鋰離子電池為便攜式醫療設備供能的優勢
消費(fei)電子(zi)產(chan)品以(yi)(yi)及許多其他行(xing)業都越來越講(jiang)究產(chan)品的移動性(xing),醫(yi)療(liao)器(qi)(qi)械(xie)制(zhi)造(zao)商(shang)也不(bu)(bu)例外(wai),這樣的趨勢提高(gao)了(le)現場救助設(she)備、監(jian)控設(she)備和固定醫(yi)療(liao)設(she)備的性(xing)能,進(jin)而推動了(le)醫(yi)療(liao)保健行(xing)業的發展。不(bu)(bu)過除(chu)了(le)便攜性(xing)以(yi)(yi)外(wai),醫(yi)療(liao)器(qi)(qi)械(xie)制(zhi)造(zao)商(shang)當然還(huan)希望能夠制(zhi)造(zao)出可靠性(xing)高(gao)的器(qi)(qi)械(xie),因為人們的生命(ming)往往命(ming)懸(xuan)一線。手(shou)機壞了(le)固然是惱人的事(shi),但如(ru)果便攜式心臟監(jian)控儀(yi)或者輸液(ye)泵由(you)于電池耗(hao)盡(jin)而停止(zhi)運作(zuo),終端用戶――及病人――面臨(lin)的問題(ti)則嚴重得多。 一種能量密度更(geng)高(gao)的技術可以(yi)(yi)釋放比其它(ta)同類產(chan)品更(geng)多的能量,且體積更(geng)小。
幾年前,醫療專業人員還無法將救助生命的設備帶到現場;因為那時便攜式儀器的技術尚未成熟。但如今,大量的監控儀器、超聲設備和輸液泵可在遠離醫院的場所使用――甚至戰場。便攜式設備的移動越來越方便。正是由于諸如鋰離子電池等技(ji)術的(de)應(ying)用,重(zhong)(zhong)達50磅的(de)笨(ben)重(zhong)(zhong)除顫器(qi)才可(ke)以(yi)被更輕便、更緊(jin)湊的(de)用戶友好型裝置(zhi)取代,也(ye)不會造成醫(yi)護人員肌肉的(de)拉傷。
病(bing)(bing)人的(de)移(yi)(yi)動性也變得越來越重要。如今(jin)的(de)病(bing)(bing)人可能會從放(fang)射科轉(zhuan)移(yi)(yi)到重癥監護病(bing)(bing)房,從救(jiu)(jiu)護車(che)轉(zhuan)移(yi)(yi)到急救(jiu)(jiu)室(shi),或用(yong)救(jiu)(jiu)護車(che)從一家(jia)醫(yi)院轉(zhuan)移(yi)(yi)到另一家(jia)醫(yi)院。同樣地,便(bian)攜(xie)式家(jia)用(yong)儀器(qi)(qi)和移(yi)(yi)動式監控(kong)設備的(de)普及(ji),使病(bing)(bing)人可以待在喜歡的(de)地方,而(er)并不一定要留在醫(yi)療機構中。便(bian)攜(xie)式醫(yi)療器(qi)(qi)械必須真正實現完全意義上的(de)便(bian)攜(xie),為病(bing)(bing)人提供最佳服務。
對更(geng)小(xiao)型、更(geng)輕便的(de)醫(yi)(yi)療器械的(de)需求也因此(ci)(ci)顯著(zhu)增加,這大大激發了人(ren)們對更(geng)高(gao)能量密度、更(geng)小(xiao)巧的(de)電(dian)池(chi)組(zu)的(de)興趣。筆記本電(dian)腦和手機(ji)使用(yong)的(de)鋰(li)離子(zi)電(dian)池(chi)技(ji)術已經有了許多突破,醫(yi)(yi)療設備設計工程師可以對此(ci)(ci)加以創(chuang)新利(li)用(yong)。
與其它傳統(tong)技術相比,鋰離子(zi)電池在便攜式醫療設備的應用中具有很多(duo)優勢。這包括能(neng)量密(mi)度更高、重量更輕、循環壽命更長、電池容量保持特性更好(hao),以及適用溫(wen)度范(fan)圍更廣。
由于(yu)化(hua)學性質(zhi)獨特,鋰(li)離子技術對設計方(fang)面的限制(zhi)與先前的電(dian)(dian)池(chi)技術如鎳氫化(hua)合物(NiMH)、鎳鎘(NiCd)和密封鉛酸(SLA)不同。同時,醫療(liao)設備在某些方(fang)面比消費電(dian)(dian)子產品有(you)更嚴(yan)格的操作要(yao)求;由于(yu)可(ke)靠(kao)性非(fei)常重要(yao),因此需要(yao)有(you)功能強(qiang)大的電(dian)(dian)池(chi)組,帶有(you)精確(que)電(dian)(dian)量監(jian)測且可(ke)靠(kao)的電(dian)(dian)池(chi)。
