When I worked on the hybrid battery pack and left the hatch door open, the 12v battery slowly drained. This was because of learning and taking the time to perform the work correctly. However, when it was ready to perform a start test, the car would not start. I measured across the 12v battery and measured around 5vdc. Note, I couldn't even access the secret menu. Anyhow, my brother knew of an access to a booster charger. Just connect the black and red cables to the right terminals and the car started. No need to trickle charge and wait hours. This happened two times and the second time was my error. I could have disconnected the ground terminal to prevent the battery from discharging. Anyhow, does anyone looked into different brands and acquired a booster for emergency conditions?
I would look into chargers too, smart charger around 3.5* amp rating. * Recommended max charger rating, per North American Owner's Manual (2nd gen) Too: shouldn't 12 volt battery be disconnected (neg lead of the post and isolated) when working on hybrid battery?
I've never actually used mine, but I picked up an Audew jump starter based on this Project Farm youtube review.
When the battery was not used for some time, I disconnected the minus terminal. When I returned, I reconnected the minus terminal. Had no problems doing this.
Most any of them sold by a reputable company, even WalMart, should be OK. The Prius doesn't require much power from it's 12 V battery. BUT............. Now that you have "killed" it twice and apparently still have not recharged it fully, the odds are great that you now need a new 12 V battery too. And a small automatic tender type charger is highly recommended too. That does NOT mean a cheap trickle charger. Those are two different things.
What you need is an automatic battery maintainer (tender). The is quite different from a "trickle charger".
Yet. As your battery gets older, the time that it will hold a charge when not in use slowly goes down. With a brand new fully charged battery, it might hold up for 6 months. 3 years old, maybe 3 months. 6 years old, maybe 30 days. Over 6 years old......it's a crap shoot.
