現代通訊設備、便攜式電子產品、筆記本電腦、電動汽車、小衛星等普遍使用蓄電池作為電源,應用非常廣泛。然而大多數設備中的蓄電池,只能使用專用的充電器,而且普通的充電器大多充電時間長,無法判斷其充電參數和剩余的充電時間。
　　本文介紹一種基于單片機的通用智能充電器的設計。充電器可以實時采集電池的電壓、電流,對充電過程進行智能控制,計算電池已充的電量和剩余的充電時間;還可以通過串口和上位機進行通訊并給用戶顯示必要的信息,有虛擬儀表的作用;另外,它也可以改變參數,適應各種不同電池的充電。這里列舉幾種不同的電池充電試驗,來說明智能充電器的實用價值。
1 智能充電器的硬件設計
　　智能充電器如圖1所示(shi)。主(zhu)要包(bao)括(kuo)電(dian)源(yuan)變換電(dian)路、采(cai)樣(yang)電(dian)路、處理器、脈寬調制控制器和電(dian)池組等,形(xing)成了一(yi)個閉環(huan)系統[4]。下面對系統的工(gong)作原理分(fen)幾個部分(fen)進(jin)行簡述。

圖1智能充電(dian)器(qi)電(dian)路模塊圖

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   1．1 處理器
　　處理器采用51系列單片機89C51。單片機內部有兩個定時器、兩個外部中斷和一個串口中斷、三個八路的I/O口,采用11.0592MHz的晶振。單片機的任務是通過采樣電路實時采集電池的充電狀態,通過計算決定下一階段的充電電流,然后發送命令給控制器控制電流的大小。單片機通過串口RS232和上位機相連,用于存儲數據和虛擬顯示。
    1．2 采樣部分
　　電壓和電流采樣采用模/數轉換器AD574。AD574為±15V雙電源供電,12位輸出,最大誤差為±4bit,合計電壓0.01V。
　　充電(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)流(liu)通過(guo)電(dian)(dian)(dian)(dian)(dian)流(liu)傳感器MAX471轉換為電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)值。電(dian)(dian)(dian)(dian)(dian)流(liu)采樣的電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)值和(he)電(dian)(dian)(dian)(dian)(dian)池組的端電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)值兩(liang)者經過(guo)模擬開關CD4051,再經過(guo)電(dian)(dian)(dian)(dian)(dian)壓(ya)(ya)跟隨器輸入到AD574,分別進行(xing)轉換,其結果由單(dan)片(pian)機讀(du)取,并進行(xing)存儲和(he)處理。主要的電(dian)(dian)(dian)(dian)(dian)路連接如圖2所示。

圖2 采樣(yang)電路

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   1．3 控制器
　　控(kong)(kong)制(zhi)(zhi)(zhi)器采用脈寬調制(zhi)(zhi)(zhi)(PWM)方式(shi)控(kong)(kong)制(zhi)(zhi)(zhi)供電(dian)(dian)(dian)電(dian)(dian)(dian)流(liu)的(de)(de)大小(xiao)。PWM發生器由另(ling)一個20MHz的(de)(de)單片機構成(cheng),主(zhu)控(kong)(kong)制(zhi)(zhi)(zhi)器和它采用中(zhong)斷的(de)(de)方式(shi)進(jin)行通訊,控(kong)(kong)制(zhi)(zhi)(zhi)其增大或減小(xiao)脈寬。PWM信(xin)號通過光(guang)電(dian)(dian)(dian)隔離(li)驅動(dong)主(zhu)回路(lu)上(shang)的(de)(de)MOSFET。開關管、二極管、LC電(dian)(dian)(dian)路(lu)構成(cheng)開關穩壓電(dian)(dian)(dian)源(yuan)。用PWM方式(shi)控(kong)(kong)制(zhi)(zhi)(zhi)的(de)(de)開關電(dian)(dian)(dian)源(yuan)可以減小(xiao)功耗,同(tong)時便于進(jin)行數字化控(kong)(kong)制(zhi)(zhi)(zhi),但母線(xian)的(de)(de)紋波系數相對較大。PWM控(kong)(kong)制(zhi)(zhi)(zhi)電(dian)(dian)(dian)路(lu)如(ru)圖3所(suo)示。

圖(tu)3 PWM控制電路(lu)

