PV Systems

Conference Paper

Use of Ni-Fe Batteries in Solar PV Systems

Abstract and Figures

In this paper we investigate the potential and suitability of today's nickel-iron (Ni-Fe) batteries for applications in stand-alone PV systems. To achieve this, we evaluated the efficiency of Ni-Fe cells from two different manufacturers using two different charging regimes. The first regime involved charging the cells according to the recommendations of the respective manufacturers. This meant charging them with an amount of charge equivalent to 1.6 times their nominal capacities. The second regime involved charging the cells only to their nominal capacity. Both these charging regimes were carried out on cell level at different charging rates. An further experiment was conducted to investigate the battery performance while it is charged by the variable output power of a PV module. The result is an increase of the efficiency at lower charge capacity and low charge rates. Cutting off the discharge at a certain voltage has a positive effect as well. Loss of water through gassing (emission of oxygen and hydrogen) was found to be a significant issue, particularly when the cells were overcharged. Excessive gassing can be avoided by implementing a suitable charge control. We discuss such an approach and give suggestions for future research topics.
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USE OF NI-FE BATTERIES IN SOLAR PV SYSTEMS
Christoph Luerßen1,2,a, Timothy M. Walsh1,b, Peter Adelmann1,2,c
1Solar Energy Research Institute of Singapore (SERIS), National University of Singapore (NUS), Singapore 117574
2Ulm University of Applied Sciences, Germany
achristoph.luerssen@web.de, btim.walsh@nus.edu.sg, cpeter.adelmann@beutelreusch.de
ABSTRACT: In this paper we investigate the potential and suitability of today’s nickel-iron (Ni-Fe) batteries for
applications in stand-alone PV systems. To achieve this, we evaluated the efficiency of Ni-Fe cells from two different
manufacturers using two different charging regimes. The first regime involved charging the cells according to the
recommendations of the respective manufacturers. This meant charging them with an amount of charge equivalent to
1.6 times their nominal capacities. The second regime involved charging the cells only to their nominal capacity.
Both these charging regimes were carried out on cell level at different charging rates. An further experiment was
conducted to investigate the battery performance while it is charged by the variable output power of a PV module.
The result is an increase of the efficiency at lower charge capacity and low charge rates. Cutting off the discharge at a
certain voltage has a positive effect as well. Loss of water through gassing (emission of oxygen and hydrogen) was
found to be a significant issue, particularly when the cells were over-charged. Excessive gassing can be avoided by
implementing a suitable charge control. We discuss such an approach and give suggestions for future research topics.
Keywords: Batteries, Battery Storage and Control, Stand-alone PV Systems, Nickel-Iron Battery, Ni-Fe
1 INTRODUCTION
An estimated 1.5 billion people worldwide have no
access to electricity [1]. These often live in remote areas
of poor countries, which have no opportunity to build a
reliable electricity supply including grid, even in future
decades. Solar PV stand-alone systems can improve life
quality and safety. In such a system, storage of energy is
the key issue.
Ni-Fe batteries were independently invented by
Edison and Jungner in the early 1900s [2] and are said to
be indestructible. In such a battery, iron (Fe) and
nickeloxyhydroxide (NOOH) plates are arranged
alternately with separators between them. This package is
placed in a container filled with an aqueous solution of
potassium hydroxide (KOH). A vent (for escaping gases)
and two external electrodes complete the Ni-Fe cell [3].
2 BACKGROUND
Batteries have the ability to supply power over a time
period and to store energy. There are two types of
batteries distinguished between their rechargeability,
namely primary and secondary batteries. Primary
batteries can only be discharged once and have to be
disposed afterwards. Secondary batteries, e.g. Ni-Fe
batteries are rechargeable.
2.1 Electrochemical fundamentals
The electrolyte of Ni-Fe cells is a solution of 30%
potassium hydroxide (KOH) and 70% water (H2O). Also
a small amount of lithium hydroxide (LiOH) is added to
the solution. According to [4], the additional LiOH
improves the performance of Ni-Fe cells and decreases
the carbonation of the electrolye.
NiOOH is used for the anode and Fe for the cathode.
