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Get a LiFe with LiFePO4 Cells This heads-up on safe LiFePO4 (Lithium Iron Phosphate) rechargeable cells outlines the possibilities and the drawbacks. Why be limited by Nicads or NiMH cells when you can get far more grunt from LiFePO4 cells? by Stan Swan D uring early 2013 an alarming outbreak of fire occurred in the first batch of Boeing’s 787 Dreamliner super jumbo planes. A second plane reported “smoke in cockpit”. These were traced to the two 32V lithium battery packs, each containing eight 4V lithiumion cells apiece. The suitcase-sized batteries are kept fully charged by the plane’s main generators for standby use. Fortunately no crashes or fatalities occurred but the adverse publicity and subsequent global grounding of the entire 787 fleet (rumoured to have cost Boeing US$50 million a week) has served as a great wake-up call on lithium cell types and associated issues. By chance, IATA postal and air travel regulations have recently been upgraded too and some countries now prohibit air-mailing isolated lithium cells, no doubt for fear that they may short circuit en route. So are today’s lithium batteries simply too hot to handle? Praised for their light weight and high energy and now abundant in numerous consumer devices (especially cameras, mobile phones and portable computers), some indeed may be potential fire hazards, even if treated with care and respect. There have been several “product recalls” for notebook computers, for example, where battery packs overheated to the point of distortion and in some cases, caused fires. The situation may have parallels in the volatile aviation gasoline that WW2 era aircraft carriers stored for their high performance piston engined planes. In spite of robust precautions, battle damage could puncture even well-protected shipboard tanks or fuel lines and lead to explosive fumes wafting below decks. A stray spark could then ignite gas vapours and explosively destroy the entire vessel. This tragically happened many times, notably with the USS Lexington during the 1942 Coral Sea battle and was a significant reason why carriers later became eager users of non-volatile kerosene-fueled jet aircraft. siliconchip.com.au So there is a corollary between the fuel problems of “real” aircraft carriers and model aircraft lithium-polymer (LiPO – as distinct from LiFePO4) battery fires reported in Bob Young’s Electric Model Planes article in the October 2012 issue of SILICON CHIP. Basic electrochemistry So-called “static” electricity has been known since ancient times but the fact that two different metals in a conductive liquid could also generate electricity was only stumbled upon two centuries ago. Huge efforts subsequently went into brewing up combinations of electrodes and electrolytes that generated higher voltages and currents, while being cost effective, long lived, light weight and with few side effects. There are two types of electrochemical cells: primary (use once, discard when flat) and secondary (rechargeable) and they’re usually known by the chemical symbols of their Reproduced from our October 2012 issue, this photo shows the dangers of using Li-Po batteries (this one in a model plane transmitter). Less than a year later, LiFePO4 batteries are a much safer proposition – but there are still some issues for the unwary! June 2013  13 While there are countless types of electro-chemical cells, operation is basically the same. A transfer of ions under chemical reaction causes a differential in voltage between the anode and the POSITIVE cathode. ANODE (typically Conventional carbon current can flow [graphite]) externally between them. In primary cells once the chemical reaction is completed the battery is exhausted. In secondary cells the chemicals can be rejuvenated by “charging”, or applying an external current and the battery can be used over and over. CURRENT FLOW A ELECTRODES + + + + + CONDUCTIVE + ELECTROLYTE + – may be a + paste or gel + + + + + ION FLOW + K – NEGATIVE CATHODE – – – – – – – – – – – – – after hydrogen and helium. So it’s very lightweight, with appealing electrode potential and being a metal, it is conductive. In ionic form its tiny atoms can easily migrate across atomic obstructions and burrow into crystal lattices. Given such features and its global abundance (being found in sea water and brine ponds), naturally lithium had long caught battery researchers attention! Although