鋰電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)必須考慮充(chong)電(dian)(dian)(dian)(dian)、放電(dian)(dian)(dian)(dian)時的安全性,以(yi)(yi)防止特性劣化。但鋰離子電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)能量密度(du)高,難以(yi)(yi)確保電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)的安全性,在過(guo)度(du)充(chong)電(dian)(dian)(dian)(dian)狀態(tai)下,電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)溫度(du)上(shang)升后(hou)能量將(jiang)過(guo)剩,于是電(dian)(dian)(dian)(dian)解(jie)液(ye)分解(jie)而產(chan)生氣(qi)體,容易使內(nei)壓上(shang)升而產(chan)生自(zi)燃或破裂(lie)的危險;反(fan)之,在過(guo)度(du)放電(dian)(dian)(dian)(dian)狀態(tai)下,電(dian)(dian)(dian)(dian)解(jie)液(ye)因分解(jie)導致電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)特性及耐久(jiu)性劣化,降(jiang)低可充(chong)電(dian)(dian)(dian)(dian)次數。因此鋰電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)的過(guo)充(chong)、過(guo)度(du)放電(dian)(dian)(dian)(dian)、過(guo)電(dian)(dian)(dian)(dian)流及短路(lu)保護(hu)很重要,所以(yi)(yi)通常都會在電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)包內(nei)設計保護(hu)線路(lu),用以(yi)(yi)保護(hu)鋰電(dian)(dian)(dian)(dian)池(chi)(chi)(chi)(chi)(chi)。

1 電路設計

1.1 電路概述

鋰離子電池保(bao)護(hu)電路(lu)包(bao)括過度充電保(bao)護(hu)、過電流/ 短路(lu)保(bao)護(hu)和過放電保(bao)護(hu)等,該電路(lu)就是要確保(bao)這樣(yang)的(de)過度充電及放電狀態時(shi)的(de)安全,并防止特性(xing)劣化(hua)。它主(zhu)要由集成保(bao)護(hu)電路(lu)IC、貼(tie)片電阻(zu)(zu)、貼(tie)片電容、場效應管(MOSFET) 、有的(de)還有熱敏電阻(zu)(zu)(NTC) 、識(shi)別(bie)電阻(zu)(zu)( ID) 、保(bao)險絲( FUSE) 等構成。

其(qi)中集成保(bao)(bao)護電路IC 用(yong)來(lai)(lai)檢測保(bao)(bao)護電路當前(qian)的電壓、電流(liu)、時間等(deng)參數以此來(lai)(lai)控制(zhi)場效應(ying)管的開關狀態;場效應(ying)管(MOSFET) 則根據保(bao)(bao)護IC 來(lai)(lai)控制(zhi)回路中是否有需開或關; 貼(tie)片電阻用(yong)作(zuo)限流(liu); 貼(tie)片電容作(zuo)用(yong)為(wei)濾波(bo)、調節延遲時間;熱敏(min)電阻用(yong)來(lai)(lai)檢測電池塊(kuai)內的環境(jing)溫度; 保(bao)(bao)險(xian)絲防止流(liu)過(guo)電池的電流(liu)過(guo)大,切斷電流(liu)回路。

1.2 電路原理及參數確(que)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(shang)升而(er)導致內(nei)壓上(shang)升,需終(zhong)止(zhi)(zhi)當前充電(dian)的(de)(de)狀(zhuang)態。此時(shi)(shi),集(ji)成(cheng)保(bao)(bao)護電(dian)路IC 需檢(jian)測電(dian)池電(dian)壓,當到達4.25V 時(shi)(shi)(假設(she)電(dian)池過充電(dian)壓臨界點為4.25 V) 即(ji)激活過度充電(dian)保(bao)(bao)護,將功率MOS 由(you)開(kai)轉為切斷,進而(er)截止(zhi)(zhi)充電(dian)。另外,為防止(zhi)(zhi)由(you)于噪(zao)音所產生(sheng)的(de)(de)過度充電(dian)而(er)誤判為過充保(bao)(bao)護,因(yin)此需要設(she)定延遲(chi)(chi)時(shi)(shi)間(jian),并(bing)且(qie)延遲(chi)(chi)時(shi)(shi)間(jian)不能短(duan)于噪(zao)音的(de)(de)持續時(shi)(shi)間(jian)以免誤判。過充電(dian)保(bao)(bao)護延時(shi)(shi)時(shi)(shi)間(jian)tvdet1計算(suan)公式(shi)為:

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

式中: Vdd為(wei)保護N1 的(de)過(guo)充電檢測電壓(ya)值。

簡便計算(suan)延時(shi)時(shi)間: t = C3/ 0. 01 ×77 (ms) (2)

