keyboard batteries low...but keyboard isn't wireless

I have an older Mac from 2009 so I've had it for quite a long time and it has been in constant use. Recently every time I log on I get a "keyboard batteries are low, recharge or replace soon" . I do have wireless mouse and replace the batteries regularly, but I don't have a wireless keyboard. I've checked all connections and I've checked system preferences which tells me that my batteries are at 17% (the number has been going down).

I have no idea what to make of this as there isn't even an area for batteries on the keyboard. Does anyone have any suggestions? I can't believe I'm really just now depleting batteries in a keyboard I've had for 7 years.

However, when I first started checking it was reporting 20%. I'm tempted to just order a new keyboard, but thought I would ask first if there's something I'm missing. Sorry I'm not very computer savvy.