本文結(jie)合醫療器械的要求及鋰(li)離子技(ji)術的特性,概述(shu)了便(bian)攜式電源系統設(she)計的注意事(shi)項。并比較(jiao)了鋰(li)離子電池和其它化學電池的特性和容量。
能量密度(du)和電(dian)壓
鋰離子電池技術的主要優勢在于其能量密度的顯著提高。相同體積和重量時,鋰離子電池可儲存和釋放的能量(liang)比其它充電(dian)電(dian)池更高(gao)。能量(liang)密(mi)度以(yi)體(ti)積和質(zhi)量(liang)兩種方(fang)式測量(liang)。鋰(li)離子(zi)技術現可以(yi)提(ti)供近500Wh/L的體(ti)積能量(liang)密(mi)度和200Wh/kg的質(zhi)量(liang)能量(liang)密(mi)度(見圖1)。
圖1.鋰離子(zi)的能量密度遠(yuan)高于鎳化合物。
與其(qi)它(ta)技術(shu)相比(bi),鋰(li)離子可以釋放更(geng)多(duo)的(de)(de)能量,而且(qie)體(ti)(ti)積更(geng)小,質量更(geng)輕。鋰(li)離子電(dian)(dian)(dian)(dian)池(chi)的(de)(de)工作電(dian)(dian)(dian)(dian)壓比(bi)其(qi)它(ta)充電(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)池(chi)更(geng)高,通常約為3.7V,而NiCd或NiMH電(dian)(dian)(dian)(dian)池(chi)為1.2V。這意(yi)味(wei)著需要(yao)使用(yong)多(duo)節其(qi)它(ta)電(dian)(dian)(dian)(dian)池(chi)時,僅需一(yi)節鋰(li)離子電(dian)(dian)(dian)(dian)池(chi)即(ji)可滿(man)足使用(yong)要(yao)求。便攜(xie)式儀器(qi)設計中使用(yong)的(de)(de)電(dian)(dian)(dian)(dian)池(chi)能量密度(du)越(yue)高,其(qi)產品(pin)的(de)(de)體(ti)(ti)積越(yue)小,便攜(xie)性也越(yue)好。
電(dian)池(chi)組(zu)體(ti)積的(de)縮小(xiao)表示(shi)工程師(shi)可(ke)以利用多余空間,在同一產品中(zhong)增(zeng)添更多新功(gong)能(見圖2)。
圖2.鋰離子電池有圓柱形(xing)和(he)棱(leng)柱形(xing)兩(liang)種形(xing)狀,可(ke)制成不(bu)同尺(chi)寸(cun)和(he)容量的電池。
自放電(dian)
充電(dian)電(dian)池的容量會不斷(duan)損失。該現象(xiang)即稱為自放電(dian)。但若儲存得當,其(qi)損失的大部分容量仍可恢復。
所有電(dian)(dian)池(chi)(chi)均應在室溫(25°C或更低(di))下儲(chu)存(cun),以(yi)(yi)保持最大的(de)電(dian)(dian)池(chi)(chi)容量(liang)。終端用戶須將(jiang)SLA電(dian)(dian)池(chi)(chi)放(fang)在低(di)溫下儲(chu)存(cun),并盡可能每次充(chong)電(dian)(dian)量(liang)接近其(qi)容量(liang)的(de)100%,以(yi)(yi)保持最佳性能。密(mi)封鉛電(dian)(dian)池(chi)(chi)在25°C下放(fang)置6個月(yue)后(hou)自放(fang)電(dian)(dian)容量(liang)約(yue)為(wei)(wei)(wei)20%;但40°C放(fang)置6個月(yue)后(hou)該值則增加(jia)到約(yue)30%。NiMH電(dian)(dian)池(chi)(chi)也(ye)應遵循類(lei)似的(de)建議,避免長期儲(chu)存(cun)使反應物(wu)失活。NiCd和NiMH電(dian)(dian)池(chi)(chi)在25°C下放(fang)置1個月(yue),其(qi)自放(fang)電(dian)(dian)率約(yue)為(wei)(wei)(wei)20%,隨(sui)后(hou)自放(fang)電(dian)(dian)率的(de)增速顯(xian)著減慢。