heats up..... volts down

[Prerunner1982]

I have been having this problem for quite a few months... my volts seem fine until the truck warms up or it get hot under the hood and then they drop below 13 volts. About the time the temp gauge reaches operating temp, the volts drop off... my truck also seems to run a smidge warm, but thats a different store that i will address by changing the therostat first. At anyrate I beleive its my battery, in need of a new one.. just havent saved up for an Optima. Any ideas?