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| 2008-11-30 15:34:33 | NiMH电池充电（Real-World NiMH Charging） |
During bike trips， Ed uses Icom handie-talkies for communication. To keep the radios‘ batteries working， he uses a three-pack charger that he designed at his workbench. The charger has three 250-mA channels and one 100-mA channel so he can charge different packs. 
Because NiMH cells have a charging efficiency of about 2/3， injecting a full charge requires 150% of the nominal capacity. Charging a cell in 15 minutes means you must deliver： 
The data plate embossed on the back of the charger states that it delivers 7.5 A to each AA cell； it can also charge AA cells at 3.0 A. Working backwards， that gives a charging time of： 
If you ignore the charging efficiency，that‘s 17.6 minutes. If you also figure that most cells aren't completely discharged，the usual recharge time should be under 15 minutes. Just for fun， I discharged one of the 2.2 cells at 2.2A·h down to 0.5 V，well below the usual 0.9-V limit， and then recharged it. The discharge required 54 minutes and delivered 2.0 A·h，which is quite good performance at that current. The recharge took 17 minutes until the green “charged”indicator blinked on and 29 minutes until the fan stopped. The instructions tell you to wait 10 minutes after the green indicator comes on for a full charge， so the actual times are pretty close to my estimates. They are， however， somewhat longer than the advertised "15 minutes，" so perhaps it‘s no surprise that the specs for the same charger now indicate that it ships with four 1.85-A·h AA cells. That lower rating means they recharge in exactly 15 minutes at their nominal capacity and 22 minutes if you allow for charging efficiency. However， that data applies to a single cell： charging four AA cells simultaneously requires four times the power. The charger‘s data plate claims an output of 5.6 V at 7.5 A，which is 42 W， and the unit's wall wart delivers 40 W （10 V at 4 A）。 So，in round numbers， the charger dumps 10 W into each AA cell. Remember that NiMH cells mark their end-of-charge state by heating up， to the extent that you can actually build a good charger by noting the rapid temperature rise. Missing that signal， however， will allow a rapid charger to overheat the cells or，worse， burst them. That Energizer charger has a fan for a good reason： the cells get rather warm！ The eight-cell packs for our radios use 2.6-A·h cells， so a true “15-minute”charger would require： 
The Energizer charger evidently uses power FET switches to connect the cells in series to its power supply and switch them out as they‘re charged. I'm certain the charger adjusts its nominal 5.6-V supply to match the number of cells in series. In round numbers， therefore， an eight-cell charger would require a 12-V source that could deliver nearly 16 A：nearly 200 W. Charging three packs at once becomes a 600-W proposition，which seems aggressive even to a guy who built a kilowatt resistance soldering unit. A 1-h charger draws a quarter of that power and takes four times longer， but that‘s still a 150-W supply for three packs. I decided that a slow three-pack charger made a whole lot more sense： it can work while I sleep without fuss， bother， fans， or risk of damage. DEAD-SIMPLE CHARGING The dumbest of dumb NiMH chargers consists of a current source set at 10% of the battery's nominal capacity：260 mA for a 2，600-mA·h battery. After 15 h， more or less， that current will bring a completely dead cell back to life. While further charging isn‘t recommended， contemporary NiMH AA cells include sufficient catalyst to recombine all the oxygen liberated by low-rate overcharging without venting. The cells become warm， but will not deteriorate due to overheating. Photo 2 shows what I came up with in the week before we left： three 250-mA and one 100-mA current sources in a box， with Powerpole connectors on short pigtail leads. The thin black wire supplies 14-V power from a surplus laptop charger. 
A slightly less dumb charger would incorporate a 15-h （more or less） timer to cut off the current after the maximum allowed charging time. An even brighter charger could monitor the pack temperature to detect the reliable end-of-charge heating. A truly smart charger would monitor the charging voltage to detect the end-of-charge plateau. All of those frills aren't really necessary if your plans include daily charging and constant moving. Figure 1 shows the trivial schematic for a single charger channel. The power source， common to all the channels， is a surplus laptop charger that delivers a regulated 14 V at up to 3.5 A. The 2-A fuse will blow at about twice the maximum total charging current. 
