Figure 3.1 - REBATEM block diagram by qcq15579

VIEWS: 38 PAGES: 11

									3. Approach

The REBATEM is a rechargeable battery system that communicates with external devices and monitors
internal cell voltages, temperatures, and current. If any of these three parameters exceed the limits
established by the technical design constraints, protocols disconnect the battery system from the load it is
powering.

3.1 Hardware

The central components of the device are the cells. To keep the cells in optimal condition, a monitoring
interface keeps track of their health. When in the charging state, a device controls the rate at which the
cells are charged and also maintains the charging voltage and current at constant values. A
microprocessor with a CAN-bus module allows for communication of cell info with external devices.
Figure 3.1 displays a block diagram layout of the REBATEM.




                                  Figure 3.1 – REBATEM block diagram

3.1.1 Cells

Lithium Polymer (LiPo) cells are the cells chosen for the REBATEM. The particular LiPo cells chosen
are shown in the following figure:




                                        Figure 3.2 – Chosen cell [1]
LiPo cells have a higher energy density and discharge rate compared to regular lithium-ion cells. Because
of the limit of using only eight cells, LiPo cells are being used because they meet four out of eleven
constraints. First, the constraint to yield an output voltage between 14 and 16 volts is met because each
cell can yield between 3.6 and 4.2 volts and the REBATEM's output voltage is given by taking the
voltage across four of these cells in series. The chosen LiPo cells output 38 amps and have a 1.9 amp-
hour rating; therefore, placing two of these cells in parallel yield the desired output current range of 60 to
80 amps and the desired current-hour range of 3.4 to 3.8 amp-hours. Second, the LiPo cells aid in meeting
costs constraints because there is a $200.00 budget, and the cells will cost more than half the budget
(approximately $116.00). Third, LiPo cells satisfy the environmental constraint because they contain
nontoxic materials. LiPo cells are an upgrade from the regular lithium-ion cells and are still new
technology but are becoming more attractive to the hybrid electric vehicle industry which can benefit
from a product like the REBATEM.

3.1.1.1 Cell Geometry

Two types of lithium ion cell geometries were looked at for potential use in the REBATEM: cylindrical
and prismatic. Cylindrical cells have the advantage of being more mechanically stable than prismatic
cells, with a higher energy density, as well. Also, they can withstand higher pressures than that of their
prismatic counterparts, due to their self-resealing venting systems. Despite those advantages, prismatic
cells have a much better heat dissipation rate than cylindrical cells, and they can be packaged more
efficiently to meet a manufacturer's size constraints. The REBATEM uses prismatic cells mainly because
of their higher heat dissipation rates, even though they tend to cost more than cylindrical cells.

3.1.1.2 Lithium Ion Chemistry

Four types of cell chemistry were considered: cobalt, manganese, phosphate, and polymer. All of these
types of cells have similar output voltages, except for phosphate, which is rated at about 0.5 volts less
then the others. Despite the lower output voltage rating, phosphate cells have a higher stability, with an
average cycle that is relatively close to polymer. Cobalt, manganese, and phosphate lithium cells have
lower cycle, compared to that of polymer. The REBATEM uses lithium ion polymer cells because this
chemistry has the highest cycle life out of the four considerations, in addition to having a better range of
voltage output compared to phosphate, despite that chemistry's higher stability.

3.1.1.3 Cell Configuration

When designing the physical layout of the cells, four different configurations were considered: a series of
eight, eight in parallel, four series of two in parallel, and two series of four in parallel. For testing
purposes, the REBATEM is powering an inverter. The configurations of eight in parallel and four series
of two in parallel do not supply enough voltage to power the inverter, which requires 14 to 16 volts. The
configurations of a series of eight and two series of four in parallel have enough voltage to power the
inverter, but the two series of four in parallel provide a higher current hour rating than the series of eight.
Therefore, the REBATEM uses two series of four in parallel.

3.1.2 Monitoring

The REBATEM monitors the voltage, current, and temperature of the cells. In order to do this initially,
the system used a battery management system (BMS) chip to handle all of these functions. Now the
monitoring is handled by an analog front end chip because it can both provide the benefits of a BMS chip
and balance the voltage levels of the cells.
3.1.2.1 Voltage

Voltage monitoring prolongs the lifespan of the REBATEM by preventing overcharging and deep
discharging of cells. To monitor the voltage, the REBATEM uses the analog front end chip, Intersil's
ISL9208IRZ, which is given in the following figure.




                             Figure 3.3 – Pin diagram of the ISL9208IRZ [2]

This device is more scalable than the other type of monitoring device researched, which was Maxim IC’s
DS2760 because it only offers single cell monitoring. That chip is given in the following figure.




