「need 」は 「 to 」を付けなければならないけど…

The red thing in the picture is a taillight.

• taillight 尾灯 (definition from Eijiro on the Web)

I was writing about it a few days ago.

× I need put fresh batteries in it. <- "need" isn't a 助動詞, so you need to use "to"
◯ I need to put fresh batteries in it.

The white one is a headlight.

• headlight 前灯 (definition from Eijiro on the Web)

× I needn't to put fresh batteries in it, though.
◯ I needn't put fresh batteries in it, though. <- UK style?
◯ I don't need to put fresh batteries in it, though. <- more commonly said by Americans

That's weird, isn't it? Even though we need "to" in the first example, don't use "to" with "needn't". I don't know why that is. Instead, I recommend that you use "don't need to" because it follows the rules nicely.

The reason I don't need to put fresh batteries in the white one is that I left it on my bicycle during heavy rain. Let me give you some advice: you need to take the lights off the bicycle in heavy rain. If you don't, the rain will ruin it. That's what happened to this one. However, I've already bought a new one, so I don't need to do anything about this one but throw it away.