The possibility of ESD in an isolated, mobile system
I know of an example where ESD was an issue on a battery powered device.

I used to work at a company where they once had an issue with failing hospital pagers. These pagers are the small battery powered devices carried around by doctors so they can be reached anywhere in the hospital. These pagers had charging contacts at the bottom so that they could be charged at night in a special stand.

The failure related to these battery contacts. It appeared that in a certain hospital there was an elevator which had a (metal) buttons. Sometimes these buttons somehow collected a charge so if you'd press them with your finger you would receive a small shock.

So what did the doctors do? They used the bottom of their pager to press the button! Then sometimes the button would discharge through the charging contacts damaging the electronics.

Fortunately it was quite easy to fix this (in newer designs) by adding some ESD protection at the battery charging contacts.

So, being battery powered does not mean that a device is immune to ESD. An ESD event can still happen through external contacts or at any other point in the design where electrical isolation is not good enough.