相反,當鋰離子電(dian)(dian)池(chi)(chi)儲(chu)存(cun)時的(de)充(chong)電(dian)(dian)電(dian)(dian)量(liang)為(wei)(wei)(wei)30-50%時,可獲得最佳循環壽(shou)命(ming)。鋰離子電(dian)(dian)池(chi)(chi)25°C儲(chu)存(cun)6個月(yue)后(hou)的(de)自放(fang)電(dian)(dian)容量(liang)僅為(wei)(wei)(wei)10%。
倍(bei)率特(te)性
選擇材料時,應考(kao)慮到(dao)其終端裝(zhuang)置的涌流和最(zui)大放(fang)電(dian)率(lv)。電(dian)池或電(dian)池組以高倍率(lv)放(fang)電(dian)會造(zao)成電(dian)壓(ya)下降。如果設計(ji)時沒有(you)考(kao)慮到(dao)這方面,終端裝(zhuang)置可能會由(you)于電(dian)壓(ya)不足(zu)而關閉。
高倍率的(de)NiCd電(dian)(dian)(dian)(dian)池(chi)(chi)的(de)連續(xu)放電(dian)(dian)(dian)(dian)率可達2C(電(dian)(dian)(dian)(dian)池(chi)(chi)額定容量的(de)兩(liang)倍)甚至更高,具(ju)體取決于電(dian)(dian)(dian)(dian)池(chi)(chi)原(yuan)料及內部(bu)阻抗。許多(duo)SLA電(dian)(dian)(dian)(dian)池(chi)(chi)的(de)連續(xu)放電(dian)(dian)(dian)(dian)率可達3C甚至更高。大部(bu)分的(de)鋰離(li)子電(dian)(dian)(dian)(dian)池(chi)(chi)的(de)連續(xu)放電(dian)(dian)(dian)(dian)率僅為1C,但采用該技(ji)術的(de)新電(dian)(dian)(dian)(dian)池(chi)(chi),其連續(xu)放電(dian)(dian)(dian)(dian)率極高,達到80A,可持續(xu)30秒,在(zai)與(yu)NiCd和SLA電(dian)(dian)(dian)(dian)池(chi)(chi)的(de)競(jing)爭中具(ju)有很大的(de)優勢。
循環壽命
電(dian)池(chi)的循(xun)環壽(shou)命是電(dian)池(chi)容(rong)量(liang)下(xia)降到(dao)其(qi)(qi)原(yuan)始容(rong)量(liang)的某(mou)一(yi)規定(ding)(ding)百分比之前,電(dian)池(chi)所經歷的充(chong)放電(dian)循(xun)環次(ci)數。鉛酸電(dian)池(chi)的循(xun)環壽(shou)命約為(wei)250至(zhi)500次(ci),具體取決(jue)于制造商的產(chan)品質量(liang)和放電(dian)深(shen)度(du)(放電(dian)容(rong)量(liang)最多至(zhi)額(e)定(ding)(ding)容(rong)量(liang)的60%)。NiCd、NiMH和鋰離子電(dian)池(chi)通(tong)常可(ke)耐(nai)受(shou)500-700次(ci)充(chong)放電(dian)循(xun)環,其(qi)(qi)容(rong)量(liang)僅下(xia)降為(wei)額(e)定(ding)(ding)容(rong)量(liang)的80%。不管使(shi)(shi)用(yong)何種化學物(wu)質,電(dian)池(chi)放電(dian)深(shen)度(du)越(yue)深(shen),用(yong)戶可(ke)使(shi)(shi)用(yong)的循(xun)環次(ci)數就越(yue)少。
充電差異(yi)
