Gen II Prius Individual Battery Module Replacement

THIS! NiMH batteries can be seriously damaged if you discharge below 6 volts. Fast discharge is also dangerous because if one of the 6 cells is weaker or has a faster self-discharge, you can actually 'empty' that cell (and the other 5 cells keep delivering current, *reverse charging* the empty cell). This kills that battery module in an instant.

This is also a reason to drive the car more gently as the battery gets older. If you have real-time battery monitoring, a pair of blocks easily reaches 12.0 volts under hard acceleration. If you have a single weak cell in the two modules (which is a total of 12 cells), then the cell gets reverse, and game over - red triangle. Worse, the car will try to 'recondition' that cell by charging and discharged the pack repeatedly (it can only charge or discharge the pack as a whole). This heats up the battery, and lots of heat is released by the dodgy cell, damaging the other module in the module-pair, and generating extra heat that heats up the modules on either side too.

 

Beautiful, thanks for this. /im about to put the first module on cycle. But Ive found that youre right, the icharger is the only one that will present the mAh after each cycle. So Im left with not really seeing the proof of improvement, except for the final discharge mAh, which would be the true capacity of the module in question. right?

How can this be? I've read almost everywhere that a module should never go below 6 V or serious damage will be done to it. Is this wrong?

what about delta peak senstivity? and cutoff limit? My charger is notorious for missing the delta peak wave and as far as i can tell thats what tells the charger to stop charging. But isnt the cutoff charge a fail safe , pressumably 6.5 Ah, but ive seen people set them as high as 8500 mAh.??

 

I don't know, I guess it depends who on the internet you are reading. It is not really a question of wrong or right, but what it is is super cautious.

There are those people who don't specifically know the answer to a question so make something up that seems to work for them.

The source of my information comes from Hybrid Automotive, who make the Prolong charger/discharger system so I figure they should know a thing or two about all this.

Having said that, the less healthy a module is, the riskier it will be to go lower, but my counter to this is if the module was going to be killed by going to 3 V, then you likely didn't want that module anyway and it would be no big deal because you already have the battery apart and, as you are dealing with the individual modules, it would be better to kill it and replace it. If you had the battery fully assembled and were grid charging, that might make you want to be more cautious if you were not in a position to open up the battery at that time.

[EDIT]
One thing I should stress, that I did not mention above, when discharging below 6 V/module (or charging above 7.2 V/module) the amperage should be around 0.05C (or 0.325 mA) to limit the possiblity of a single cell within a module reversing.

 

In what state the battery is assembled is a problem. The voltage balance at 7.xV does not seem to guarantee the battery balance.
1. Measure the capacity of each battery, and it can be used if it is greater than 3Ah. The larger the condition, the better. Dissatisfied elimination. In addition, pay attention to the self-discharge rate, and eliminate the modules with too fast self-discharge.
2. Discharge to a voltage lower than 7V. This may need to be determined by testing, not necessarily a fixed number, that is, when the battery is discharged until the battery is basically dead, the load current is greater than 6A, and the voltage is lower than 7V, before the rapid decline The moment is that state, ensuring that each battery is installed in a dead state.
3. It can also fully charge all modules. The voltage may be greater than 8.4V. There is no fixed number. After charging to 8.2V, it can be charged in series with trickle charging for a certain period of time to ensure that the modules are assembled in a full state.
4. It is best not to choose the state of 7.xV, it cannot guarantee the same power.
5. I personally feel that the installation is good when the battery is discharged to empty, so that the voltage of the battery can be reduced as much as possible to reduce the heat generation of regenerative charging.
This is just a personal point of view, and there may be many errors. It takes a lot of time to test the effects of various state installations, which is beyond my ability, so the above method may have many errors. I prefer to see someone who can provide a more secure approach and point out my mistakes.
Love your friends

 

Battery modules vent through the top through the 2 rubber tubes attached and routed to the floor of the trunk.

 

*** IMPORTANT NOTE ***
When working on a Prius battery check the connection between the voltage harness and the ECU.
Ideally, replace the voltage harness with a new one (ie part number is 82165-47040). There's also the black cable below it - the part number for that is G9242-47090.
I've gotten away with not checking or replacing these parts so far, but these cars are getting old now and it's much more likely now that they will have corrosion in that connection.
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For some more information and photos have a look at this thread: Photos - P3030 - Burnt Wiring Harness | PriusChat
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Last week I rebuilt a Prius battery for a friend of mine. I knew to check the voltage harness to ECU connection... and there was some minor corrosion.
I replaced the ECU computer with a spare one that I had, and fitted a new voltage harness.



There was very slight corrosion on one of the pins on the inside of the ECU:


The matching corrosion spot on the voltage harness connector:

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I only figured this problem out recently. A few months ago I rebuilt a battery for a Prius of mine, and now it's giving a P0AFA code.
The battery was removed today and the voltage harness connector checked... only to find substantial corrosion.
Luckily I had another spare ECU and managed to find a reasonable condition voltage harness. I then rang a Toyota dealership and ordered four new voltage harnesses for future use.





The pins on the inside of the ECU are clean, so I'll try cleaning the inside of the connector later to see if it cleans up properly.

 

Are there enough reports to suggest re-balancing your battery as a preventative measure will extend it's service life?

I've got a low mileage 3rd Gen (48k) that is coming up on it's 10th birthday from date of manufacture.
I got techstream running a while back to make a new key. I just ran a scan and no codes. Still ~49mpg but starting to think about battery replacement options for the future.

