Nickel Metal Hydride 3.0 Nickel Metal Hydride (NiMH) 3.1 NiMH Priciples of Operatio The priciples i which NiMH cells operate are based o their ability to absorb, release, ad trasport (move) hydroge betwee the electrodes withi the cell. The followig sectios will discuss the chemical reactios occurrig withi the cell whe charged ad discharged ad the adverse effects of overcharge ad overdischarge coditios. The success of the NiMH battery techology comes from the rare earth, hydroge-absorbig alloys (commoly kow as Misch metals) used i the egative electrode. These metal alloys cotribute to the high eergy desity of the NiMH egative electrode that results i a icrease i the volume available for the positive electrode. This is the primary reaso for the higher capacity ad loger service life of NiMH batteries over competig secodary batteries. 3.2 Chargig Chemical Reactio Whe a NiMH cell is charged, the positive electrode releases hydroge ito the electrolyte. The hydroge i tur is absorbed ad stored i the egative electrode. The reactio begis whe the ickel hydroxide (Ni(OH) 2 ) i the positive electrode ad hydroxide (OH ) from the electrolyte combie. This produces ickel oxyhydroxide (NiOOH) withi the positive electrode, water (H 2 0) i the electrolyte, ad oe free electro (e ). At the egative electrode the metal alloy (M) i the egative electrode, water (H 2 0) from the electrolyte, ad a electro (e ) react to produce metal hydride (MH) i the egative electrode ad hydroxide (OH ) i the electrolyte. See Figure 3.2 Chemical Equatios ad Figure 3.3 Trasport Diagram. Because heat is geerated as a part of the overall chemical reactio durig the charge of a NiMH cell, the chargig reactio described above is exothermic. As a cell is charged, the geeratio of heat may ot accumulate if it is effectively dissipated. Extreme elevated temperatures may be experieced if a cell is excessively overcharged. See Sectio 3.4 Overcharge ad 3.5 Overdischarge. Figure 3.2 Chemical Equatios charge Positive Electrode: Ni(OH) 2 + OH - NiOOH + H 2 O + e - discharge charge Negative Electrode: M + H 2 O + e - MH + OH - discharge charge Overall Reactio: Ni(OH) 2 + M NiOOH + MH discharge
Figure 3.3 Trasport Diagram 3.3 Discharge Chemical Reactio 3.4 Overcharge Whe a NiMH cell is discharged, the chemical reactios are the reverse of what occurs whe charged. Hydroge stored i the metal alloy of the egative electrode is released ito the electrolyte to form water. This water the re leases a hydroge io that is absorbed ito the positive electrode to form ickel hydroxide. See Figure 3.2 Chemical Equatios ad Figure 3.3 Trasport Diagram. For NiMH cells, the process of movig or trasportig hydroge from the egative electrode to the positive electrode absorbs heat ad is therefore edothermic. Heat cotiues to be absorbed util the cell reaches a state of over discharge, where a secodary reactio occurs withi the cell resultig i a rise i temperature. See Sectio 3.5 Over discharge. Nickel Metal Hydride cells are desiged with a oxyge-recombiatio mechaism that slows the buildup of pressure caused by overchargig. The overchargig of a cell occurs after the positive electrode 1) o loger has ay ickel hydroxide to react with the hydroxide from the electrolyte, ad 2) begis to evolve oxyge. The oxyge diffuses through the separator where the egative electrode recombies the oxyge with stored hydroge to form excess water i the electrolyte. If this oxyge-recombiatio occurs at a slower rate tha the rate at which oxyge is evolved from the positive electrode, the result is i a buildup of excess oxyge (gas) resultig i a icrease i pressure iside the cell. To protect agaist the first stages of overcharge, NiMH cells are costructed with the egative electrode havig a capacity (or active material) greater tha the positive electrode. This helps to slow the buildup of pressure by havig more active material available i the egative electrode to effectively recombie the evolved oxyge. See Figure 3.4, Useable Capacity Diagram.
