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Battery Available Capacity Meter Built into an AC/DC Telecom Power Supply System

Authors:
  • National Institute of Telecommunications, Poland
  • The National Institute of Telecommunications

Abstract and Figures

The article describes the results of testing a Benning AC/DC power supply, with integrated NIT TBA-ST meter. Such integration enables accurate and energy-efficient measurements of available capacity of individual 48-voltage VRLA/AGM batteries with remote management possibilities. The full compliance with the requirements for telecommunication power systems, the great functionality, upgrade ability, immunity to user errors and the high accuracy of capacity and voltage measurement of battery monoblocks of proposed solution are presented.
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Paper Battery Available Capacity Meter
Built into an AC/DC Telecom
Power Supply System
Paweł Godlewski, Ryszard Kobus, and Paweł Kliś
National Institute of Telecommunications, Warsaw, Poland
Abstract—The article describes the results of testing a Ben-
ning AC/DC power supply, with integrated NIT TBA-ST me-
ter. Such integration enables accurate and energy-efficient
measurements of available capacity of individual 48-voltage
VRLA/AGM batteries with remote management possibilities.
The full compliance with the requirements for telecommuni-
cation power systems, the great functionality, upgrade ability,
immunity to user errors and the high accuracy of capacity
and voltage measurement of battery monoblocks of proposed
solution are presented.
Keywords—battery capacity tester, battery discharge tester,
VRLA real capacity.
1. Introduction
In AC/DC power supply systems, during voltage inter-
ruption energy backup is provided by lead acid batteries,
mainly of the VRLA/AGM type. They are inexpensive,
safe and their high fault current enables fast tripping of
fuses. However, owing to certain unfavorable phenomena,
i.e. premature capacity loss (PCL), VRLA/AGM batteries
require periodical capacity testing.
The most commonly operated telecommunication power
systems (Fig. 1a), contain at least two 48-voltage batter-
ies B connected by a common disconnector switch RGR,
system bus with + on the ground with rectifier outputs
PS and DC load R. The operation is managed by the ST
controller, which also ensures communication with the SN
supervisor center. Under normal operation conditions, the
rectifiers feed load R and charge batteries keeping them
fully charged.
When AC mains failure occurs the power supply to load
is taken from batteries, and after voltage is restored, rec-
tifiers again feed load and charge batteries. The circuit
also has a common high-current disconnector switch RGR,
which protects batteries from deep discharging. It breaks
the circuit when battery voltage drops below 40 V in the
long absence AC mains network. In such a power sys-
tem, the measurement of available capacity of any battery
can be performed by portable instruments, e.g. RRR shown
in Fig. 1a, or electronic loads as well as TBA-IŁ [1]. The
measurements require disconnection of the battery from the
system bus by service operator. While testing, the neces-
sary energy reserve is provided by the other batteries.
In AC/DC power systems with TBA-ST meter built-in
(Fig. 1b), the RGR disconnector switch and PN low-current
contactor, are connected in series with each battery Bn [2],
and to the meter output. First, the instrument discharges
the battery for test purposes, next recharges it and calcu-
lates the capacity. After that the battery is connected back
to the system bus and capacity data is transferred to SN.
2. TBA-ST Unit
The TBA-ST meter has been designed and made at the
National Institute of Telecommunications under the energy
reserve checking system for telecommunication sites project
(SKOT), co-financed by the European Union with European
Regional Development Funds (ERDF). The meter occupies
half of the 19”/1U rack frame (Fig. 2a).
The circuit is based on a bi-directional boost-buck DC/DC
converter allowing increase or decrease input voltage, with
control circuit and voltage and current sensors (Fig. 2). The
converter can deliver up to 50 A output current and oper-
ates similarly to the TBA-IŁ devices. By battery discharg-
ing with preset current, it transfers energy to the system
bus, which temporarily sheds load from the rectifiers. By
charging the battery with preset current to a programmed
voltage, the unit draws energy from system bus and continu-
ously measures voltage on the power system and the battery
under test, as well as voltages of monoblocks of all present
system batteries. The “LEM” current transducer allows
battery current measurement and thus capacity. The con-
figuration parameters and measurement results are stored in
controller non-volatile memory. The communication with
the power system controller is DC-based, and data exchange
with the SN takes place via the RS485 and the ZSN-5 com-
munication module of the SCS Win system or by a built-in
Ethernet interface.
2.1. Operation
The battery test is initiated remotely the controller SN.
First the TBA-ST unit checks if the test can be performed.
If yes, it sends a command to the power system con-
troller. Test begins after battery reconnecting to the test
unit. The discharging phase ends when a predefined charge
(Ah) has been sink and voltage have been reduced on bat-
54
Battery Available Capacity Meter Built into an AC/DC Telecom Power Supply System
Fig. 1. AC/DC power system with battery capacity meter: (a) as portable “RRR” instrument and (b) as built-in “TBA-ST” unit.
Fig. 2. VRLA battery capacity meter TBA-ST: (a) overview,
(b) block diagram.
tery monoblocks to a programmed value. Charging phase
takes place for given time calculated from the moment the
battery voltage preset is reached. The test is ended with
battery reconnecting. An initiated test can be remotely
canceled or break with a stop button. The cycle is then
interrupted, the battery is fully charged and reconnected to
the power system as soon as possible.
3. AC/DC Power System
In conducted measurements the Benning AC/DC power
supply system “SBE200SL version SKOT” was used. It
was configured in accordance with requirements received
from the National Institute of Telecommunications for op-
eration with the TBA-ST. The solution is protected by
a European patent of the National Institute of Telecommu-
nications.
The system was housed in a PSJ1866 600 ×600 ×1870 mm
cabinet (Fig. 3a). It contains three 48 V/1500 W rectifier
modules (PS) with AC current protection devices, a distri-
bution panel for 15 loads with DC current protection de-
vices (F), a current protection supervisor card, three high
current disconnectors and three contactors (P). MCU2500
controller (ST), a signaling module, two input/output cards
(IO), three 48 V/100 Ah (4 ×12 V) battery strings (B), as
well as connectors. There is a free space for the TBA-ST
meter and ZSN-5 communication module in enclosure.
The block diagram shown in Fig. 3b presents the TBA-
ST meter circuit. The contact components (switches and
contactors) are shown in which battery B1 is tested. The
power system ensures automatic operation with temperature
compensation of floating voltage and programmable bat-
tery charging current (via resistor R1). The standard power
system operation program featuring macro instructions of
non-standard use of the inputs/outputs of the IO/02 card.
The power system was not equipped with battery voltage
measurement cards and thus the battery “auto test” func-
55
Paweł Godlewski, Ryszard Kobus, and Paweł Kliś
Fig. 3. AC/DC power system equipped with 3×48 V/100 Ah batteries and modified to use TBA-ST unit: (a) overview, (b) block
diagram.
tion was not checked, because such measurements are per-
formed with greater accuracy by the TBA-ST meter. During
testing, worn battery banks (HZB12-100FA) were used, as
this reduced the time of energy reserve, and consequently
the time of testing the individual functions.
3.1. Operation without the TBA-ST
If the circuit breaker f.Tba is open, the power system oper-
ates in standard mode [3] with the one difference that three
RGR disconnectors are used instead of one. They are con-
trolled by the K4 relay located on the IO/01 card. The relay
contactors k1, k2, k3 located on the IO/02 card remain in
non-active state. With such settings, the TBA-ST measures
only battery monoblock voltages.
3.2. Operation with TBA-ST
If the circuit breaker f.Tba is closed (as shown in Fig. 3b)
and the G.TBA signal appears at the input of IO/02 card,
then in response to the signal demanding the connection
of battery BAT-1, BAT-2 or BAT-3 (e.g. BAT-1 for bat-
tery B1) to the TBA-ST unit for testing, the IO/02 card
closes k1 relay. As a result (for battery B1), relay RGR-1
disconnects battery B1 from the system bus, and the con-
tactor K31 connects it to the meter. After testing, when the
TBA-ST, in the presence of the G.TBA signal switches off
the signal demanding battery connection, and the IO/02
card restores the non-active state of the relay k1. In re-
sponse, the contactor K31 disconnects the battery from
the meter, and RGR-1 connects it to the power system bus.
The operation for other batteries is proceed in the same
way. In case of meter power supply failure as well as
other fault the state of relays k1–k3 and RGR-1–RGR-3
is unchanged. During switching there is no significant cur-
rent flow in the RGR-1–RGR-3 disconnectors and K31–K33
contactors.
4. TBA-ST Meter Test Results
The TBA-ST capacity meter was tested in a circuit con-
tains a power supply, battery simulator, with real batteries
as well as power system simulator (Table 1). Column with
56
Battery Available Capacity Meter Built into an AC/DC Telecom Power Supply System
TBA-ST/22 header is related to data for a meter installed
in the power system. The “other” column shows values
available for all versions. The result of testing the elec-
tromagnetic disturbances level generated by the TBA-ST is
shown in Fig. 4.
Table 1
The TBA-ST meter parameters
Parameter TBA-ST/22 Other
Rated voltage battery 48 V (54 V power system)
Operating programmable current 2–20 A 2–50 A
for discharging and charging
Number of battery strings Up to 3 Up to 6
Number of controlled Up to 4 Up to 24
monoblocks of each battery
Capacity consumption as
End of charge criterion percent or final
monoblock voltage
Accuracy of voltage, current, Better than 1%
charge and time measurement
Maximum energy losses on 5.3%
discharging/charging
Communication with power DC voltage (Fig. 5)
system
Remote management RS485 or Ethernet
(Modbus RTU protocol) interface
Fig. 4. EMC disturbance level generated by the TBA-ST/22
during battery discharging at rated current.
5. Power System Testing with TBA-ST
Function Disabled
The scope and testing results of system with the TBA-
ST disabled are presented in Table 2. The column “result”
contains reference to the sections with further details.
5.1. Power System Operation
The power system indicates operational status and emer-
gency states. The monitoring and power system program-
ming is possible from the signaling module or from a PC
Table 2
Issues to check in the SBE200SL power system
Issues to test Result
Supply voltage 230 V or 3×230/400 V +
Floating voltage 48–56 V +
Automatic charging 53–58 V +
Power system equipment, protection devices 3.1
Easy access to batteries and fuses +
Signaling and programming 5.1
Battery disconnection at low voltage 5.2
Charging after AC mains failure 5.2–5.3
Voltage drop in battery circuits 5.4
TBA-ST function on/off +
via RS232 serial interface or as well as by Ethernet based
network and the web server. The available programming
includes value of floating voltage, charging current and tem-
perature correction of battery charging voltage, as well as
RGR control voltage threshold. Such settings are password
protected. In addition, certain options are available only
for the maintenance engineers.
5.2. Battery Disconnection at Mains Failure
Figure 5 shows the voltage and current waveforms recorded
during long-lasting voltage interruption in the AC mains.
While AC mains failure (AC off), the system bus voltage
Us1 and battery voltage Ub lowers, so loads (Io) the cur-
rent fed with power Ib1 (drawn from the battery) increases.
When voltage on the system bus (and the battery voltage)
drops to programmed 43 V value, the controller switches
off power supply to the RGR. The RGRs disconnect (RGR
off) all batteries from the system bus and DC loads lose
power.
When AC mains is back (AC on), the PS rectifiers power
system bus Us2. The controller closes the RGRs (RGR
on) and Ub voltage is fed to the batteries from the system
power bus. Initially, the charging is high current power Ib2
and is limited by maximal output current (90 A) and load
Fig. 5. Long-lasting voltage interruption in the AC mains. (See
color pictures online at www.nit.eu/publications/journal-jtit)
57
Paweł Godlewski, Ryszard Kobus, and Paweł Kliś
current. Then the rectifiers decrease the voltage, hence bat-
tery charging after approximately 1 min reach programmed
20 A value. While battery is charged, the Ib2 current de-
creases to the low maintenance level (approx. 0.2 A).
5.3. Battery Charging After Mains Return
On short mains voltage interruption, which does not cause
the RGRs trip, the battery is charged to the floating volt-
age with programmed 20 A value. Next it is charged with
decreasing current. An automatic charging feature can be
programmed after each AC voltage interruption with a dura-
tion exceeding the preset time. The battery will be charged
with voltage increased to a programmed value for the en-
tered period. In the tested Benning power supply system,
battery charging after mains interruption to floating value
(54.4 V at +20C), and periodic battery charging with boost
voltage is performed by the TBA-ST meter.
5.4. Voltage Drops in Battery Circuits
A voltage drop in the distribution powerlines should not
exceed 0.5 V. The total voltage drop between battery termi-
nals and the load connector located on telecommunication
equipment should not exceed 1.2 V.