Would it be worth it to try to DIY balance with a hobby charger? It's not a daily driver and even if it's out of comission for a few weeks I can drive something else. Seems like it could be an interesting project but only if there is some actual benefit. Would replacing any weak modules early potentially extend the overall lifespan?

 

One of the major causes of NiMh HV battery problems is that an imbalance of the cells/modules can result in a cell reversal when the battery is discharged to minimum. This destroys that cell and your Prius display lights up with maintenance warnings.

Rebalancing will remove this failure mechanism and extend the life of your battery. As your battery ages, the overall energy storage capacity does decrease, but not evenly across the 288 cells (28 modules, 6 per cell). The lower capacity will not noticeable affect your Prius, but a failed cell does.

JeffD

 

wouldn’t recommend this. all of your modules are aging under similar conditions. If you don’t have any codes currently, swapping modules prematurely would introduce more variation among the health of your modules

 

Thanks for the feedback.

Maybe I'll at least try cycling the modules and see how they perform. Is there any current charger that is recommended and will show current dischaged? The iChargers are a bit pricey but they also seem to have a feature to log to an SD card. Might be interesting to compare the discharge graphs similar to what the commerical pack rebuilding software seems to. Has anyone tried one of the cheaper single channel iChargers like the S6 or X8? They also seem to have this feature where you can add extra resistance which increases their discharge capacity.

As an aside, I was at the local Toyota dealership today to pick up some parts and they quoted me around $3000 CAD for a oem replacement which isn't too bad. The parts guy also said the battery failures they were seeing currently were in like 05s and 06s which surprised me. The weather here (Vancouver Island) is very temperate so maybe that is a major factor in battery longevity. He also said the local taxi cabs were using replacement modules instead of full OEM packs, suggesting that it is in fact the cheaper option and not just a rabbithole of whack a mole and wasted money. He also said some of them had > 1 million km on the odometer!

 

I wouldn't recommend removal, disassembly, and charging with hobby chargers at your stage. In addition, if you have no codes or any signs of battery problems, I definitely would not be creating a 'frankenbattery' by replacing modules. That is just asking for trouble.

My recommendation would be to grid charge in place.

Once you have the harness in place, a one-time effort, you can quickly set up and be going in 5-10 min.

I started doing this at the same point in time (i. e. at 10 years of age) with two Prii and have had no problems and so far both batteries continue to perform. Mine are now 15 and 16 yo respectively. I charge as required rather than on a set schedule. My indicators for when it is required are based on my fuel economy is not as good and noticing a 'softness' in acceleration and performance. The batteries never (so far) have shown any symptoms of charging/discharging quickly and usually still operate in the 6 bar range.

A grid charge session takes me about 5 days with the battery in place.

You can find a couple of turnkey chargers that work well (I use the Hybrid Automotive product) and also there is a thread here that details how to build a charger for under US$100.

I hope that helps.

 

I've been considering the grid charge option. How often would you say you end up charging the modules? Is the car out of service during the 5 days?

From what I have seen it appears the middle of the battery pack tends to fail first so even just opening up the pack and reshuffling the modules would probably result in a longer lifespan.

Can grid chargers balance the modules as effectively as charging them individually?

 

Is anyone using an external discharge bank with a 2+ channel charger? From what people are saying discharging the cells at 10A is the limiting factor in these cycles. By hooking one up to the output channel of an icharger dual channel charger you can discharge at up to 40A. Is there any limit to how fast you want to discharge when rebalancing the modules?

 

Here's a video about building a load bank:

 

Generally speaking, yes. We have another car to use when I do the maintenance, but if something unforeseen comes up you can suspend the process and put the car back into service. Once the emergency is over you can recommence the process.

In my opinion, balancing using a grid charger is superior to charging individually with hobby chargers if only because it charges all modules at once. Keep in mind grid charging is a maintenance process and not a repair process.

Maybe, but unless you are monitoring the individual block voltages and seeing some sort of issue, I still would not mess with disassembling the pack.

 

@dolj Wymień szynę zbiorczą. M=2011

 

Would you say that you do the procedure every 6 months, once a year, every few years? I guess I am a bit nervous to be messing with the battery in it's fully wired together state. The appeal of the hobby charging route is that the safety plug is out and you're just dealing with low voltage individual modules. I am a bit leery at the thought of working with a harness wired into the whole enchilada.

 

With this cycling procedure on individual modules, is the procedure primarily just top - balancing the modules? Or is balancing also happening at the low end of the discharge cycle? What is the fastest rate you can discharge these things (and still have the cycling be effective at increasing capacity, obviously they can handle massive discharge rates when in the Prius).

 

Lots of info in Wiki DiY Grid Charger
DiY Grid Charger | PriusChat

TMR-JWAP threads
Just Another HV Battery Thread and Experiments | PriusChat
Dorman HV Battery with a new lease on life..... | PriusChat
Dorman Battery Experiences | PriusChat

 

I found that it is the slow, slight overcharging of a module that gets the cells within the module balanced. The cells with a higher State of Charge (SOC) dissipate the extra energy as heat while the cells with a lower SOC catch up.

The discharge cycle give you a measure of the module capacity after each charge/discharge cycle. Of course, finish with one more charge cycle. I found that going past 3 full cycles was diminishing returns for NiMh modules.

JeffD