What could cause a low battery reading in a Mazda b3000?

Simply check the ground wires for a good tight clean connection. If good, meaning removed and cleaned (good idea) it probably is the battery is close to it's life expectancy or could have a dead cell, or crack plate and not charging up to capacity. Unless the alternator is not charging properly which is would have to be either bench tested at auto zone or advance auto parts for FREE. I suggest taking the vehicle to either parts stores above and have them do a check on the system for free it may indicate if the battery is going bad, or alternator are the issue. If the ground wires are clean and battery terminals are as well and tight are a good idea to do before having the check done. Easy. Note the check does not require removing the battery or alternator a machine is hooked up to do the diagnosis at auto zone or advance auto.