通過USB為NiMH電池充電

圖1. USB供電的單節NiMH電池開關模式充電器

雖然表面上Li+電池已經占據整個便攜世界，但NiMH電池并沒有被完全遺棄。令人吃驚的是，盡管單位重量的能量仍然有較大差距，但其單位體積的能量僅比Li+電池低大約15%。NiMH電池的最大缺點是自放電率較高，混合型NiMH電池在很大程度解決了這一問題，例如SANYO? Eneloop?電池，靜態下一年之后仍然能夠保留85%的電量。NiMH電池的吸(xi)引(yin)力在于(yu)成(cheng)本低、安全性高、用戶更換方便等，至少標準電池(chi)具備這些(xie)優勢。

圖1所示便攜設備由一節AA型NiMH電池供電，利用USB充電。充電器開關頻率大約為150kHz，電池充電電流為1.1A (典型AA型NiMH電(dian)(dian)(dian)(dian)(dian)池在(zai)大(da)約0.5°C條(tiao)件(jian)下)。由于(yu)降壓轉(zhuan)換(huan)器將(jiang)5V、500mA轉(zhuan)換(huan)成電(dian)(dian)(dian)(dian)(dian)池充(chong)電(dian)(dian)(dian)(dian)(dian)時(shi)的1.5V、1.1A，電(dian)(dian)(dian)(dian)(dian)路供給電(dian)(dian)(dian)(dian)(dian)池的電(dian)(dian)(dian)(dian)(dian)流(1.1A)大(da)于(yu)從USB接口獲得的電(dian)(dian)(dian)(dian)(dian)流(500mA)。需要注意的是，由于(yu)在(zai)低充(chong)電(dian)(dian)(dian)(dian)(dian)速率下不能正確判斷(duan)充(chong)電(dian)(dian)(dian)(dian)(dian)終(zhong)止，只(zhi)能采用500mA或更大(da)功率的端口進行充(chong)電(dian)(dian)(dian)(dian)(dian)。所以，當枚(mei)舉確定只(zhi)有(you)100mA電(dian)(dian)(dian)(dian)(dian)流可用時(shi)，不應(ying)激活(huo)充(chong)電(dian)(dian)(dian)(dian)(dian)。系(xi)統通過(guo)關閉(bi)TMR上(shang)的Q2，使定時(shi)器電(dian)(dian)(dian)(dian)(dian)阻懸空，停止充(chong)電(dian)(dian)(dian)(dian)(dian)。

該充電器另外一項特別有用的功能是：通過檢測電池阻抗確定接入的是否為堿性電池或故障電池，檢測到這(zhe)種狀況時將禁止充電。這(zhe)就(jiu)允許用(yong)戶在緊(jin)急情況下(xia)插入堿性電池，無需擔心意外充電。