Battery Management System - Rowan-Rocketry/SRAD-Avionics GitHub Wiki

Philosophy

One goal of this project is to easily expose battery charging from outside the rocket. To accomplish this goal requires that battery charging be integrated into the avionics system.

Perhaps the most elegant way to accomplish this is to implement USB C charging capabilities. This makes sense, because a USB C port will already be installed on the board for microcontroller programming and data acquisition purposes.

We require that batteries be rechargeable, and to keep recurring costs down, have many charge/discharge cycles. The most prevalent batteries in the rechargeable realm are Lithium-Polymer (Li-Po) and Lithium-Ion (Li-Ion) batteries. Due to safety concerns, the manner in which Li-Po and Li-Ion batteries are used has been limited. Li-Po batteries must be either installed by the manufacturer of the COTS flight computer or recommended and supplied by the manufacturer of the COTS flight computer. Li-Ion batteries, on the other hand, may be used as long as they are packaged in metallic, cylindrical casings.

Therefore, we have opted to use cylindrical-cell Li-Ion batteries.

Due to their chemistry, however, Li-Ion batteries require a battery management system (BMS). This page discusses, at length, the design of our BMS.

Hardware Search

Monday, July 8th, 2024

Upon first glance, the TP4057 (first discovered here) seems to be a popular solution for lithium-ion battery charging.

The TP4057 device is widely available and very affordable, and it may be purchased from DigiKey. Its EDA/CAD models are included in the KiCAD default libraries, so a preliminary design may be created more easily.

Saturday, July 13th, 2024

In spite of their popularity, minimal quality documentation available for TP405x devices was readily available. To ensure that the BMS system is properly set up, a device with better documentation was needed.

Further part research led us to the MCP73831 series of devices. These devices, manufactured by Microchip Technology Inc. have leagues better documentation, and are available in two packages: 8 lead DFN and 5 lead SOT-23. The DFN package also includes an exposed thermal pad, and since part of the purpose of the BMS is to provide thermal shutoff protection to the battery charging system, we opted to implement the DFN version in our drawings.

Now equipped with quality documentation, designing the rest of the BMS circuitry was a breeze. Once done, focus was moved to converting the power from the battery to power usable by the rest of the avionics system.

One design requirement is that all devices operate at 3.3v. This will greatly simplify the process of connecting the different subsystems to the microcontroller. Since the battery voltage to the operating voltage of the system are always less than 1v apart, a switching power supply was deemed not necessary. Instead, a simple low drop-out regulator would suffice.

We again opted to use a device by Microchip Technology Inc. The device chosen was the TC2185-3.3VCTTR, available on DigiKey. The circuitry surrounding the device was also quite simple. One potential drawback of using this device is the 150mA output current limit. This limit will require close attention to the power draw of all devices. If 150mA is deemed to be too restrictive, a new device may be chosen at a later time.

Both the MCP73831 and the TC2185-3.3 are used on the very popular TeleMetrum flight computer by Altus Metrum, meaning that the chosen devices have been proven to be reliable in the exact scenario in which we will be using them.