Batteries and wireless microphones

Transcription

1 Batteries and wireless micrphnes Shure Technical FAQ Batteries and wireless micrphnes Published 07/06/ :47 PM Updated 03/16/ :56 PM QUESTION: What type f battery shuld I use with my wireless micrphne? Can I use a rechargeable battery with my wireless mic? ANSWER: Batteries and Wireless Micrphnes Benjamin Franklin first cined the term "battery" during his famus electrical experiments. Then arund 1800, Alessandr Vlta, a prfessr f natural philsphy at the University f Pavia, develped the first apparatus knwn t prduce cntinuus electricity. Vlta s name was later used t describe the ptential f these cells: the vlt. Since batteries are an expendable item, users are always trying t find cheaper, better alternatives. In the audi industry, wireless micrphnes are ne f the tp users f batteries. This article answers many f the cmmn questins and explains hw and why things perate the way they d. Battery Chemistry psitive cntact insulating ring PVC insulating sleeve * & bttm insulatr steel can cathde current cllectr metal jacket ande (amalgamated zinc geii) separatr sealant cathde 4 insulating ring plastic gasket negative cntact ande current cllectr safety vent Batteries are devices that cnvert chemical energy int electrical energy. They are cmpsed f cells. Cells are cmpsed f a cathde and an ande immersed in an electrlyte. The different types f cells, such as alkaline, Nickel Cadmium (Ni-Cd), Nickel Metal Hydride (Ni-MH), all use these 3 cmpnents, but use different materials id/250/~/batteries~and-wireless-micrphnes

2 Batteries and wireless micrphnes Shure Technical FAQ fr the cmpnents. Batteries can be made up f a single cell r multiple cells. The different battery types prduce different vltages per cell, depending n their chemical reactins. Fr instance, an alkaline battery prduces 1.5 vlts per cell, and Ni-Cd batteries prduce 1.2 vlts per cell. Fr a AA r AAA battery, just a single cell is used per battery. Fr a 9 vlt battery, 6 alkaline cells are used, wired in series. Cells can be separated int tw grups. Primary cells are ne time use nly, such as mst alkaline s and the ld Zinc/Carbn type. Secndary cells are types that can be recharged, such as Ni-Cd, Ni-MH, and Lithium In (Li+). There are many mre battery types, but this paper will nly discuss the cmmn nes applicable t wireless micrphnes. Battery Capacities The mst imprtant specificatin fr a battery is the capacity. The capacity is measured in, i.e. current (specified in milli-amps) multiplied by time (specified in hurs). If yu knw hw much current a device takes t perate, it is pssible t determine hw lng a battery will last. Current Density () Current (ma) = Time (hurs) The table belw gives typical capacities f ppular AA batteries. Nn-Rechargeable Type Rechargeable Type Alkaline 1200 Lithium 3000 Ni-Cd Ni-MH The table belw gives typical capacities f ppular 9V batteries. Nn-Rechargeable Type Rechargeable Type Alkaline 500 Lithium 1200 Carbn Zinc 300 Li+ 500 Ni-Cd 120 Ni-MH 150 Nte: The capacity f alkaline batteries changes with current draw, thus yu might see ther surces with a different capacity fr alkaline batteries. The capacity listed abve is fr the typical current draw f a wireless micrphne transmitter. Rechargeable Batteries Many peple want t use rechargeable batteries with wireless systems. Rechargeable AA batteries tend t wrk quite well in wireless micrphnes. Hwever, there are dwnsides t using certain types f rechargeable 9 vlt batteries. The first prblem is the lw capacity f Ni-Cd and Ni-MH 9 vlt batteries. Ni-Cd and Ni-MH 9 vlt batteries will id/250/~/batteries-and-wireless-micrphnes

