Conditioning/Forming Nimh packs

[sikeston34m]

I have read that alot of Brand New Nimh Battery Packs won't live up to their mah ratings until they have been conditioned or formed.

On how to do this, I found this on another forum: "The charge should be C/10- for 14 to 16 hours. The discharge should be C/5 to .9 volt per cell."

They suggest cycling using this method for at least 3 cycles and one person said sometimes even more cycles are needed before the pack will deliver it's rated mah capacity.

They also suggest that this type of cycling will "bring back to life" older packs that are losing their capacity from age or sitting and especially works well with packs that have been fast charged alot.

Can anyone add to this?

I would also like to find more information on matching/zapping Nimh cells.

Any help would be appreciated.

[lincpimp]

Most high end chargers have a auto cycle function. I usually required a big power supply to do it. The ICE charger can do cycling but has to have a 15amp or bigger power supply to do so. I originally bought my ICE when I still did nimh batteries for the cycle function.

Why are you going to nimhs? Please tell me it is for a non rc related project! I am sure that you must have fooled with nimhs before, and you know that they are less than optimum for the rc enviorment, mainly due to weight, and poor discharge capabilities compared to lipo, etc.

[sikeston34m]

Originally Posted by lincpimp

Most high end chargers have a auto cycle function. I usually required a big power supply to do it. The ICE charger can do cycling but has to have a 15amp or bigger power supply to do so. I originally bought my ICE when I still did nimh batteries for the cycle function.

Why are you going to nimhs? Please tell me it is for a non rc related project! I am sure that you must have fooled with nimhs before, and you know that they are less than optimum for the rc enviorment, mainly due to weight, and poor discharge capabilities compared to lipo, etc.

Don't worry Linc. I am not giving up on Lipos. That's for sure. They are FAR superior to NIMH packs anyday.

I do want to learn more about the Characteristics and Behavior of Nimh's over their cycle life. The ever Quest for more knowledge. Plus, I'm trying to help out a friend that isn't ready to take the Lipo plunge just yet.

Yes, I did run Nimh's before Lipo. I always thought they tended to run a tad on the HOT side and was rather disappointed at the runtime I got.

Also it's kinda hard to run a really demanding setup with them since their amp delivering ability is somewhat limited.

[BrianG]

I've had some NiMH AA's that sat around for too long and they were at ~.5v and wouldn't hold their mAh rating. Doing the c/10 charge method worked, but did take a few cycles to get them back up to par.

Don't know muhc about matching/zapping, but IIRC, zapping is charging at high currents (C*2 or something like that) discharging at ~30A, and then charging again at high currents. Matching is simply grouping together the cells with the same zapped voltage.

[sikeston34m]

Originally Posted by BrianG

I've had some NiMH AA's that sat around for too long and they were at ~.5v and wouldn't hold their mAh rating. Doing the c/10 charge method worked, but did take a few cycles to get them back up to par.

Don't know muhc about matching/zapping, but IIRC, zapping is charging at high currents (C*2 or something like that) discharging at ~30A, and then charging again at high currents. Matching is simply grouping together the cells with the same zapped voltage.

Good! So the c/10 method does work. That's great. I have some 7 cell packs that I'm cycling on a MRC 989 Superbrain now. It just takes so darn long to charge/discharge at such low rates though. Results are still a day or two out. LOL

I was reading about zapping. There is some sort of chemical compound "strings" that form between the anode and the cathode internally. These compound strands inhibit the cells ablity to perform and also promote quicker self discharge.
To zap a cell, they use a LARGE Capacitor type setup. They charge the Capacitor and unload it's entire charge all at once into a single Nimh cell. This "burns" away these compound "strands" and frees up the cell to perform better.

Matching is done not only by tested capacity, but also the voltage at which a cell carries a "standardized" amperage load.

A group of cells are tested and the results of each cell is recorded. Then the cells are divided into groups according to the test results.

The cells that perform and carry an amp load the best are made into packs.

The idea behind this is, if all the cells in a pack perform the same, the pack is much more balanced. It will not only provide more cycle life, but will deliver more total pack mah.

Cycle life is greatly decreased in unmatched cells, when the first "weaker" cells become discharged. It is at this time that the surrounding cells still have "some" charge. They attempt to backwards charge, the discharged cells in the series. Cell reversion. This is bad.

Anyways, there is alot of information out there.

Hmmmm, I wonder what that zapper looks like? LOL "still digging for info here"

[BrianG]

The zapping method you speak of is something that has been done to NiCds in the past, but never heard of it being done to NiMH. Back in high school, I built a little 555 timer oscillator device that did just that; charged a cap and then dumped it across the cell. The supply voltage had to be about 50% higher than the cell voltage for it to work. Using a cap limits the amount of time that high current pulse lasts. This was actually called cell "renewal" and was used to bring some life back to a NiCd - a cheap way to extend its life a little.

However, with NiMh, the term zapping usually refers to conditioning a brand new cell to supply a high voltage under load. AFAIK, it's just charging at 1.5-2C and discharging at about 10C a few times.

[sikeston34m]

On the MRC Superbrain 989 Charger, What is the best cut off peak mv setting for 7 cell Nimh packs?

I've charged several times with it set to the default setting of 5. It appears to be terminating the charge too early.

I changed this to 6 and will try again.

Any thoughts?

[hoober]

5 mv is about right for subC packs. Here's a recipe:

For brand new packs or nice packs that have sat for a long time. 1-2 slow c/10 charges will form and balance the cells.

For packs that have sat or self-discharge at varying rates or for some other reason are imbalanced , no amount of cycling will bring the cells back to balance. A c/10 charge should be done for 15 hrs to balance them once.

Although "cycling" may help the pack as a whole it will not ever bring the pack to even balance. What is needed is a "forced overcharge" and normally a peak charger is a very poor choice unless it has a trickle charge that is allowed to overcharge an amount for a lenth of time that is equalk to the difference in capacity between the highest and lowest cell.

An example may be a single peak charge with a 100 ma trickle allowed to full charge then trickle well overnite. If the pack is already balanced then it makes no difference.