鋰(li)(li)電(dian)池(chi)(chi)(chi)必(bi)須考慮(lv)充(chong)(chong)電(dian)、放電(dian)時的(de)(de)安全性,以防止特性劣化(hua)。但鋰(li)(li)離(li)子電(dian)池(chi)(chi)(chi)能量密度高,難以確保(bao)(bao)電(dian)池(chi)(chi)(chi)的(de)(de)安全性,在(zai)過(guo)(guo)(guo)度充(chong)(chong)電(dian)狀態下(xia),電(dian)池(chi)(chi)(chi)溫度上升后能量將過(guo)(guo)(guo)剩,于是電(dian)解液分解而產生(sheng)氣體,容(rong)易使內壓上升而產生(sheng)自燃或破(po)裂的(de)(de)危(wei)險;反之,在(zai)過(guo)(guo)(guo)度放電(dian)狀態下(xia),電(dian)解液因分解導致電(dian)池(chi)(chi)(chi)特性及耐久性劣化(hua),降低可充(chong)(chong)電(dian)次數。因此(ci)鋰(li)(li)電(dian)池(chi)(chi)(chi)的(de)(de)過(guo)(guo)(guo)充(chong)(chong)、過(guo)(guo)(guo)度放電(dian)、過(guo)(guo)(guo)電(dian)流及短路(lu)保(bao)(bao)護(hu)很重要(yao),所以通(tong)常都會(hui)在(zai)電(dian)池(chi)(chi)(chi)包內設計(ji)保(bao)(bao)護(hu)線路(lu),用以保(bao)(bao)護(hu)鋰(li)(li)電(dian)池(chi)(chi)(chi)。

1 電路設計

1.1 電路概述

鋰離子電池保(bao)(bao)(bao)護(hu)電(dian)(dian)(dian)(dian)(dian)路(lu)包括過度充(chong)電(dian)(dian)(dian)(dian)(dian)保(bao)(bao)(bao)護(hu)、過電(dian)(dian)(dian)(dian)(dian)流/ 短路(lu)保(bao)(bao)(bao)護(hu)和過放電(dian)(dian)(dian)(dian)(dian)保(bao)(bao)(bao)護(hu)等(deng),該電(dian)(dian)(dian)(dian)(dian)路(lu)就是要(yao)確保(bao)(bao)(bao)這(zhe)樣的過度充(chong)電(dian)(dian)(dian)(dian)(dian)及(ji)放電(dian)(dian)(dian)(dian)(dian)狀態時的安全,并防止特性劣化(hua)。它(ta)主要(yao)由集成保(bao)(bao)(bao)護(hu)電(dian)(dian)(dian)(dian)(dian)路(lu)IC、貼片(pian)電(dian)(dian)(dian)(dian)(dian)阻、貼片(pian)電(dian)(dian)(dian)(dian)(dian)容、場效(xiao)應管(MOSFET) 、有(you)的還有(you)熱敏(min)電(dian)(dian)(dian)(dian)(dian)阻(NTC) 、識(shi)別電(dian)(dian)(dian)(dian)(dian)阻( ID) 、保(bao)(bao)(bao)險絲( FUSE) 等(deng)構成。

其中集(ji)成保(bao)(bao)護(hu)電(dian)(dian)(dian)路IC 用來檢(jian)測保(bao)(bao)護(hu)電(dian)(dian)(dian)路當前的電(dian)(dian)(dian)壓、電(dian)(dian)(dian)流(liu)、時間等參數以此來控制(zhi)場效應管的開關狀態;場效應管(MOSFET) 則根據保(bao)(bao)護(hu)IC 來控制(zhi)回路中是(shi)否有需開或關; 貼片(pian)電(dian)(dian)(dian)阻用作限流(liu); 貼片(pian)電(dian)(dian)(dian)容(rong)作用為濾波、調節延遲時間;熱敏電(dian)(dian)(dian)阻用來檢(jian)測電(dian)(dian)(dian)池(chi)(chi)塊內的環(huan)境溫(wen)度; 保(bao)(bao)險絲防止流(liu)過(guo)電(dian)(dian)(dian)池(chi)(chi)的電(dian)(dian)(dian)流(liu)過(guo)大,切斷電(dian)(dian)(dian)流(liu)回路。

1.2 電(dian)路原理(li)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,鋰電池會因溫度(du)上(shang)升(sheng)而導致內壓(ya)上(shang)升(sheng),需(xu)終止(zhi)當(dang)前(qian)充(chong)電(dian)的(de)狀態。此(ci)時(shi)(shi),集成保(bao)護(hu)(hu)電(dian)路IC 需(xu)檢(jian)測電(dian)池(chi)電(dian)壓(ya),當(dang)到達4.25V 時(shi)(shi)(假設(she)電(dian)池(chi)過充(chong)電(dian)壓(ya)臨界點為(wei)(wei)4.25 V) 即激活過度(du)充(chong)電(dian)保(bao)護(hu)(hu),將(jiang)功(gong)率MOS 由開轉為(wei)(wei)切斷,進而截(jie)止(zhi)充(chong)電(dian)。另外(wai),為(wei)(wei)防(fang)止(zhi)由于噪(zao)音所產生(sheng)的(de)過度(du)充(chong)電(dian)而誤(wu)判(pan)(pan)為(wei)(wei)過充(chong)保(bao)護(hu)(hu),因此(ci)需(xu)要設(she)定延(yan)遲時(shi)(shi)間,并且延(yan)遲時(shi)(shi)間不(bu)能短于噪(zao)音的(de)持(chi)續時(shi)(shi)間以免誤(wu)判(pan)(pan)。過充(chong)電(dian)保(bao)護(hu)(hu)延(yan)時(shi)(shi)時(shi)(shi)間tvdet1計(ji)算(suan)公式為(wei)(wei):