3 Batteries and wireless micrphnes Shure Technical FAQ last 1/4 f the time that an alkaline battery wuld. S, if yur wireless bdypack lasts 8 hurs with an alkaline battery, it wuld nly last 2 hurs with a Ni-Cd r Ni-MH 9 vlt rechargeable battery. The secnd prblem is the vltage r rechargeable 9 vlt batteries. Unfrtunately, althugh batteries are packaged in the "9 V" size, they may nt have a ptential f 9 vlts. Obviusly, batteries that are the "9 V" size cntain multiple cells. Since alkaline cells are 1.5 V, it takes 6 cells t make ne 9 V battery. Six Ni-Cd cells (1.2 V per cell), thugh, are nly 7.2 V. It is pssible t get a "9 V" sized Ni-Cd with 7 r 8 cells t make 8.4 r 9.6 V respectively. This shws that when lking at batteries, attentin must be paid t their actual vltage, nt just their physical appearance. Wireless transmitters like t see 9 V, s the 7.2 V rechargeables are generally nt acceptable. Als, the 9.6 vlts f an 8 cell battery can be t much fr a wireless transmitters. The latest secndary cell is the rechargeable alkaline. The rechargeable alkaline is a gd chice fr a rechargeable battery. Unfrtunately, n 9 vlt versin is available yet. While its initial capacity is within 10 percent f a standard alkaline, the discharge capacity fades with each cycle, with the majrity f the fade ccurring in early cycles. The best chice fr a rechargeable 9 vlt battery is the Lithium In. This type r rechargeable battery has a similar capacity and discharge when cmpared t the alkaline. AA rechargeable batteries can ften have a larger capacity than their alkaline equivalent, making them an excellent chice. The Lithium nn-rechargeable AA battery will last abut twice as lng as an alkaline fr thse events that need extended battery time. Charging Rechargeable Batteries If treated prperly, secndary cells can be discharged and recharged hundreds f times and last many years. By far, the largest reasn that rechargeable batteries fail t hld their charge is abuse. The largest cause f abuse is imprper charging techniques. Bks can and have been written abut the charging techniques f secndary cells, s the infrmatin prvided here will be an verview. Memry Effect First, the term "memry effect" needs t be addressed. Mst peple incrrectly use the term "memry effect". Memry effect is als synnymus with "vltage depressin." Memry effect is a temprary decrease in capacity. Nte that it is temprary. The best explanatin fr vltage depressin is frm an article abut Ni-Cd batteries by Ken Nishimura, University f Califrnia, Berkley. Let us define memry as the phenmenn where the discharge vltage fr a given lad is lwer than it shuld be. This can give the appearance f a lwered capacity; while in reality, it is mre accurate t term it vltage depressin. Originally, memry effect was seen in spacecraft batteries subjected t a repeated discharge/charge cycle that was a fixed percentage f ttal capacity (due t the earth s shadw). After many cycles, when called upn t prvide the full capacity, the battery failed t d s. Memry can be attributed t changes in the negative r cadmium plate. Recall that charging invlves cnverting Cd(OH) t Cd metal. Ordinarily, and under mderate charging currents, the cadmium that is depsited is micrcrystalline (i.e. very small crystals). Nw, metallurgical thermdynamics states that grain bundaries (bundaries between the crystals) are high energy regins, and given time, the tendency f metals is fr the grains t calesce and frm larger crystals. This is bad fr the battery since it makes the cadmium harder t disslve during high current discharge, and leads t high internal resistance and vltage depressin. id/250/~/batteries~and-wireless-micrphnes