鋰離(li)(li)子電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)的充(chong)電(dian)(dian)(dian)(dian)(dian)方(fang)式(shi)(shi)不同于其(qi)它電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)。SLA電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)最好采(cai)用恒壓(ya)充(chong)電(dian)(dian)(dian)(dian)(dian),速率通常為(wei)額定容量(liang)的1/10(C/10),充(chong)電(dian)(dian)(dian)(dian)(dian)時(shi)(shi)間為(wei)14-16小(xiao)(xiao)時(shi)(shi),或涓(juan)流(liu)充(chong)電(dian)(dian)(dian)(dian)(dian)或浮充(chong)充(chong)電(dian)(dian)(dian)(dian)(dian),充(chong)電(dian)(dian)(dian)(dian)(dian)速率為(wei)C/20至(zhi)C/30。NiCd電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)充(chong)電(dian)(dian)(dian)(dian)(dian)的終止推(tui)薦采(cai)用-△V的方(fang)式(shi)(shi),此時(shi)(shi)充(chong)電(dian)(dian)(dian)(dian)(dian)器(qi)的電(dian)(dian)(dian)(dian)(dian)壓(ya)到達峰值。NiMH電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)由(you)于其(qi)發熱的特(te)(te)點,充(chong)電(dian)(dian)(dian)(dian)(dian)過程中要求采(cai)用溫(wen)度檢測,ΔT/Δt為(wei)其(qi)首選方(fang)式(shi)(shi)。特(te)(te)制的快(kuai)充(chong)式(shi)(shi)NiCd和NiMH電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)可在(zai)C/2-C/3的速率下充(chong)電(dian)(dian)(dian)(dian)(dian)4-6小(xiao)(xiao)時(shi)(shi)。極低阻尼鎳(nie)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)作為(wei)快(kuai)充(chong)式(shi)(shi)電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)的一(yi)種(zhong),可在(zai)1C速率下充(chong)電(dian)(dian)(dian)(dian)(dian)1小(xiao)(xiao)時(shi)(shi)。最后(hou),鋰離(li)(li)子電(dian)(dian)(dian)(dian)(dian)池(chi)(chi)建議采(cai)用恒流(liu)/恒壓(ya)充(chong)電(dian)(dian)(dian)(dian)(dian)(CC/CV)。
通常,鋰離子(zi)電池(chi)供電的(de)器械以1C的(de)速率充電60-75分(fen)鐘至(zhi)4.1V后,即可(ke)從原來的(de)低能狀態(tai)達到(dao)電量(liang)80-90%的(de)狀態(tai)。其它電池(chi),除(chu)可(ke)采(cai)用高電流(liu)充電的(de)特制電池(chi)外,同樣充電至(zhi)80-90%時可(ke)能需要(yao)更多的(de)時間。
鋰離子電池還需慢充(chong)4-5小時至4.2V,獲得剩余的(de)(de)10-20%的(de)(de)電量(liang)。這種充(chong)電方式有(you)兩個好(hao)處。用戶可以在(zai)極短的(de)(de)時間內獲得接(jie)近滿充(chong)的(de)(de)電量(liang),且充(chong)電完成后(hou)的(de)(de)實際電壓絕不會超過4.2V。
