鋰(li)電(dian)(dian)(dian)池(chi)必(bi)須考(kao)慮(lv)充(chong)(chong)(chong)電(dian)(dian)(dian)、放電(dian)(dian)(dian)時的(de)(de)安全性(xing),以防止(zhi)特(te)性(xing)劣(lie)化。但鋰(li)離子電(dian)(dian)(dian)池(chi)能量密度(du)高,難以確保電(dian)(dian)(dian)池(chi)的(de)(de)安全性(xing),在(zai)過(guo)(guo)(guo)度(du)充(chong)(chong)(chong)電(dian)(dian)(dian)狀態(tai)(tai)下(xia),電(dian)(dian)(dian)池(chi)溫度(du)上升(sheng)后能量將過(guo)(guo)(guo)剩,于(yu)是電(dian)(dian)(dian)解(jie)液分(fen)解(jie)而(er)產生氣體(ti),容易使內壓(ya)上升(sheng)而(er)產生自燃或破裂的(de)(de)危險(xian);反(fan)之(zhi),在(zai)過(guo)(guo)(guo)度(du)放電(dian)(dian)(dian)狀態(tai)(tai)下(xia),電(dian)(dian)(dian)解(jie)液因分(fen)解(jie)導(dao)致電(dian)(dian)(dian)池(chi)特(te)性(xing)及(ji)耐久性(xing)劣(lie)化,降(jiang)低可(ke)充(chong)(chong)(chong)電(dian)(dian)(dian)次數。因此鋰(li)電(dian)(dian)(dian)池(chi)的(de)(de)過(guo)(guo)(guo)充(chong)(chong)(chong)、過(guo)(guo)(guo)度(du)放電(dian)(dian)(dian)、過(guo)(guo)(guo)電(dian)(dian)(dian)流及(ji)短路(lu)保護(hu)(hu)很重要,所以通常都會在(zai)電(dian)(dian)(dian)池(chi)包內設計保護(hu)(hu)線路(lu),用以保護(hu)(hu)鋰(li)電(dian)(dian)(dian)池(chi)。

1 電路設計

1.1 電路概述

鋰離子電池保(bao)護電(dian)(dian)(dian)路(lu)(lu)包括過(guo)(guo)(guo)度充(chong)電(dian)(dian)(dian)保(bao)護、過(guo)(guo)(guo)電(dian)(dian)(dian)流/ 短路(lu)(lu)保(bao)護和(he)過(guo)(guo)(guo)放(fang)電(dian)(dian)(dian)保(bao)護等,該(gai)電(dian)(dian)(dian)路(lu)(lu)就是要(yao)確保(bao)這樣的過(guo)(guo)(guo)度充(chong)電(dian)(dian)(dian)及放(fang)電(dian)(dian)(dian)狀(zhuang)態(tai)時的安全,并(bing)防(fang)止特性劣化。它(ta)主要(yao)由(you)集成(cheng)保(bao)護電(dian)(dian)(dian)路(lu)(lu)IC、貼片電(dian)(dian)(dian)阻(zu)、貼片電(dian)(dian)(dian)容、場效應管(MOSFET) 、有(you)的還有(you)熱敏(min)電(dian)(dian)(dian)阻(zu)(NTC) 、識(shi)別電(dian)(dian)(dian)阻(zu)( ID) 、保(bao)險(xian)絲( FUSE) 等構(gou)成(cheng)。

其中(zhong)集(ji)成(cheng)保護(hu)(hu)電(dian)路IC 用(yong)來檢(jian)測(ce)保護(hu)(hu)電(dian)路當(dang)前的電(dian)壓、電(dian)流(liu)(liu)、時(shi)間(jian)(jian)等參數以(yi)此來控(kong)制場效應管的開關狀態;場效應管(MOSFET) 則根據(ju)保護(hu)(hu)IC 來控(kong)制回(hui)路中(zhong)是否(fou)有需開或關; 貼片電(dian)阻(zu)用(yong)作限流(liu)(liu); 貼片電(dian)容作用(yong)為濾(lv)波、調節延遲時(shi)間(jian)(jian);熱(re)敏電(dian)阻(zu)用(yong)來檢(jian)測(ce)電(dian)池塊(kuai)內的環境溫度; 保險絲(si)防止(zhi)流(liu)(liu)過(guo)電(dian)池的電(dian)流(liu)(liu)過(guo)大,切(qie)斷電(dian)流(liu)(liu)回(hui)路。

1.2 電路原(yuan)理及參數確定

1.2.1 過度充電保護

當充電器對鋰電池過度充電時,鋰電池會因(yin)溫度上升而導致內(nei)壓上升,需終止當前充電(dian)的(de)狀態。此時(shi),集成(cheng)保護(hu)電(dian)路IC 需檢測電(dian)池電(dian)壓,當到達(da)4.25V 時(shi)(假設電(dian)池過充電(dian)壓臨界點為(wei)4.25 V) 即激活過度充電(dian)保護(hu),將功(gong)率MOS 由開轉為(wei)切斷,進而截(jie)止充電(dian)。另外,為(wei)防止由于噪(zao)音所產生的(de)過度充電(dian)而誤判為(wei)過充保護(hu),因(yin)此需要設定延(yan)遲(chi)時(shi)間,并(bing)且(qie)延(yan)遲(chi)時(shi)間不能短于噪(zao)音的(de)持續時(shi)間以免(mian)誤判。過充電(dian)保護(hu)延(yan)時(shi)時(shi)間tvdet1計算公式為(wei):

t vdet1 = { C3 ×( Vdd - 0. 7) }/ (0. 48 ×10 - 6 ) (1)

