Car will not start, is the main battery dead ?

Measure the voltage of the newly installed 12V battery. A fully-charged AGM battery will measure ~13.0V. If the voltage is good, you need to retrieve the diagostic trouble codes. Toyota Techstream is one way to do so. Mini VCI is a clone product.

If you determine the traction battery has failed, I would suggest you buy a new Toyota battery unless your idea of fun is a new hobby of having to periodically work on battery module replacement.

 

Agreed. Reconfirm and report back here what voltage of the auxuauxil battery you purchased.

To check the hybrid battery you would need some tools which would cost about $75.
- Android phone/tablet ~$50
- Elm 327 Bluetooth OBDII adaptor ~$20
- Torque Pro app ~$5

Not sure you want to spend that kind of money.
May just want to send it to hybrid mechanic to have it diagnosed.

 

Absolutely. IF the new battery was not fully charged before being installed, that might be the problem. Many/most new batteries come less than 50% charged.

And while it is true that a good AGM will read 12.8 to 13 when fully charged with no load, when you try to put it in READY mode it might be less than that. Down to 12.4 should be enough to get it going.

 

Reluctant to recommend this -- so many people seem to muck it up -- but seems to me an easy check would be to try to jumpstart the Prius.

If that goes well, well....

 

Letting a 14 year old hybrid battery sit for 4 months I’m pretty sure it’s the hybrid battery. It’s dead Jim.

If you have dash icons that usually means the 12 volt battery is ok enough to get the system up.
But without hybrid battery power it will not turn over the engine.

Hybrid battery powers one of the motor generator mg1 to spin up the motor and then with ignition on the engine fires up. No hybrid battery power no spin.

Your not quoting any voltages which tells me your not an electronics guy if you were
I would get you to open the relay cover on the battery and measure the battery pack output.

You can measure the pack output with various obd devices but your looking at a new hybrid pack or begin the long journey of fixing it yourself I’m afraid.

You never mentioned the miles on the car. What’s she got?

 

At 12.4 volts it looks pretty good imho the 12 is fine disregard it.

You could buy a Prolong Charger and charge up the hybrid battery but the sad fact is your hybrid battery is very old and that would only be a temporary fix. At the very least the connections inside that battery will be corroded and will require servicing. Since Your an electronics guy pull that battery out Asap and see how bad it really is.
Go to hybridautomotive.com and look at there install pictures for there charging harness they have step by step instructions on how to safely remove the hybrid battery from your car.

After removing it take many pictures of it before tearing into it. Use the search forum button up top and search

Hybrid battery

You have a lot of reading to do.

 

The main battery can be charged.
BUT... Remember, your are dealing with lethal levels of DC voltage.

In a sense it is like charging any battery.
Positive to positive, Negative to negative.

But most people don't have a high voltage charger.
(As opposed to nerds like me that have sonmany you can't keep track of them all).