These were established well over 50 years ago. There were defined A and B sizes.

在50多年前，這些型號(hào)的名字建立之初只有A和B兩種。

But these were not popular, probably because the size difference from C was not that great.

可能是因?yàn)锽與C的大小差異不是那么大，所以B的使用并不廣泛。

However as electronics grew less power hungry and smaller (walkman devices etc), equipment makers clamored for smaller batteries to make their equipment smaller. So they came up with standards for even smaller batteries. Since they already started with A and can’t go lower, they became the sub-A batteries, AA and AAA and even AAAA can sometimes be found.

然而，隨著電子產(chǎn)品的耗電量越來(lái)越少，產(chǎn)品的體積越來(lái)越小(如隨身聽(tīng)設(shè)備等)，設(shè)備制造商們迫切需要更小的電池來(lái)使他們的設(shè)備變得更小。所以出現(xiàn)了更小的電池，也就需要為這些更小的電池起名。因?yàn)橐呀?jīng)開(kāi)始使用A級(jí)電池這個(gè)名字了，A之前沒(méi)有字母了，沒(méi)有辦法再低了（名字由D到A，尺寸越來(lái)越?。?，所以就變成了“AA”、“AAA”，有時(shí)還會(huì)出現(xiàn)“AAAA”。

via quora