EM Battery Pack

This page corresponds to the battery test plan document and shows the pack assembly, procedure of the tests and the test results.

Battery Purchase Documents

Batteries part number is The batteries were purchased from Battery space with Order# 448605 and Order date: August 1, 2019. The manufacturing date of the cells is 03/06/2019. A total of 24 battery cells were purchased, that is, 6 cells for EM (Engineering Model), 6 cells for QM (Qualification Model), 6 cells for FM (Flight Model), and 6 cells for FM backup. Iris P/Ns are: UMS-0036-EM (EM Pack), UMS-0037-QM (QM Pack), UMS-0038-FM (FM Pack), UMS-0039-FM (FM Pack backup). Email records below provide more info about battery purchase.

Battery Pack Assembly

The assembly of the EM battery pack was done on a 3D printed saddle shown in Figure 1. Epoxy was used to hold the cells in place. After the cells were secured on the saddle, the connections were added by soldering wires to the soldering tabs of the cells. The final assembly is shown in Figure 2. This battery pack is assembled in a 2S3P configuration and has a nominal output voltage of 6.4 V and a nominal capacity of 4500 mAh. The EM battery pack tests such as charge-discharge cycles, battery protection and battery interface were performed on this pack.

Test Procedures

Battery Interface:

The battery interface test is related to the battery gas gauge IC (A.K.A. Coulomb counter IC) which monitors the state of charge (SOC) of the battery by counting the charge entering or exiting the battery pack. The first design of the EPS EM board had the LTC2942 as the coulomb counter but it was out of stock by the time we ordered our board. The design is now using LTC4150 as the Coulomb counter which is shown in Figure 3. For EM tests, we have this IC connected on a breakout board and on a bread board. This will be integrated to the board for FM. The output data from the Coulomb counter is fed to an Arduino Uno which runs a simple program that reads the amount of coulombs and mAh going in/ out of the battery, voltage of the battery pack and an estimation of the current, and displays it in the serial terminal.

Battery Protection Circuit Test

To ensure the safe charge/ discharge process of the batteries, we have two stages of protection implemented in EPS board. The primary protection stage happens inside the EPS controller microcontroller. The secondary stage happens on the EPS board using a specially designed protection circuit. In this section, the secondary protection circuit is tested and the results are shown.

To ensure that the batteries are safely charged/ discharged, we have tested the protection circuit based on the AP9101 LiFeO4 battery protection IC (Figure 4).

As stated in the datasheet, this IC provides protection features like: Overcharge Detection, Overdischarge Detection, Discharge Overcurrent Detection, Short Current Detection, Charge Overcurrent Detection.

The circuit of Figure 4 was assembled on a bread board to allow for tests. Figure 5, shows the circuit under test attached to an electronic load to discharge the battery pack and test the over-discharge protection feature of the circuit.

To make sure our breadboard prototype didn't produce false results due to loose wires and high connection resistance, the same circuit was soldered on a protoboard and test results were observed.

The results of the tests for the protection circuit based on AP9101 IC is as follows:

• The overcharge cut-off voltage is: 7.3 V

• The over-discharge cut-off voltage is: 4.4 V

• The overcurrent limit is: 3.5 A

• Short circuit cut-off

Battery Insulation

The batteries come in PVC insulation from the factory. This material is not allowed for space applications so we are going to remove the battery sleeves and cover the bare parts with Kapton tape to provide protection for the batteries against accidental short circuits.

The Kapton tape selected for this purpose is a 1" wide by 36 yards tape from ULINE Canada (Model No. S-11730) https://www.uline.ca/Product/Detail/S-11730/High-Temperature-Tape/Kapton-Tape-1-Mil-1-x-36-yds .

The batteries' body will be covered in Kapton tape before assembly. After assembly of the battery saddle, the bare contacts will also be covered to provide more protection against accidental shorts.

Video below shows the process of removing the PVC protection for a cell. This was done on one of the EM cells in our lab.

Kapton tape insulation was applied on all the flight model cells in the cleanroom as shown in the pictures below.