widely used in stable compounds in greases, glasses and alloys, isolated metallic lithium however can be dangerously reactive. It’s less so than other alkali metals (sodium and potassium) but when exposed to air it rapidly tarnishes and even reacts with the air’s nitrogen and carbon dioxide. It’s potentially explosive in contact with water and (as it floats in oils) lithium metal usually has to be stored under a cover of viscous petroleum jelly. Once ignited, lithium burns with great heat and becomes molten, with such fires often difficult to halt, requiring containing and smothering by dry powder fire extinguishers. Lithium battery types electrodes. Thus the common primary dry cell is C-Zn (carbon–zinc), a popular secondary cell is the NickelCadmium (Nicad) or now more likely Nickel-Metal-Hydride (NiMH). A myriad of primary and secondary types has evolved with the most common rechargeables being both “wet” lead acid (used in their millions in vehicles) and sealed lead acid (SLA “gel cel”) which have been very popular in demanding consumer applications as well as for commercial and industrial uses. None of these secondary cells (or batteries) have been without limitations, with low terminal voltage, short cycle life, poor standby and cold performance, weight and toxic internal materials being factors. As power hungry portable devices began their rise some 30 years ago, more suitable batteries were called for! Incidentally, a battery (from an artillery term for a cluster of guns) strictly means a group of cells connected together, although modern usage is quite forgiving (slack?) with single cells often referred to as batteries. Those who paid attention during Periodic Table school chemistry lessons may recall lithium as the third element, Dozens of lithium cell variations have arisen (refer to http://en.wikipedia.org/wiki/Lithium_battery) and are named after the cathode material (graphite anodes are traditional), along with a variety of conductive electrolytes. Although appealingly energy-dense, offering long life and light weight, many are primary types and may offer only “low” cell voltages (1.5V-3V). Coin-sized CR2032 and lithium AA types are typical. In 1979 oxide chemist John Goodenough at Oxford University perfected a higher voltage (~3.6V) rechargeable lithium-ion type using Lithium Cobalt Oxide (LiCoO2) and Lithium Manganese Dioxide (LiMn2O4), which Sony went on to commercialise in 1991. This “Li-Ion” (not IRON!) type is still the most common rechargeable lithium cell, although its slim Li-Po (lithium-ion polymer) offspring is increasingly ubiquitous in consumer products. It’s the laminated nature of today’s sleek and powerful Li-Po batteries, along with their (often) negligible outer protective casing that is of increasing concern. Many YouTube video clips dramatically confirm the explosive nature of Li-ion/Li-Po cell contents when abused, exposed, shorted or overheated, or if charge/discharge circuitry is overwhelmed. (Again, we draw readers’ attention to Bob Young’s article mentioned above). The discharge curves of similar capacity 12V lead-acid (6 cells) and Lithium-Iron-Phosphate (4 cells) batteries show the superior performance of the latter. LiFePO4 types are also only half the volume and a third the weight and have a much steadier output under varying loads. Here’s a more graphic comparison of the various types of commonly-available secondary batteries. As you can see, the venerable Lead-Acid variety doesn’t compare particularly well to even NiCad, while the Lithium varieties far outshine the rest. Enter lithium 14  Silicon Chip siliconchip.com.au Although awarded a lucrative prize, Goodenough didn’t benefit financially from his Li-Ion discovery. But in 1996, while in his mid-seventies and back in his native USA, he patented a more stable and cheaper Lithium Iron Phosphate (abbreviated to LiFePO4, LFE or even LiFe) cathode type. It’s this battery that this article is most focused on, and given his surname - no doubt the gifted researcher (now in his 90s and still working as a professor at the University of Texas, Austin) endures ongoing puns that his first lithium rechargeable wasn’t “good enough”! The dates mentioned may be very relevant for the Dreamliner’s woes, as the radical new aircraft’s prolonged development meant Boeing’s early 2000s choice of (Yuasa’s) Lithium Cobalt Oxide batteries was seemingly made well before safer LiFePO4 types became commercially