DIY Prius HV battery rebuild

Hi guys, first post here but I've been lurking since I was considering buying a Prius.
I was hoping that some of you guys could offer some advice for my situation.

I bought an 06 with 216k miles on it and what seemed to be a bad battery pack, the warning lights were on and the battery read at 1 bar. We left it idling to charge for an hour before I attempted to drive it home, an hour down the park way. On the way back the battery entirely cut out and I only had the motor to power me, according to the info screen the battery was not being used at all, and the car could barely make it up to speed with my foot to the floor perpetually.

Once I got it home took the battery out of the car and disassembled it and saw that a cell was reading at 6.8 volts so I replaced it with another I bought off ebay then reassembled the pack and put it into the car.
The car drove fine for about 50 minutes until the warning light went on and it quit using the battery entirely again. During the 50 minutes that it did work the mileage seemed to be lower then it should be, around 36 mpg.

I've noticed that the car perpetually thinks it had a full tank since I've filled it up when I got it, even though I've burned at least two gallons. The 12v battery also appears to be a Toyota one, is it even possible that it's the stock one?

I limped the car home and pulled the pack out of the car, read the voltage of every cell and then did a load test with a 55w automotive headlight bulb. All of the numbers seem normal so I'm not really sure where to go from here.
here are the cells and their results
cell #, unloaded V, loaded V
1, 7.99, 7.91
2, 8.03, 7.80
3, 8.03, 7.85
4, 8.03, 7.89
5, 8.03, 7.95
6, 8.05, 7.91
7, 8.03, 7.94
8, 8.10, 7.93
9, 8.03, 7.87
10, 8.04, 7.82
11, 8.03, 7.85
12, 8.03, 7.92
13, 8.04, 7.90
14, 8.03, 7.92
15, 8.03, 7.85
16, 8.03, 7.82
17, 8.04, 7.92
18, 8.03, 7.82
19, 8.04, 7.92
20, 8.03, 7.89
21, 8.03, 7.80
22, 8.04, 7.87
23, 8.03, 7.90
24, 8.03, 7.85
25, 8.03, 7.87
26, 8.03, 7.90
27, 8.03, 7.86
28, 8.02, 7.87

After searching and reading whatever I could find online I think the next step is to balance charge them? So I bough 6 DC6 chargers on amazon. here's their title since I can't post a link
"SKYRC 100% Original iMAX B6 OEM Balance Charger for LiPo, 1s-6s Li-ion,1~15cell NiMH Akku 50W Balancer Ladegerät Netzteil"
The car has been throwing a P3000 code but that's all my code reader will say.
Thanks for the help in advance!

 

Thanks!

 

The battery is out of the car now so I can work on it.
The safety lock was certainly pushed down because it wasn't originally, and it took me four hours to figure out it was causing the car not to start lol.

The thin wires running to the bus bars all appear to be in good shape, and I cleaned all of the buss bar's copper plates and nuts the first time I had the battery out of the car to clear off any corrosion. Nice and shiny now
The nuts were all torqued to whatever "26" is on my cordless drill's torque setting, which was snug but not over tightened when I checked by hand.

I have the scanner that they lend out at pepboys, it says it reads DTC codes but it would only show the generic p3000 high voltage code
As for the temperature sensors I've only removed one, it was the one on the bottom farthest from the ecu end of the battery on the last cell, but I put it back in place before reinstalling the battery the first time. I haven't touched the other temperature sensors so I assume they're still installed the way they should be.

I forgot to mention that once the car went into safe mode when I was driving it, the battery fan stayed on high. Not sure if this is standard or a sign of overheating problems?

I've been suspicious of the battery's ecu but if that was bad the car wouldn't run at all right?

Thanks for the quick response and all the help!

 

I thought I did load test them? I hooked a 55 watt automotive headlight to each cell, one at a time and measured their voltage drop, that is the data that I posted in my first post. Unless there's another test I could do?
I don't want to take it to a stealership because even after knowing the mechanic they still want 114$ just to read the codes.

 

Hey guys, so I left the batteries in parallel over night then put them back into the car and took it on a test drive.
I averaged 43 mpg which gives me some confidence, but during the beginning of the ride the battery fluctuated a lot going from purple to green in the span of five minutes. So I'm a little worried about that, the fan never came on loud enough for me to hear from the driver's seat though. Towards the end of the ride it stabilized around 2/3 full on the display.
My balance chargers are supposed to be delivered tomorrow so I'm debating just returning them or going through what appears to be an arduous process of balance charging all of the 28 cells

 

The car threw the red triangle of death again, but thankfully this time my code reader said that block 6 became weak.
I have an extra good cell I bought off eBay so all I have to do now if figure out which of the two in the block is back then swap it out and put the cells in parallel for the night. Any tips for identifying which cell is bad?

 

Holy crap! The connection between cell 17 and 18 melted the orange bus bar plastic to the terminals. Which is odly not block 6

 

Now it just so happens that the melted terminal ends would be block six if I counted from the non ecu end, maybe it's a coincidence but I'm suspicious. The copper plate that formed the connect where the melting occurred in now heavily corroded. Which is super weird considering I cleaned every copper terminal bridge before reassembling the pack the first time.
The entire pack is now measuring between 8.05 and 8.25 volts
There seems to still be liquid around the melted terminals, battery acid?

 

If I count from the fan end then the suspect cells would be block six, and yes the plastic on the cells are melted and there seems to be battery acid running down them, here's a pic.
I see liquid on the the ends of the modules under where the terminals melted.
I'm about to go out and take the pack apart to see the sides of the cells

 

That's the thing, they both read 8.1x and both do fine under a headlight load test.