Gen II Prius Individual Battery Module Replacement

Most Gen 1 batteries in 2021 are now junk. It's common for Gen 1 to self discharge if left to sit for months or a year. Any module that has fallen below 7 volts from self discharge cannot be brought back to a useable condition.

Try any method of charges and discharges you like until you come to the same conclusion.

 

I will do! I just looked at the readings, most of them were 1-2 volts, with the exception that the first two modules at either end were better (one was 7.5v each for the block, the other was 6.3 volts each for the block - an extreme example of the cells getting damaged in the centre!

 

Hey guys, So after my traction batt died, I rebuilt it with a few replacement modules, cycle charged and rebalanced as per everyone's recommendations. I reassembled and it worked great for about 2 weeks, then died again with codes P0607 and P0AA6. I'm assuming some of the other modules have leaked and caused another short. I just ran the DrPrius diag on it to get the codes and attached is the battery status. Once I find the obvious leaker(s) what else should I look at based on this chart? I assume blocks 6 and 10 for sure? Thanks everyone.

 

What stands out to me is blocks 13 and 14 being 0.09-0.14V higher than the next highest block. All the other blocks are within 0.9V of each other.

 

Between the Gen 2 and Gen 1 cells, I’ve found an interesting pattern; the cells charged up (even Gen 1), and the capacity is good overall (proper cycling not yet done but slow charge and then measured discharge shows good capacity).

The pattern I have found is basically the modules are either good, or..... no matter what I try, a small subset will drop down to 6.98 volts after a few minutes even after slow charge for several hours. This is unrelated to whether they had self-discharged (only 3 modules of the 38 Gen 1 do this, and a small number of Gen 2 modules).

It seems as if the modules are all pretty good, and a small group have one ‘fried’ cell. I assume this is due to those cells getting ‘reversed’ at maybe 30-100 amps briefly while in the car and therefore being destroyed in an instant. Not sure what to try to fix this (the NWH20 modules, about 5 of them across two batteries, have this exact problem; the module took a good charge otherwise... basically 5 good cells and 1 ‘dead’ cell

 

As dolj noted - blocks 13 and 14 are at a higher voltage. What modules / blocks did you replace? One thing to do is to see whether blocks 13 and 14 are stronger (eg if they hold voltage better while parked with air conditioner and headlights on and battery is draining), or if they are weaker (faster voltage drop than the others).

Do check the battery ECU once you are in the pack again but it probably isn’t that. The leak should be fairly obvious hopefully?

 

Thanks Guys, Yes I believe modules 25-28 were replaced as well as #15 so those were "new" ones I put in. Maybe that's why they are showing a bigger charge. I will have to do more testing once I reassemble I guess. Right now it's disabled and won't start or get out of neutral.

 

FYI just took your advice and went out and ran the a/c and lights for about 10 mins while watching the monitor. Didn't seem to change much.

 

Another thing that can happen is that those 2 blocks have a higher charge than the rest; this can actually ‘fix’ itself in the car; basically when cruising on the highway, the car charges everything a bit, discharges everything a bit. It does it slowly and ends up balancing all the cells this way (higher charge == faster discharge etc). I would watch the temperature and pattern of cell voltages when driving; also maybe discharge those two blocks a bit with a hobby charger / discharger (slow discharge till same voltage as at rest).

I hope the other issues are fairly simple once the battery case is open!

 

Me too, thanks!
Yes, I just rebuilt this battery and balanced all of them before putting it back in so they were all equal a couple of weeks ago.

 

I would suggest that you get a scantool that is capable of reading the sub code (aka the inf or detail code) for P0AA6 as this will help direct you to the problem area. Many here purchase a mini VCI cable with a (pirated) copy of Techstream (the OE diagnostic software) from ebay or amazon and install it on an old windows laptop.

Your problem certainly could be in the high voltage battery but without the sub code that's a guess. If it is there then you have to disassemble and inspect. Also carefully measure for voltage from one of the battery bus bars to the car body. (Normally there should be none) A seeping module that can throw an isolation fault often does not show any unusual voltage readings on a scantool or app.

For any "internal fault" code on any ecu, the first step I would take is to charge and load test the 12V battery - or just replace it if it's more than a few years old. A weak 12V can/will cause lots of strangeness on a Prius.

Posted via the PriusChat mobile app.

 

Hello guys!
Because the english is not my native language, I have to ask what do you mean by "balanced" the modules?
Thanks!

