Keith and I have discovered a change in the behavior of the protection circuits integrated in the LiPo batteries we sell for use with Altus Metrum products that poses a risk for our customers. This post is meant to document what we now know, communicate changes we're planning as a result, and explain what we think flyers of our existing electronics and batteries may want to do to maximize their chances of successful flights.

Background

Choosing batteries and designing pyro circuits for high power rocketry avionics involves a variety of trade-offs. Reliability is the highest concern, both because nobody wants to lose an airframe due to a failed pyro event, and also because airframes recovering anomalously have safety implications. But we also care about other factors including cost and weight, and usually want to minimize the complexity of both the electronics and the overall installation.

The objective of a pyro circuit is to dump enough energy rapidly into an electric match to cause it to catch fire. We need batteries both to power the electronics that decide when to fire the charge, and to provide the energy that actually ignites the match.

The two most common battery types seen in the rocketry hobby are alkaline cells, often the nominal 9V rectangular variety, and rechargeable batteries based on Lithium Polymer (LiPo) chemistry. LiPo cells are 3.7 volts per cell nominal voltage, are very light, and have a high energy density.

LiPo capacity is measured in units of current times time, so an 850mAh cell should be able to deliver 850 milliamps for an hour. The battery industry also uses something called a "C rate" to describe how fast the battery can be usefully discharged, wich is a multiplier relative to the battery capacity. So a battery with 850mAh capacity and a "2C" rating can deliver current at a sustained rate of 1700mA until discharged, while the same capacity at a "5C" rating can deliver 4250mA.

By comparison, most 9V alkaline batteries are actually composed of 6 individual 1.5V cells enclosed in a wrapper. It's hard to get hard numbers for capacity and discharge rate, since in an alkaline cell the two are not independent, and the discharge rate is related to the volume of each individual cell. The data sheet for an Energizer 522 shows just over 600mAh at a 25mA discharge rate, dropping to about 300mAh at a 500mA discharge rate.

Importantly for use in pyro circuits, LiPo cells have a very low source impedance, which means they can source immense amounts of current. It's not unusual for cells in the 1000mAh range to have ratings in excess of 30C! Because this rapid discharge ability can pose a risk of fire, it's common for LiPo cells to come with a "protection board" integrated into the battery assembly that is designed to limit the current to some rate such as 2C continuous duty.

In large airframes, or projects that involve staging, air-starts, or other complex pyro event sequences, the most reliable approach will always be to use separate batteries for the control electronics and the source of pyro firing power. In the limit, having separate pyro batteries for each pyro charge with the control electronics only providing the switching to connect the batteries to the charges could even make sense. But for most airframes, this is overkill, and the increases in mass, volume, and wiring complexity just don't make sense.

The challenge, then, is how to design electronics that will robustly initiate pyro events without negatively affecting the rest of the electronics when operating from a single shared battery.

Altus Metrum Pyro Circuits

The very first prototypes of TeleMetrum were designed to use a single-cell LiPo battery, and had an on-board 100mA charging circuit. Because we needed 5 volts to power the accelerometer, we had a small switching regulator that up-converted the LiPo voltage, and we used some of that regulator's output to charge a 1000uF capacitor. The pyro circuit used Fairchild FDN335N N-channel MOSFET switches in a low-side switching configuration to dump the energy stored in the capacitor through an attached ematch. Those FETs had an on resistance of under a tenth of an ohm in our operating conditions. The circuit worked very reliably, but the 1000uF electrolytic cap was huge and we struggled with the mechanics of such a large part hanging off the board...

It turns out that 3.7 volts is way more than enough to fire a typical low-current e-match or equivalents like the Quest Q2G2 igniters. In fact, bench testing with a good digital oscilloscope showed that a typical e-match with resistance of 1.3-1.8 ohms will fire in approximately 13 microseconds when hit with the nominal 3.7 volts from a LiPo.