                               Figure 3.4 – Pin diagram of the DS2760 [3]

First, the ISL9208IRZ has the ability to monitor four to seven cells in series and give their corresponding
voltages. The DS2760 can only monitor one cell at a time and measures an individual cell's temperature.
Second, the DS2760 does offer individual temperature monitoring of cells, but the ISL9208IRZ only
offers temperature monitoring of a group of cells, with the help of a thermistor input. This offers
individual cell temperature monitoring by multiplexing thermistors to individual cells, but it was decided
that monitoring the temperature of individual cells is not necessary because it would be costly to
implement on a larger scale. To be scalable, the REBATEM monitors the temperature of a group of cells
because, if a thermistor or a DS2760 is used for each cell, then for a large scale implementation of the
REBATEM, in which there could be 200 or more cells, there would be a total of 200 thermistors or chips.
This would not be cost effective in comparison to using only 30 ISL9208IRZ chips along with 30
thermistors. Third, unlike the DS2760, the ISL9208IRZ offers some type of cell balancing but only
during charging. Cell balancing is the ability to keep each individual cell at the same voltage level.
Fourth, the DS2760 offers overcurrent, overvoltage, and undervoltage protection, but the ISL9208IRZ
only offers overcurrent protection. It depends on a host microcontroller to maintain overvoltage and
undervoltage protection of cells. Finally, the ISL9208IRZ works better with a microcontroller because it
can monitor up to seven cells and communicate all of its data to the microcontroller via an I2C interface.
Monitoring seven cells with the DS2760 requires the microntroller to talk to seven different DS2760
chips over the I2C line. Overall, the scalability of the ISL9208IRZ makes it more desirable than the
DS2760.

3.1.2.2 Current

Current monitoring is important to prevent damaging the cells from supplying too much current while
recharging. It is also important to measure how fast the cells are discharging. Sustained discharging at
rates higher than the suggested manufacturer discharge rate can damage the cells. Both cases damage cells
by lowering their maximum cell capacity through increased internal temperature.

Several current sensors were considered, including the DS2760 and the INA219. The latter device is a
chip built specifically to monitor current via measuring the voltage drop across a shunt resistor. It also has
an I2C interface, but since the chosen analog front end monitors current, these considerations became no
longer necessary.

3.1.2.3 Temperature

The REBATEM system will be monitoring the temperature of the cells. There are many sensors that can
monitor temperature. The ones investigated were resistance temperature detectors (RTDs),
thermocouples, thermistors, and integrated circuits (ICs) built specifically for temperature sensing. Out of
those four types of sensors, the integrated circuit and thermistors are more ideal to use. They have the
temperature range that is specified by the design constraints and are less expensive than both RTDs and
thermocouples. Despite that, however, the REBATEM will be using the thermistors, since the analog
front end chip requires thermistors in order to monitor temperature.

3.1.3 Charging

The initial consideration when constructing the charging circuit was to build a charge distribution device
with discrete components. This consists of manually wrapping a transformer to distribute the charge
voltage across the individual cell terminals. It was later learned that this method is unsafe. Also, this
functionality has already been implemented in a smaller, simpler way, which is the charging integrated
circuit. With an integrated circuit charging interface, the charging circuit itself is composed of
independent voltage and current loops. Also, it is easy to interact with a typical charging integrated circuit
via a microcontroller, and some are standalone. These chips also have built-in power regulation, and, for
safety purposes, can monitor charge current to prevent overcurrent conditions.

When deciding on a charging integrated circuit, a few characteristics of the IC were considered. First,
there are three types of charging the IC offers: linear, pulse, and switch-mode. Linear charging provides
cells with constant current constant voltage (CCCV). Constant current is applied for a majority of the
charging period, and constant voltage is applied toward the end of the charging period to top off a cell’s
voltage. The downfall to linear charging is that it has higher energy dissipation. Pulse charging pulses
current in order to charge cells. The disadvantage to pulse charging is that this type of charging is better
for cells that are at low voltage (roughly 3.0 volts). Switch-mode charging is the best among the three for
the REBATEM. The only negative aspect is that this type of charging has more complex external
circuitry. The second consideration is the amount and type of cells that the IC can handle. Most of the ICs
researched could handle from one to six lithium-ion or lithium-ion polymer cells. The final consideration
is the type of protection that the IC offers. The charging ICs researched generally offered pre-charge
protection, overvoltage protection, current protection, and temperature protection. If a cell's voltage is
very low (around 2.5 volts), pre-charge protection introduces a small amount of charging current to that
cell. A charging IC with overvoltage protection stops charging to keep a cell's voltage from becoming
greater than 4.2 volts. Temperature protection deals with the IC’s ability to receive a cell's temperature
and slow the rate of charge if that cell is too hot or cold.

Taking into consideration the previously mentioned characteristics, the MAX1758 is the charging IC
chosen for the REBATEM. A diagram of the MAX1758 is given in the following figure.