 

Each Prius NIMh module has 6 individual cells connected in series to have a 7,2 nominal voltage and there are 28 modules connected in series in the HV battery to obtain the nominal 201.6 volts. You want all of the cells in each modules at the same State of Charge (SOC) which is a "balanced module" and all of the modules at the same SOC in the battery.

As you use your Prius the modules (and the cells within each module) will tend to drift apart in their SOC. Your Prius Battery Management System (BMS) only monitors the Modules in pairs and will set an alarm if their paired SOCs differ by more than about 0.3 volts. You can "rebalance" a module by slowly overcharging it (careful, do not overheat it and keep it clamped as there can be a build up of internal pressure and cause the sides to bulge). This allows a weaker cell to catch up as the stronger cells will dissipate the excess charge as heat. The same process can be done on the whole battery at once with a high voltage charger (one is available on this web site) as long as you again watch out for over heating the modules.

 

Thanks Jdenenberg.
In case of Prius 2 modules, what it means "slowly overcharged" ?
Over 6500 mah? Over 9000 mah?
I charged them with 1,5Ah until 6500 mah...
Is correct or I messed them up?

PS: On my charsoon I have the option: "forming charge" which is described as " aiming to eliminate the imbalance between cells" ....

 

Thanks, yes I've been thinking about getting one of those scantools but not sure which one to get or how to get the Techstream software.
It does have a brand new AGM 12v battery.

 

How accurate would detection of bad cells be if we were to mark the voltages when battery is "fully" charged and then remeasuring after battery is "fully" discharged (as far as the toyota ecu lets us discharge it), then seeing which cells module has the lowest voltage and replacing it?

 

If you search amazon or ebay for "toyota mini vci cable" you will get a bunch of results for a cable that comes with a cd. Bootleg software can be fiddley to install but the seller might give support. If you are concerned about malware don't connect the laptop to the web, or install onto a virtual machine.

Posted via the PriusChat mobile app.

 

I finally had time to pull the battery today. Before removing, I checked voltage from the terminals where the plastic connector goes. The top terminal to ground showed 1.69v and the bottom terminal to ground showed 4.32v. I checked battery voltages and removed each one for inspection. As suspected, the 4 "new" modules which were located next to the ECM (Is that #28 or #1?) were at 7.8v and the rest were at 7.74v. I'm assuming they are in better condition than the others. I had all of them cycle charged at least 3x and then connected all of them in parallel for a day or so and had all voltages equal. I did not see any signs of leaking acid in the tray, like I did the first time. However, the 2 modules farthest from the ECM (again, is that #28 or #1?) had a spot on the bottom where it looks like a short circuit charred them where the steel bar runs under them. (see picture) All of the others look fine. So, should I get 2 more modules and try again?

 

More experienced members will likely reply, but...

The ‘charred’ modules don’t look ‘that bad’... it is weird how they burnt - a circuit must have formed and briefly ‘short-circuited’. Regarding the modules, a charge / discharge cycle would be interesting (and, after that, if they aren’t leaking, they are probably ‘good’.

The next issue is figuring out what actually happened to cause the short circuit in the first place... basically ‘how did the bars at the bottom become ‘live’?

There has to be a circuit from a terminal of one cell, which then somehow reaches the bars at the bottom of the pack; it is likely the final part of the circuit will be between that ‘burnt’ area, and one of the terminals of another cell.

As a warning, there seems to be a ground fault; the battery case could easily be high voltage.

A leak from the top of a module could easily get into the metal plates on the sides, and from there just a small jump from the bottom to the bar the module rests on.

Assume the casing is ‘live’ etc; check for electrolyte leaks; also... check everywhere for evidence of condensation (which would ‘rehydrate’ any minor deposits of electrolyte, causing a short circuit).

Did you keep the car dry inside? I run the AC all the time on my NWH20 after experiencing the ‘short’ in my battery ECU; and original battery - no problems at all - and it is a few years later!

 

Can you check the voltage between the metal plates on the side of the module, and + / - of each module? The 4.32 and 1.69volt readings (to ground) seem as if a module is connected to ground - but connected *between* the cells inside the module?

I hazard a guess that the burnt module (or one adjacent to it) has a leak connecting one of the 6 cells of the module, to the metal plate? That could cause the sorting you saw - and a ground fault (and the NWH20 refuses to start if it detects a ground fault)