During discharging, electrons move from the cathode to
the anode. At the anode H2O is absorbed and hyroxide
(OH-) is emitted for transforming NiOOH to nickel
dihydroxide (Ni(OH)2). At the same time there is OH-
absorbed on cathode. The material changes in two steps
from Fe to iron dihydroxide (Fe(OH)2) and from Fe(OH)2
to iron oxyhydroxide (FeOOH). H2O is used up during
discharge at the anode. OH- ions are used for charge
transport from the anode to the cathode through the
electrolyte. The cell reaction happens in two steps [5]:
2NiOOH + 2H2O + Fe → 2Ni(OH)2 + Fe(OH)2 (1)
NiOOH + Fe(OH)2 → Ni(OH)2 + FeOOH (2)
During charging, the chemical reactions are reversed
and H2O is recovered.
2.2 Electrochemical side effect: Gassing
Side effects are processes, that occur on top of the
fundamental reactions – they are generally not useful.
When the cell is fully charged and the power supply
is still connected (overcharge), a side effect called
gassing occurs [5]. Because the anode is fully NiOOH
already, a different reaction has to occur. In this case, it is
the electrolysis of water. OH- ions react to form oxygen
(O2) and H2O and to release electrons at the anode:
4OH- → O2 + 2H2O + 4e- (3)
The O2 gas leaves the cell as gas bubbles. The
electrons flow from the anode terminal to the cathode
terminal into the cathode itself. The electrons react with
hydronium (H3O+) ions to form H2O and H2:
2H3O+ + 2e- → 2H2 + 2H2O (4)
The H2 gas leaves the cell as gas bubbles. The OH-
and the H3O+ originate from the autoprotolysis of water.
The reaction is irreversible (water is consumed).
Containing (3) and (4), the overall reaction is
2H2O → O2 + 2H2. (5)
2.2 Manufacturers
Nowadays most battery manufacturers do not
produce Ni-Fe batteries, but there are still some
manufacturers in China, Ukraine, Russia and USA that
do. In general, cell capacities from 10 to 1200Ah are
available. The price is comparable to high-end lead acid
and lithium iron phosphate (LiFePO4) batteries because
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of high costs for processed nickel and small production
scale.
2.3 Installation and maintenance
Ten cells are connected in series to form a nominally
12V battery. The installation requires according to EN
50272-2 [6] an appropriate cabinet or room, especially to
deal with the potential spillage of electrolyte. The chosen
housing has to assure protection from external hazards:
hazards generated by the battery, access from
unauthorized personnel and extreme environmental
influences.
In the European Standard Ni-Fe batteries are not
included. Nevertheless, it is the standard document where
ventilation for battery rooms is discussed. The first
choice should be natural ventilation. When the air flow
rate cannot be obtained by natural ventilation, a
supervised forced ventilation has to be installed. The
ventilated air shall be released into the atmosphere
outside the building [6].
Generally cells are delivered in dry condition with
dry electrolyte. The electrolyte has to be prepared first,
which requires laboratory environment. A suitable scale
for weighing chemicals and containers which are resistant
to corrosive chemicals are needed. Heat is generated
when the chemicals are added to the distilled water.
Maintenance is unlikely to be carried out frequently,
if at all, for solar stand-alone systems in rural remote
areas, because these areas are typically difficult to access.
Nevertheless, because of the water consumption due to
gassing, regular top up of distilled water is required.
Distilled water may be hard to come by in these areas.
During operation, the electrolyte can absorb carbon
dioxide from the air and build potassium carbonate (2K+
+ 3CO2 → K2CO3). The effect is known as poisoning of
electrolyte and decreases the capacity of the battery. The
LiOH additive decreases the carbonation [5]. This
process takes years and can be prevented when there is
no possibility for the CO2 or air to enter the cell.
However, when a capacity decrease is noticed, an
electrolyte replacement is recommended.
