Project 1 Mechanical design of battery pack
Q. TO PREPARE A DETAILED BATTERY PACK DRAWING ALONG WITH ITS ENCLOSURE USING ANR26650M1-B CYLINDRICAL CELLS OF PACK CAPACITY 18KWH.
SOLUTION:-
Theory
Battery
An electric battery is defined as a device that makes use of either one or an assembly of electrochemical cells that convert the stored chemical energy within them into electrical energy. The individual cells are connected in various combinations of series and parallel connections to get a battery of desirable characteristics.
A battery pack is a set of any number of preferably identical batteries or individual battery cells. They may be configured in a series, parallel or a mixture of both to deliver the desired voltage, capacity, or power density. Components of battery packs include the individual batteries or cells, and the interconnects which provide electrical conductivity between them. Rechargeable battery packs often contain a temperature sensor, which the battery charger uses to detect the end of charging. An advantage of a battery pack is the ease with which it can be swapped into or out of a device. This allows multiple packs to deliver extended runtimes, freeing up the device for continued use while charging the removed pack separately.
some basic terms related to the battery be :
Voltage: total voltage, voltages of individual cells, minimum and maximum cell voltage, or voltage of periodic taps. In order to maximize the battery's capacity, and to prevent localized under-charging or over-charging, the BMS may actively ensure that all the cells that compose the battery are kept at the same voltage or the State of Charge, through balancing.
Temperature: average temperature, coolant intake temperature, coolant output temperature, or temperatures of individual cells
State of Charge (SOC) or depth of discharge (DOD), to indicate the charge level of the battery
State of health (SOH), a variously-defined measurement of the remaining capacity of the battery as % of the original capacity
State of power (SOP), the amount of power available for a defined time interval given the current power usage, temperature, and other conditions
Current: current in or out of the battery
CELL USED TO MAKE BATTERY PACK:
ANR26650M1-B cells manufactured by A123 systems were used for the battery pack. It is a type of Lithium-ion battery, the lithium iron phosphate battery (LiFePO4), using LiFePO4 as the cathode material, and a graphitic carbon electrode with a metallic backing as the anode. Because of the low cost, low toxicity, well-defined performance, long-term stability, etc. LiFePO4 is finding a number of roles in-vehicle use, utility-scale stationary applications, and backup power.
Cell data:
ANR26650M1-B Specifications
Nominal Ratings
Voltage: 3.3 Volts
Capacity: 2.5 Ah
Energy: 8.25 Wh
Specific Power: 2600 W/kg
Impedance (1KHz AC Typical) 6 mΩ
Cycle Life at 1C/1C, 100% DOD: > 4000 cycles
Cell Type: 26650 Lithium Ion Power Cylindrical Cell
Discharging
Max Continuous Discharage: 50 A
Max Pulse Discharge Current >50% SOC (10s): 120 A
Minimum Voltage: 2 Volts
Temperature: -30ºC to 55ºC
Charging
Recommended Standard Charge: 2.5 Amps
Max Charge Rate: 10 Amps
Max Pulse Charge Current <50% SOC (10s): 25 A
Float Voltage: 3.45 Volts
Recommended charge V & Cut-off Current: 3.6 Volts, taper to 125mA
Temperature
(reduce charging current to 250mA when under 0ºC): 0ºC to 55ºC
Mechanical
Diameter: Ø25.96 +/- 0.5 mm (1.0")
Length: 65.15 +/- 0.5 mm (2.6")
Mass: 76 g
The LithiumWerks ANR26650M1-B is the next generation of A123 Systems’ pioneering 26650 cylindrical cell, now with greater power and energy density and lower impedance. This versatile lithium ion cell is suitable for a wide variety of applications and system designs. Proven performance in the toughest conditions, combining durability, reliability, and safety, the ANR26650 cell offers an excellent combination of price-performance.
Battery pack construction:
Voltages and Capacity:
Cells can be combined together either in series or in parallel to achieve higher operating voltages and capacities, respectively.
Series Strings:
Cells combined in series strings can achieve higher operating voltages by connecting the positive terminal of one cell to the negative terminal of the next cell. Connect strings of series cells using their current collection tabs in a manner similar to that illustrated in Figure,
Parallel Strings:
Cells combined in parallel strings can achieve higher operating capacities by connecting like-polarity terminals of adjacent cells to each other. Connect strings of parallel cells using their current collection tabs in a manner similar to that illustrated in Figure,
Charging Cells:
Charging or Recharging Cells or Strings When charging or recharging Nanophosphate cells, the charger should apply a constant current (CC) charge followed by a constant voltage (CV) charge.