available. The rival Airbus A380 and A350 of the same era more cautiously settled on heavier (but safer) NiCad/NiMH types, as did early hybrid vehicles such as Toyota’s Prius. LiFePO4 features LFP (we’ll use the shorter abbreviation for convenience) cells are still quite new and their long-term features have yet to be fully verified. But their claimed characteristics include: • An output of ~3.2V, which remains quite steady under load, only falling in the last 5% of capacity (Li-Ion starts near 4.2V but falls progressively to ~3V, while lead-acid is nominally 2V and NiCad/NiMH is only 1.2V) • Lightweight and compact – extremely good power-to-weight ratio (appealing for motorcycles etc) • Require constant current (CC) charging, which then tapers off as 3.6V constant voltage (CV) is reached. Note – LFP cell voltage settles back after full charging to ~3.3V. • A specialised (but cheap) charger should be used, although simpler approaches may suffice in a pinch. • No memory effect – cells can be charged/discharged at any state. • Extremely low standby losses. • Modest but appealing Ah (Amp Hour) capacity (but lower than comparable Li-Ions) • A cycle life of several thousand times (and far greater than Li-Ion’s annoying and costly hundreds of cycles) • Can be near fully discharged (although 2.5V is the recommend cutoff) but will probably be ruined if totally discharged. • High charge (~1C) and discharge (~10C) rates – both however are lower than comparable Li-Ions. (“C” refers to the capacity in Ah, with 700mAh being 1C for the AA cell type) • Quite safe for all discharge applications, as the cathode is non flammable and stable. • Excellent sub-zero and elevated temperature performance. • Environmentally benign (“green”) in manufacture, usage and disposal - no hazardous internal contents. No lithium remains in the cathode of a fully charged siliconchip.com.au A 12V, 18.4Ah LiFePO4 motorbike battery from Ever Power Energy Tech Corp, Taiwan. LFP cell. • Capable of even further performance enhancement when doped with Yttrium – Y – (pronounced “it-tree-um” and a common element – found apparently in cabbages!). Such cells are titled LiFeYPO4 (LFYP). Obtaining LFP cells At the time of writing (April 2013) LFP cells and batteries are still elusive at most outlets. Specialists are beginning to stock them, especially as 12V LFYP batteries for performance motorcycles or demanding standby solar power applications. Usefully, four cells (x 3.2V) gives 12.8V and smart LFP charging at 14.4V (4 x 3.6V) is comparable to traditional 12V lead-acid systems. This review now focuses on small and cheap individual “AA” sized cells that most users hopefully will soon encounter. My selection of cells and dedicated LFP charger were obtained from a specialist NZ firm but prices were noted far cheaper via direct imports from Hong Kong/Chinese outlets, which focus on global battery sales. Although concerning for international air freighting, feedback from radio-controlled-plane enthusiasts indicates battery orders thankfully arrive in very rugged protective packaging. Note: Electrochemical cells, quaintly still known as A,B,C,D variants from the ancient valve electronics era, are increasingly being titled by their metric dimensions. The common AA is 14500 sized since it’s nominally 14mm wide x 50mm long. You’ll occasionally see 14505 which are 14mm x 50.5mm – that extra 0.5mm can be crucial for making reliable battery contact. AAA sized cells, which are also available in LiFePO4 chemistry, are known as 10440 (10mm x 44mm). More powerful 18650 offerings (thus 18mm x 65mm and often used to stuff laptop battery packs and power tools) are of course much larger and hence will not fit common AA sized devices, battery holders or chargers. 18650 LFPs are currently being offered on ebay with up to 1800mAh capacity. It’s not difficult to obtain 700mAh AA cells for less than $3.00 each (including postage) out of China. AAA are even cheaper and larger cells (with higher capacities) are LiFePO4 batteries are now being used in e-bikes due to their increased performance. June 2013  15 dummy link for circuit continuity. Although readily made (eg, with a nail inserted in a suitable piece of bamboo!), such dummy cells can be purchased cheaply. Of course no doubt many dummies will be considered flat batteries, especially on April Fool’s Day, and consigned to the bin before their true nature is apparent! All cells received had terminal voltages