須注意的是:如果僅將(jiang)鋰(li)(li)離子電(dian)池(chi)充(chong)至4.1V而非4.2V,可(ke)(ke)延長其循環壽命(ming);但(dan)其每(mei)次可(ke)(ke)用(yong)的電(dian)量將(jiang)會下(xia)(xia)降(jiang)(jiang)。在(zai)某些(xie)醫(yi)療(liao)器械(xie)中電(dian)池(chi)是一種后備(bei)裝(zhuang)置,它始終保持充(chong)電(dian)的狀態,以保證隨時可(ke)(ke)用(yong)。鋰(li)(li)離子的化學性(xing)質決定(ding)其不適合采(cai)(cai)用(yong)涓(juan)流充(chong)電(dian);鋰(li)(li)離子電(dian)池(chi)不能采(cai)(cai)用(yong)恒定(ding)浮充(chong)充(chong)電(dian)。但(dan)有幾種方(fang)法可(ke)(ke)以在(zai)不損害電(dian)池(chi)或(huo)影響醫(yi)療(liao)器械(xie)的基礎上(shang),有效(xiao)降(jiang)(jiang)低鋰(li)(li)離子電(dian)池(chi)過(guo)度(du)充(chong)電(dian)的可(ke)(ke)能性(xing)。方(fang)法之(zhi)一是在(zai)觸發電(dian)池(chi)再(zai)次充(chong)電(dian)前確保電(dian)池(chi)放電(dian)量至少為(wei)20%,隨后進行標(biao)準充(chong)電(dian)。鋰(li)(li)離子技術與(yu)SLA相比顯著提高了能量密度(du),在(zai)大多數情(qing)況下(xia)(xia)足以防止鋰(li)(li)離子電(dian)池(chi)電(dian)量完(wan)全充(chong)滿。
安全(quan)電路
每種(zhong)電(dian)(dian)(dian)(dian)池(chi)技術(shu)均有(you)其(qi)一整套安(an)(an)全考慮(lv)。NiCd電(dian)(dian)(dian)(dian)池(chi)組帶(dai)有(you)某種(zhong)電(dian)(dian)(dian)(dian)流(liu)(liu)分(fen)斷(duan)設(she)(she)備以防止發生嚴(yan)重故障,這是(shi)優秀(xiu)的電(dian)(dian)(dian)(dian)池(chi)設(she)(she)計必不(bu)可(ke)少的。NiMH具有(you)發熱的化學性(xing)質(zhi),因(yin)此電(dian)(dian)(dian)(dian)池(chi)中(zhong)需配(pei)有(you)熱量感應設(she)(she)備,與充(chong)電(dian)(dian)(dian)(dian)器(qi)相聯系,防止過度充(chong)電(dian)(dian)(dian)(dian),電(dian)(dian)(dian)(dian)池(chi)組本身(shen)還帶(dai)有(you)電(dian)(dian)(dian)(dian)流(liu)(liu)分(fen)斷(duan)設(she)(she)備。在鋰離子電(dian)(dian)(dian)(dian)池(chi)組中(zhong),一旦發生過電(dian)(dian)(dian)(dian)壓,即有(you)鋰金屬產生。這說明電(dian)(dian)(dian)(dian)池(chi)中(zhong)應使用(yong)安(an)(an)全電(dian)(dian)(dian)(dian)路(lu),使充(chong)放電(dian)(dian)(dian)(dian)過程中(zhong)電(dian)(dian)(dian)(dian)池(chi)電(dian)(dian)(dian)(dian)壓保持(chi)在特定的范(fan)圍內(見圖(tu)3)。
圖3.鋰(li)離子電(dian)(dian)池(chi)組設計中需要采用電(dian)(dian)子安全裝置。電(dian)(dian)池(chi)組內還包含電(dian)(dian)量監(jian)測裝置和充電(dian)(dian)電(dian)(dian)路(lu)。
雖然(ran)SLA電池(chi)(chi)一般不需要外(wai)部安全元件,但許多(duo)醫(yi)療(liao)設(she)備制造商仍(reng)堅(jian)持將不可復位保(bao)險絲(si)(si)置(zhi)于電池(chi)(chi)內部或周圍。由于大部分(fen)SLA電池(chi)(chi)都帶(dai)有(you)突出的(de)正負極板(ban)(ban),如果沒有(you)保(bao)險絲(si)(si),當其置(zhi)于金屬板(ban)(ban)上(shang)時,就(jiu)很容易發(fa)生短路(lu)(lu),而金屬板(ban)(ban)大量存在于醫(yi)療(liao)保(bao)健設(she)備中。這些電池(chi)(chi)還可能出現其它(ta)短路(lu)(lu)的(de)危(wei)險。如果發(fa)生短路(lu)(lu),裝置(zhi)即有(you)爆(bao)炸的(de)可能。鋰離子電池(chi)(chi)組短路(lu)(lu)的(de)危(wei)險較小,安全電路(lu)(lu)主要用于保(bao)護電池(chi)(chi)。