To achieve maximum life, reliability, and safety, A123 Systems recommends using cell balancing circuitry during cell charging to prevent an increasing spread between highest and lowest battery voltages. Refer to Cell Balancing on page 41 for more information. Stop the charging process when either:
• The string voltage across the pack has exceeded the recommended maximum charging voltage for the string, or
• Any one cell in the series string, has exceeded its maximum recommended charge voltage, or
• The temperature measured in the pack has gone outside the recommended range for charing.
Determine the charge current for a string of cells by multiplying the number of parallel cells in the string by the recommended charge current for a single cell. Note that this calculation does not take into account limitations imposed by any protection electronics or any other features of the battery pack assembly.
Eq. 1 (Number of cells in parallel) x (Recommended Charge Current, Cell) = Charge Current, String
Determine the end of charge voltage for a string of cells by multiplying the number of series elements in the string by the recommended charge voltage of a single cell.
Eq. 1 (Number of cells in series) x (Recommended Charge Voltage, Cell) = Charge Voltage, String.
Discharging Cells:
Recommended Discharge Method for Strings Determine the maximum continuous discharge current for a string of cells by multiplying the number of parallel cells in the string by the maximum continuous discharge current for a single cell. Note that this calculation does not take into account limitations imposed by any protection electronics or any other features of the battery pack assembly. Eq. 1 (Number of cells in parallel) x (Max Discharge Current, Cell) = Max Discharge Current, String
Additionally, correctly size the cell-to-cell current collection tabs to carry the maximum design current. Discharges at currents higher than the tab design limit risks damaging these tabs, as well as possibly overheating cells. Discharge Cell Temperature Limits For optimum life, do not continuously discharge the strings of cells faster than the maximum continuous discharge current. Do not allow the string of cells to self-heat beyond the maximum recommended cell temperature of 70o
C for discharge or 60o C for recharge. Operation above the maximum recommended cell temperature will result in accelerated performance degradation during its service life. At low temperatures, the maximum available discharge current will decrease due to markedly increased internal impedance at these lower temperatures. Discharge String/Cell Cut-Off Voltage Limits Discharge should be cut off whenever any one cell in the string reaches its lowest
recommended discharge cutoff voltage. Stop discharges when any of the following occurances happen:
• The string of cells reaches the recommended discharge cut-off voltage
• Any one cell in the series connection reaches its minimum allowable cut-off voltage
• The maximum allowable cell temperature.
Mechanical Connection and Protection:
Design mechanical interconnects to prevent accidental short circuits. Size these mechanical interconnects to carry the expected maximum rated current for both the
maximum time and ambient temperature in which the pack is expected to operate.
Cell Interconnects:
Size cell interconnects for the expected maximum current carrying capability. Improperly sized tabs could heat up excessively, resulting in even higher resistivity. For reliable welded connections at the terminals, A123 Systems recommends either pure nickel tabs or nickel-clad copper tabs. In addition, other alloys afford both the weldability, strength and low resistance, which are all desirable features of a good interconnection. Cell interconnects (tabs) should be neither soldered on the end caps nor attached using extreme heat. A123 Systems recommends tabs be resistance or laser-welded to both ends of the cells. Resistance welding provides a constant current for a specified period of time through a salient feature in the weld tab. These parameters can be specified and regulated for a consistent high-quality weld every time.
Allowing the Cell to Vent in a Fault Condition:
The cell vents, located on the end cap(s) of the cells (on the POSITIVE side of the cell), should not be blocked by any mechanical means. Blocking the cell vent and then
sufficiently abusing the cell so as to build up pressure in the cell prevents the cell from properly venting. An ESS designed to install cells end-to-end needs at least 1 mm of space between the cells to allow the vent to open under fault conditions. Refer to Figure,
Cell Insulation:
The outside case of the 26650 and 32113 is electrically connected to the positive end (cathode) of the cell. Take care to keep this surface electrically-isolated from any
electrical bus bar or mechanical support that may be of different voltage potential. Insulation, such as tapes, shrink wraps, or sleeves, must have at least a 150 °C melting point. This helps ensure that the cells do not short circuit to each other in a high temperature fault condition, which could cause even more widespread damage.
Cell Support Secure the cells in place by supporting their outside cases, not their terminal ends. The vibration induced between the terminal ends and the rest of the case has been shown to be detrimental to the life of the cell, causing internal and external cell damage. The intercell terminations must be light enough not to cause vibration-induced damage to the cell.
