Don't discharge below 6 volts!

From what I can see the biggest driver is cost, but the trade-off is time. You can get some time back by spending more on multiple chargers.

The other advantage is the granularity of data. working with individual modules you can get a lot more data.

It is just another (and cheaper) way to skin the cat. The purpose is load testing. Fixed load, fixed time. Record start and end (under load) voltages. You calculate the elapsed time and use that to compare modules. The ones that are similar in elapsed time are keepers. The ones where the elapsed time is greater are suspects that require work or throwing in the trash. Simple as that. You can evaluate purchased modules using this same methodology to compare them to your existing modules. If the time is similar to your existing modules, use them. If they're way off, don't use them. Again simple.

 

Individual NiMH cells can be discharged to zero volts and so can multiple cells in series, provided that the capacity of the cells are closely matched. The problem comes when cells in series are not matched or have become unbalanced over time. In that case, it is possible for one of the cells to go to zero before the others have finished discharging. Current continues to flow through the cell at 0v and it can reverse polarity. Once this happens the module is toast!
Personally I think that the discharge voltages recommended by the Prolong system are too agressive. If I was discharging the pack as a whole, I would be monitoring individual module voltages in addition to the total pack voltage and if any individual module fell below 6V, I would halt the discharge of the entire pack.
With the battery in the car, maybe it is possible to monitor block voltages during discharge using PriusApp or similar OBD2 scanner?

 

After enough years of NiMH pack rebuilding you learn alot... I've managed to successfully restore reversed polarity modules, but once you load test after restoration they never test strong enough to risk putting in a pack.

Also in the early years I was so hyper vigilant when discharging with 50w 12v lightbulbs because I didn't want to damage anything. But then inevitably I fell asleep while working and when I woke up I forgot I was working on a pack until about noon the next day. When I finally realized it all 7 modules had been sitting still connected to lightbubls and at 0 volts for nearly 12 hours. But soon as I disconnected them and started charging the voltages went back up real quick and these modules load tested fine. Was surprised about that.

I suspect had I been using a solid state resistor rather than lightbulbs I wouldn't of been so lucky...

But in general NiMH sucks and after 5 years or battery rebuilding for friends and family I'd rather talk people into buying Project Lithium packs were better by every measure, especially MPG. Free unlimited tech support if you use my affiliate link too: https://projectlithium.com/?ref=9qLPw

 

I imagine shipping to the other side of the Pacific would make PL economically unviable. New OEM is still really the only option here.

 

Nope... Not only 1/2 the weight of OEM packs, but Project Lithium is distributing world-wide and is getting popular in Europe. There was a time a 1/2 dozen years ago when shipping LiFePo4 was not easy, especially because of Boeing including them in their newest airplanes that caused a couple fires. But as the years go by some many lithium batteries are getting shipped world-wide every day without issue that the rules have become way more lax. Of course there are some countries and some carriers that are still creating a hassle. And I bet New Zealand probably has more strict standards than most.

 

A module being under a load won't reverse polarity, within 24 hours at least. If you leave it to self-discharge, then a cell that has discharged to ~0 volts will reverse polarity.

 

So you are saying the current from adjacent cells that are still discharging will not cause polarity reversal in a cell at 0V, but self discharge current will? Do you have a technical reference to back this statement up?

 

Then you would be missing 99% of the value of using the Prolong System.

 

A previous discussion:

Inconsistent Hybrid battery readings | PriusChat

 

I see the value of the Prolong System, I just don't see the necessity of discharging the lowest capacity cells to 0V and risking those cells becoming reversed.

 

I came to this conclusion by building a recovery station for a single module and I tried to restore modules of varying degrees of degradation. You most likely will not find a technical reference for this online, i couldn't, but if you will search for it in paper books i think you'll find it.

 

<shrug>

I did a DIY "refurb" on my modules when I first got my 2006. A few modules didn't do much when cycling down to 6.0V.

They improved to 5500mAh in line with the rest after I went down to 5.0 to 4.5V. Don't know the theory, just know it worked.

That pack was ok for another three years and 50k.

Posted via the PriusChat mobile app.

 

Thanks for the info. I assume that you were discharging one module at a time.
The first cycle discharge value (134V) for the Prolong system is 0.8V per cell or an average of 4.8V per module. But that is the average across 28 modules, so depending on their capacity, some modules will be significantly above or below this value. Especially on the first cycle.
My point is that we should be monitoring individual module (or at least block) voltages during the discharge cycle and not relying on the total pack voltage alone.