2.4 Properties
In general, Ni-Fe cells are mechanically durable and
safe to use. Its competitive technologies are nickel
cadmium (Ni-Cd), lithium iron phosphate (LiFePO4) and
high end lead acid batteries. Ni-Fe cells have a nominal
voltage of 1.2 V and a comparatively wide voltage range
from 1.0 V to more than 1.7 V. Because of the
insensitivity to overcharge and over discharge, no sudden
breakdown is possible. Even when the charge control
breaks down, the system can still run without service,
which is a huge advantage over other technologies. Ni-Fe
cells can be discharged to 0 V without becoming
damaged. The efficiency of Ni-Fe cells is lower than the
efficiencies of other technologies. The self-discharge is
not important in a stand-alone PV application, because
there is ideally one cycle every day. The lifetime can
about 20 years [2], but there are much older batteries
known, which still work or are working again after
replacement of the electrolyte [7]. However the number
of cycles of these cells is unknown. According to [2], the
cycle life is 3000 cycles. The performance of Ni-Fe cells
decreases at low temperatures and they are operable up to
more than 50̊C [5]. Lead and especially Cadmium are
very toxic; Ni-Fe batteries are a safe and environmentally
friendly alternative.
3 METHODS
In order to conduct a comprehensive investigation of
Ni-Fe cells, we analyzed the charge and discharge
characteristics of cells from two different manufacturers
at various charging regimes. To include real application
conditions, cells are tested also under outdoor conditions
– charged by a PV module and discharged by a load.
3.1 Initial charge/discharge and following cycles
Initial charge/discharge, as recommended by the
manufacturers and a certain number of normal cycles
(0.2C for 8h and 1.0V/cell discharge end voltage) are
conducted.
3.1 Charge and discharge characteristics
Figure 1: SFC describes the procedure for recording the
charge/discharge characteristics
The operation of recording the charge and discharge
characteristic is visualized in Figure 1 by using a
sequential function chart (SFC). The third and fourth
steps represent the charge/discharge characteristics.
Current and voltage are recorded every second.
The procedure is conducted for four different
charge/discharge rates (0.05C, 0.1C, 0.2C and 0.4C) and
two different charge capacities, recommended charging
(16Ah) and charging by nominal capacity (10Ah) for
two different manufacturers.
3.3 Outdoor performance
Figure 2: SFC describes the procedure for recording the
outdoor test
The battery is charged by a PV panel during daytime.
During nighttime the discharge runs until the battery
voltage is lower than 0.9V/cell or the daytime starts as
visible in Figure 2. PV voltage, battery voltage, charge
current and discharge current are recorded once a minute.
Tests were run for 25 days.
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4 EXPERIMENTAL SETUP
For implementation of the methods described in
Chapter 3, the batteries themselves, a laboratory battery-
testing station and an outdoor testing station are used.
4.1 Batteries
The elecetrolyte was prepared from dry chemicals.
Ten 10Ah cells of manufacturer one (Man1) and twenty
10Ah cells of menufaturer two (Man2) were filled with
electrolyte.
4.2 Battery testing station
A National Instruments PXI-1042 device was
extended by two PXI-4130 SMU cards in parallel, an
external electronic circuit and auxiliary power supplies,
in order to supply constant charge/discharge current. The
software including user interface, which is able to
implement the methods, was developed in LabView.
4.3 Outdoor testing station
For testing the batteries under outdoor conditions, a
measuring circuit is needed in addition to the load circuit.
In Figure 3 the load circuit and the corresponding
measurement points are shown.
Figure 3: Outdoor load circuit and measurement points
The measurement points are connected to analog
inputs (AI) of a data logger (Gantner IDE100). The
discharge control is operated by the same device. Ten
10Ah cells from Man2 are connected in series and
discharged over a 5.6Ω resistor during the night. During
daytime the cells are charged by a PV panel (UMPP =
23.7V; IMPP = 2.8A). Between PV module and Battery a
Schottky diode is located to prevent backward current
into the solar module during nighttime.
5 RESULTS
5.1 Initial charge/discharge and following cycles
The initial charge of both manufacturers’ batteries
was conducted as recommended by the manufacturers. It
was noticed that the cells of Man1 needed 10 cycles after
conduction of initial charge to reach the full capacity.
The cells of Man2 reached their nominal capacity from
the beginning. Both manufacturers’ charge and discharge
characteristics keep on changing during the first few
cycles after the initial charge. During these cycles,
Man1s’ battery efficiency increases constantly until the
final efficiency of 50% ±2%, while Man2s’ batteries
obtain an efficiency of 60% ±2%.