Cell Support:
Secure the cells in place by supporting their outside cases, not their terminal ends. The vibration induced between the terminal ends and the rest of the case has been shown to be detrimental to the life of the cell, causing internal and external cell damage. The intercell terminations must be light enough not to cause vibration-induced damage to the cell.
Cell Protection:
In addition to being supported, protect cells from exposure to corrosive substances and oxidizing catalysts, such as dust and moisture. The necessary level of protection for cells in a battery pack varies depending on the intended application. For example, a battery pack designed for use in an HEV must have an enclosure that isolates cells from shock and vibration, protects them from dirt and debris, as well as shielding them from other environmental dangers, such as salt spray. Unless it is hermetically sealed, even a sealed enclosure is subject to pressure differentials between its insides and the ambient, causing minute amounts of air exchange. Therefore, over time, some moisture may accumulate and condense on inside surfaces. An ESS designed to be sealed from the environment (from dust, moisture, and VOCs) must have a way to benignly drain off whatever condensate does manage to leak into it. In addition, enclosures protecting cells must work with the thermal management system to achieve optimum durability and safety of the battery pack. For example, a poor choice of materials for the enclosure, combined with insufficient cooling and controls, may cause the battery pack to overheat.
Thermal Management:
A123 Systems’ cells operate throughout a wide temperature range; however, they are most effective between 10°C and 50°C. The temperature differential between the coolest cells and the hottest cells should be no more than 10°C. Careful attention to thermal management is necessary to keep the cells operating at peak efficiency and avoiding fault conditions. In most cases, this implies a cooling system. There are certain applications - such as HEV vehicles operating in cold climates - where a heater isbeneficial to keeping the cells operating in their optimal range.
Electrical:
When joining cells together, A123 recommends using a Battery Management System responsible for carefully monitoring cell voltage, current, and impedance. The BMS may be implemented as discrete circuitry or through a microcontroller.
Cell Monitoring:
To ensure optimal performance, safety, and durability of the pack, the Battery Management System must monitor the voltage of each individual series cell in a battery
string.Take care to place the voltage monitoring connections in such a place that they are not affected by high currents going through the interconnection elements.
The diagram below depicts an optimal positioning of the voltage monitoring contact points in an idealized setting:
In this case, Vm1 = Vcell1 – 2 x i x Rs and Vm2 = Vcell2 – 2 x i x Rs. The result of Vm1 – Vm2 would be exactly what is desired, and that being Vcell1 – Vcell2.
If the contact points are placed asymmetrically, such as shown below:
Vm1 = Vcell1 – 3 x i x Rs and Vm2 = Vcell2 – i x Rs. The result of Vm1 – Vm2 would contain an undesirable offset in it: Vcell1 – Vcell2 + i x Rs where Voffset = i x Rs.
This offset in the voltage readings would cause the BMS to read that there is more charge in one cell while the current is flowing in one direction, and have less charge in it while current is flowing in the opposite direction. So while the battery is discharging, the BMS would balance some of the cells, and while it is recharging, the BMS would balance the others. This results in a great deal of wasted heat and energy that contributes to a reduced performance and sevice lifetime of the battery. Ensure that balancing currents do not share the voltage sensing leads. Otherwise, the balancing currents will affect the voltage reading proportionally, increasing the time needed to achieve proper cell balancing, if not making it impossible.
The following circuit depicts a condition in which the measured voltages would be affect by balancing currents going through the sense leads:
In such a case, if Rb were connected to cell 1, Vm1 would read Vcell1 – ib x 2 x (Rbs + Rs). Having a separate wire for balancing current from the sensing wires eliminates a good portion of the error.
Nickel strips:
Conductive metal strips were spot welded betweeen all the cells in battery pack.Nickel is used due to its low relative resistance and ease of spot welding.
Battery pack production line:
Model of battery pack:
Cylindrical Cell Primary Applications:
- Portable high power devices
- Stationary battery backup systems
- Grid stabilization energy storage systems
- Commercial truck and bus hybrid electric vehicles (HEVs)
Lithium Ion Cell Benefits:
- Power: Over 2,600 W/kg and 5,800 W/L
- Safety: Excellent abuse tolerance and environmentally friendly
- Life: Excellent calendar life, 10X cycle life vs. conventional lithium ion
CONCLUSION:-
While completing this challenge we learnt every single aspect required to connect battery with electric vehicle, its total capacity, theoritical values by calculation and full BMS. This challenge is the summary of whole battery discusions.