Has anyone tried going straight into a deep discharge?

Hi there folks,

This question is in regards to reconditioning NiMH packs. I've seen some people (S Keith and Jeff from HA come to mind) suggest grid charging > deep discharging > grid charging, but I was wondering what the point o the initial grid charging is. Why not just go straight into a deep discharge > grid charging?

The reason I ask is based off what I was reading in this article Study of parameters causing plateau effect on nickel hydrogen cells (Sorry about the lack of a link. I'm new here so can't post one, but you can find the DOI pretty easily and find the article on Sci Hub if you're interesting in reading it). If I'm understanding the paper correctly, they conclude that gamma-nickel oxyhydroxide is responsible (at least in part) for voltage depression and that deep discharging below 0.8V/cell reduces that phase back beta-nickel oxyhydroxide, the phase we want for the proper functioning o the positive electrode of NiMH cells. They also conclude that this gamma-nickel oxyhydroxide phase is created during overcharging > 1.4V/cell.

Anyway, it got me thinking. If the paper is correct, perhaps doing an initial grid charging is creating more of this unwanted gamma-nickel oxyhydroxide phase. It may even be possible that the top balance grid charging after the deep discharge is also needlessly creating this undesirable phase.

It got me wondering, so I thought I'd ask - has anyone experimented with just doing a deep discharge followed by a grid charge or perhaps even just a pack charge to just 80% SoC. I admit I'm not completely certain about whether or not the "grid charging" term refers to the setup or the procedure or both, so sorry about the confusion if I'm using the term wrong.

Thanks!

 

But as I understand it, people who do grid charging are working at the pack level, not the module/block level. Any kind of capacity data you get from discharging an entire pack, won't help you with determining the capacity of a module/block, right?

 

Thanks! Good to know.

Not sure I understood what you meant by "this." Difference between grid charging-->discharging vs grid discharging/grid charging?

 

Thanks! I try.

Sure, I'd love to hear more details. I guess a good question to start with is... what is your method for reconditioning?

 

No, I meant I literally don't know what you were referring to with the word "this."

 

Yup, that makes sense. But if I'm not mistaken Truth Seeker, you don't do deep discharges, correct? By deep discharge, I mean <1.0V/cell (or in some research papers <0% nominal state of charge/ >100% depth of discharge, whichever one of these you prefer).

In the end, my question isn't so much about pack vs module level reconditioning, so much as it is about the sequence of charging and discharging. It would seem that those that do pack level work say to overcharge, then deep discharge, and if needed, repeat. I'm just wondering what the purpose of the initial overcharge is, especially when every research paper i've read so far says that overcharging is what changes the active nickel oxyhydroxide, on the positive electrode, to the inactive gamma-NiOOH.

Btw, I've loved reading some of your threads on PC (because I'm the kind of weirdo that loves reading forums). Your diligence and candor are awesome. You're a legend!

 

Ahh Gotcha. Yea, you right. In the end, like you said, batteries are different with different mileages and conditions. I imagine you'd have to do it more than just once to start seeing a pattern emerge. So I just thought I'd see what the more experienced people have noticed.