Some Thoughts on a CubeSat in Trouble

NASA is reporting that it has lost contact with another CubeSat that is part of the ten that were launched on the Artemis mission, CuSP. https://blogs.nasa.gov/sunspot/2022/12/08/artemis-i-payload-cusp-cubesat-mission-update/

While we do not definitively know what caused CuSP to fail (and the CuSP team may regain control of CuSP), there is some data re anomalous battery operation that perhaps gives us some insight and can be used as a teaching moment. "An unexplained battery anomaly also occurred at the end of the initial data collection period. Two minutes prior to the end of the data collection period, one of the battery cells suddenly experienced a temperature spike – jumping from 34 degrees Celsius to more than 168 degrees Celsius in under a minute. The temperature of the anomalous cell subsequently increased from approximately 34 degrees Celsius to about 80 degrees Celsius before loss of contact"

Most CubeSats today use Lithium-based rechargeable batteries, primarily because of their high energy density, and because Lithium batteries are so prevalent in mass-produced consumer products like laptops, smartphones, power tools and cars. Rechargeable (also called secondary) Lithium cells are available in many different chemistries, and one can generalize that most commonly-used Lithium cells like to operate in a relatively narrow temperature range of roughly 0C to +40C. Most cells can discharge (albeit with lower performance) at temperatures below freezing, down to -20C or maybe a bit colder. Similarly, some chemistries work even more efficiently at temperatures above +40C to +50C, but major manufacturers like Panasonic and LG generally recommend not charging above +45C or so. Most CubeSat batteries are composed of multiple cells, in series and in parallel, to achieve the desired voltage range and current delivering capability, and stored energy; 2S2P (an "8.4V, 50Wh battery") and 4S2P (a "16.8V, 100Wh battery") are common cell configurations.

The report from CuSP that a single cell's temperature rapidly climbed from +34C to +168C tells us a couple of things. First, +34C is a reasonable operating temperature for a cell. CuSP's internal temperature might have been around +34C and very little current was flowing through the battery. Or perhaps CuSP was at a colder temperature but the battery was being charged --  charging inherently causes cells in the battery to heat up, with higher charge currents resulting in more cell self-heating. Either way, +34C is a perfectly reasonable operating temperature for Lithium-based cells; +168C is not. It's not clear what type and size of Lithium cells CuSP used. But if the +168C reading is correct, and if the cell in question is a typical 18650 (cylindrical Li-Ion) cell, in rough numbers it took around 5,000 Joules (or 5,000 Watt-seconds) to raise the cell's temperature by 134C. Given that the change in temperature happened "in under a minute,"  then that reflects a hefty dose of energy that is in excess of the roughly 50-60W of power that comprise CuSP's solar array, applied over that minute. Therefore, it seems unlikely that CuSP's EPS failed to the point where it was attempting to overcharge the cell, and suggests that another source of energy is being responsible for the cell's temperature rise. That is probably a failure in the cell itself. This kind of failure (typically an internal short circuit) may be localized to the cell itself if each cell is individually fused. If not, then connected cells may also dump their charge into the failed cell, heating it up further. When a runaway event like this happens, one can generally assume that the affected cell is no longer functional. At a minimum, the individual cell no longer provides any energy to the battery (a pack of interconnected cells), and it may or may not inhibit current flowing through any series-connected cells elsewhere in the battery.

When a cell failure like this happens, several issues rise to the fore: First, the battery voltage will have fallen by one Lithium cell's worth (typically 4.2V when fully charged). In, say, a 4-cell battery with all the cells connected in series, the battery is now operating at 3/4 of its normal operating voltage. In a  16.8V Li-Ion battery, that means that the maximum voltage achievable is now just 12.6V, which is very close to the undervoltage limit of the three remaining cells. At best, you might end up with a battery that now has an operating voltage of 12V to 12.6V, which is a tiny fraction of the total energy of a 4S, 16.8V battery. This narrow operating range would be difficult for most EPS and battery systems to handle correctly. Second, the battery failure may take down the entire power system, depending on how the EPS is architected. Some EPSes can operate without batteries, so a working ADCS system could conceivably keep CuSP alive if there are no loads on the system that exceed the solar array's power output. Conversely, a battery failure could even physically harm the EPS, ending the mission. Lastly, if the battery has cells connected in parallel, and one cell has failed, then the current through the remaining cells will be limited by the remaining parallelism; i.e., a single cell's failure (with automatic isolation via a blown fuse) in a 4S2P battery yields a 3S2P + 1S1P battery, which is effectively a 4S1P battery from the standpoint of how much current can be drawn from the cells. For most EPSes, this will also lead to difficulty in charge balancing, end-of-charge threshold issues, etc.

In summary, from the contents of the NASA blog, it appears that there was a failure internal to a battery cell inside CuSP, and this may have taken CuSP's EPS down with it. If NASA is able to establish communications with CuSP, then that may suggest that CuSP's battery is no longer functional, but that the EPS is able to operate without it.

Pumpkin's approach to power systems has been quite different from the usual CubeSat offerings. We've applied automotive-grade multi-level safety, redundancy and intelligence to our BM2 intelligent battery module (a 4S2P Lithium-Ion battery), and our EPSM1 is completely capable of operating with two, one or zero active BM2s attached to it. Morehead State's Lunar Ice Cube (one of the Artemis 1 CubeSats) has been successfully commissioned on-orbit, with Pumpkin's EPSM1 and BM2s aboard.

While it is not unheard of for Lithium-based cells to fail internally and thereby cause problems that ripple through spacecraft power systems, Pumpkin's EPSM1 and BM2 are designed to give the host CubeSat a fighting chance at continued operation in these conditions.