Project 1 Mechanical design of battery pack

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.

CAD file:

TECHNICAL DATA FOR THE CELL BE:

requirements of battery pack=18kWh

taking ideal factors and environments into mind, I aimed for a capacity of 20kWh (which has the factor of safety included)

To get a total of 20kWh energy for the battery pack a cell configuration of 45s56p was used which gives an equivalent nominal voltage of 148.5V and an equivalent capacity of 140Ah which gives total energy of 20.79kWh.

(148.5/3.3=45) because in series voltages are added up.

for parallel 140 Ah/2.5Ah=56.

BATTERY HOLDER:

It is made of a blend of PC/ABS (Polycarbonate/Acrylonitrile Butadiene Styrene) providing a combination of the high processability of ABS with the excellent mechanical properties, impact, and heat resistance of PC. It is heat resistant and cracks resistance, making it good for holding multiple cells in close proximity where temperatures may rise to very high levels due to the heat produced by current. Other useful properties are

• high impact strength even at low temperatures – this allows operation in a wide temperature range
• High stiffness – allows the battery pack to be compact and
• solid Low overall shrinkage and high dimensional accuracy
• They can be 3D printed or made from traditional processes like injection molding which give high dimensional accuracy and high efficiency of manufacturing with minimum finishing requirements.

The holder was designed keeping in mind that the cells need to be press-fit into the holder so that the cells don’t get loose. A minimum distance of 1mm was kept at the boundaries and was gradually increased to 4mm in the middle of the battery pack as in that place the heat generation would be maximum due to surrounding cells in all directions. The larger gaps allow convective currents to dissipate heat more effectively and to prevent a dangerous situation where excessive temperatures could lead to explosions of the cells.

A subassembly of the holder has been used as linear pattern in the main assembly.

top view of battery holder

NICKEL STRIPS:

Nickel strips are used for connecting individual cells and are called interconnects. Most cells are nickel coated and thus using nickel strips prevent galvanic corrosion which takes place when one metal corrodes another dissimilar metal when it is in electrical contact with it, in the presence of an electrolyte. They have high tensile strength of 345-1000MPa, low electric resistivity of 6.99e-8Ω•m and easy and quick soldering by spot welding. Spot welding is a type of electric resistance welding used to weld sheet metals, through a process in which contacting metal surface points are joined by the heat obtained from resistance to electric current.

CAD:

A subassembly showing the parts mentioned above:

FINAL ASSEMBLY SHOWING 45s56p:

depending upon thermal modeling of the battery placement of cooling fans can be decided. and further safety can be added by making a covered enclosure for the battery pack which can be easily injection molded.