式中: Vdd為保護N1 的過充電(dian)檢測電(dian)壓值。

簡便(bian)計(ji)算延時時間: t = C3/ 0. 01 ×77 (ms) (2)

如若C3 容值(zhi)為0.22 F ,則(ze)延(yan)時值(zhi)為:0. 22 /0. 01 ×77 = 1694 (ms)

1.2.2 過(guo)度放(fang)電(dian)保護(hu)

在過度放電的情況下,電解液因分解而導致電池特性劣化,并造成充電次數的降低。過度放電保護IC 原理:為了防止鋰電池的過度放電狀態,假設鋰電池接上負載,當鋰電池電壓低于其過度放電電壓檢測點(假定為2.3 V) 時將激活過度放電保護,使功率MOS FET 由開轉變為切斷而截止放電,以避免電池過度放電現象產生,并將電池保持在低靜態電流的待機模式,此時的電流僅0.1μA 。當鋰電池接上充電器,且此時(shi)鋰(li)電(dian)(dian)池(chi)電(dian)(dian)壓高于過(guo)度放電(dian)(dian)電(dian)(dian)壓時(shi),過(guo)度放電(dian)(dian)保(bao)護(hu)功能方可解除。另外,考慮到脈(mo)沖放電(dian)(dian)的情(qing)況,過(guo)放電(dian)(dian)檢測電(dian)(dian)路(lu)設有延遲時(shi)間(jian)以(yi)避免產生誤(wu)動作。

1.2.3 過電(dian)流及短路電(dian)流保護

因為不明原(yuan)因(放(fang)電(dian)(dian)時(shi)或(huo)(huo)正負(fu)極遭金(jin)屬物誤觸) 造成(cheng)過(guo)電(dian)(dian)流或(huo)(huo)短路(lu),為確保安全,必(bi)須使(shi)其(qi)立(li)即停止(zhi)(zhi)放(fang)電(dian)(dian)。過(guo)電(dian)(dian)流保護(hu)(hu)IC 原(yuan)理為,當放(fang)電(dian)(dian)電(dian)(dian)流過(guo)大或(huo)(huo)短路(lu)情況產生時(shi),保護(hu)(hu)IC 將激活(huo)過(guo)(短路(lu)) 電(dian)(dian)流保護(hu)(hu),此時(shi)過(guo)電(dian)(dian)流的(de)檢測(ce)是將功率MOSFET 的(de)Rds (on) 當成(cheng)感應(ying)阻抗用以監測(ce)其(qi)電(dian)(dian)壓的(de)下降情形,如果比所定的(de)過(guo)電(dian)(dian)流檢測(ce)電(dian)(dian)壓還高則停止(zhi)(zhi)放(fang)電(dian)(dian),運(yun)算(suan)公式為:

V_ = I ×Rds ( on) ×2 ( V_為過電流檢測(ce)電壓(ya), I 為放(fang)電電流) (3)假設V_ = 0. 2V , Rds (on) = 25 mΩ,則保護(hu)電流的大小為I = 4 A 。

同樣,過(guo)電(dian)(dian)流檢測也(ye)必須(xu)設有(you)延遲時間以(yi)防有(you)突發電(dian)(dian)流流入時產(chan)(chan)生誤動作(zuo)。通常(chang)在過(guo)電(dian)(dian)流產(chan)(chan)生后(hou),若能去除過(guo)電(dian)(dian)流因素(例如馬上(shang)與負載脫離) ,將(jiang)會(hui)恢復其正(zheng)常(chang)狀態,可以(yi)再進行正(zheng)常(chang)的充(chong)放(fang)電(dian)(dian)動作(zuo)。

2 結束語

在(zai)進(jin)行保護電(dian)路(lu)設(she)計(ji)時(shi)使電(dian)池充(chong)電(dian)到(dao)(dao)(dao)飽滿的(de)狀態是使用(yong)者很關心的(de)問(wen)題,同(tong)時(shi)兼(jian)顧(gu)到(dao)(dao)(dao)安全(quan)性問(wen)題,因此需(xu)要在(zai)達到(dao)(dao)(dao)容(rong)許電(dian)壓時(shi)截止充(chong)電(dian)狀態。要同(tong)時(shi)符合這兩(liang)個條件,必(bi)須(xu)有(you)高精(jing)密(mi)度的(de)檢測(ce)(ce)器,目前檢測(ce)(ce)器的(de)精(jing)密(mi)度為25 mV 。另外還必(bi)須(xu)考(kao)慮到(dao)(dao)(dao)集成保護電(dian)路(lu)IC 功耗、耐高電(dian)壓問(wen)題。此外為了使功率MOSFET的(de)Rds ( on) 在(zai)充(chong)電(dian)電(dian)流(liu)(liu)與放電(dian)電(dian)流(liu)(liu)時(shi)有(you)效應(ying)用(yong), 需(xu)使該阻抗(kang)值盡量低, 目前該阻抗(kang)約為20～30 mΩ,這樣過電(dian)流(liu)(liu)檢測(ce)(ce)電(dian)壓就可較低。