For load up to 90 A, with both (B1, B2) connected batteries,
and disconnected B3 battery while rectifiers are off, the
voltage drop in the power system (tested between the system
bus ground and the battery negative terminal) is less than
180 mV. The voltage drop between the system bus and the
battery fuse terminal does not exceed 65 mV.
6. PS Testing with a TBA-ST
Table 3 presents list of Benning power supply system issues
being tested with the TBA-ST, with reference to subsections
and detailed description or test result.
Table 3
Power system functions with active TBA-ST meter
Issue Result/
section
All power system functions maintained +
Installation and replacement of TBA-ST meter 6.1
Configuring and programming 6.2
Communication with management system 6.3
Meter integration with the power system 3.3 and 6.4
Test initial conditions 6.5
Remote disconnection of batteries +
Battery capacity measurements 6.6
Influence of AC mains voltage interruptions
on measurements 6.7
Remote operation hold and continue +
Battery test by cable with remote readout 6.8
Tolerance of user faults +
6.1. Meter Installation and Replacement
The TBA-ST meter is installed inside the power system
enclosure. Together with the ZSN-5 communication con-
troller, it occupies 1U×200 mm. The instrument is con-
nected to the power system by cable with DB-37 connector.
In case the ZSN-5 controller is not used, the communica-
tion with the management system (e.g. PC computer) takes
place via the internal Ethernet interface module with RJ-45
port. In case of system maintenance, the meter replacement
requires only few manual operations: opening of the circuit
breaker f.TBA, overcurrent breakers in the battery circuits
(F31-33) and power system (F57), 4 screws and DB-37
connector removal. Such maintenance does not influence
the power system operation.
6.2. Configuring and Programming the TBA-ST Unit
The internal settings as well as operational currents are
set by PCB jumpers. The threshold voltages can be soft-
ware programmed after pressing a button on front panel.
The parameters of the batteries being test (i.e. number of
cells, storage capacity, block voltage, installation date, dis-
charging and charging currents, charge to be drawn, final
charging as well as discharging voltage) can be modified
locally from dedicated PC software or remotely through the
ZSN-5 controller (password protected). Remote monitoring
and battery test initiation are performed in a similar way.
6.3. Communication with the Host Management System
The data transmission between the TBA-ST unit and the
management system uses Modbus RTU protocol. The
TBA-ST meter accepts the following groups of commands
of the protocol: 0×03 i and 0×04 – read multiple registers;
0×06 – read single register; 0×10 – write multiple regis-
ters; 0×11 – report controller ID. Physical communication
can use RS232, RS485 or Ethernet (M2M) interfaces. In
Table 4 all recognized commands are summarized.
6.4. Meter Integration with the Power System
The communication of TBA-ST meter with MCU-based
controller is implemented by using IO/02 card using
slowly-variable DC signals (Fig. 3). A detailed description
of both units’ interoperability is provided in Subsection 3.3.
The used method of information exchange is presented in
Fig. 6. The active state of the signal (high level) G.MCU
is “+power system”, and for the other ones, feeding the
internal reference potential “MS” of card IO/02 to its re-
spective input, which is performed by MOSFET relays in
the TBA-ST meter circuit.
6.5. Test Initial Conditions
The TBA-ST meter can request any battery bank discon-
nection from the system bus “–power system” and to switch
it on to the meter input when all of the following conditions
are fulfilled simultaneously:
58
Battery Available Capacity Meter Built into an AC/DC Telecom Power Supply System
Table 4
Modbus RTE protocol commands used in TBA-ST meter
Description Type Address range
Configuration parameters R/W 400–465
Values from external W 500–524
measurements
TBA-ST current state R 100
Battery monoblock voltages R 110–133
Commands concerning B1/2/3 W 300–302
batteries)
Result of current test R 170–179
Results of last battery test R 600–616
Measurement result for each R 200–244
battery
)Commands to the meter include:
battery equalization charging,
control battery discharge followed by charging return,
charging equalization followed by discharging and
charging return,
battery disconnection from power system bus,
signals test by MCU controller.
Fig. 6. DC communication method with TBA-ST device.
it has received the instruction from the supervisory
computer,
the switch “f. TBA-ST” is on,
it is not in “stop” state,
all batteries are connected to the system bus, by
means their voltage difference is less than 0.2 V.