                               Figure 3.5 – Schematic of the MAX1758 [4]

The MAX1758 is a standalone charger and requires no interaction with a host microcontroller. The IC
offers switch-mode charging and is accompanied by complex external circuitry. The MAX1758 can
monitor up to four LiPo cells in series and is more suitable, given the REBATEM's current cell
configuration. The IC offers pre-charge protection, overvoltage protection, temperature protection, and
current protection. Implementation with the REBATEM's current eight cell limit and current
configuration will only require two of the MAX1758 ICs. On a larger scale, a REBATEM that consists of
roughly 200 cells would only require 50 of the MAX 1758 charging ICs and is therefore scalable.
3.1.4 Microcontroller

The microcontroller first chosen for the REBATEM was the PIC18F458 because it has a built-in CAN-
bus module. Later it was learned that the PIC24HJ32GP202 has a CAN-bus module and also has better
local support.

3.2 Software

The two software modules compose the central operations for which the REBATEM is responsible. One
module monitors the cell parameters of the battery, and the other translates the information obtained by
the monitoring module into a format suitable for CAN-bus communication. These two modules are
explained in more depth in the following sections. Below are general physical diagrams of the two core
components of the REBATEM.




                         Figure 3.6 – Physical Diagram of the controller interface




                              Figure 3.7 – Physical diagram of the cell packs

3.2.1 Monitoring Software

3.2.1.1 Power State

While in the powering state, in which the device is serving as a power source, the monitoring module
receives the output given by the battery management system chips once every five seconds. This output
consists of voltage, temperature, and current measurements. The microcontroller checks the received data,
and if any values are beyond its specific safety limits, a signal is sent to the two contacts at the positive
and negative terminals to break the circuit and prevent a potential disaster. Basic functionality of the
power mode monitoring is shown in the figure below.




                                Figure 3.8 – State diagram when powering

The next two figures display "sunny" and "rainy" day cases of the REBATEM when operating in power
mode.




                              Figure 3.9 – “Sunny day” case when powering
                             Figure 3.10 – “Rainy day” case when powering

3.2.1.2 Charging State

When connected to a 120 VAC outlet, the REBATEM is in the charging state. While in this state, the
previously mentioned contacts at the terminals of the battery are opened. Output from the battery now
only consists of voltage and temperature, as the charging circuitry handles overcurrent protection. Upon
encountering a voltage or temperature value beyond the voltage or temperature safety margins, the
charging circuit is disconnected from the battery. This is shown in the following state diagram.




                               Figure 3.11 – State diagram when charging
The "sunny" and "rainy" day cases when in the charging mode are displayed in the following figures.




                             Figure 3.12 – “Sunny day” case when charging




                             Figure 3.13 – “Rainy day” case when charging

3.2.2 Communication Software

When transmitting data via a CAN bus interface, there are two possible framing formats: base frame
format and extended frame format. The main difference between the two is that the base frame format
uses 11 identifier bits, and the extended frame format uses 29 identifier bits. Since a typical CAN bus
network applicable to the REBATEM's use requires only the base frame format. In the table below, the
sequential bits inside of a CAN-bus packet and their descriptions are listed.
                            Table 3.1 – Bit descriptions for CAN-bus frames [5]
           Field name              Length (bits)                             Purpose
Start-of-frame                            1           Denotes the start of frame transmission
Identifier                               11           A (unique) identifier for the data
Remote transmission request               1           Must be dominant
(RTR)
Identifier extension bit (IDE)            1           Must be dominant
Reserved bit (r0)                         1           Reserved bit but it must be set to dominant
Data length code (DLC)                    4           Number of bytes of data (0-8 bytes)
Data field                            0-8 bytes       Data to be transmitted (length dictated by DLC field)
CRC                                      15           Cyclic redundancy check
CRC delimiter                             1           Must be recessive
ACK slot                                  1           Transmitter sends recessive and any receiver can
                                                      assert a dominant
ACK delimiter                             1           Must be recessive
End-of-frame (EOF)                        7           Must be recessive
REFERENCES:

[1] “Hi-Power Polymer Li-Ion Cell.” [Online] Available:
http://www.batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=4391.

[2] “ISL9208IRZ Datasheet.” [Online] Available: http://pdf1.alldatasheet.com/datasheet-
pdf/view/179933/INTERSIL/ISL9208IRZ.html.

[3] “DS2760 High-Precision Li+ Battery Monitor.”[Online] Available: http://datasheets.maxim-
ic.com/en/ds/DS2760.pdf.

[4] “Stand-Alone, Switch-Mode Li+ Battery Charger with Internal 28V Switch.” [Online] Available:
http://datasheets.maxim-ic.com/en/ds/MAX1758.pdf.

[5] “CANBUS Tutorial.” [Online] Available: http://www.cbb-
software.de/technicalinformation/canbustutorial/index.html.

								
To top