2007 Prius - trickle charging 12v battery overnight

If you're ever shopping chargers, searching, avoid the word "trickle", it's implies an old-school, very low amperage, steady charge rate charger.

"Smart charger" is more succinct, and you need something around 4 amp, I believe 5 amp is the ceiling commencing with 4th Gen.

 

Yeah I have an older Scumacher and it has a 2 amp or Start function. The 2 amp which is basically trickle takes forever to reach 13.8.
From 12.3 to 13.8 is hours and hours of trickle.
I need to buy a new charger thats at 4 amps.

 

With the condition you described, I would have left the charger on for at least several days, and possibly a week or two. I've been using a smart 3a trickle charger for more than a decade, alternating for 2 to 6 week periods between two different cars. About a year ago the car I was driving was left parked for several days, then wouldn't start, and the battery was down to 6 volts. The drain turned out to be a faulty gauge plugged into OBD1. I managed to jump the car, drove it into the garage, and connected the smart charger overnight. The 7 year old battery has been fine ever since and keeps a good charge, ~ 12.6v after sitting with no charge for a week or so. If you don't give the battery a full charge, way past green, it won't maintain that high of a voltage.

My neighbors had a old car they hadn't driven for a long time, wanted to sell it but the battery was dead. I put on the charger and the next day the battery was up to 9 volts, and the second day 11.x volts. I don't recall the exact figures. I recommended a few more days, but they got the car started and were satisfied. A good modern smart charger can be left on a 12v car battery with good results for indefinite periods.

I used to use a watt meter, and could tell the state of charge by the watts that were being used by the charger. It would start at 35 watts, go up to 52w, then gradually come back down depending on battery condition. The green light would come on at 17w, but removing the charger at that point, the battery could go below 12.4 in a couple of days. Leaving the charger on to 2w on the meter would result in the battery retaining 12.6v capacity for much longer when the charger was removed. Often at 2w, I would just leave the charger on the battery.

 

Thanks for all the help in this discussion. My 2011 goes into what is apparently accessory mode if it's not driven for several days, and/or cold. Patience and fiddling with it will get it to start, but will replace the battery. Not worth the aggro.