Excessive overchargig of a NiMH cell ca result i permaet loss i capacity ad cycle life. If a cell is overcharged to the poit at which pressure begis to build up, elevated temperatures are experieced ad ca cause the separator to lose electrolyte. The loss of electrolyte withi the separator (or separator dry out ) ihibits the proper trasport of hydroge to ad from the electrodes. Furthermore, if a cell is severely overcharged ad excessive amouts of oxyge (gas) are evolved, the pressure may be released through the safety vet i the positive termial. This removes elemets from withi the cell eeded for proper fuctio. To protect agaist the damagig effects of overchargig, proper charge termiatios must be used. See Sectio 3.8.2 NiMH Charge Termiatio. Figure 3.4 Useable Capacity Diagram 3.5 Over Discharge There are two phases to the over dischargig of a NiMH cell. The first phase ivolves the active material of the positive electrode becomig fully depleted ad the geeratio of hydroge gas begis. Sice the egative electrode has more active material (metal hydride), it has the ability to absorb some of the hydroge gas evolved by the positive electrode. Ay hydroge ot absorbed by the egative electrode begis to build up i the cell geeratig pressure. The secod phase begis whe the etire egative electrode is fully depleted of active material. Oce both electrodes are fully depleted, the egative electrode absorbs oxyge cotributig to the loss of useable capacity. Extreme over discharge of a NiMH cell results i excessive gassig of the electrodes resultig i permaet damage i two forms. First, the egative electrode is reduced i storage capacity whe oxyge permaetly occupies a hydroge storage site, ad secod, excess hydroge is released through the safety vet reducig the amout of hydroge iside the cell. To protect agaist the damagig effects of over dischargig, proper ed of discharge termiatios must be used. See Figure 3.7.4 NiMH Low Voltage Discoect or Voltage Cutoff.
3.6 Rate Capability The maximum rate a cell is able to achieve for a short burst is depedet o the cell costructio, temperature ad maer i which it is assembled ito a pack. This will be clearer after lookig at Sectio 3.7.1 NiMH Capacity. 3.7 NiMH Discharge Characteristics The discharge characteristics of NiMH batteries (both cells ad battery packs) deped o may factors. These factors iclude capacity, voltage, discharge rate, discharge termiatio (or voltage cutoff), matchig (of cells withi a battery pack), iteral resistace, ad temperature. 3.7.1 NiMH Capacity Egieers ad desigers are usually most iterested i how log a battery will supply the curret eeded to ru a piece of equipmet or a device. The legth of time is directly proportioal to the capacity of the battery ad discharge rate. Defied as C, capacity is the electric curret cotet of a battery expressed i amperehours (Ah) or milli-ampere-hours (mah). The capacity of a battery is determied by dischargig the battery at a kow costat curret util a predetermied ed voltage is reached. The amout of time it takes to discharge a battery to the ed voltage multiplied by the rate of curret at which the battery was discharged is the rated capacity of the battery. Therefore, a battery would be rated at 1500 mah if it was discharged at a rate of 150 ma to a ed voltage of 1.0 volt per cell ad it discharged for 10 hours. For clarificatio, the rate of curret (charge or discharge) that is applied to a battery is ofte defied i terms of the rated capacity C of a battery. For example, a battery rated at 1500 mah that is discharged at a rate of C/2 (or 0.5C) will have 750 mah discharged from the battery per hour. Thus, a discharge rate of C/2 of a 1500 mah battery is 750 ma, but this does ot mea the battery will last for 2 hours! Oe of the biggest miscoceptios regardig NiMH cells is that rated capacity is the capacity that will be received by the user. This would oly be true if the user charged ad discharged at the same rates of curret at which the cell was graded. Rated capacity has bee defied by the Iteratioal Electrotechical Commissio (IEC) i documet #61436.1.3.4, as a charge at a rate of 0.1C for a period of 16 hours. This is followed by a discharge of 0.2C to a voltage of 1.00V per cell. However this umber sometimes ca be covoluted by Maximum, Typical, ad Miimum cell ratigs. For example, a lot of 1000 cells might rage from 1000 mah to 850 mah. The maximum capacity would the be 1000 eve though oly a small umber of the cells attai this capacity. The omial capacity would be 900 mah, ad the majority of the cells tested would attai this capacity. All those cells must coform to the miimum of 850 mah. We have foud that to be cosidered with other maufacturers, we must also coform to this cell ratig procedure Results derived whe testig NiMH techology battery packs for capacity ca chage dramatically with variatio i: l Temperature l Charge Rate l Discharge rate l Number of Cells i the Pack / Icrease i Voltage Cutoff per Cell