The TBA-ST will refuse return to non-active state without
performing the test if the correct battery voltage (43–57 V)
does not appear on its input within 2 minutes from send-
ing the command, or the voltage difference between any
12-voltage blocks of the battery is greater than 1.8 V.
6.6. Battery Capacity Measurements
The process of B1 and B2 battery capacity measurement as
voltage waveforms for each block is shown in Fig. 7. One
can see, that no AC mains interruption occurred during the
test. Discharging was performed by 10 A current (10 hour,
0.1 CA) until the 10.80 V voltage is reached on the B1
battery case, or the declared charge of 80 Ah (80% Q) has
been consumed – battery B2 case. After discharging, the
batteries were immediately charged back with 10 A current
to 56.5 V end voltage.
During testing battery block voltage measurement errors
were below 0.5%, and discharging current fluctuations
and its measurement error was under 1%. The instrument
showed that battery B1 has a 52 Ah capacity, and battery B
is over 80 Ah. Interestingly, after 4 h of discharging, the
worst block (C = 52% Q) of the bad B1 had a higher volt-
age than the best B2 (C >80% Q), although both batteries
are of the same type, and have been used in a similar bad
condition for 3 years.
During charging, when battery voltage reaches the system
bus voltage, a voltage and current swing occurred (see 3
in Fig. 7). It is a result of voltage drop on the battery
(HZB12-100FA) during switching internal converter from
buck to boost mode. This effect does not influence on
battery charging level, the charge delivered to the battery
or the battery charging time.
6.7. Influence of AC Mains Voltage Interruptions on
Capacity Measurements
A mains voltage interruption can occur after battery bank
disconnection from the power system. The voltage and
current diagrams for such case are shown in Fig. 8. The
used notation is: 1 – voltage of battery being charged, 2 –
voltage of battery being discharged, 3 – voltage of system
bus, 4 – voltage of loads with constant power demand of
1500 W, 5a – current of battery being charged, 5b – current
of battery being discharged, 5c – battery current during
backup, 6 – battery voltage during backup, 7 – moment
of switching to battery backup mode, 8 – current of other
batteries connected to system bus “–power system”.
Figure 8a shows the effect of long-lasting AC mains volt-
age interruption during the discharging operation, while
Fig. 8b shows the same case during equalization B2 bat-
tery charging. When voltage on the system bus drops be-
low 51 V, charging is stopped but discharging is contin-
ued. While voltage on the system bus lowers below 48.5
V, the meter switches into backup mode “7” in Fig. 8b.
The meter circuit contains power converter with current
limiter. It draws energy from the battery being tested and
transfers it to power system bus. While AC voltage ap-
pears, the battery being tested is recharged and connected
to the system after its voltage and system bus becomes
equal. This operation may few repetition, as V–I character-
istic varies in time from varies programmed values. Battery
current is not constant, and the Ah drawn may be greater
than preset.
The effect of short AC mains voltage interruption while
B1 battery is tested, in case the voltage on the system
bus exceed 48.5 V threshold, is illustrated in Fig. 8c. Dur-
ing battery charging, the meter stops the process for total
power failure period. If voltage interruption occurs dur-
ing battery discharging, the meter continues operation with
uncharged current. Due to described purpose of operation
the measurements are fully reliable – the measured avail-
able capacity of batteries for current of 0.2 CA = 20 A was
46 Ah (C5= 46 Ah) or 2.22 kWh, which corresponds to
50 Ah (for 0.1 CA), and during return charging, a charge
59
Paweł Godlewski, Ryszard Kobus, and Paweł Kliś
Fig. 7. Voltages during battery test process on different block: 1 – worst block, 2 – best block, 3 – fluctuations during charging.
Fig. 8. Battery voltage and current at interruption in phase of: (a) discharging, (b) equalization charging, (c) full testing.
of 54 Ah was delivered to the batteries (about 10% greater
than drawn).
6.8. Remote Readout of Battery Test Results
The testing results can be downloaded to remote com-
puter by the ZSN-5 communication controller or (alterna-
tively) via built-in Ethernet interface. For testing purposes
and at the system commissioning stage, the results can be
read remotely and recorded on a PC with the Windows
by using of the TBA Starter and TBA Reporter programs
(Fig. 9).
