My 12 Volt battery is completely discharged after sitting unused for a few weeks, I want to jump start it but need to know if I can use a small battery do so it. I tried charging the battery in place but the charger shows about 20% then all zeros so maybe the battery is finished.…….. This battery is supposed to be new according to the guy from whom I bought the car in September. I am concerned that the big battery might also discharge and then I might have bigger problems. Any advice on the best way to address this situation will be greatly appreciated. Thanks again for all the knowledge amd wisdom. Michael Crestohl.
The big hybrid battery should be fine, since when the car is not on and in 'Ready' mode, it's physically disconnected by a relay. The 12V battery closes this relay when starting the car. If the 12V is completely discharged, you'll need to charge the battery before the car can even be jump started. If it won't hold a charge, it's time to get a new battery.
The question in the thread title was investigated here: Prius Startup Current which ended up suggesting you might need to source a very brief peak of 60 amps, more like 30 for a couple seconds, coming out to a total on the order of 0.025 amp-hours, or a third of a watt-hour, before the converter comes online and takes over supplying power.
