Don't discharge below 6 volts!

I have been studying NIMH battery tech in order to learn how to properly rebuild these packs.

A lot of good info can be found here: https://en.globtek.com/nimh-battery-safety-notes

Basically, the most important figure to determine the health of the battery is the rated capacity displayed after a discharge cycle. This is determined by the hobby charge applying a load, and measuring the amount of time it takes to hit a cutoff voltage.

Almost ALL NIMH batteries, including multi cell packs, are tested at the factory using a cut off voltage of 1v per cell.

Why is this important? Well some people are out there discharging to as little as 3 volts per module, or 0.5v per cell. This WILL NOT give you an accurate reading of capacity. The MAh capacity shown after a charge cycle is pretty much a pass or fail for the module, but pretty meaningless for measuring capacity as capacity counts during discharging, not charging.Same is true for car batteries when they advertise cold cranking amps. No capacity on any battery that I know of is derived from charging.

That site, as well as numerous other sites I have been studying for the past couple days state that discharging NIMH batteries below the cutoff voltage..which is 6V for a Prius module, is a bad idea. In fact the cars onboard computer systems will turn on the engine to prevent the hybrid battery pack from discharging below this. This is why when a pack starts reaching it's end of life, MPG suffers as the ICE is kicking on more often to keep the battery pack from falling below a cutoff voltage (not sure what that cutoff is on it's own onboard battery management system). But basically the computer will prevent a pack from falling below a cutoff, it's nearly impossible to exceed this unless something is amiss electrically within the car.

Below a certain voltage, cell polarity reversal can occur, since these packs are made of 6 cells. I am not 100% sure if cell polarity reversal can occur in the Prius NIMH pack, but can in other multi-cell NIMH battery packs.

Nearly all of the Youtube videos I've watched on HV pack rebuilding show the discharge cycles to no lower than 6 volts.

Now if anyone has any information to prove to the contrary, I'm all ears.

While I could not find this online, I am interested in knowing what the bars on the battery indicator mean as far as voltage goes. It is my understanding that the battery management system operates the pack at 40% to 80% state of charge, with 40% being purple and 80% being green on the MFD (although not sure what voltage this translates to).

By reconditioning the cells using multiple charge/discharge cycles that take the battery down and then back up to levels that go below and above the levels on the BMS, it is supposed to breathe in new life into the modules.

 

I am not really sure what benefit is provided by discharging to 3 volts per module.

Honestly I don't see the benefit to the equipment that Hybrid Automotive tries to peddle. Neither their charger or discharger shows the capacity provided when a full discharge cycle is completed to the industry standard of 1v per cell.

The capacity provided when the discharge cycle is complete is extremely helpful in determining which cells need replacing. When I tried calling Hybrid Automotive to ask some questions about their process of reconditioning, they acted like they didn't want to give me the time of day.

So that makes me think what they are offering is electronic snake oil. $519.99 for equipment that has no way of determining which individual cells are beyond saving. It seems like a giant waste of time discharging/charging all cells at once by the way of a grid charger if some cells are beyond hope, and no way to tell with that equipment either.

 

Apologies if I sound argumentative, I'm just trying to get a clear picture of this.

I don't really understand the point of grid charging and discharging. From this thread:
https://priuschat.com/threads/prius-traction-battery-repair-summery.191078/
Which is a summary of a 103+ page thread.

It is my understanding that by doing load testing PER MODULE, and not discharging the entire HV pack as one unit, you can see which modules need replacement after several cycles.

Or is the grid charging/discharging going on AFTER what faulty modules have been determined?

Also..

I understand that using a hobby grade charger is not going to show a super accurate capacity as I am not using Mil-Std or industry grade equipment with ±1% tolerances on components.

However the capacity shown after a discharge cycle could be off by 200MAh or more, but that isn't important. These readings are needed for a comparative analysis between modules, because as mentioned prior, if you have 27 cells at 5000MAh and one very bad cell at 1500MAh, your capacity greatly suffers.

I cannot see how you can make any determination of remaining life of individual modules by going whole-hog and discharging and charging all 28 modules at once, unless as I said earlier..this is to speed things up..AFTER a determination of failed modules has been made. If one uses the prolong system without any other form of measuring remaining life..you could very easily be spending $500+ and sticking the HV pack back into your car with 1 or more modules that didn't come back up in capacity and be right back at square one again.

Honestly..if I had the cash I would of just bought the $600 HV module that was pulled from a 2015 with 96k miles at my local pick-n-pull, as I'm sure a 2015 should have all good modules at under 100,000 miles. Shell is different, I would need to swap all the modules out.

 

So if I'm understanding correctly, if the capacity of a module turns out fine after say 1-2 discharge cycles, then the "hardcore discharging" would not be necessary, correct?

Would this method be like a "last resort" for cells that show signs of being near their end of life, but possibly harmful if the cell doesn't have much of a crystal structure built up?

The other issue I am trying to figure out..is when I did a "hard core" discharge on a couple modules, to 5v, let them rest, and then started recharging again..the modules swelled up quite a bit, about double as much as they did when I took them down to 7 volts.

I was only charging at 2.5A...other people are feeding 4-5 amps with minor heat issues. The issue is if the charger misses the delta peak on a lower voltage discharge, and the only thing it has to rely on is capacity cutoff..but if you discharge below 6v, the capacity needed by the charger to take it up to a full charge will be higher than a normal charging cycle from the 6v cutoff, which I think is somehow confusing the charger and allowing it to continue to charge once the battery is done, and once the battery itself is charged, the excess current is dissipated as heat.

I think in order to rectify that, you'd need a charger with a high voltage cutoff, rather than just a programmable low voltage cutoff, but it's my understanding these chargers don't have a voltage cutoff on the charging side, just discharging.

 

Is there a hobby charger that can slow charge to around 1v, then fast charge to the 1.44 and trickle back down again?

 

Did you discharge to 6 volt after going to 5 and 4v? Because if you discharge to 5 or 4 volts using the hobby charger, of course the discharge capacity is going to be higher as you've decreased the cutoff voltage which will take more time at a fixed current load, which is how capacity is measured.

 

So what is the most cost-effective way to recondition these modules without spending $500+ on a pro long charger and discharger, or waiting weeks for modules to trickle charge?

 

So by using the dedicated discharger what is the max discharge current I can safely use on a module? 6.5A?

 

Apologies, I'm half asleep and just now saw it after re-reading your post.
How long would discharging take at only 325MAh?
I noticed when I discharged at 2.5A, the battery really didn't get that warm, it seems charging generates more heat than charging, at least with my charger.

 

So is there any benefit to using hobby chargers over a grid charger like the prolong, or are people doing it because of the lower cost?

I've also seen people use a headlight bulb and record the before and after voltages after 2 minutes, what exactly is being done here?