June 23, 2015

NiMH cells delta peak

mostly, we are using Lipo batteries with some of LiFePO4 nowadays. and dealing with NiMH cells was not a pleasant experience too me.

NiMH cells are more powerful and have bigger capacities than NiCd cells. but, NiCd cells require less maintenance as we know

a friend found me 3 packs of NOS Panasonic 3000mAh batteries along with 1 pack of zapped cells. then I opened the packs and started to recycle them one by one.

I never charged or discharged the cells individually before. and forgot the delta peak setting of NiMH cells. I didn't like NiHH this much. I wanted to forget about them.

I tried this document from Novak electronics which I trust
it says do not discharge the cell below 0.9V each. it's correct for the most of the NiMH cells. but few cells from GP and Intellect were optimised for 0V or less than 0.3V discharge. I got Panasonic cells here, and 0.9V limit is pretty much right for me

and this documents says set the delta-peak to 4mV per cell, you can set it to 6mV if charging stops occasionally. and do not exceed 10mV per cell.

I got Team, Race, Sport, and Zapped packs with difference prices. it's around $130, $110, $90, $70 from the top

The cells are more than 10 years old. so, I cannot trust the labels anymore, but it still was good to test them with the original data tags. and the tags worked somehow.

higher grades are more expensive because they are only a few from the lot. and the worst ones sold as Zapped batteries without any data tag.

from this experimental with Panasonic 3000mAh cells. I noticed a few things

Better cells have lower internal resistance - less than 25m Ohm during 3.0A charging (18~25m)
Some of the cells are like new. charged up to 3800 to 3900 at 3.0A charge only from the 2nd cycling
Some of the cells work with 6mV delta-peak, and some are working with 8 to 10mV, and it keeps changing with more charging-discharging cycles
6 of 18 cells still are fresh from the 1st or 2nd charging.
20 to 25% of them are bad from now. I'll try more with them, then will update too.
The rest of them are different, some still has high capacities along with higher resistance (over 38m Ohm), some have low resistance with low capacities. I need to try more with them too

I already got 6 of best cells just like I bought them 10 years ago. and working with the rest of them. few of them have 38m to 51m Ohm resistance. and I believe higher resistance are worse than low capacity. higher resistance make the cells hot and make them dead fast

The recommended delta peak setting is around 30mv for 6 cell pack. but it's for one pack. some of the single cells are too sensitive. and single cell charging can require bigger threshold than 5mv which is 30mv/6. it's my experience from these 10 year old 18 of Panasonic 3000mAh cells

will update soon with the discharging rate. I'm working at 5.0A and collecting the data

[update after 5 days of cycling - Jun 28th, 2015]
All of the cells had around 5 times of charging-discharging cycling.
none of them are hot, up to 19V, nor has 50m Ohm resistance.
some of the cells were good as I mentioned before. and they have only 3 to 4 times cycling. some cells were not that good. and had 1 to 2 more cycling.

I kept researching about the Delta Peak. and got this conclusion of my own. some documents say 10mV is fine per cell, some documents say 6mV. and I noticed both are right.

Each cell in the same pack has different delta peak point. and, it was the reason for the matched pack. but, it's still true each cell meets the delta peak point at the different moment.
so, it's fine the set delta peak to 10mv for individual cell. and it's correct you need to set 5~7ml per cell when you charge a pack. if it's 30mV/pack, some of the cells are not fully charged, and some of them are slightly overcharged. and it's still fine I believe. but if you set 60mV for a pack. some of the cells in the same pack would be seriously overcharge.

but I'm running with Lipo, no more with NiMh batteries which need some special care.

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