5.2 Charge/discharge characteristic under recommended
charge
Considering the 16Ah charge, the voltage of the cell
of Man1 (Figure 4) and Man2 (Figure 5) is fairly constant
by the end of the charge procedure. In general, the rule is
that the higher the charge rate, the higher the charge end
voltage. A significant difference between both
manufacturers is visible when the value of the charge end
voltage is considered. The charge end voltage of the cell
of Man1 is higher.
The cells were discharged to 0.0V. The voltage, at
which the nominal discharge capacity of 10Ah is reached,
is higher for Man1s cells. When the cells are discharged
to 0.0V, the maximum capacity is up to 31% higher and
47% higher for the cells from Man1 (Figure 4) and Man2
(Figure 5) respectively.
Figure 4: Man1 10Ah cell charge/discharge
characteristics (16Ah charge) at various rates
Figure 5: Man2 10Ah cell charge/discharge characteristic
(16Ah charge) at various rates
Figure 6: Energy efficiency at various charge rates and
two regimes
The energy efficiencies were calculated by dividing
the integral of the area below the discharge characteristic
by the integral of the area below the charge characteristic.
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The efficiencies (Figure 6) at 0.2C are fairly similar for
the 16Ah charge. At the other charge rates higher
efficiencies are provided by Man1 (from 2% up to 7%).
In general, the efficiencies decrease with increasing
charge rates.
5.3 Charge/discharge characteristic under nominal charge
in comparison to the recommended charge regime
Figure 7: Man1 10Ah cell charge/discharge characteristic
(10Ah charge)
Figure 8: Man2 10Ah cell charge/discharge characteristic
(10Ah charge)
The charge characteristics of the 10Ah charge and the
16Ah charge have similar patterns: The charge end
voltage rises when the charge rate increases. The charge
end voltage of both batteries is very similar.
Similar patterns are apparent in the discharge
characteristics as well: Faster voltage drop and sharp
bend towards the end of discharge at Man2s’ (Figure 8)
cell. The calculated efficiencies of Man1 (Figure 7) are
from 6% up to 15% higher than for Man2, see Figure 6.
The efficiency difference between the manufacturers is
much larger at the nominal capacity charge than at the
recommended charge. The energy efficiency of Man1s
cell increases by 14% at 0.2C, when it is charged by its
nominal capacity instead of the recommended charge
capacity. The efficiency of Man2s cell does not increase
significantly at 0.2C, but up to 10% at 0.4C (see Figure
6).
The repeatability of the exact characteristics is only
given with Man1s cells – at both 10 and 16Ah charge.
5.4 Comparison between 0V/cell and 1V/cell discharge
end voltage
In this section ten cells are connected in series. The
voltage level during discharging is in general higher
when a discharge voltage limit is applied in previous
discharge cycles, as can be seen in Figure 9. The voltage
of Man1 is constantly 0.3V higher when discharged to
10V than when discharged to 0V in the previous cycles.
Towards the end of the discharge curve, the difference
increases up to 1.2V at 10Ah discharge capacity. Man2’s
characteristics diverge even more, up to 2.5V. The
characteristic of Man2, which was discharged to 0V, has
dropped below 10V at 6.7Ah.
In the charge characteristic the charge end voltage
drops for both manufacturers by 0.3V when the discharge
is limited.
Figure 9: Comparison of charge/discharge characteristics:
discharge to 0V/cell and 1V /cell
5.4 Outdoor performance
Figure 10: Energy efficiency against daily charged
capacity and median charge current
In the outdoor test, the charged capacity is dependent
on the solar energy during daytime. The efficiency
decreases linearly when the daily charged capacity
increases, as can be seen in Figure 11. The same relation
is visible when the median charge current is plotted
against the efficiency. The efficiency of Man2s battery is
around 50% when operated under reasonable daily
charge.
If median charge current and daily charge is plotted
against median battery voltage, another relation is
apparent, as can be seen in Figure 11. Even if the daily
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charge is comparatively high, the median battery voltage
increases only slightly. This confirms the charge
characteristics, which were recorded in the lab under
constant current. The same relation exists between
median charge current and median voltage. The highest
measured voltage during 25 days outdoor test was 16.9V
and an instantaneous charge rate of 0.33C.
