My truck has a hard time starting up. It seems like a weak battery..

Once it turns over and starts, I'm getting full power so I"m thinking it's the battery.

I just don't want buy a new battery when it might be the alternator...

Obvious battery..

or go to a gas station garage and get a used battery, theyre alot cheaper and work just as good because the mechanics recharge used batteries to full power.

Yep, defin. the battery.. Just drive up to like Auto Zone (if yall have one of those) or a parts store and they can check your battery for you, probably for free..

When I turn my key the gauge for the battery is very low.. then once the engine starts, the gauge is fine.

So it's probably my battery.

It's more than likely your battery...

Your vehicle relies on the battery to start initially. Once started, it really relies more on the alternator because if you didn't have an alternator, the battery would slowly die.. As long as your battery gauge isn't dropping when your driving around, then your alternator is fine...

Does your gauge drop at all when you turn on the headlights or anything??

Also, check to make sure you have no corrosion around the battery terminals and cables.. And that the cables are tightly secured to the posts.