All of these coditios must be take ito to accout whe a compariso is beig made from pack to pack, ad cell-to-cell. Polarizatios Whe a NiMH techology cell is subjected to a flow of curret a chemical chage occurs withi the cell. The obstacles to curret flow are kow as polarizatios. The polarizatios are: Ohmic- Ohmic polarizatio is the iteral impedace of the cell agaist curret flow. The impedace has a correspodig voltage drop, which ca be see i the voltage profile of ay NiMH cell. Durig charge curret this voltage is added to the overall voltage of the cell. However durig discharge this voltage is subtracted from the overall voltage of the cell. The magitude of the voltage drop will be directly proportioal to overall impedace iteral to the cell ad the rate of curret (charge/discharge) the cell is subjected to. If a cell has higher impedace, the drop will be greater. As the curret applied to the cell is icreased the voltage drop will icrease. This is critical whe a predetermied termiatio voltage is set. If a cell is discharged at a high eough curret it ca istatly force the cell voltage below 1.00V, eve though there is almost 100% capacity remaiig. Cocetratio- Cocetratio polarizatio is directly proportioal to the surface area of the aode ad cathode of the cell. The greater the surface area of these active plates, the greater the reductio i this polarizatio. This is determied durig the egieerig ad maufacturig of the cell itself, ad little ca be doe after that. Activatio- Activatio polarizatio is the amout of eergy expeded to cause the chemical reactio. Nothig is 100% efficiet, so ay chemical chage expeds eergy. At a molecular level temperature has a great effect o the amout of eergy it takes to make a reactio traspire. Geerally all ratig is performed at 20-25 C, as the temperature icreases, or decreases the amout of eergy o discharge will chage. Kowig what affects the NiMH chemistry, we ca fashio testig to isolate ay variable that may cause icosistet results. For example, a cell is charged at a 0.5 C with a dv termiatio. Whe capacity testig is performed at a 0.5C discharge expect to see betwee 5-7% reductio i cell capacity over the IEC ratig. This is explaied first by the Ohmic polarizatio o discharge, ad secodly due to the variatio i charged eergy usig a dv charge regime. I order for a dv to be see o a NiMH cell it must be subjected to a small margi of overcharge. This overcharge coditio forces the voltage depressio. It occurs whe all of the active material i the cell is chemically reacted, ad oxyge evolves ad the diffuses ito the egative electrode. The egative electrode is desiged to be larger tha the positive so it accepts a portio of this oxyge diffusio, however pressure is still geerated withi the cell, which geerates heat. The heat the causes the cell voltage to drop. A direct compariso of the eergy iput durig charge ad output durig discharge caot be made because a portio of the iput eergy was wasted o oxyge evolutio ad was ever recombied. I order to derive a % of rated capacity, we must calculate the eergy received durig discharge of our test, with that cell s IEC rated capacity 3.7.2 NiMH Voltage The discharge voltage profile of a NiMH battery is cosidered flat (see Figure 3.7.2 C/10 Discharge Profile @ 25 C) ad varies with the rate of discharge ad temperature. As a fully charged battery is discharged the voltage begis at about 1.5 volts followed by a sharp drop to aroud 1.3 volts. The voltage remais betwee 1.3 to 1.2 volts for about 75% of the profile util a secod sudde drop i voltage occurs as the useful capacity of the battery begis to deplete. At this poit is where the discharge curret (or load) is termiated at a safe voltage level (see Sectio 5.4 Low Voltage Discoect or Voltage Cutoff). With elevated discharge rates, the etire discharge profile is lowered by losses i ohmic polarizatios (iteral resistace). At high temperatures, the discharge profile is raised by a icrease i potetial (voltage) betwee the electrodes. At temperatures below 10 C (50 F), cocetratio polarizatio sigificatly lowers the voltage ad the useable capacity. This is caused by a icrease i eergy required to trasport molecules withi the battery. See Sectio 2 Priciples of Operatio ad Costructio. The idustry stadard for the rated voltage of a NiMH cell is 1.2 volts. This value is the omial voltage of a cell that is discharged at a rate of C/10 at a temperature of 25 C (77 F) to a ed voltage of 1.0 volt. This idustry stadard is used primarily to call out the rated voltage of battery packs. For example, a battery pack made of three cells i series would be rated as a 3.6-volt battery pack. Figure 3.7.2, C/10 Discharge Profile @ 25 C, shows that the omial voltage of a Quest NiMH cell is just above 1.2 volts.