A venerable LM317 three-terminal voltage regulator acts as the current source. The regulator adjusts its output voltage to maintain 1.25 V between its Out and Adj terminals， so the parallel combination of R2A and R2B determines the output current： 
I picked 250 mA because it requires a 5-Ω resistor that‘s merely a pair of 10-Ω resistors. The 100-mA channel，chosen to charge the slip-on batteries I rebuilt in the original Icom packs，calls for 22 Ω in parallel with 30 Ω。 The main disadvantage of this circuit comes from its profligate power use： each LM317 regulator must dissipate the product of its load current and the voltage difference across it. The lowest voltage across a six-cell battery can be under 6 V at the start of charging， R2 always drops exactly 1.25 V，and D1 accounts for about 0.5 V： call the total 8 V. The regulator will thus see about 6 V， making its power dissipation： 
The full-charge voltage for a six-cell pack is about 9 V， so the regulator dissipation eventually drops to： 
I selected a 14-V laptop power supply from my parts heap to ensure the regulator had （just barely） enough voltage headroom for a fully charged eight-cell pack at 12 V. A rackmount server power supply in my heap disgorged the four TO-220 heatsinks and mounting hardware appearing in Photo 2. They were designed for use in a forced-air environment：the LM317s run much hotter than I prefer， despite the rows of vent slots in the aluminum box I used. A thermocouple poked into a blob of heatsink grease on one of the regulators showed that it hits 80°C at the beginning of the charge cycle. LM317 regulators shut down when their junction temperature reaches 125°C. The TO-220 package has a 4°C/W junction-to-case thermal resistance，so the internal temperature at 1.5 W falls well under that limit. It‘s too hot for long-term reliability， but at least the regulator won't incinerate itself. The charging process requires essentially no thought at all. At the end of the day‘s ride， I unplug three battery packs from our bikes and plug them into the charger's three 250-mA connectors. Because even a full day‘s use doesn't completely discharge the pack，it‘s generally ready for use in much less than 15 h. Simply touching each pack tells me when to unplug it as it reaches full charge， but no harm ensues if I do nothing until the next morning. A friend points out that building a battery charger with a touch-dependent charge termination requires a certain，um， personality. All I know is： it works for me！
In March 2006， one of those cells provided just over 1.5 A·h at 500 mA，its C/4 rate， as shown by the black trace （use the right-side Y scale）。 The stairstep effect reveals the 10-mV voltage resolution of my West Mountain Radio CBA II battery tester. A cell‘s capacity strongly depends on the discharge current， with lower currents extracting more energy from the cell. Different manufacturers may use C/10 and C/20 rates， but typical uses require far more current than that； after all， NiMH cell advertising touts their "high drain" abilities. The remaining traces show the capacity of a six-pack made from those Lenmar cells， but tested in recent months. These traces use the left-side Y scale， where the CBA II's resolution isn‘t nearly so obvious. The blue trace corresponds to the first discharge of the pack after I pulled it from my battery FIFO. I rotate the packs so that that they're neither freshly charged nor totally dead. NiMH cells self-discharge up to about 1% per day， so I estimate this pack has been on the shelf about three weeks. The red trace shows roughly the same capacity for a 1-A discharge as it does at 500 mA， which is a good sign. However， the other traces （made at 500 mA） show that the cells have lost 10% of their capacity during the last two or three years. Figure 3 shows the results of a yearold six-pack made from Tenergy 2.6 A·h at 500 mA. I ran the test that produced the black trace shortly after removing the pack from the bike； you can see that one cell was almost totally discharged， even though the remainder of the pack still had nearly 20% of the rated capacity. By definition，the weakest cell goes flat first. 
The blue trace shows the other five cells can provide about 1.3 A·h after recharging. The final discharge slope has a step at 5.0 V， revealing the nextweakest cell in the pack. However，they are reasonably well matched. Recharging the weakest cell， which I labeled Q， produced the red trace（use the right-hand Y scale）。 Obviously，it was suffering from under-charging，the typical cell failure mode，rather than reaching its end-of-life point. Putting the pack back together again and charging it for a good long while produces the purple trace. The pack now has about 1.3 A·h of capacity and the final dropoff shows that all the cells go down together. Adding the GPS interface to our radios increases the current drain， so I bought more AA cells to make up the eight-cell， 9.6-V packs required for 5-W transmitter power and the TinyTrak3+ voltage regulator. Surprisingly enough， the new cells sport the same Tenergy brand and the same 2.6-A·h rating as before. I have no way to tell if they also have the same internal construction. The traces in Figure 4 document the first six charge-discharge cycles applied to one cell. Common knowledge has it that NiMH capacity increases during the first few cycles，but that‘s certainly not obvious from the data. 
The first discharge， shown by the black trace， had the lowest capacity，but the other five traces don't show any particular progression. In fact，under these conditions， the cell delivers only about 70% of its rated capacity. While that‘s better than the yearold Tenergy cells， it's not really up to spec. Figure 5 shows that this need not be the case. Earlier this year I made up an eight-cell battery from Duracell 2.65-A·h cells; the additional 0.05 A·h has no significance outside marketing staff meetings. Although I cannot vouch for how long this pack sat in my rotation， the initial discharge and subsequent cycle deliver nearly the entire rated capacity. Notice that this is at 1.0 A， rather than the 500 mA used in the other tests. 
The bottom line， I think， is that there‘s a difference between Tier-1 batteries and the rest， but that there's an even larger difference due to the care and feeding the cells receive during their lifetime. CONTACT RELEASE When we‘re not on vacation， we spend about an hour a day riding on errands， to stores， and making other trips around the neighborhood. I change the battery packs on Friday afternoons so that we're all charged up for our longer weekend rides， and carry a spare pack. Under those conditions，all of the packs continue to provide good service， but I plan a major weak-cell purge when I get all three GPS units running. Even such a simple project requires analog design， power management，and EMI control. When you‘re handed a new project， remember that it's never as simple as it seems and you can‘t work on any of the pieces in isolation！ Ed Nisley is an EE and author in Poughkeepsie，NY. Contact him at ed.nisley@ ieee.org with "Circuit Cellar" in the subject to avoid spam filters. PROJECT FILES To download code, go to ftp://ftp.circuitcellar.com/pub/Circuit_Cellar/2008/221. RESOURCES E. Nisley, "Battery Capacity" Circuit Cellar 199 & 201, 2007. ---, "Battery Power, Feeding the Z3801A," Circuit Cellar 155, 2003. PowerStream Technologies, "NiMH Battery Charging Basics," www.powerstream.com/NiMH.htm. SOURCES Powerpole connectors Anderson Power Products www.andersonpower.com/products/singlepole-connectors.html TinyTrak3+ Tracker and GPS receivers Byonics www.byonics.com Energizer CH15MNCP4 15-minute charger with four NiMH AA batteries Eveready Battery, Inc. www.energizerbatteries.com/productdetail.asp?device=DIGCAM&prodcode=15505 2.0-A•h Cells Lenmar Enterprises, Inc. www.lenmar.com(Distributor: www.batteries.com) 2.6-A•h Cells Tenergy Corp. www.all-battery.com Computerized battery analyzer West Mountain Radio http://westmountainradio.com |
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