如若C3 容值(zhi)為0.22 F ,則延時值(zhi)為:0. 22 /0. 01 ×77 = 1694 (ms)

1.2.2 過(guo)度放電保護

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,且此時鋰(li)電池(chi)電壓高于過(guo)度(du)放電電壓時,過(guo)度(du)放電保護功能方可(ke)解(jie)除。另(ling)外(wai),考(kao)慮到脈沖放電的情(qing)況,過(guo)放電檢測電路設有延(yan)遲(chi)時間以避免產生誤動作(zuo)。

1.2.3 過電流(liu)及短路電流(liu)保(bao)護(hu)

因為(wei)不明原(yuan)因(放(fang)電(dian)(dian)(dian)時或(huo)正負極(ji)遭(zao)金屬物誤觸) 造成過(guo)電(dian)(dian)(dian)流(liu)或(huo)短(duan)路,為(wei)確保(bao)安全(quan),必須使其立即(ji)停止放(fang)電(dian)(dian)(dian)。過(guo)電(dian)(dian)(dian)流(liu)保(bao)護IC 原(yuan)理(li)為(wei),當放(fang)電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)過(guo)大或(huo)短(duan)路情況產生時,保(bao)護IC 將(jiang)激活過(guo)(短(duan)路) 電(dian)(dian)(dian)流(liu)保(bao)護,此時過(guo)電(dian)(dian)(dian)流(liu)的(de)(de)檢測(ce)是將(jiang)功率MOSFET 的(de)(de)Rds (on) 當成感應(ying)阻抗用以監測(ce)其電(dian)(dian)(dian)壓的(de)(de)下降情形,如果(guo)比所(suo)定的(de)(de)過(guo)電(dian)(dian)(dian)流(liu)檢測(ce)電(dian)(dian)(dian)壓還高則(ze)停止放(fang)電(dian)(dian)(dian),運算公式為(wei):

V_ = I ×Rds ( on) ×2 ( V_為(wei)過電(dian)(dian)流(liu)檢測電(dian)(dian)壓, I 為(wei)放電(dian)(dian)電(dian)(dian)流(liu)) (3)假設V_ = 0. 2V , Rds (on) = 25 mΩ,則保(bao)護電(dian)(dian)流(liu)的大小為(wei)I = 4 A 。

同樣(yang),過(guo)電(dian)流(liu)(liu)檢測也必須設有延遲時(shi)間以(yi)防有突發電(dian)流(liu)(liu)流(liu)(liu)入時(shi)產生(sheng)誤動(dong)(dong)作。通常(chang)在過(guo)電(dian)流(liu)(liu)產生(sheng)后,若(ruo)能去除過(guo)電(dian)流(liu)(liu)因(yin)素(例(li)如馬(ma)上(shang)與負載脫離(li)) ,將會恢復其正常(chang)狀(zhuang)態,可以(yi)再進行正常(chang)的充放(fang)電(dian)動(dong)(dong)作。

2 結束語

在(zai)進行保護電(dian)(dian)(dian)路設計(ji)時(shi)使(shi)(shi)電(dian)(dian)(dian)池充電(dian)(dian)(dian)到(dao)(dao)飽滿的狀(zhuang)態是(shi)使(shi)(shi)用者很(hen)關心(xin)的問題,同時(shi)兼顧(gu)到(dao)(dao)安全(quan)性問題,因此需(xu)要在(zai)達(da)到(dao)(dao)容(rong)許電(dian)(dian)(dian)壓(ya)(ya)時(shi)截(jie)止充電(dian)(dian)(dian)狀(zhuang)態。要同時(shi)符(fu)合這兩個(ge)條件,必須(xu)有高(gao)精密(mi)度(du)的檢(jian)測(ce)器,目(mu)前檢(jian)測(ce)器的精密(mi)度(du)為25 mV 。另外(wai)(wai)還必須(xu)考慮到(dao)(dao)集(ji)成保護電(dian)(dian)(dian)路IC 功耗、耐高(gao)電(dian)(dian)(dian)壓(ya)(ya)問題。此外(wai)(wai)為了(le)使(shi)(shi)功率MOSFET的Rds ( on) 在(zai)充電(dian)(dian)(dian)電(dian)(dian)(dian)流與放電(dian)(dian)(dian)電(dian)(dian)(dian)流時(shi)有效應用, 需(xu)使(shi)(shi)該阻抗值(zhi)盡量低(di), 目(mu)前該阻抗約為20～30 mΩ,這樣過電(dian)(dian)(dian)流檢(jian)測(ce)電(dian)(dian)(dian)壓(ya)(ya)就(jiu)可較低(di)。