7. Conclusions
The results of testing the SBE200SL power supply system
with a built-in TBA-ST meter showed that it can be used for
powering telecommunications devices at maintenance-free
60
Battery Available Capacity Meter Built into an AC/DC Telecom Power Supply System
Fig. 9. Battery test results as represented in the TBA Reporter
software.
sites. A minor upgrade of a typical power supply system
provides the capability to measure the available capacity of
individual battery banks and remote management.
The tested SBE200SL power supply system can be addi-
tionally equipped, with a second set of rectifiers, which
increases the efficiency. In order to extend the time of
energy autonomy, without changing power system compo-
nents, three 100 Ah batteries in an external rack housing
could be exchanged with a higher capacity ones. The power
system could also be additionally provided, without signif-
icant changes in equipment and functioning, with a fourth
RGR disconnector switch and battery contactor, which will
make it possible to individually test the available capacity
of four battery strings (the TBA-ST meter can control up
to 6 batteries of 1,000 Ah each).
References
[1] R. Kobus, P. Kliś, and P. Godlewski, “Maintenance of lead-acid bat-
teries used in telecommunications systems”, J. of Telecommun. and
Inform. Technol., no. 4, pp. 106–113, 2015.
[2] P. Godlewski et al., “Method and system for remote measurement of
available capacity of the batteries in the telecommunications power
system”, Patent PL 219471, Patent Office of the Republic of Poland,
2015 (in Polish).
[3] “Telecommunications Power System 48 V type SBE300SL,
SBE200SL – O&M Manual”, version R-60704635-4, Benning Power
Electronics (in Polish).
Paweł Godlewski received his
B.Sc. degree from the Fac-
ulty of Electronics of the War-
saw University of Technology
in 1973. He has been work-
ing at the National Institute
of Telecommunications since
1973. He is the designer of
many devices, and co-author of
a system for the assessment of
quality of telecommunications
services, a well as AWP-IL and TBA-IL ATE equipment.
He is the author of numerous scientific publications and
co-author of patented solutions. His research interests in-
clude: visualization systems used in the telecommuni-
cations sector, programmable measurement devices for
telecommunications.
E-mail: P.Godlewski@itl.waw.pl
National Institute of Telecommunications
Szachowa st 1
04-894 Warsaw, Poland
Ryszard Kobus received his
B.Sc. and M.Sc. degrees from
the Faculty of Electronics of
the Warsaw University of Tech-
nology in 1975. Mr. Kobus has
been working at the National
Institute of Telecommunica-
tions since 1975. He is a mem-
ber of the Expert Technical
Committee CEN/TC 331 spe-
cializing in postal services,
and the deputy chairman of the Postal Service Com-
mittee PKN/TC 259. He is a co-author of many patented
telecommunications solutions. His research interests in-
clude: telecommunications, measurements and evaluation
of quality of telecommunications services, quality sur-
veys, evaluation the quality of postal services, standard-
ization.
E-mail: R.Kobus@itl.waw.pl
National Institute of Telecommunications
Szachowa st 1
04-894 Warsaw, Poland
Paweł Kliś received his B.Sc.
degree from the Faculty of
Electrical Engineering of the
Opole School of Engineering
in 1976. He has been work-
ing at the National Institute
of Telecommunications since
1976, formerly in the Power
Systems Department, currently
in the Central Chamber for
Telecommunications Metrol-
ogy. He is a co-designer of numerous telecommunications
power systems and devices. He is a co-author of several
scientific publications and co-author of several patents.
His research interests include: telecommunications power
systems, metrology of basic electrical parameters.
E-mail: P.Klis@itl.waw.pl
National Institute of Telecommunications
Szachowa st 1
04-894 Warsaw, Poland
61
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Article
Full-text available
The article presents numerous problems with standby batteries used in telecommunications systems, with a particular emphasis placed on the assessment of their real capacity. The methods used to evaluate the technical condition of batteries and to measure their real capacity are presented. Also, the a new test device which measures the actual battery capacity is presented. The said measurement is based on the discharge test method and is performed with the use of a new TBA-A automated test unit. The article is targeted for electronic designers, managers and telecommunications hardware maintenance personnel, as well as for other telecommunications systems experts.
Method and system for remote measurement of available capacity of the batteries in the telecommunications power system
  • . P Godlewski
P. Godlewski et al., "Method and system for remote measurement of available capacity of the batteries in the telecommunications power system", Patent PL 219471, Patent Office of the Republic of Poland, 2015 (in Polish).