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2 智能充電器的軟件設計
    2．1 數據測量
　　在單片機的測量中,電池電壓值和電流測量值經過多路選擇器進行選擇,然后通過A/D轉換器轉換為16進制數,直接存入單片機。電池電容量C則需要間接計算,由于每個循環 周期檢測電流一次,故可以利用電流值的積分求出電容量C。考慮電池內阻r的影響,可以得到計算電容量的計算公式為:
　　Cn+1=Cn+I?t-I2?r?t
　　充電時間和剩余充電時間由上位機進行計算,剩余充電時間等于預設的充電時間與已充電時間的差值。其中,預設時間可根據電池的型號預先得到。
    2．2 單片機控制程序設計
　　對于不同(tong)(tong)的(de)電(dian)(dian)(dian)池(chi)和不同(tong)(tong)的(de)參(can)數,單片機需(xu)要設定不同(tong)(tong)的(de)充(chong)電(dian)(dian)(dian)參(can)數,選擇不同(tong)(tong)的(de)充(chong)電(dian)(dian)(dian)策(ce)略。另外,程序(xu)需(xu)要在(zai)電(dian)(dian)(dian)池(chi)過電(dian)(dian)(dian)流(liu)(liu)、過電(dian)(dian)(dian)壓等異常情況下強制終止充(chong)電(dian)(dian)(dian)。以鋰離子電(dian)(dian)(dian)池(chi)為(wei)例,一般(ban)采用恒(heng)(heng)流(liu)(liu)-恒(heng)(heng)壓充(chong)電(dian)(dian)(dian)方式,其(qi)充(chong)電(dian)(dian)(dian)過程包括(kuo)小電(dian)(dian)(dian)流(liu)(liu)預充(chong)電(dian)(dian)(dian)、大(da)電(dian)(dian)(dian)流(liu)(liu)充(chong)電(dian)(dian)(dian)、恒(heng)(heng)壓充(chong)電(dian)(dian)(dian)等幾部(bu)分。其(qi)充(chong)電(dian)(dian)(dian)控制程序(xu)流(liu)(liu)程圖如(ru)圖4所示。

圖4 充(chong)電控制策略(lve)程(cheng)序

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在控制恒定電流和恒定電壓的過程中,采用比例控制,即如果充電電流I大于設定電流Is,就按照比例減小脈寬;反之按照比例增大脈寬。單片機還需要接收和處理上位機的命令,并根據上位機的要求將數據實時回送給上位機。兩者的通訊協議要在程序中預先設定。
    2．3 上位機處理程序設計
　　上位機程序由VisualC++編寫。其任務是每隔1秒鐘向串口發送一個查詢命令,并讀取單片機回送的信息,提取充電電流、充電電壓、工作狀態等參數。參數經過數制轉換和計算后進行顯示。軟件有著良好的用戶界面,可以方便地觀測電池目前的工作狀態以及剩余充電時間等信息。上位機程序會同時把讀到的數據存儲到文件中,這些數據可以利用其它數學軟件(如Matlab)進行處理。
　　另外,程序在初始化時要把充電電池的型號參數發送給智能充電器,參數一般包括充電電池的種類(鋰離子電池、鎳鎘電池)、充電電池的容量(單位為mAh)等。根據不同的電池型號,單片機可以設定不同的充電參數,程序可以直接控制單片機的運行與停止。
3智能充電器的應用試驗
    3．1充電性能試驗
　　這里選(xuan)用型號為(wei)US18650的SONY鋰離子電(dian)池,其額定(ding)容量(liang)為(wei)1800mAh;經過測量(liang),電(dian)池在(zai)4.2V左右時的內阻約為(wei)0.3Ω。取恒流充(chong)(chong)(chong)電(dian)電(dian)流為(wei)1/3C=0.6A,截(jie)止(zhi)電(dian)壓為(wei)4.2V,充(chong)(chong)(chong)電(dian)結束標志電(dian)流為(wei)0.06A,進行充(chong)(chong)(chong)電(dian)試驗。　圖5為(wei)充(chong)(chong)(chong)電(dian)過程的電(dian)壓、電(dian)流和電(dian)容量(liang)的曲線。

圖(tu)5 鋰離子電池充電性能試驗

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　充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)時(shi)間約為(wei)(wei)240分鐘,如果需要進一步縮短充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)時(shi)間,只需在初(chu)始化時(shi)設定更大的充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)即可。因為(wei)(wei)采用PWM控制(zhi)器(qi),所以電(dian)(dian)(dian)(dian)(dian)(dian)源供電(dian)(dian)(dian)(dian)(dian)(dian)的效率高,從供電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)源到充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)池(chi)的工作(zuo)效率,最低時(shi)在85%左右。充(chong)(chong)電(dian)(dian)(dian)(dian)(dian)(dian)電(dian)(dian)(dian)(dian)(dian)(dian)流(liu)波(bo)動較大,

 3．2 智能充電器通用性試驗　選用NOKIA6100鋰離子電池(額定容量為550mAh),用恒流-恒壓充電方式進行充電,取恒流充電電流為0.15A,截止電壓為4.2V,充電曲線如圖6所示。
　　從充電曲線來看,電池電壓達到3.96V時就不再上升了,充電電流也不再下降了。可以判斷商用電池內部有保護電路,將多余的電流旁路了,這樣的保護電路使充電過程中能量損耗很大。試驗曲線顯示在四小時時電池電量已經達到550mAh,但實際上并沒有達到滿充。此實驗證明,此充電器可以作為一般的商用電池的通用充電器,充電速度快,效果良好。不足的是它與實際的充電電池在機械接口上還不能匹配,需要進一步改進。
 

圖(tu)6 NOKIA商用電池充電試驗(yan)

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