Use of Icons within Excel

'Icons' are graphical objects. You can turn them into pictures or shapes, but you can't reference them in formulas. At least, not that I know of.

'Icon sets' are used in conditional formatting, but there aren't any in the shape of batteries.

But you can use regular symbols from the Insert ribbon. They are just like any other character and you can use them in a formula as demonstrated in the picture below.

I'm on a Mac now and found the empty battery symbol. I'm pretty sure Excel for the PC has a similar one. Personally I wouldn't bother and just color the 'Replace by' cell red if the day is on or before TODAY()

EDIT: I assumed too much regarding symbols on the PC being similar to the ones on a Mac. This is the closest I could get to in Excel on a PC.

So, an almost empty battery symbol has the UNICHAR value of 129707 in Excel for the PC. And that code works also on the Mac, producing the symbol shown in the first screenshot.