For techical applicatios ad calculatios, the omial voltage of a battery pack provides a useful approximatio of the average voltage throughout discharge. The omial voltage ca be simply calculated after the battery has bee discharged. To calculate the omial voltage, divide the batteries eergy [watt-hours (Wh)], by the capacity, [ampere-hours (Ah)]. This calculatio proves beeficial whe a battery is discharged at high temperatures, sice omial voltage will icrease uder these coditios. Figure 3.7.2 C/10 Discharge Profile @ 25 C 3.7.3 NiMH Discharge Rate The delivered capacity ad omial voltage of a battery are depedet o the rate of curret at which a battery is discharged. For NiMH batteries, there is o sigificat affect o capacity ad voltage for discharge rates below 1C. A reductio i the omial voltage occurs for discharge rates betwee 1C ad 3C for all the NiMH cell sizes with the exceptio of the high rate series of cells. 3.7.4 NiMH Low Voltage Discoect or Voltage Cutoff To prevet potetial irreparable harm to a battery due to polarity reversal of oe or more cells durig discharge, the load (discharge curret) must be termiated prior to the battery beig completely discharged. Damage ca be avoided by termiatig the discharge at a poit where essetially all capacity has bee obtaied from the battery, but a safe voltage level remais. Removig the load i this maer is referred to as Low Voltage Discoect or Voltage Cutoff. Capacity of a battery is slightly depedet o the Voltage Cutoff used at the ed of discharge. Cotiuig the discharge to lower ed voltages ca slightly icrease the delivered capacity, yet if the ed voltage is set below the recommeded Voltage Cutoff the cycle life of the battery will be decreased. The followig table gives the Voltage Cutoff recommedatios for discharge rates of less tha 1C: Figure 3.7.4 Voltage Cutoff Schedule Number of Cells i Series Low Voltage Discoect/Voltage Cutoff 1 to 6 1 volt per cell 7 to 12 [(MPV-150mV)(-1)]-200mv Where MPV is the sigle cell midpoit voltage at the give discharge rate (typically 1.3) ad is the umber of cells i the pack. For discharge rates greater tha 1C the MPV will decrease.
3.7.5 NiMH Matchig Matchig refers to the groupig of idividual NiMH cells with similar capacities to be used withi a battery pack. Typically, the matchig of the cells i a battery pack is withi 2%. Matchig elimiates the potetial of reversig the polarity of oe or more cells i a battery pack due to the capacity rage of the combied cells beig too great. Matchig becomes more critical as the umber of cells i the battery pack icreases. This is due to the potetial of oe cell havig a capacity sigificatly lower tha the average capacity of the other remaiig cells. As a result, the lowest capacity cell has the potetial to reverse polarity while the other cells remai at safe voltage levels before reachig the voltage cutoff. If a battery pack has oe or more cells reversed before the Voltage Cutoff is reached, the performace ad cycle life will be reduced. 3.7.6 NiMH Iteral Resistace The iteral resistace of NiMH cells varies depedig o cell size, costructio, ad chemistry. Differet materials are used i various NiMH cells to achieve desired performace characteristics. The selectio of these materials also affects the iteral resistace of the cell. Sice NiMH cells of various sizes, costructio, ad chemistry are differet, there is o oe value of iteral resistace that ca be defied as a stadard. The iteral resistace for each cell is measured i m at 1000Hz. 3.7.7 NiMH Temperature To obtai the optimum capacity ad cycle life of a NiMH battery, the recommeded rage of temperature whe dischargig a stadard battery is 0 C (32 F) to 40 C (104 F). Due to the edothermic discharge characteristics of NiMH cells, the discharge performace is icreased moderately at higher temperatures, yet the cycle life will be lowered. At lower temperatures, performace decreases more sigificatly due to the cells polarizatio. See Sectio 3.7.7. This is caused by a decrease i trasport capabilities (the ability to move ios withi the electrodes). For most NiMH batteries, the followig chart shows the typical affect of temperature o capacity at a C/5 discharge rate. Figure 3.7.7 Discharge Capacity vs. Temperature 3.8 NiMH Charge Characteristics Overview The chargig, or rechargig, of NiMH batteries (both cells ad battery packs) is the process of replacig the eergy that has bee removed or discharged from the battery. The performace of the battery, as well as the cycle life, depeds o effective chargig. The three mai criteria for effective chargig are: Choose the appropriate charge rate Select the appropriate charge termiatio techique Cotrol the temperature 3.8.1 NIMH Charge Rates As the capacity of NiMH cells has icreased, so has the demad for faster chargig. This leads to higher charge rates, which requires care to esure a complete charge while miimizig the potetial damage of overchargig. Slow chargig is still a reliable method, but ot all batteries ca be slow charged without some type of termiatio. Some NiMH cell chemistries have higher capacities ad are better suited to the more popular fast chargig methods.
