Replace 12v Battery?

Well... Next least expensive fix after hybrid battery fan is pulling pack, getting voltage data & load testing data on a spread sheet and cleaning out all the corrosion... If those high temp readings are accurate one of your 168 cells is still passing the high-amp current but has for the most part failed, just not completely and that's the source of all your heat and warning light problems. Though I'm still unclear if your 12v battery is so far gone all of the rest is just an aberration of a weak 12v, but most likely it's time to replace a module, as well as recondition pack in order to find next bad module after that / increase packs performance. Or go with brand new pack and forget about it being an issue for another 6-10 years...

 

Just for another data point:

Highest temp I've ever seen (albeit on our 3rd gen) was around 45C (113F). Fan speed then was maybe 4. This was in extreme stop-and-go, hot day.

Driving on cool evening, easy going, it would settle in around 35C (95F). Fan speed then maybe 1.

 

Gen 1 battery cells corroded often, so Toyota made changes for Gen 2.
If Dorman uses Gen 2 or later cells when doing Gen 1 rebuilds, they are not technically lieing. They are using 'technology that prevents corrosion' it is just Toyota's technology.

Toyota Hybrids P3009: High Voltage Leak Detected, internal to battery - Luscious Garage | Hybrid Specialists

 

Nickel plated bus bars?

 

Yes, it is entirely likely your 12-Volt battery is dying. Change it soon. Get the one sold at the parts counter of your Toyota dealer, as it is guaranteed for 7 years (pro-rated, of course). Also, if you haven't changed the spark plugs yet, consider replacing them. The other thing to check is tires... Proper inflation? Correct type?

I have a couple of 2009s that get the same mileage now as they did when new.

 

Well... as for special technology to prevent corrosion... It's called di-electric grease and seals the electrical parts so they are not exposed to moisture in the open air, yet still allows electricity to flow, thus limiting the severity of corrosion over time. Issue is that stuff is a mess to work with. I only spray it on bus bars of packs that are brand new and / not going to removed and put into a different car in the future.

Rebuilt batteries are not brand new and periodic replacement of bad modules every 1-3 years is to be expected and I'm not going to grease up a rebuilt pack if there's a chance the pack will be worked on again by me or some other person that doesn't want to get their hands all greasy and nasty.

I'm not sure if this is the "technology" Dorman uses or not, but in general they're a dishonest operation selling over-priced rebuilt battery packs as if they were new and nothing could be further from the truth.

 

We make jokes about that test on Dr. Prius app... There's not enough of a frame of reference to make sense of yet, especially wen it estimates how much longer the pack will last, which is a highly problematic estimate to make. One thing it does do that's helpful is point to Block #4 as being a problem and that could be the source of a failing cell creating your high temps.

 

You're right on track with your plan... I always recommend first cleaning cooling fan and then replacing 12v to get all that out of the way before focusing on battery pack... Have you thought about what battery to buy yet?

Myself and @Britprius who is wheelchair bound are big advocates of 55amp hour wheel chair batteries.

Traditional cars use 12v batteries mostly to start the engine, aka: cold cranking amps.

But in a Prius the hybrid battery starts the engine so instead of turning over an engine with 12v you're doing it with high voltage or ~220v.

Because of this the 12v system is primarily running the computers and sensors so the most corrupt garbage battery builders like YellowTop Optima can get great reviews from misguided Prius owners and other 12v battery sellers can rip people off selling small-size batteries with high cold cranking amps in the near $300 range.

So @Britprius and I tell people to save $100+ and head for the most commonly used computer back up power supply / wheelchair battery in the 55amp-hour range (20amp hours more than the ones the auto parts stores sell for way more money / cold cranking amps). For example a year ago I bought for a friend's Prius from BatteriesPlus for $140: Duracell Ultra 12V 55AH AGM SLA Battery

These are the most common 12v batteries sold so they don't sit on a shelf growing old before you buy, they just grab 'em off the pallet from the latest delivery. The only challenge is the bolts that mount the terminals don't match up so you need to head to hardware store after buying the battery and purchase some longer bolts, as well as some correct size bushings, so the terminals mount to the battery properly.

I installed one of these in my Prius in 2013 and though I get signs it's weakening, had I spent more money on something designed for non-hybrid cars I'd already be on my next battery by now.

 

Direct fit, no terminal post modifications needed (JIS posts), 45AmpHr SKS, 12V AGM battery
$146.24 Bosch, 4yr straight warranty, no proration
$167.28 AC Delco, 2yr straight warranty, no proration

 

Pep Boys usually has the Bosch. Their website is a bit tricky tho, to enter vehicle type, find correct battery. But it's in there, somewhere. Or just phone them, let them do the checking.

 

I have the same mpg problem and could not find the Bosch any more at pep boys. Ended up buying one from the dealer for $202 - $18 for core refund = $184 total. Used a 15% off Costco coupon. Both Toyota dealerships around me participate in the Costco vehicle repair program, which is excellent by the way.

The bad: The dealership won’t warranty your Toyota brand battery unless you have a receipt. It says Toyota and the date on the battery. What more do the actually need. It has an 84 month battery but nothing says you need a receipt, online or anywhere else I could fine. Feel like I got robbed for $100 since I only used 40 months. Very disappointed in Toyota. Keep your receipts!

 

Hey Caleb, just curious, were you able to replace the 12v battery? If so, did you MPG change at all?