4 Batteries and wireless micrphnes Shure Technical FAQ The trick t aviding memry is aviding frming large crystal cadmium. Very slw charging is bad, as slw grwth aids large crystal grwth. High temperatures are bad, since the nucleatin and grwth f crystals is expnentially driven by temperature. The prblem is that given time, cadmium crystals will grw, and thus, the material needs t be refrmed. Partial cycling f the cells means that the material deep within the plate never gets refrmed. This leads t a grwth f the crystals. By a prper executin f a discharge/charge cycle, the large crystal cadmium is destryed and replaced with a micrcrystalline frm best fr discharge. This des nt mean that the battery needs t be cycled each time it is used. This des mre harm than gd, and unless it is dne n a per cell basis, the cells risk being reversed and that really kills them. Perhaps nce in a while, use the pack until it is 90% discharged, r t a cell vltage f 1.0 V under light lad. Here, abut 95% > f the cells capacity is used, and fr all intents and purpses, is discharged. At this pint, recharge it prperly, and that s it. The accmpanying graph shws an 1.35 experiment that Duracell perfrmed n a Ni-MH cell. In cycle #1, the cell was 1.25 discharged t 1.0 V then recharged. In cycles #2-#18, the cell was discharged H^ 1.15 t 1.15 V and then recharged. In the cn cs 1 05 graph, it is apparent that cycle #18 had 73 > less capacity than cycles #1-#2. In.35 cycles #19-#21, the cell was Cyde #18 discharged cmpletely t 1.0 V befre.85 recharging. The graph shws that after a cuple full discharge/charge cycles, the cell returned t its nrmal capacity Time (Hurs) This clearly shws that vltage depressin is a temprary characteristic. 1.0 Cycle #13 Cycle #20 Cde #21 Cyde #1 _ L Cycle #2 _ L Charging Techniques Basically, there are tw charging techniques: cnstant vltage and cnstant current. Lead acid batteries use cnstant vltage surces. Ni-Cd and Ni-MH use cnstant current surces. Li In use cnstant current fr mst f the charge and then switch t cnstant vltage fr the last perid. Li In is by far the pickiest abut its charging. This means that manufacturers must take mre care when develping the batteries and chargers. Irnically, this makes the Li In batteries harder t abuse because the batteries and chargers are mre intelligent. The main criteria fr effective charging are chsing the apprpriate rate, limiting the temperature, and selecting the apprpriate terminatin technique. If a secndary cell des nt last fr hundreds f discharge-charge cycles and many years, it was mst likely abused in charging. Overcharging is where the mst damage is dne t the battery. There are many ways t determine when t stp charging a cell. Depending n the charge rate and type f battery, different terminatin techniques are used. Many times, multiple techniques are emplyed t guarantee prper charge terminatin. Cmmn methds include (1) Timed Charge: charge the cell using lw current ver a set amunt f time. (2) Vltage Drp: When a Ni-Cd cell is near full charge it rises sharply in vltage and then drps in vltage. The charging circuit detects this drp and stps the charge. (3) Vltage Plateau: Ni-MH des nt have a very prnunced vltage drp, s instead the circuit detects the leveling ff f the vltage upn full charge. (4) Temperature Cutff: as the cell reaches full charge its temperature increases dramatically. The charging circuit terminates when the temperature f the cell reaches abut MOT. (5) Delta Temperature Cutff: measures the difference in temperature frm start t end. The circuit wuld stp charging when the id/250/~/batteries~and-wireless-micrphnes

