Gen II Prius Individual Battery Module Replacement

Cell polarity reversing while on discharging is just a theory. This happens when a weak module sits self-discharging for weeks.
The battery I just cycled had been discharged for about 30 hours. I forgot about it because i didn't really care. That battery came from a recycling management, pulled from prius with over 200k miles.
The voltage was 0.3 volts for all 28 modules, after discharging the whole pack for ~30 hours.
On the same battery, I left it discharged to 1.5 volts for 4 hours with no load. Now it's charged to 239.5 volts. I'll test the battery in a few days when i have time.

 

When you are discharging a series string of cells (6 cells in series for a module in my case) and their initial SOCs differ, the lowest SOC will run out of charge before the others. As the same discharge current goes through all of the cells, if you continue to discharge the module you WILL reverse that "weakest" cell. That is what happened to my 2004 Prius battery at 195k miles which started me on studying the process of "balancing" modules.

JeffD

 

Hi all, I am just calling back past here after about oh 7 and a bit years lol

So the pack I rebuilt for my daughter's 2008 back at Easter time 2017 just let go after lasting almost as long again as it did from new! 7+ years and about 130/140k km.. not bad! sorry Kiwi.

After all these years the failed pack still has capacity but the car sat for 9 months while waiting for an engine swap then soon after getting back on the road .. P0AA6 526-612 = UR STRANDED! hey it's been 7 years, I'm not complaining.

So.. I put another "tested good" spare 2008 pack aside around a month after this one got built back in 2017, I have grid charged the spare pack (along with a few others I've picked up) every 12 months since

Last couple days I have been testing the spare pack ready to put it back in service, open circuit voltage suggested every module is still in great shape with the acception of positions #1&28 which are always bad around here even on the best packs! but I found this one module in position #7 with lower capacity and higher ir than all the others, then I noticed it suffered a broken vent neck while in storage, which leads me to question! ...

How or why would mechanical failure of the vent neck cause the said module to deteriorate faster than the others, I mean the vent it's self should still work fine right? just the neck for the "stinky tubes" has snapped off, what am I missing here?

My first thought when noticing the broken neck was to simply plastic weld it back down but obviously the broken vent has compromised this module somehow, I just can't quite get my head around how unless it's let oxygen through or something?


Matt

Posted via the PriusChat mobile app.

 

Why have you never restored the capacity in this battery in 7 years and just let it reach the point of no return?

 

Hi Carall,

Not sure how you got that idea sorry, I done quite a few grid cycles on it over the past 7 years that it was in service, it ended up failing with a leaking module not low capacity!

Leaking modules are becoming very common around here! particularly on J prefix modules!

Granted I should have done a cycle or two after the engine swap but didn't have time the daughter needed her car back, seeing the pack failed with a leaking module I doubt it would have helped anyway ..

The "spare pack" that I am currently working on was also deep cycled/ reconditioned before storage and topped up annually BUT it has a broken vent neck on one module which has somehow compromised capacity of that module, I'm wondering if anyone has ideas on why the broken neck would have such effect

Posted via the PriusChat mobile app.

 

You wrote that you rebuilt the pack in 2017, 7 years ago. I didn't know the whole story.
Where has this battery been used: city traffics, country roads, highways?

 

No worries Carall

Its mostly used in the city, not a lot of highway.. I only mentioned the failed pack for interests sake, and to lead on to my question about ....

Another pack which sat in storage (with annual top ups) for near the same amount of time (2017) one module in this pack has a broken vent tit and that module now has lower capacity compared to all the others In that pack, it's doing my head in trying to figure out how a broken vent tit could compromise capacity

Cheers

Matt



Posted via the PriusChat mobile app.

 

Perhaps evaporation of the liquid electrolyte..

JeffD

 

Hi JeffD

Good thought, maybe the broken tit let's it gas out more than all the others when I do annual top ups

Posted via the PriusChat mobile app.

 

Do you do any maintenance to the 2020 NiMh battery?