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

式中: Vdd為保護N1 的過充(chong)電檢測電壓值。

簡便計算延時時間: t = C3/ 0. 01 ×77 (ms) (2)

如(ru)若C3 容值為0.22 F ,則延時(shi)值為:0. 22 /0. 01 ×77 = 1694 (ms)

1.2.2 過度放電(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)鋰電(dian)(dian)池電(dian)(dian)壓高于(yu)過度放(fang)電(dian)(dian)電(dian)(dian)壓時(shi),過度放(fang)電(dian)(dian)保(bao)護功能方可解除(chu)。另外,考慮到脈沖(chong)放(fang)電(dian)(dian)的情況,過放(fang)電(dian)(dian)檢測電(dian)(dian)路設(she)有延遲時(shi)間以(yi)避免(mian)產生誤動作。

1.2.3 過電流及(ji)短路(lu)電流保護

因(yin)為不明原(yuan)因(yin)(放(fang)電(dian)(dian)時或(huo)(huo)正(zheng)負(fu)極遭(zao)金(jin)屬(shu)物(wu)誤觸) 造(zao)成(cheng)過(guo)(guo)電(dian)(dian)流(liu)或(huo)(huo)短(duan)路(lu),為確保(bao)安全(quan),必須使(shi)其立(li)即停(ting)止放(fang)電(dian)(dian)。過(guo)(guo)電(dian)(dian)流(liu)保(bao)護(hu)IC 原(yuan)理為,當放(fang)電(dian)(dian)電(dian)(dian)流(liu)過(guo)(guo)大或(huo)(huo)短(duan)路(lu)情況(kuang)產生時,保(bao)護(hu)IC 將(jiang)激活過(guo)(guo)(短(duan)路(lu)) 電(dian)(dian)流(liu)保(bao)護(hu),此時過(guo)(guo)電(dian)(dian)流(liu)的(de)(de)檢測是將(jiang)功(gong)率MOSFET 的(de)(de)Rds (on) 當成(cheng)感應阻抗(kang)用以監測其電(dian)(dian)壓(ya)的(de)(de)下(xia)降情形,如(ru)果(guo)比所定的(de)(de)過(guo)(guo)電(dian)(dian)流(liu)檢測電(dian)(dian)壓(ya)還高則停(ting)止放(fang)電(dian)(dian),運算公(gong)式為:

V_ = I ×Rds ( on) ×2 ( V_為(wei)過電流(liu)檢(jian)測(ce)電壓, I 為(wei)放電電流(liu)) (3)假(jia)設V_ = 0. 2V , Rds (on) = 25 mΩ,則(ze)保(bao)護電流(liu)的大小為(wei)I = 4 A 。

同樣,過(guo)(guo)電(dian)流檢測(ce)也(ye)必須設有(you)延(yan)遲時間以(yi)防有(you)突(tu)發電(dian)流流入時產生(sheng)誤動(dong)作。通常(chang)(chang)在過(guo)(guo)電(dian)流產生(sheng)后,若能(neng)去除(chu)過(guo)(guo)電(dian)流因素(例如馬上與負載脫離(li)) ,將會恢復其正常(chang)(chang)狀態,可以(yi)再(zai)進行正常(chang)(chang)的充放(fang)電(dian)動(dong)作。

2 結束語

在進(jin)行保護(hu)電路設計時(shi)使(shi)(shi)電池充電到(dao)飽滿的狀態是使(shi)(shi)用(yong)者很關心的問題,同(tong)時(shi)兼顧到(dao)安全性(xing)問題,因此需要在達到(dao)容許電壓時(shi)截止充電狀態。要同(tong)時(shi)符合這兩個條件,必(bi)須有(you)高精(jing)密度(du)(du)的檢(jian)測(ce)器(qi),目(mu)前(qian)檢(jian)測(ce)器(qi)的精(jing)密度(du)(du)為(wei)25 mV 。另外還必(bi)須考慮到(dao)集成保護(hu)電路IC 功耗、耐高電壓問題。此外為(wei)了使(shi)(shi)功率MOSFET的Rds ( on) 在充電電流與放(fang)電電流時(shi)有(you)效應用(yong), 需使(shi)(shi)該阻抗(kang)值(zhi)盡(jin)量低, 目(mu)前(qian)該阻抗(kang)約為(wei)20～30 mΩ,這樣過電流檢(jian)測(ce)電壓就(jiu)可較低。