Figure 11: Comparison of charge/discharge
characteristics: discharge to 0V and 10V
6 DISCUSSION
6.1 Initial charge
Man2’s constant current initial charge procedure
worked as described in the manual. Man1 needed ten
cycles after the initial charge to reach the full capacity.
The “warm-up” period could be increased because of
long storage time. In general, there are more cycles than
just the initial charge needed to achieve a constant
charge/discharge characteristic. The performance
improves from cycle to cycle.
In solar standalone application there is no constant
current available. Though the initial charge mainly
consists of repeated overcharging and discharging to a
certain voltage, it can be conducted in PV off-grid
applications by controlled disabling and enabling of the
loads.
6.2 Efficiency
It was shown that the efficiency decreases at higher
charge/discharge rates in both regimes – charging by
recommended capacity (1.6 times nominal capacity) and
charging by nominal capacity. However, since typical
charge/discharge rates in solar stand-alone systems are
usually low.
The constant current and the outdoor test showed a
high potential for increasing the efficiency of the cells of
Man1 by overcharging them less, which can mean a
reduction of the PV costs by using a smaller panel.
Through further testing, the optimal charge capacity for
high efficiency and high energy output may be found.
Comparing the efficiencies calculated when the cells
were discharged to 0V/cell and when the cells were
discharged to 1.0V/cell show that Man2s cell efficiency
increases by 9.2% if the discharge end voltage is set (see
Figure 6). In a well dimensioned stand-alone system with
fixed loads, the voltage will never drop below that level,
but if the batteries are used in systems with indefinite
loads, problems can arise. Setting a discharge end voltage
is a simple way to make sure that the battery of Man2 is
operated with reasonable efficiencies. Man1s cells are not
susceptible to discharge to 0V/cell. The efficiency is still
the same.
The outdoor test showed that the efficiency of the
same cells (Man2) is 10% lower than in the constant
current operation – 50% instead of 60%. This has to be
considered when investigating the best operation
conditions as well. However, a discharge voltage limit
and an overcharge protection would improve the battery
performance.
6.3 Voltage range
The voltage range of Ni-Fe batteries is comparatively
high. The maximal charge voltage can be higher than
18V at 0.4C constant current in a 12V system, but the
outdoor test has shown that the voltage is typically lower
when the battery is charged by a PV panel.
During discharging the voltage can be pushed to
0V/cell, because the batteries do not suffer a sudden
breakdown. Not only the efficiency increases when a
discharge voltage limit is set, but the use of diverse loads
is also enabled due to higher operating voltage during
discharge. Many DC loads are only operable above a
certain voltage. In addition Man1 states that the lifetime
decreases with higher DoD. The overcharge cannot be cut
off at a certain voltage, because the slope of charge
characteristic is very low at the end of the charging
procedure, and at first sight there is no need to stop the
charge, because the Ni-Fe technology is resistant to
overcharging.
6.5 Gassing
While describing the electrochemical fundamentals
and the properties of Ni-Fe batteries, the problem of
gassing was explained. Gassing means that water is being
depleted. Filling up distilled water requires regular
maintenance and should be avoided if possible. During
this study, no measurement regarding gassing was
performed. The following statements are based on
observations.
Towards the end of the charging procedure more
bubbles were visible in the batteries and the escaping
gases through the vents were clearly audible, which
means that the gas flow rate is higher during
overcharging. Therefore, overcharging should be
minimized so as to reduce maintenance required.
6.6 Integration into a PV system
The power source, battery and load can be put in
parallel without any charge control. This is the simplest
solution, which can be applied when loads, which do not
work with low battery voltage, are used. In this case the
water consumption would be high and a regular fill up of
distilled water is strongly recommended (time period
needs to be determined by experience as every system is
different). Prolonging of service interval and protection
to deep discharge due to unknown loads can be provided
by implementation of a charge control as shown in Figure
12.
The deep discharge control cuts off the load at a
certain voltage. This voltage limit has to be chosen
according to the discharge rate and is lower at higher
discharge rates. According to the conducted tests,
1.0V/cell is a suitable value for a discharge rate of 0.2C.
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In order to decrease the water consumption rate, the
battery can be charged until a certain voltage, which
depends on the charge rate and on the manufacturer, and
once this border is reached, the overcharge control keeps
the voltage constant on a lower level by using pulse
width modulation (PWM). The values for the voltage
border and the constant voltage level should be
investigated for different manufacturers, cell sizes and
operation in order to find the operation points of low
gassing and high efficiency.
