Concept of a versatile and mobile charging system for different E-Bike Accumulators

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Deggenhausertal, 18.10.2021

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A technical description of an dcdc-converter for charging e-bike accumulators.

Concept of a versatile and mobile 12V to 24V
charging system for different E-Bike Accumulators
Wilhelm Richter
M.Sc.
25, July 2021,
Deggenhausertal, Germany
wilhelmrichter@web.de
Abstract-This paper introduces a concept of a versatile and
mobile charging system for E-Bike accumulators as a useful
accessory on a camping trip. It combines multiple communication
protocols of different bike manufacturers and can be powered
either by a 12V or 24V vehicle electrical system or by a pho-
tovoltaic solar panel. With the help of special intelligent cables,
unique for every accumulator, the charger can detect the correct
accumulator type and select the proper communication protocol
as well as the right end-of-charge voltages. The appropriate
charging voltage is transformed by a boost converter. By applying
Maximum Power Point Tracking (MPPT), the charger is able to
use the optimal output power of a photovoltaic power system and
can also protect a vehicle battery from being deeply discharged.
Index Terms-Charging System, MPPT, E-Bike, Li-Ion ac-
cumulator, Li-Ion charging system, BOOST Converter, analog
multiplexer
I. INTRODUCTION
Bicycles have always been a convenient way to explore
travel destinations, especially on camping trips. Over the
course of the electrification of drive systems, they are more
and more replaced by e-bikes, which allow to cover longer
distances while leaving the cars or campervans at the camp-
site. But since the different manufacturers of E-Bikes and
drive trains use various accumulators with unique plugs and
communication protocols, there are several chargers necessary
on a trip with family or friends, where not everybody has
the same bike or accumulator system. This is were a mobile
and versatile charging system would be helpful by making
it possible to charge all types of accumulators with only one
charger. To get a quick overview of the most important systems
at the market, three different manufacturers and accumulator
types are shown in Section II. Section III addresses the basics
of Li-Ion chargers in general. After that, the main parts of
the versatile mobile charger are described by covering the
intelligent cable system in combination with the analog switch
mechanism in order to detect the correct accumulator type and
to choose the proper communication protocol in Section IV,
and the power input and transformation mechanism in section
V, which allows the use of either a 12V vehicle electrical
system or photovoltaic cells as a voltage source by using
Maximum Power Point Tracking (MPPT). Finally a conclusion
together with a quick summary is given in Section VI.II. OVERVIEW OF SYSTEMS IN THEMARKET
In the current market there are several bike brands with
different drive trains from bigger manufacturers like BOSCH,
Shimano, Panasonic and also smaller ones like Stromer. They
use accumulators with nominal voltages of 26V, 36V and in
the latest versions also 42V. They all use intelligent charging
systems which communicate with the accumulator in order
to get information about the optimal charging conditions or
to monitor the current temperature for safety reasons. Fur-
thermore they mostly use different plugs for the connection.
Therefore for every system a corresponding charger is required
in order to provide the correct plug and communication
protocol. Shimano for example uses a UART communication,
while Stromer accumulators communicate via CAN bus [1].
Bosch and Panasonic accumulators on the other hand need
a 5V level on the NTC (Negative Temperature Coefficient
Resistor) pin of the accumulator in order to activate charging
mode via the internal battery management system (BMS). So
the task is to build a charger, which is able to communicate by
different protocols and also to apply different voltage levels
to some pins of the plug.
III. BASICS OFLI-ION CHARGERS
Beside the communication protocols, accumulators are
mostly build of Lithium Ion (Li-Ion) rechargeable cells. They
are commonly used in E-Bikes due to different advantages
such as light weight, small package size and a high charging
and discharging efficiency in combination with a high number
of charge cycles. They need a loading strategy which applies
a constant current (CC) to the accumulator first. With rising
state of charge the clamping voltage between the poles rises
until it reaches the cell-specific end-of-charge voltage. From
this point on, the charger must switch to a constant voltage
(CV) mode in which it supplies enough current to keep the
clamping voltage constant. The minimum amount of current
necessary for keeping up the voltage declines with time until it
reaches a certain level in which the accumulator is considered
fully charged [2]. This scheme is shown in Fig. 1.
When the higher constant current from CC mode would be
applied any longer after the end-of-charge voltage is reached,
the voltage would rise above this critical level and could
damage the cell due to chemical processes inside, which would
also heat it up.
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Current
Voltage
TimeCC
CV
Fig. 1. Li-Ion charging strategy with constant current (CC) and constant
voltage (CV) mode.
The typical nominal voltage for a Li-Ion cell is 3.6V or 3.7V,
while the typical end-of-charge voltage is 4.2V. So the usable