near 3.2V but were given a top-up on a “Powerlion” LiFePO4 AA charger. (This twin-bay smart charger handles both AA and AAA cells but in spite of the “Powerlion” title is only suitable for LFP types). It delivers about 300mA per cell, and indicates full charge by switching a red LED to green when 3.6V is reached (and held as CV). Weight Ordering LiFePO4 batteries over the ’net won’t cost you sheep stations – here’s my order for six cells, two dummy cells and a charger from www.fasttech.com (China) and it came to a grand total of $US22.27 – including postage! Prices on ebay are quite similar. also common and relatively cheap. Of course, you’re going to need a charger but these too are quite cheap (albeit of unknown electrical pedigree). If you’re looking for higher voltage/capacity LFPs, these are available but are still relatively expensive: a 12V 8Ah battery could set you back around $125, including postage. However, it weighs only 450g and measures just 60 x 60 x 100mm – compare that to a 7Ah SLA at 2.7kg and 65 x 92 x 150mm (admittedly, around $30 each). First impressions Although much of an AA cell’s weight is made up of the protective metal casing (which may be similar to the dummy’s 8 grams), it was apparent that the LFP type has an attractive power-to-weight ratio. Being rechargeable and with claimed high cycle life, the total LFP ownership cost (even with a smart charger) may be attractively very low. Tabled below are some typical AA cell weights and energy claims. Initial consumer device trials AA-powered digital cameras are valued by professionals for their “off the shelf” battery capabilities, which may arise in emergencies when well away from the mains charging that a Li-ion driven camera would require. Indeed, many cameras are supplied with an alternative “emergency” AA-battery holder for such eventualities. Using AA alkalines or lithium primaries can become costly for power hungry devices of course, making rechargeable NiMH more attractive. However, the low NiMH voltage often causes picky camera electronics to flag low batteries and (annoyingly!) cycle slowly, or even shut down totally. Fitting a Canon A530 with a single LiFePO4 and a series dummy cell immediately made the camera sit up and eagerly start snapping! Spare LFP AAs could be carried and still give an overall weight saving compared to multiple alkaline cells. Revitalising such two-AA devices as a lethargic camera and LED torch was satisfying but a shaver with three flat AA cells also was found very responsive to a single LFP and two dummies (in place of its normal three series cells). Although the 3.2V supply was below the expected 4.5V (3 x 1.5V), the LFP’s high current delivery pleasingly gave the stubble a real workout. Four AA-sized LFP cells were obtained (Coolook, Powerlion, Soshine and a nameless blue) with all clearly labeled as being LiFePO4 3.2V rechargeable. Beside modern NiMH, their capacities of ~700mAh didn’t look high, but with a cell voltage almost three times greater they’ve comparable stored energy. Multiple series NiMH cells are needed for the supply to most items, whereas just a single LFP cell will suffice. By the way, be very wary about ordering NiCd, NiMH or indeed any cells from overseas as many stories have emerged about their labelled ratings being somewhat exaggerated by unscrupulous dealers. Indeed, we’ve actually seen some “D”-sized NiCad cells labelled 4Ah which didn’t seem to deliver the goods AA cell type – and when opened up contained only and a 600mAh “AA” cell cell fitted inside! Supply over voltage alert Numerous consumer items (torches, battery shavers, digital cameras) use two AA cells in series and thus run on nominally 3V. It’s crucial to appreciate the higher (3.2V) LiFePO4 cell voltage, as two LFP in series will supply 6.4V and may destroy the equipment if inserted! Only a single LFP will be needed, along with a conducting 16  Silicon Chip V x I x t Cell Energy/ Lifetime cell cost Energy Weight Weight (considering specifications (approx) (approx) ratio recharge cycles) C-Zn (1.5V 0.7Ah) Alkaline (1.5V 2.0Ah) Lithium (1.5V 3.0Ah) NiCad (1.2V 0.6Ah) NiMH (1.2V 2.4Ah) LiFePO4 (3.2V 0.7Ah) Dummy - - - - - - 1.5 Wh 17g Modest Medium 3.0 Wh 24g High Higher 4.5 Wh 15g Very high Very high 0.75 Wh 18g Low Low 2.9 Wh 25g High Modest 2.2 Wh 17g High Very low 8g - - -  siliconchip.com.au It should be appreciated that present AA-sized LFP cells have only modest energy storage