在電(dian)(dian)(dian)池(chi)(chi)中加入安全電(dian)(dian)(dian)路增(zeng)加了器(qi)械的成本,耗費(fei)了更多的空間。設計者必須認識(shi)到(dao)(dao)這些都是電(dian)(dian)(dian)池(chi)(chi)的選擇過程中會考慮到(dao)(dao)的權(quan)衡因(yin)素。總的來說(shuo),盡管(guan)有安全電(dian)(dian)(dian)路的存(cun)在,鋰(li)離(li)子電(dian)(dian)(dian)池(chi)(chi)仍可以縮(suo)小電(dian)(dian)(dian)池(chi)(chi)組體(ti)積(ji),減輕其重量,并能釋放更多的能量。
電量監(jian)測
來越(yue)(yue)多的醫(yi)療器械(xie)制(zhi)造商開(kai)始采用(yong)(yong)鋰離子技術,電(dian)(dian)(dian)池管理(li)特性(xing)在行業中也越(yue)(yue)來越(yue)(yue)常(chang)見。電(dian)(dian)(dian)量監(jian)測設備可以為終(zhong)端(duan)用(yong)(yong)戶(hu)提供(gong)一些信息,如(ru)電(dian)(dian)(dian)池預計使用(yong)(yong)時(shi)間等。管理(li)特性(xing)的引(yin)入,很大程度上明確了電(dian)(dian)(dian)池電(dian)(dian)(dian)量評估及充電(dian)(dian)(dian)方(fang)案的執行。
就電(dian)池(chi)管理而言,使用(yong)鋰離(li)子電(dian)池(chi)的設計者有(you)(you)(you)多種(zhong)選擇方案(an)。例如,一些(xie)鋰離(li)子電(dian)池(chi)電(dian)量(liang)監測(ce)設備中含有(you)(you)(you)信息特征,可以報告已經過(guo)的充放電(dian)循環次(ci)數。此類(lei)信息在一些(xie)重要的醫療設備中具有(you)(you)(you)重要作(zuo)用(yong)。電(dian)量(liang)監測(ce)基本方法有(you)(you)(you)兩種(zhong):基于電(dian)壓和庫侖(lun)計數。將兩種(zhong)技術相(xiang)結合的解決方案(an),其準確(que)性(xing)高達99%。
耐高溫(wen)性
鋰離(li)子電池(chi)在40°-45°C的(de)高溫條(tiao)件下(xia),性能(neng)優于(yu)其它電池(chi)。SLA和NiMH電池(chi)在高熱(re)量環(huan)(huan)境下(xia)無法正常工作。這成為(wei)其在急救工具中使用的(de)一個(ge)限制因素,因為(wei)此時,使用者無法將他們的(de)便攜式(shi)器械保存在低溫環(huan)(huan)境下(xia)。
在為(wei)便攜(xie)式(shi)器械選擇最佳電源方案(an)時,必須(xu)對其總成本和整體性能進行評估。
鋰離子技(ji)術的高電(dian)壓特(te)性可(ke)以減少電(dian)池使(shi)用量,由此降低了電(dian)池組的成(cheng)本,使(shi)之(zhi)與(yu)使(shi)用鎳技(ji)術的電(dian)池大(da)致相當。此外,鋰離子電(dian)池供應商(shang)不斷使(shi)用新(xin)材料(liao),以降低電(dian)池成(cheng)本。
鋰離子電池體積小、重量輕、能量高(gao)、循環(huan)壽命長、耐(nai)久(jiu)性(xing)好(hao)、電(dian)壓高(gao)及耐(nai)熱(re)性(xing)好(hao)的(de)特點使其具(ju)有潛在的(de)優(you)勢(shi)。醫療(liao)電(dian)子(zi)產品(pin)制造商可以利用這些特性(xing),拓寬產品(pin)市場,并最終(zhong)給(gei)消費者、醫療(liao)專業人員和病人帶(dai)來治療(liao)等方面的(de)好(hao)處。