range is between this charge voltage of 4.2V and the typically
final-discharge voltage of 2.8V per cell [3]. To get the typical
nominal system voltages of the E-Bike accumulator of i.e.
36V, several cells are arranged as a package with 10 cells in
a row and multiple cells in parallel, depending on the capacity
of the package. This makes a voltage range between 28V and
42V for a 36V nominal accumulator. The ranges for the others
nominal voltages are given in table I respectively.
TABLE I
NOMINAL VOLTAGEVNOM,MINIMAL VOLTAGEVMIN,AND MAXIMAL
VOLTAGEVMAXFOR THE DIFFERENT PACKAGES.
V
NomV
MINVMAXCells in Row26V19.6V 29.4V 7
36V28V 42V 10
48V36.4V 54.6V 13
Considering this voltage ranges, the charger must be able
to provide a voltage between 19.6V and 54.6V while being
powered by a 12V battery of a campervan or car or by a
photovoltaic cell with a typical voltage between 10V to 20V
[4].
An easy, cost efficient and space saving design for this task
is based on a boost or also called step up topology [5], which
basic schematic is shown in Fig. 2. It can handle a wide input
range as long as it is lower than the output voltage. Its basic
function is to increase the input voltage.DCDC-StepUp Converter
C1Q1D1
L1
UinUout
DCDC-StepUp Converter
C1Q1D1
L1
UinUoutFig. 2. Basic schematic of the boost topology.
The boost converter is controlled by a pulse width modula-
tion (PWM) signal which drives transistor Q1. With the dutycycleDof the PWM signal, which is defined by the ratio of
its on-time (Ton) to its period-time (T)
D=TonT
,(1)
the output voltageVoutcan be set into relation with the input
voltageVinby the equation
V
out=11-D·Vin.(2)
The PWM signal is controlled by a microcontroller in order
to generate the required voltage or current according to the
charging mode (CC or CV) as described in Fig. 1.
IV. INTELLIGENT CABLE ANDCOMMUNICATION
PROTOCOL SWITCH MECHANISM
If several accumulators should be supported by the charger,
a mechanism to switch between the different communication
protocols is required.
Communication Interface
MUX2
Logic
S0
S2
MUX2
Logic
S0
inputs to the output. With two multiplexers per accumulator
two-wire protocols like UART or CAN could be selected.
The outputs of each multiplexers would be wired to the
accumulator plugs, which are realized by an intelligent plug
system which is shown in Fig. 4.Cable-Connector
Memory
-EEPROM
-FRAM
-...
Memory
-EEPROM
-FRAM
-...
SDA
R1
R2
R3
R4
HW-Coding
VDDVDD
R1
R2
R3
R4
HW-Coding
VDDVDD
HW1
HW2
Accu+
Accu-
DATA1
DATA2
VDD
VDD
NTC
Cable
Accu-
ConnectorCable-Connector
Memory
-EEPROM
-FRAM
-...
SDA
R1
R2
R3
R4
HW-Coding
VDDVDD
HW1
HW2
Accu+
Accu-
forming circuit. So the microcontroller must drive the Mosfet
Q1 from Fig. 2 in order to keep the load in the maximum
power point. This leads to the drawback of powering the
charger by a solar module. If the radiation is too low, the
accumulator can"t be charged with the maximum charging
current, which leads to longer loading times. In the constant
voltage section, there is also a lack of efficiency since the
current for the maximum power point could be too high which
might lead to a less efficient operating point, especially near
the end of the charging process.
When powered by a car battery, the controller can also monitor
the input voltage and turn off the charge process in case that
the voltage drops below a critical level, to protect the car
battery from being deeply discharged.
In Fig. 6 the complete system from the source to the accumu-
lator is shown.
VI. CONCLUSION
This introduced mobile and versatile E-Bike charger would
be a useful device for a camping trip with several E-Bikes
from different manufacturers. Combined with a solar panel
module it can also be used for trips in the outback, were no
electricity is available. The boost topology allows relatively
small spacing with a good efficiency and with the accu-
specific intelligent plug system, the charger can detect and load
different accumulators automatically. It can also be designed
with two boost converter circuits, which makes it able to
charge two accumulators at the same time. This makes it very
convenient especially for a couple on a camping trip.
REFERENCES
[1]
myStromer A G.,"Instruction Manual: CR 246", 
https://www.fietsaccuwinkel.nl/wp-content/uploads/Strom
er-CR246-Gebrauchsanleitung-Instruction-Manual.pdf, accessed
on July 21, 2021
[2]
W eixiangShen, "Char gingAlgorithms of Lithium-Ion Batteries - an 
Overview", https://ieeexplore.ieee.org/document/6360973, accessed on
July 21, 2021
[3]
Electronics Lab .24 March 2016, "Ho wto reb uilda Li- 
Ion pack", https://web.archive.org/web/20120217001508/http:
//www.electronics-lab.com/articles/LiIonreconstruct/How%20to%
20rebuild%20a%20Li-Ion%20pack.pdf, accessed on July 21, 2021
[4]
M. Se yedmahmoudian,B. Horan, "Ef ficientPhoto voltaikSystem 
Maximum Power Point Tracking Using a New Technique", https:
//www.researchgate.net/publication/296686425EfficientPhotovoltaic
SystemMaximumPowerPointTrackingUsingaNewTechnique,
accessed on July 21,2021
[5]
A. B. Kancherla, "DESIGN OF SOLAR-PV OPERA TEDFORMAL 
DC-DC CONVERTER FED PMBLDC MOTOR DRIVE FOR REAL-
TIME APPLICATIONS", https://ieeexplore.ieee.org/document/9358813,
accessed on July 22, 2021
[6]
N. Femia, G. Petrone "Optimization of Perturb and Observ eMaximum 
Power Point Tracking Method", https://ieeexplore.ieee.org/document/
1461481, accessed on July 22, 2021Cable-Connector
Memory
-EEPROM
-FRAM
-...
Memory
-EEPROM
-FRAM
-...
SDA
R1
R2
R3
R4
HW-Coding
VDDVDD
R1
R2
R3
R4
HW-Coding
VDDVDD
HW1
HW2
Accu+
Accu-
DATA1
DATA2
VDD
VDD
NTC
Cable
Accu-
ConnectorCable-Connector
Memory
-EEPROM
-FRAM
-...
SDA