Need advice on Prius with dead batteries

the only thing i can determine from your info is that you need a new 12v.

if it started, the hybrid battery is working, because it has to crank the engine to start.

to find out why it died, if it wasn't low on gas, you would need to have the trouble codes read.

i don't see why it wouldn't pass emissions

 

How many miles on the vehicle, and is this a 'garage queen' ?

 

It won't run if the hv battery is truly dead. As stated above, the jump start would not have worked without the hv battery spinning the gas engine. So the $64,000 dollar question is.... did the car run ok when you parked it two years ago AND did the engine start when you jumped it?

 

Coudl be something as simple as old gasoline.....
Can you post a picture of how may bars are displayed for the HV charge level?

 

Two years will kill any 12v battery left in the vehicle. The car has a continuous draw on the battery that is small enough to allow a start after three or four weeks. After that damage to the 12v battery begins. An agm 12v battery can take moderate discharges but two years is stretching it.

On some occasions a high amp jump star will start a car with a ruined 12v battery. Toyota's instructions say to to connect a jump and wait 5 minutes. This usually brings up the discharged battery enough to start, under normal "left the lights on" conditions.

On occasions it won't work because the bad battery sucks the jump voltage down too far. Finally any battery requires more than twenty minutes to recharge even if its good.

A new battery may be enough since you started it once recently. Long storage can be facilitated with a staged battery maintainer like the Battery Tender Jr, currently $35 at Walmart or Lowe's.

 

Also not a bad idea to remove the air filter housing and clean the throttle body bore around the throttle blade. Definitely add fresh gas.

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If using the built-in prius 12v charging circuit, it could take up to 10 hours at 4amps to recharge a depleted 12v battery.

Before throwing in a new 12v battery, haul the existing 12v battery to an auto parts chain store and they will test and charge it up for free.

 

get a smart trickle charger, and let it work it’s magic overnight like you asked. Probably closer to 30 dollars, but if you ever have another period of low driving you can leave it hooked up for awhile

 

Smart charger might work, but it’s had a long hiatus, very likely not much life left.

for downtime like that, best to either disconnect 12 volt, or leave it permanently connected to a smart charger. Around 3 amp rating is recommended for 2nd gen.

long term parked with neither of the above tactics employed, you’re pretty much guaranteed to kill the battery.

 

All kinds of good advice above on the 12V. Once you get it charged up or replaced, don't forget to check the oil, brake fluid, etc. to make sure it's at the right level and hasn't accumulated moisture over the past two winters. Especially the motor oil. Also, check the cabin and engine air filters to make sure mice haven't been homesteading in your car. Those are favorite places.

 

I'm currently headed down this same path - to revive a dead 12V.

When i pulled it out of the car it was measuring about 2V. I connected jumper cables to another vehicle for 20-30 minutes and got the voltage up to 10V and then put it on a trickle charger over night. These trickle charges won't work if the battery voltage isn't in the correct range. Using the jumper cables will at least get it in that range pretty quick. Pretty sure its not great for the battery but you really don't have much choice. Guessing it will take days with the trickle charger (mine only charges at .75A) to get it fully charged if it even works at all.

 

if it was at 2 volts you should replace it, it is going to be nothing but trouble for you going forward, especially on a prius

 

At first glance the HV battery seems to be OK. The 12V battery should be damaged. Fresh gas and a 12 battery is needed at a minimum. Without a functional 12V battery error codes will not be stored to help out troubleshooting. A plus would be to change the engine oil.

A trickle charger may not do the trick because the battery will rise to a high voltage as soon as the charger is powered on. The charger will confuse that high voltage reading with "charged" when the battery is actually "sulfated"

If you can find gas without ethanol that would be a plus. You can also use the ethanol free gas cans that are sold at walmart or lowes for lawnmowers, make sure you use the ones without the 2 stroke engine oil. The cans are *quite expensive*

I am not sure if it is feasible: I did bring back a 12V battery that was left discharged for 6 months. The dome light discharged it. The battery was removed from the car then charged via a 1uF capacitor, it took almost one week while I monitored via a DM. Allowed the battery to rise all the way to 14.5V. Note: The first day the battery read 16+ volts, then dropped down all the way to 10+ and slowly started to climb up to 14.5V and that value I stopped. Added distilled water before charging. That battery belongs to a 2014 Prius and it is still working. It is the Panasonic OEM battery. If what I just said does not make sense please ignore the paragraph.

 

10 year old battery. Yikes. I would just buy a new 12Volt battery. About $200 when we bought our new ones a few years ago.
I would put a tank of 91 or 93 octane without ethanol if you can find it. Rcheck, clean and replace both cabin air filter and engine air filter for sure. Check coolant levels in both tanks, check brake fluid, then Check the oil and top off as needed. Once it starts up. Drive it for an hour. longer if you can. Then get the oil changed. ASAP. Try to use the OEM toyota oil filter(amazon) and Mobil 1 oil (wal-mart) That should get the system up and running. After that, a MAF cleaning, new spark plugs and transmission drain and refill might be in order.