I’ve also got a multimeter in my truck. Cheap way to know what’s going on 

 

beware of auto parts store diagnostics. I went to an auto zone once and they said my battery was dead. Then went 150 feet down the road to advance auto and they said the battery was fine. I asked them what kind of portal I must have driven thru in that 150 feet that changed my fate 

 

Batteries?  We don't need no stinkin' batteries!

 

I change mine when the car strains to start.  I change it at the first sign trouble because when the battery starts to go out it won't last the day.