5 Batteries and wireless micrphnes Shure Technical FAQ temperature is abut 27 F higher than the start temperature. This minimizes ambient temperature prblems that the standard temperature cutff exhibits. (6) Rate f Temperature Increase: measures hw fast the temperature increases. If fast charging, the charger will stp when the rate f increase is abut 1,8 F per minute. This als helps minimize ambient temperature effects. Nte that sme f these terminatin methds are better fr Ni-Cd cells and sme are better fr Ni-MH cells. Fr mre infrmatin, see Duracell s paper n Ni-MH batteries available n their web site. The life f a secndary cell can easily be up t 500 cycles. If a battery des nt hld a charge after 50 r s cycles, it is nt because f memry effect. It is because f a breakdwn f sme cmpnent f the battery, usually caused by abuse. Operatin r strage at extreme temperatures, vercharging, cell venting and abusive use will reduce battery life. When a cell is vercharged excessively, expsed t extreme high temperatures, r therwise abused, gas builds up in the cell. This excess pressure is released thrugh a safety vent that is designed int the cell. Heat and vercharging are a rechargeable battery s wrst enemies. Battery Physical Size Besides different vltages in batteries, batteries can als have different physical dimensins. This is true f bth AA and 9 vlt batteries. The dimensins f batteries can easily vary by ver 1/16". While that may nt sund like much, 1/16" can be a 13% change in the dimensin f the battery. T the mechanical engineer that is designing the battery cmpartment, it makes a wrld f difference. Typically, the rechargeables are larger. There have been instances where battery cmpartments were t small t fit the larger batteries. Battery Discharge Of curse every wireless transmitter will have a different battery life depending n the current draw f the circuit. There is ne difference in circuit design, thugh, that can largely effect battery life: crystal cntrlled vs. frequency synthesized. Crystal cntrlled and frequency synthesized are the tw circuit types that are used t create an RF frequency Crystal cntrlled The circuit design fr a crystal cntrlled system is fairly simple and uses a minimum amunt f cmpnents. This design is fairly efficient in peratin s battery life can be quite lng. The disadvantage t this methd is that the system is limited t a single frequency. Frequency Synthesized A SC > Wireless Battery Life 1 I I I I I I I I I I I I I I I I I I I I I I CM CO Time (Hurs) Frequency synthesis circuits can prduce many frequencies frm a single reference crystal, allwing fr multiple perating frequencies. Since the frequency synthesis circuit is mre cmplex, the cmpnents used draw mre current and therefre shrten the battery life f the transmitter. As electrical cmpnent manufacturers develp mre efficient cmpnents, the battery life f these tw circuit O O in r-~ c cri UC1 UT1 id/250/~/batteries~and-wireless-micrphnes

6 Batteries and wireless micrphnes Shure Technical FAQ designs will grw clser tgether. The graph shws a crystal cntrlled UT transmitter versus a frequency synthesized UC system, bth 9 vlt pwered transmitters. Which Battery t Use Many times the Applicatins Engineering Department at Shure is asked fr battery recmmendatins. The accmpanying graph f a UT transmitter helps summarize the perfrmance f different 9 vlt batteries with wireless micrphnes. The first battery n the graph is an alkaline type. The 9 vlt alkaline battery is the best battery fr the jb. It prvides very gd perfrmance time a> d TO > y Life f Different Battery Types l l i l l l I i l l CD CN CO i n CD r- 00 CD CD Time (Hurs) Alkaline NiMH 8.4V NiCd 7.2V alng with decent vltage levels fr a minimal amunt f mney. Alkaline batteries are the recmmended chice fr nearly all wireless systems and applicatins. There are ther batteries, such as Lithium, that will last lnger, but cst much mre. The secnd battery n the graph is an 8.4 vlt versin f a Ni-MH battery. During it s usable life, the vltage f the 8.4 battery tracks the alkaline quite well. An 8.4 vlt versin f a Ni-Cd wuld die ut abut minutes befre the Ni-MH did. The third battery shwn is a 7.2 vlt versin f the Ni-Cd. Whether Ni-Cd r Ni-MH, 7.2 vlt batteries d nt have adequate perfrmance t prvide the prper vltage fr the wireless transmitter. Batteries that are nly 7.2 vlt are nt recmmended fr wireless systems. Cnclusin By knwing the current draw f the wireless transmitter alng with the capacity f the battery, it is pssible t knw the life f the battery. Wireless transmitters that use AA batteries, rechargeables and alkalines bth perfrm well. Fr lder transmitters that use 9 vlt batteries, it is easy t shw that rechargeable 9 vlt Ni-Cd and Ni-MH batteries have a much shrter life than standard alkaline types. Fr this reasn, 9 vlt Ni-Cd and Ni- MH rechargeables are typically nt well suited fr wireless transmitter applicatins. Therefre, 9 vlt alkaline batteries are the best chice because f their perfrmance fr the cst. If an applicatin des require rechargeable batteries, use the 9 vlt versin f the rechargeable Li+ batteries. Until smene finishes Nikla Tesla s experiments n wireless electricity, batteries will be a majr part f ur lives. Learning hw t prperly chse and use these batteries will make the experience as pleasant as pssible. id/250/~/batteries~and-wireless-micrphnes