 

My 2004 (NiMh) battery lasted 195k miles and 8 years without any maintenance. It would have lasted longer with a periodic rebalancing, but I did not know that at the time. My 2020 AWDe is now 4 years old and I no longer drive as much as I used to (about 12k miles per year) so I should be good for another 6-8 years without any battery maintenance. In 2030 I could consider getting or borrowing a 200v battery charger to rebalance the battery if the price is right.

JeffD

 

I think I have a couple of modules on their last legs. I’ve had the P0A80 code a few of time, always at slow speeds when it’s hot. After monitoring with Dr. Prius I think it’s a couple of the modules in the middle of the pack. But my gas mileage appears to be as good as it ever was according to the car. I plan to start tracking the gas mileage manually and the battery temp with Dr. Prius as I drive. I think I’m only getting the errors when the batteries get hot.

I’m considering a few options:

1. Do nothing, drive the car to failure. But I want to be confident in the car and not get stranded.

2. Buy a new OEM battery. $$$ but the original lasted 10+years/150k+ miles. If the replacement does the same the car will probably be near the end of its life.

3. What I’m currently leaning towards, buying some reconditioned modules. This is about $800 and 1/3 of what I think would be a good price for a new OEM battery. This would give me time to recondition my current modules and then I would have a spare set. I could also use something like a DC6 charger since time wouldn’t be an issue.

Feedback appreciated.

 

Is there another big thread (like this one) which discusses "reconditioning" existing NiMH modules?
With that said, I observed a great deal of useful information in this thread.
So I'd like to add some bits from my own experience with "cycling" NiMH batteries to improve their mAh:

First of all, both charge and discharge must be performed very slowly in order to prevent permanent damage.

Fully charging NiMH cells to the very top may require more than 1.5V per cell.
However, charging above 1.4V per cell converts most of the incoming energy into heat.
One possible method (I haven't tried this yet) is to apply charging current in pulses.
Perhaps a high charging current (5A for a Prius module) could be applied for 1-5 minutes followed by 20 minutes of allowing the module to cool?
The goal would be to reach 1.5V per cell without raising module temperature more than 10 or 20 degrees.


Fully discharging ("deep discharging") presents the issue of "cell reversal" for sealed multi-cell modules such as Prius modules.
I've read several reports that a "deep discharge" down to 0.4V per cell may initiate chemical changes inside the cell which are capable of increasing cell mAh capacity by a considerable amount compared to terminating the discharge cycle at 1.0V per cell.
However such a deep discharge must be performed at very low currents in order to prevent permanent damage from cell reversal.


I'm beginning to experiment with discharging in stages:

For example:

• First stage: Discharge to 1.2V per cell at a rate around 0.5C. This would be 1.5 to 2A for Prius modules.
• Then pause for several minutes.
• Second stage: Discharge to 1.0V per cell at rate of 0.1C (or even lower). For a Prius module 0.1C is 650mA.
• Another pause
• Third stage: Discharge to 0.4V per cell at an extremely low rate. Perhaps only 50 to 100mA for a Prius module? Because the cells are mostly depleted after they reach 1.0V per cell, this final "deep discharge" won't take forever.
• When testing this "deep discharge" I watch for a sudden voltage drop which would indicate one cell has rapidly discharged to 0V.

At this time I'm testing uncased strings of small cylindrical NiMH cells at my bench, such as the sub-C variety commonly used in small handheld power tools. I continuously monitor each individual cell for "reversal." My thinking is that if a cell discharges to 0V and remains at 0V that is probably OK. What must be avoided is allowing a cell to actually "reverse" where the voltage across it becomes negative.

If slow (low rate) charge/discharge cycling actually works to increase mAh capacity, then it seems feasible that one could apply this same process to a full Prius pack of 28 modules.

So-called "grid chargers" appear to perform OK with the charging portion of this cycling process.
However, it seems the "deep discharging" portion of the cycle is often performed by loading the pack with several incandescent light bulbs wired in series (two 120V incandescent bulbs in series can handle 240V). However I doubt that such an uncontrolled discharge will prevent cell reversal.

Thoughts?