3.8.2 NIMH Charge Termiatio Properly cotrollig the chargig of a NiMH battery is critical to achievig optimum performace. Charge cotrol icorporates proper charge termiatio to prevet overchargig the battery. The overchargig of a battery refers to the state at which the battery ca o loger accept (store) the eergy eterig the battery. As a result pressure ad temperature builds up withi the cell. If a cell is allowed to remai i the overcharge state, especially at high charge rates, the pressure geerated withi the cell ca be released through the safety vet located withi the positive termial. This may cause damage to the battery reducig cycle life ad capacity. To prevet damage occurrig to the battery, charge termiatio is oe of the most critical elemets to be applied to ay method of charge cotrol. Charge cotrol may utilize oe or more of the followig charge termiatio techiques. The three primary techiques of charge termiatio are time, voltage, ad temperature. 3.8.2.1 Time Time-based charge cotrol techiques termiate chargig of the battery after a predetermied legth of time. This techique should be used whe slow chargig to avoid excessive overcharge, ad used as a backup secodary termiatio for all fast charge methods. 3.8.2.2 Voltage Charge cotrol techiques that are voltage-based are attractive because of the predictable charge voltage profile of a NiMH battery (see Sectio 3.8.3 Charge Termiatio Nomeclature). The charge voltage profile of a NiMH battery is cosistet regardless of the batteries state of charge. However, the voltage-based charge termiatio techiques geerally occur after a battery has already reached the overcharge state. I additio, the voltage-based techiques may ot be applicable at rates below C/4 ad are proe to false termiatio due to RF oise. It is also ecessary to iclude temperature-sesig devices to termiate the charge if the temperature becomes too high. Such devices iclude thermostats ad PTC resettable fuses. Peak Voltage Detect (PVD) The recommeded techique for voltage-based charge termiatio is peak voltage detect or PVD. This techique ivolves sesig the drop i voltage after the battery has reached its peak voltage ad becomes overcharged (see Sectio 3.8.3 Charge Termiatio Nomeclature). This techique is recommeded because it reduces the risks of overchargig from that of other voltage charge termiatio techiques. To prevet substatial damage to a battery, a maximum drop i voltage of 3 mv per cell before termiatio is recommeded to limit the amout of overcharge o the battery. Additioally, the samplig rate of the charger IC is more frequet to icrease sesitivity. 3.8.2.3 Temperature Negative Delta V (-ÎV) Negative delta V (-ÎV), like PVD, follows the same cocept of sesig the drop i battery voltage after the battery has reached its peak voltage. The differece is the chage or drop i voltage is icreased to 3 to 5 mv per cell before the charge is termiated. This techique allows the battery to be exposed to loger periods of overcharge ad is ot ormally recommeded. See Sectio 3.8.3 Charge Termiatio Nomeclature The exothermic ature of NiMH batteries uder charge refers to the geeratio of heat as the battery is charged especially just before ad durig overcharge. The temperature-based charge termiatio seses this temperature rise ad termiates the charge whe the battery has reached a temperature that idicates whe full charge is beig approached. This type of charge termiatio is recommeded because of its reliability i sesig overcharge, yet it requires care i the selectio of set poits i the charge circuitry to avoid premature charge termiatio or failure to detect the overcharge whe the battery is exposed to extreme temperature eviromets.