Figure 12: Approach for charge control including PWM
overcharge control and cut off discharge control
The use of a maximum power point tracker (MPPT)
would open the opportunity to PV panels with a
standardized number of cells. The MPP tracking has to be
evaluated for every system/project to determine whether
it can reduce the cost of the system. The high voltage
range of the battery from charge end voltage to discharge
end voltage has to be considered in the evaluation as
well. MPP tracking has the potential to decrease the PV
costs by decreasing the needed module power.
The high voltage range could also be a problem for
loads. The loads may break because of high voltage or do
not work at low voltage anymore. If this is the case, an
implementation of a DC/DC converter with constant
voltage output between battery and load should be
considered.
The state of charge (SoC) cannot be determined by
using the cell voltage. When the ampere hours are
balanced for indicating the SoC, many parameters have to
be considered: temperature influence on efficiency,
efficiency dependence on SoC, efficiency dependence on
charge rate, the variability of charge/discharge rate in PV
application etc.
6.7 Useful application cases
Based on the information from Chapter 2 to 6, useful
applications can be devised. Firstly the main problem is
still the necessity of replenishing the distilled water. In
this case, the batteries can be used in places where
regular service is already required, such as
telecommunication stations. In these places, the
additional effort of replenishing distilled water is not
significant. A charge control is recommended, because
these systems have to operate without any interruption.
However, even if the charge control fails, the system can
keep on operating and will not suffer a sudden
breakdown.
In the rural electrification market, there is a trend
towards mini grids. When a mini grid is sold, the owner
is likely to feel more responsible for the system. It should
be comparatively easy to advise the person to maintain
the system regularly, especially when the person gets
income from the users.
For use in solar home systems, the need for water
replenishment is a big disadvantage. But the battery
technology can convince users because of the simplicity
of the system. Loads and PV modules have just to be
connected in parallel to the battery.
To sum up, there are two types of systems in which
Ni-Fe batteries should be considered, interruption-free
long life systems and simplest systems.
7 CONCLUSION
The implementation of Ni-Fe batteries in solar PV
stand-alone systems may not reduce costs, because nickel
is expensive, but it makes sense for building durable
systems with long lifetime in rough conditions.
7.1 Future prospect
In case of prohibition of toxic lead and cadmium in
batteries, Ni-Fe batteries have the potential to replace Ni-
Cd and partly lead acid batteries. The world is becoming
more environmentally conscious in recent years and this
may support the use of Ni-Fe batteries instead of cells
containing toxic metals. Low PV module costs can
invalidate the disadvantage of lower efficiency of Ni-Fe
compared to other battery types. Further investigations in
the field of Ni-Fe batteries will be beneficial for the
industry.
7.3 Future research
Relations between gassing and charge rate, voltage
and overcharging can determine points of minimum
gassing, which translates to high efficiency operations.
The leftover gases could be recombined by
recombination plugs. Research on this may lead to
maintenance-free operation, which means that Ni-Fe
batteries would be directly competitive to all sealed
battery types.
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[1] International Finance Corporation (IFC), \From gap to
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[3] Association of Edison Illuminating Companies,
Manual of Storage Battery Practice, 1914
[4] D. Linden, T. B. Reddy, Handbook of Batteries, 3rd
edition, 2001
[5] A.K. Shukla, Nickel/Iron batteries, Journal of Power
Sources, 51 (1994) 29-36
[6] EN 50272-2, Safety requirements for secondary
batteries and battery installations – Part 2: Stationary
batteries, 2007
[7] P. J. DeMar, Nickel-Iron, This all but forgotten
technology has a very important place to occupy with
users that desire very long life and the ability to suffer
abuse in their battery systems, IEEE (2011)
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... 41 In addition, a variable output power source can adequately charge it at different charging rates, such as a PV module. 42 While other batteries are "consumables" because of their shorter life span, the IE storage batteries have no harmful heavy metals and outlast any HRES. Another reason for exploring the IE battery in the present study is the widely used LA batteries' contribution to environmental contamination and lead poisoning among workers and children. ...
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