ratings (typically 700mAh at 3.2V), meaning more frequent charging may be needed for power hungry devices. However, with their claimed thousands of cycles the cells look easily up to this. And like early NiCad, NiMH and other secondary batteries, LFPs are expected to get higher capacities as manufacturing process – and demand – improve. Conclusion Lithium Iron Phosphate (LiFePO4/LFP) rechargeable cells look to have a very bright future ahead. Their cheapness, light weight, high cell voltage, steady discharge level and abuse tolerance make them attractive in numerous applications where other primary and secondary cells are presently used. On safety grounds alone, they may well become preferred to Li-ion/Li-Po types, especially where case damage or overheating may occur. Although not so much of an issue with cell phones and tablets (where rapid upgrading is the norm) LiFePO4’s claimed thousands of charge/discharge cycle life may further appeal for demanding electric and hybrid vehicle use, as Li-ion battery packs for electric cars One of the perceived “disadvantages” of LiFePO4 cells is that they cannot be charged with simple chargers such as used for NiCad or NiMH. Indeed, many “professional” chargers separately monitor each cell in a battery. For AA/AAA cells, though, purpose-made chargers (like this “Coolook”) are cheap! and bikes can be both costly and short-lived. User awareness may be the key to LiFePO4 AA cell uptake, as series dummy place holders must be specified with great certainty. With each LFP cell delivering 3.2V at high currents, yet in appearance similar (in AA form) to normal 1.2 -1.5V cells, particular care should be taken to avoid accidentally over-supplying devices! The likes of 6.4V (2 x 3.2V) in a digital camera designed for only 3V (2 x 1.5V) will almost certainly give it a “bad hair day”. Resources: For convenience these are hosted at: www.picaxe.orcon.net.nz/LFP.htm SC LFP powered electronic circuitry PROGRAMMING (TO PC SERIAL PORT) SUITABLE ANTENNA ~170mm LONG 2 22k 3 5 10k 7 IC1 PICAXE -08M2 8 SC Ó2013 6 VCC DATA 5 ANT WHITE LED l K ANT ON 3.2V (SINGLE LiFePO4 CELL) GND 100nF BATTERY TEST 10k 4 1 (RECHARGE IF LED DIM) DORJI DATA TRANSMITTER USING L IF EPO4 CELL LiFePO4 powered - single 3.2V AA cell Smart charge 3.6V DO NOT allow supply to fall below ~2.5V RECHARGE PROMPTLY IF TEST LED IS DIM The 433MHz “fox hunt” transmitter built on a protoboard and housed in a 3 x AA battery box – with one LiFePO4 AA cell instead of three! We also included a white LED which serves as a battery level meter – press the switch and if the LED lights, the battery still has plenty of charge. siliconchip.com.au 3 4 8 DORJI 433MHz DATA TRANSMITTER 1 2 A DATA VCC GND As a “proof of concept” trial a PICAXE/ Dorji 433MHz beacon transmitter circuit was developed, powered by a single AA LFP and housed in a partially gutted 3 x AA switched battery box. Assorted low voltage cutoff sensors and PICAXE-driven software (especially the 08M2 “CALIBADC” command) were considered but initially just a dumb test LED was used for simplicity. In fact, it reminded me of the external “see the power” power-check strips featured on some Duracell alkaline AAs. I had only 5mm white LEDs on hand (although naturally a smaller white could be used) but all showed significant dimming below 3V and most usefully for LiFEPO4, a total light cutoff by 2.5V – a near perfect matching! Such a simple battery state test could also be included with dumb circuitry (discretes, 555 etc) housed in a similar partially gutted LiFePO4 powered box. The breadboard section shown makes for great initial convenience but naturally Veroboard/Kiwi Board/PCB etc could be used for final soldered versions. A PCB “finger” or dummy cell would be needed for a single AA space in a 2-AA battery box. The circuit simply sends an occasional Morse ID beacon tone transmission on the 433MHz ISM band and then sleeps at very low currents for an adjustable time (ie, a classic “fox hunt” transmitter). Battery life of the single LFP is estimated as being several weeks due to the low duty cycle. Further details and the controlling code (which features low battery alerts) are hosted at the article resource site (see above). Quite aside from the LiFePO4 insights the layout was motivated by the “potential” of the switched AA battery box, as discrete switches and suitable project cases can otherwise end up costing more than the internal electronics! June 2013  17