Chage i Temperature (ÎT) Chage i temperature or ÎT is the techique that measures the differece of the rise i battery temperature above the startig (ambiet) temperature durig charge. The charge is termiated whe the rate of chage i temperature reaches a predetermied value. See Sectio 3.8.3 Charge Termiatio Nomeclature Chage i Temperature/Chage i time (dt/dt) The recommeded techique for temperature-based charge termiatio for all fast-chargig methods is dt/dt (see Sectio 6.3 Charge Termiatio Nomeclature). This techique moitors the chage i temperature T verses the chage i time t, ad is cosidered most accurate because it seses the start of overcharge earlier tha other techiques. Baselie dt/dt temperature termiatio is 1 C per miute, but varies accordig to pack desig. Whe usig a dt/dt termiatio, a top-off charge is suggested i order to fully charge the battery (see Sectio 3.8.6.5 Top-Off Charge). Temperature Cut Off (TCO) Temperature cut off or TCO is a secodary termiatio required for all fast-chargig methods usig dt/dt ad/or PVD. This techique is based o the absolute temperature of the battery ad is recommeded oly as a fail-safe strategy to avoid destructive heatig i case of failure of ay or all other charge termiatio techique(s). See Sectio 3.8.3 Charge Termiatio Nomeclature 3.8.3 NIMH Charge Termiatio Nomeclature Figure 3.8.3 NiMH Charge Termiatio Nomeclature 3.8.4 NIMH Temperature ad Charge Efficiecy The recommeded chargig temperature is betwee 10 C (50 F) ad 40 C (104 F). If a NiMH battery is exposed to high temperatures (above 40 C, 104 F) due to overchargig or exteral heat sources, the charge efficiecy (icrease i stored cell capacity per uit of charge iput) will be decreased. I order to avoid decreased charge efficiecy, batteries should have charge cotrol methods applied to limit the amout of overcharge heat that is geerated. I additio, it is critical to ot place batteries i close proximity to other sources of heat or i compartmets with limited coolig or vetilatio. At temperatures below 10 C (50 F) charge efficiecy will also decrease resultig i a icrease i the amout of time eed for chargig. Low temperatures ihibit trasport capabilities (the ability to move ios withi the electrodes) causig the low charge efficiecy (see Sectio 3.7.1, NiMH Capacity ad Costructio; Rate Capability). Chargig below 0 C (32 F) is ot advisable
3.8.5 NIMH Charge Methods Not all charge methods are recommeded for all NiMH cell chemistries, seeig they are ot desiged the same. Differet materials are used i various NiMH cells to achieve certai desired performace characteristics. The selectio of these materials also affects the chargig characteristics of the batteries. Therefore, ay method that could cause problems with some batteries has bee oted for each charge method. See Figure 3.8.5 Charge Method Specificatios for recommeded charge currets ad charge termiatios. Figure 3.8.5 Charge Method Specificatios Charge Charge Charge Method Curret Termiatio Commets Slow 0.02-0.1C 1. Noe 1 or Timer Timer rated at 160%C. Stadard 0.1C 1. Timer Timer set for 16 hours. Time 0.1-0.2C 1. Timer, ad Timer rated at 160%C @ 0.1C to 120%C @ 0.2C. 2. TCO = 55 C Rapid 2 0.25-0.5C 1. PVD, or dt/dt, or PVD = -ÎV of 3-5 mv/cell ÎT, ad dt/dt = ~1 C/1 mi rise. 2. Timer, ad Timer rated at 140%C @ 0.2C to 120%C @ 0.5C. 3. TCO = 55 C Fast 2 0.5-1.0C 1. PVD, or dt/dt, or PVD = -ÎV of 3-5 mv/cell ÎT, ad dt/dt = ~1 C/1 mi rise. 2. Timer, ad Timer rated at 125%C. 3. TCO = 55 C Maiteace 0.002-1. Noe 5-10%C per day at C/128 to C/512 pulse 0.008C recommeded. 1 Not all batteries ca be charged without a termiatio. 2 See Rapid/Fast Chargig Procedure (Sectio 3.8.6) 3.8.5.1 Slow Charge Whe charge time is ot a issue ad maximum rechargeable capacity is desired, the slow charge method is ofte used. This method uses a charge that is less tha 0.1C ad for more tha 16 hours (see Figure 3.8.5 Charge Method Specificatios). Yet, with the recet developmets of some NiMH cell chemistries to be better suited to faster chargig methods, slow chargig is ot recommeded for all NiMH batteries. 3.8.5.2 Stadard Charge This method ca be used for most of the NiMH cell chemistries. The stadard charge is a simple system usig a charge rate of 0.1C for 16 hours (see Figure 3.8.5 NiMH Charge Methods). Sice the charge rate is low ad the charge is termiated after 16 hours, there is less risk of overcharge ad icreased temperatures. The dowside to this method is the iability to detect how much charge a battery has at the time chargig begis. Thus, a battery that is at a 60% depth of charge (DOD), or 40% state of charge (SOC), which is charged usig this method will see the same amout of charge as a fully discharged battery. This leads to the overchargig of the partially discharged battery before the time termiatio occurs. 3.8.5.3 Time Charge For the faster timed charge method, batteries ca typically be charged i 6 to 16 hours. This method of chargig NiMH batteries requires the most attetio before selectio. Sice this method uses higher rates of curret (see Figure 3.8.5 Charge Method Specificatios), two methods of termiatio are eeded: timed ad TCO. The later of the two termiatios would require the battery to iclude a thermistor to detect the temperature durig the charge cycle. If oly a timed charged termiatio was used, the battery may be pushed ito overcharge, especially if a partially discharged battery was charged usig this method. For some NiMH cell chemistries this would sigificatly deteriorate battery performace.
