4 weeks ago I changed a 12V battery on my Toyota Prius auto. But after being in for two weeks the battery was discharged again. I tried to recharge it, but it did not work - the charger does not start charging - though the battery voltage was 0.1 V. Now I want somehow to start the car and go to the service and check the electronics. To do this, I have bought a new battery.

  1. What should I do with the first battery? Is it possible to recover it or not? Has anyone encountered such a strong discharge in a relatively short period of time? Does this mean the battery status?

  2. How should I prepare a new battery: are they sold already charged or is a pre-charge needed before installation?

2 Answers 2


IIRC, the Prius has a step down voltage regulator which charges the 12v battery from the main power. I'd bet this system is having an issue and is not charging the battery.

As for the 12v battery. I'd bet the old battery (the first replacement?) is good, but has dropped in voltage below what the charger can deal with. To overcome this, you need a second 12vdc battery source tied to the first (in parallel) which is used as reference voltage so the charger will pick it up. Once the charger starts charging, it is usually okay. I'm not sure why the chargers do this, but having a second 12v source which the charger can actually read allows it to charge. You need to get the battery charged sooner than later as the longer it sits without a charge, the more sulfation will occur on the battery plates. Sulfation is the process of sulfate crystals forming which occurs as a battery is deeply discharged.

  • 1
    Smart chargers need to read a voltage as safety feature to prevent arching if the electrodes are shorted. The amount of voltage they need to read changes from brand and model, can be a PITA, I wish they had a "smart/dumb" switch. If memory serves me well since on the Prius the charge to the 12V battery is taken from the hybrid system you should need to just start that through a jump and disconnect the jump wires. Mar 9, 2016 at 18:56
  • @ErikvanDoren - You are right sir! Thanks for the assist :o) Mar 9, 2016 at 23:20

Don't leave the proximity key anywhere near the vehicle. It wakes the computer which will slowly drain the battery.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .