From a704327cccf17ab19385304c64b93b02cd134e53 Mon Sep 17 00:00:00 2001 From: Bdale Garbee Date: Wed, 10 Apr 2013 18:11:53 -0600 Subject: [PATCH] capture current draft of post on batteries and pyro circuits --- .../posts/Batteries_and_Pyro_Circuits.mdwn | 207 ++++++++++++++++++ 1 file changed, 207 insertions(+) create mode 100644 bdale/blog/posts/Batteries_and_Pyro_Circuits.mdwn diff --git a/bdale/blog/posts/Batteries_and_Pyro_Circuits.mdwn b/bdale/blog/posts/Batteries_and_Pyro_Circuits.mdwn new file mode 100644 index 0000000..ee977e5 --- /dev/null +++ b/bdale/blog/posts/Batteries_and_Pyro_Circuits.mdwn @@ -0,0 +1,207 @@ +[[!tag tags/rockets]] +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](http://altusmetrum.org) 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](http://data.energizer.com/PDFs/522.pdf) +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](http://altusmetrum.org/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](http://www.questaerospace.com/cgi-bin/commerce.exe?search=action&category=OJ) +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. + +So, starting with our v0.2 boards, we switched to using the LiPo battery +voltage directly to fire the pyro charges, eliminating the clunky electrolytic +capacitor entirely. We also switched to the Fairchild FDS9926A dual N-channel +MOSFET whose specs are better in all regards for our application. The on +resistance is down around 40 milli-ohms in our circuit, such that the current +ratings are much higher (FET current limits are primarily driven by how much +heat is generated due to current flowing through the channel's on resistance). + +Because using the LiPo voltage directly means we're effectively temporarily +putting a very low resistance across the battery during the pyro events, the +input voltage to the voltage regulator gets pulled down. To ensure the +processor could "ride through" these events, we added a 100uF surface mount +bulk capacitor on the 3.3 volt regulated voltage rail, which bench testing +demonstrated was more than sufficient to maintain processor operation through +pyro events. And that is essentially the same pyro circuit on all the boards, +both [TeleMetrum](http://altusmetrum.org/TeleMetrum) and +[TeleMini](http://altusmetrum.org/TeleMini), that we have shipped to date. + +What's Changed +============== + +The LiPo batteries we source and sell with our electronics come with a +protection board and cable terminated in a 2-pin, 2mm "JST" connector. The +specs on the protection board have always been "2C continuous", but we +observed the +ability to source much higher currents for short periods such as the 13 +micro-seconds or so required to fire an e-match. Thus these protection +circuits seemed just fine .. we could fire e-matches with a burst of current +but were protected against short circuits in the wiring or our boards by +the 2C continuous limit. + +Unfortunately, the most recent batch of batteries we sourced seem to have a +much "twitchier" protection circuit. We can draw more than 2C for short +bursts, but not as much as with prior boards, and not for as long an interval. +With a 1 ohm power resistor on the pyro terminals of one of our boards, we +get about 9 milliseconds of power before the protection circuit cuts in and +shuts the battery down. The power stays down until *all* load is removed, +which at least means the board is turned off and back on again, and in some +cases could even mean the battery is unplugged and re-plugged since we draw +trace current to keep the GPS memory alive even when the power is turned off, +and at least some of the new batteries see that as enough to keep the power +turned off after an over-current event. + +For many e-matches, this isn't an issue, since 9 milliseconds is *way* longer +than the 13 microseconds needed to fire the charge. Unfortunately, we've +discovered that many of the e-matches bought and used in the rocketry hobby +are actually made for use in the fireworks industry, where it is desireable +to retain continuity +after firing so that series connections of e-matches all can fire even if +some fire faster than others! This means that while the resistance goes +up some after firing, *sometimes* the drain on the battery remains sufficient +to cause the protection circuit to kick in even after the pyro charge has +fired. + +What We're Doing About It +========================= + +If we remove the protection circuit from the LiPo (or jumper around it), all +existing Altus Metrum products will operate successfully with pyro charges +thave have an effective resistance of as low as 1 ohm. That's lower than +any e-match or Q2G2 we've ever seen, so in effect what this means is that +if you have an existing Altus Metrum flight computer, and you remove the LiPo +protection circuit, you're good to go. This does not really make things any +more "dangerous", since our battery chargers are all current limited and our +discharge patterns will never cause heating of the battery. Frankly, in a +rocket, the most likely way to cause a problem with a LiPo is by smashing or +puncturing the actual battery during bench work or during a crash... and +those cause the same problems with or without a protection board present. + +In the future, we will ship batteries that have either a much higher C rating +on the protection circuit, or have no protection circuit at all. + +The number of problems reported by actual customers that we think should be +attributed to LiPo protection circuit boards is *very* low, and we +suspect most of our customers who are happily flying their boards can continue +to do so. Ground testing where you fire pyro charges (or at least e-matches) +using RF to issue the commands (not USB, since the LiPo charger is running +any time USB is connected!) will confirm whether there's a problem. If the +board resets (does startup beeps) after a pyro event, or shuts down completely +(no LED activity), then you have a problem. If the matches light and the +board keeps running, you're good to go. + +However, *any* Altus Metrum customer with LiPo batteries sourced from us or +our distributors who is worried about this problem (even if you haven't seen +problems in ground testing or previous flights), and who doesn't want to try +soldering on the battery circuit board yourself, is welcome to contact us +about removing the protection circuit for you. We won't charge anything +other than shipping. + +To take advantage of this offer, just send email to +fixmybattery@altusmetrum.org telling us how many of which capacity batteries +you have that you'd like updated, and we'll respond with an RMA number and +shipping details. + + [ make sure Keith is ok with working it this way ] + +Going Even Further +================== + +As previously indicated, with the LiPo protection circuits removed, all of +our current products will work reliably with at least 1 ohm across the pyro +terminals. That should cover all real-world flying conditions just fine, +but we're not satisfied yet. + +We've designed a new pyro control circuit that transfers the maximum possible +energy to the load regardless of battery voltage without ever allowing the +voltage to the processor to droop at all. We're testing this new circuit in +various prototypes now, and if it pans out it will probably show up first in +[MegaMetrum](http://altusmetrum.org/MegaMetrum) and then trickle down to +[TeleMetrum](http://altusmetrum.org/TeleMetrum) and +[TeleMini](http://altusmetrum.org/TeleMini) as those products +are updated later this year. The new pyro circuit tolerates 0 ohms (dead +shorts) on the pyro terminals for as long as the battery can provide +current, which is as good as it gets. We think the circuit is clever enough +that we'll probably write more about it once we're finished validating it. + -- 2.30.2