3.8.5.4 Rapid Charge The Rapid Charge method is good for applicatios eedig a faster charge time, but the battery compartmet does ot allow for good heat dissipatio. Rapid charge methods typically charge i 2.5 to 6 hours usig charge rates of 0.25C to 0.5C (see Figure 3.8.5 Charge Method Specificatios). This method of chargig uses PVD, - ÎV, dt/dt or ÎT with time backup. For further chargig details see Sectio 3.8.6 Rapid/Fast Chargig Procedure. This system usually uses a temperature backup to esure agaist overchargig. The advatage of this charge method is the ability to safely charge batteries that are at ay state of charge. I other words, a partially discharged battery ca be charged without the risk of beig overcharged. The disadvatage to this type of system is the added complexity ad expese of the charger. 3.8.5.5 Fast Charge Whe time is a limited resource ad there is good heat dissipatio, Fast Charge methods are the best approach. Fast Charge methods will charge batteries i 2.5 hours or less. Like the Rapid Charge method, this method has icreased charge rates ad requires three separate charge termiatios (see Figure 3.8.5 Charge Method Specificatios). Some of the higher capacity NiMH batteries will ot hadle a costat 1.0C charge rate. Presetly, a good rule to follow is o costat chargig above 1.0C or 3.0Amps. For further chargig details see Sectio 3.8.6 Rapid/Fast Chargig Procedure. As with the Rapid Charge, the advatages of the Fast Charge method is the ability to safely charge batteries that are at ay state of charge i a short period of time. The disadvatages are, agai, the added complexity of the charger ad expese. 3.8.5.6 Maiteace Charge Ulike the previous six methods the Maiteace Charge method is ot cosidered a meas of chargig a discharged battery to full capacity. Rather, this method is used to couteract the occurrece of battery selfdischarge whe the battery is ot i use. See Sectio 3.9 Battery Storage. 3.8.6 NIMH Rapid/Fast Chargig Procedure The followig procedure outlies the six steps for fast or rapid chargig a NiMH battery. These steps will provide isight ito what the charger chip maufacturers have tried to icorporate ito their chips. 3.8.6.1 Iitializatio Charge Before fast or rapid chargig a battery, a trickle charge rate is recommeded. Startig with a pulsed C/10- C/50 trickle charge is good for two reasos. First, to brig up the temperature if the batteries are cold, ad secod, to verify there are o problems with the batteries or chargig circuitry. 3.8.6.2 Temperature Measuremet Before fast or rapid chargig ca begi, the temperature must be betwee 10 C ad 40 C. This is doe as part of the trickle charge step. If a battery has bee exposed to lower temperatures, the battery temperature must be brought up to above 10 C before fast chargig ca begi. Furthermore, take ote that the dt/dt parameters will be reached at the begiig of a fast charge o a cold battery, thus resultig i the premature termiatio. May chargers icorporate a "low temperature ihibit" to ullify this evet. 3.8.6.3 Pack Voltage Measuremet (PVM) The measuremet of the battery pack voltage is also part of the trickle charge step. A pack voltage measuremet (PVM) ca be used to verify that the battery is at the proper voltage level ad to verify there is curret available for chargig. Time (few secods to 10 miutes) ad voltage (1.1 volts x # of cells) are depedet o the type ad umber of cells used. If the voltage of the battery is ot reached i the set time (usually about 20 miutes) the charge is termiated. For PVM a pulse charge at a rate of C/10 to C/50 is recommeded, but a costat charge rate of C/10 to C/50 ca be used.
