HV Battery Sudden Discharge

This is a known problem with older Gen2 Prius that's starting to show up in Gen3. We used to think they fixed this problem in Gen3, but as those cars get older it turns out to not be true.

Basically, the antiquated NiMH battery chemistry seeps out of the seals at the terminals in super tiny amounts that creates corrosion that damages the voltage sensor system. Specifically, the corrosion builds up inside the BMS where the voltage sensor harness plugs into the pins that connect to the circuit board. Once the corrosion is thick enough, there will be tiny micro-arcs of electricity between pins via the corrosion causing the BMS to suddenly think the battery has lost alot of its charge.

If you got some spare time you can fix it for free by pulling the pack and cleaning everything, though replacing with brand new bus bars, nuts and voltage sensor harness might last longer. But not necessary. Plenty of youtube videos will show you how.

Also learn more about restoring capacity of older NiMH packs: FAQ - Hybrid Automotive

 

Has this been confirmed by someone in practice or is it just another someone's theory?

 

Lol, If you want to confirm it to be true simply look inside the battery ECU for corrosion. Or alternatively just ignore it... That works for most people for a while. That's what I did 2014-2018 in my Gen2.

Or most expensive alternative is to wait for it to get bad enough for warning lights and take it to a Toyota Stealership where they'll rob you for as much as $6K telling you battery pack and ECU failed because corrosion is part of their planned obsolescence profits even though I've many times repaired this problem in people's Prius for a few hundred dollars.

 

I don't have direct experience with a failing Aqua/Prius c battery, but "sudden charge plunge" is a pretty common descriptor that comes up in Prius forum posts shortly before "I need a new HV battery."

Going by the calendar I'd think it's plausible that the battery is not long for this earth.

As always, check the battery cooling fan underneath the left rear seat. Keep that thing clean for the longest battery life and best health.

 

The quotes are going to be super expensive and the Toyota Stealerships refuse to clean stuff, only replace the pack and the ECU with brand new at a hyperinflated price, which is uneccessary and super expensive and wasteful.

There's plenty of youtube videos on how to do every step. I suggest you spend a fraction of the repair quotes on paying a friend or family member with basic mechanical skills to watch the YouTube videos and do the job with you. That way in the future, if you run into other problems, you'll have more skills and will be more confident in your DIY ability. This is something that can be done in a couple-few hours and I'm sure you have a friend or family member that would greatly appreciate a hundred dollars for their time.

 

There is a member @dolj that might know someone near you that can help with your battery

 

You're basically saying:
Electrolyte seeps from the terminal and causes corrosion on the voltage sensing tab.
It seeps along the length of the wire(s) and also corrodes the pin(s) at the ecu plug.
?

Cut and inspect the wire down a bit from the sensing tab.

 

I'm not exactly sure how corrosion travels... I don't think it travels in a straight line under the insulation, so much as it builds up in bare areas where exposure to the air gives it enough humidity to grow well. It doesn't take much corrosion to cause this sudden drop in state of charge problem.

Here's an ECU that had this failure in a prototype Lithium pack that I didn't use brand new busbars and voltage sensor harness, just cleaned them. In October 2020 this ECU was clean. But October 2022 it had this much corrosion, which was enough to cause these symptoms, which is one of a few problems that caused Jack to have to add more overcharging protection to his module design. He always tells people to use new voltage sensor harness and bus bars if you own a gen2. And I just replaced my first Gen3 ECU with failed pins from corrosion this winter.

 

And check any other connectors and boxes wherever it grows or magically appears out of nowhere.
So many boxes and connectors, so little time, most of the time these days.

Rememberin the pics ( probably in the wayback machine by now ) of peeps that wanted to grid charge faster than was recommended by the charger manufactures and without enough cooling even for the much slower charging rates. - >