Super Brain 989

I suggest that you match them as closely as you can. I go for within a couple of hundredths of a volt. That, of course means that you adjust every module in the battery.

JeffD

 

Bob, I was wondering if your lost capacity happens to match the color map in Figure 9 of this

(i don't know the numbering of where they were in the pack, so I mean do the modules on the furthest left by the ECU still have more capacity then those on the far right, all other things being equal.)

 

Not only get the voltage to match, but check the voltage again after all the modules are the same temp. You might also monitor the voltage after 24 hrs to see if there is any self-discharge that is much higher than other modules.

 

Hi Joe,

The cell numbering starts from "1" closest to the control electronics. So when I removed my NHW11 modules from the old traction battery case, I numbered them. They already had red and black dots showing the polarity that is embossed in the case.

I had the paper "Thermal Evaluation of Toyota Prius Battery Pack", Zolot M., Pesaran, A., and Mihalic, M., National Renewable Energy Laboratory, 2002-01-1962. In "Prius Technical Stuff" I had collected and plotted the voltages of failed traction battery packs as a function of position. With one exception, they were "U" shaped with the strongest (aka., highest rest voltages) on the ends going to weaker modules in the middle.

Bob Wilson

 

Looking for "formation" charge protocols for NiMH batteries, I came across:

"INVESTIGATION OF THE RESPONSE OF NIMH CELLS TO BURP CHARGING", Darcy, Eric C., Doctor of Philosophy in Chemical Engineering, University of Huston, December 1998.​

Lessons learned:

• Excellent review of electro-chemical reactions - oxygen generation and transport is the key to cell heating and charge limits. In the past, I had thought both O{2} and H{2} might be generated but the primary, parasitic reaction generates O{2} which makes a lot of sense. H{2} has an amazing ability to permeate through just about anything and if it had been a primary gas, these modules would fail too soon relative to their observed life.
• "burp", "pause", and "galvanostatic" charge characteristics - it turns out O{2} formation is 'the enemy' and the "burp" charge mechanism, a brief, rapid discharge during the charging cycle, results in lower O{2} formation. In effect, the brief discharges shrink and reduce O{2} formation. Eric Darcy quantified the effect giving insights as to how a smarter, NiMH charger should work.
• ICS1702 charger IC for NiMH/NiCd cells - this IC can be used for a multi-cell stack such as our Prius modules. An interesting device, I wasn't aware of it until this paper mentioned it.
• O{2} generation mostly occurs in the last half of the charge cycle - this is when the brief discharge is most effective. Although the charge time is directly impacted by the 'burb', reduction in O{2} masking reduces 'flaking' of the nickel electrode and more than compensates for the improved NiMH capacity and life. There are charts clearly showing how galvanostatic leads to significantly shortened life, ~200 cycles, and capacity but use of the "burp" actually increased the cell capacity over the 400 cycles of the lab test.

This changes my NiMH battery charger design. I won't be using the ICS1702 but will accomplish the same effect using other circuits.

Bob Wilson

 

Let me try to clear this up for you b/c you're getting conflicting info. Ted is correct that the module has to be w/in .3V of the rest. Anything more will trigger an error/DTC.

Jeff is giving you excellent advice to get them as close to each other as possible to avoid future problems (if they are just w/in .3 right now there is a good chance they will go outside of .3 in the future That's why Jeff is recommending .02 for all modules (to limit the potential for future problems). This is excellent advice.

And... JoeDirte is giving you good advice to test them all at the same temp b/c there are voltage variances at different temps (which is another good reason to heed Jeff's advice).

And... Bob is talking over my head again. Does he not realize I'm from KY? We don't have any rocket scientists here (like Huntsville) but, we do have a lot of people like Jed Clampett!!!!!. (lol).

So, to sum it up, discharge/charge all modules to w/in .02 (if possible) of each other (slow discharge/charge to limit heat) and check them again after about an hour (they should all be at room temp by now). If they are still w/in .3 (which they def should be), reinstall your pack and enjoy.