3.8.6.4 Rapid/Fast Charge Rapid charge or fast charge methods require three modes of termiatio: 1. PVD, or ÎV, or dt/dt, or ÎT 2. Timer 3. Temperature Cut Off (TCO). See Table 3.8.5 Charge Method Specificatio for charge rate iformatio. 3.8.6.5 Top-Off Charge Top-off chargig is oly used if the fast charge or rapid charge does ot fully charge the batteries. This occurs with some dt/dt ad T termiatios. Before usig a dt/dt or T fast chargig, the chargig ad termiatio parameters eed to be tested iside the device. Sice a costat curret top-off charge has a tedecy to remove eergy, the top-off charge is best to be doe as a pulse charge. The top off charge is termiated by time ad is C/10 to C/40 of the rapid charge rate. 3.8.6.6 Maiteace A maiteace charge will retai a full charge o the battery util it is removed from the charger. Durig the first 24 hours after a battery has bee charged, it will lose about 5% of its eergy caused by the battery s selfdischarge. A stadard maiteace charge at a C/128 rate is desiged to couter this self-discharge. Some NiMH chemistries are able to hadle maiteace charge rates up to a C/64 rate. Cotiuous chargig at low rates is ot very efficiet, therefore, pulse chargig is recommeded. 3.9 NiMH Battery Storage Overview Over time capacity ad voltage of NiMH rechargeable batteries will decrease whe stored or left uused. This is caused by a chemical reactio that takes place withi the cells, commoly referred as self-discharge. The affects of self-discharge will be miimized if uused batteries are properly stored. Proper storage of NiMH batteries requires both temperature cotrol ad ivetory maagemet. 3.9.1 NiMH Storage Temperature ad Battery Cyclig Temperature is the major factor affectig the rate of self-discharge of a uused battery. As storage temperatures icrease, the self-discharge rate icreases thus causig the maximum storage time of a battery to decrease. It is best to store batteries i a temperature-cotrolled eviromet so that the maximum storage time ca be accurately determied. Figure 3.9.1 Storage Temperature vs. Storage Time shows the rage of storage temperatures that NiMH batteries ca be stored ad the maximum legth of time a battery ca be left uused before havig to be cycled. Figure 3.9.1 Storage Temperature vs. Storage Time Storage Temperature Maximum Storage Time (Frequecy of Cyclig) 40 C to 50 C (104 F to 122 F) Less tha 30 days 30 C to 40 C (86 F to 104 F) 30 to 90 days -20 C to 30 C (-4 F to 86 F) 180 to 360 days The eergy storage capability of a battery will be decreased if the battery is allowed to completely selfdischarge. The affects of self-discharge ca be corrected if the batteries are subjected to cycles of chargig ad dischargig. O the iitial charge/discharge cycle, the battery will achieve approximately 95% of rated capacity. Full capacity will be achieved o the secod ad third cycles.
3.9.2 NiMH State of Charge The state of charge (SOC) of a uused NiMH battery has o ifluece o the required storage temperature or the maximum storage time (see Figure 3.9.1 Storage Temperature vs. Storage Time). A fully discharge battery will last i storage as log as a fully charged battery. Therefore, NiMH batteries ca be stored at ay state of charge. 3.9.3 NiMH Storage Guidelies The key to properly storig NiMH rechargeable batteries is establishig good ivetory maagemet practices. To prolog the cycle life ad maitai battery performace adhere to the followig five guidelies: 1. Practice FIFO (First I First Out) ivetory rotatio. 2. Never store batteries uder load. 3. Store batteries i a temperature-cotrolled eviromet (see Figure 3.9.1 Storage Temperature vs. Storage Time). 4. Cycle batteries (see Figure 3.9.1 Storage Temperature vs. Storage Time). 5. Store batteries at 65% (±20%) humidity.