Battery System for an Electric go-Kart
OVerview
Provided below is a summary of the battery system design including the general wiring configuration and system functionality. It should be noted that I worked extensively in a team setting for the battery project highlighted on this page. Special thanks to Jack Webster, Daniel Martin and the other battery team members for their help with this project.
The Battery System
Relectric’s go-kart battery system consists of a 51.2-volt battery pack that is composed of 16 LiFePO4 prismatic cells wired in series. These are used to power a 48V HPEV AC-9 motor.
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To ensuring the health and safety of the battery, a battery management system (BMS) balances the cell voltages, enable discharge and charge modes, and monitor current output and cell temperatures. Since this was the team’s first major battery system, a third party BMS (Orion Jr 2 BMS) was used. This allowed the team to get familiar with the control logic and operation of a battery management systems.
The control Circuits
To protect against excessive peak discharge rates and ensure that the current does not exceed the motor controller’s rated current, all current out of the battery pack must first pass through a 400 A fuse before reaching any loads in the circuit. To initiate discharging or charging, economizer contacts are activated by the BMS to close the respective circuits. Economizer contacts are advantageous for this operation as they require minimal current to maintain a closed position compared to standard contacts, thus reducing the potential for overheating during periods of extended use.
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However, before the BMS will initiate discharging or charging, the BMS will first ensure that each cell in the pack is balanced. Unbalanced cells pose a risk during both charging and discharging and can cause cell damage, overcharging and overheating. The BMS detects unbalanced cells by measuring the difference in potential between the positive terminals of adjacent cells. For the first cell, the difference is taken between the positive and negative terminal. The wires that connect to these terminals are referred to as cell taps, for which my team used small gauge lug connectors that were soldered to the BMS cell tap wiring harness.
This wiring diagram was created by teammate Jack Webster using the team's hand-drawn diagrams sketched during meetings.
A Hall current sensor is located between the battery pack and fuse and allows the BMS to detect peak current and continuous current and open the contacts in the even that they exceed their respective threshold limits.
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Due to the higher activation current of the contactors, the BMS has to control the contactors indirectly using relays and a 12 V DC-to-DC converter. The relays are connected between the main pack fuse and the BMS discharge enable pin which drains to ground when activated. The contactor is powered by the DC-to-DC converter and the circuit is complete once the relay is activated. This allows the BMS to open and close the contactors indirectly. The 12 V DC-to-DC converter also acts as an accessory power source for the battery pack cooling fans and other kart accessory systems.
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The motor controller has two separate power inputs for high and low current. The low current input provides power to the controller’s logic board and accessory systems, while the high current input connects to the controller’s inverter. Like the BMS, the motor controller also controls a contactor to enable discharge, allowing it to control when the inverter is connected to the battery pack.
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All high current carrying wires were specified as 2/0 wire to ensure that it could handle a continuous current of 140 A with minimal heat production due to wire resistance.
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Shown below is a video of Jack and I getting the motor operational for the first time after spending a couple hours resolving motor controller error codes. A very exciting milestone for the team! Also included as some photos from the middle of our electrical assembly process, therefore please excuse some of the tangled wires.
(Click for audio)
