How To Tell If Prius 12v Battery Is Bad. If your car displays an erratic charge or it indicates that your battery is fully charged in a little while and then extremely low after a while then your vehicles battery could be overcharging or unable to hold onto a charge. If on a short trip the 8-bar State of Charge SoC changes more than 23 bars often.
Check the voltage between the battery posts while the vehicle is running. This indicates my battery probably needs replacing soon. Very low MPG during moderate acceleration.
When its starting to go bad you may notice that your miles per gallon MPG is decreasing or perhaps your state of charge is fluctuating.
This indicates my battery probably needs replacing soon. Very low MPG during moderate acceleration. That charges the 12V battery. But when it reads 124 volts the battery has attained a 75 charge level and it could still be charged further there would be no danger.
