A fully charged 12-volt lead-acid battery should read at 12.6 volts on a multimeter. However, its worth noting that a healthy, fully charged lithium iron phosphate battery, such as the Renogy lifepo4 battery, should read between 13.4 volts and 13.6 volts at rest. Its important to keep in mind that the voltage readings for a lithium iron phosphate battery will fluctuate as the battery is discharged and charged. For example, a half-charged battery may read between 13.1V and 13.2V, while a quarter-charged battery may read between 12.8V and 12.9V. If a 12-volt battery reads below 12.0 volts, it is considered to be fully discharged or "flat" and should be recharged as soon as possible to avoid damage.