NiMH Discharge Tray and/or Equalizer Tray

[Rinkrat99]

I was reading throug the many threads posted here but didn't see much on this concept.

It is true that you should not discharge NiMH batts to 0 Vdc. It is true based on a NASA report that the ebst storage method for NiCd batts is to throw a dead short across ecah cell for a prolonged storage.

But what about NiMH's? After a race, the pack should be at a voltage of high 7's without a load. There is probably no charge left to turn into power. But if an equalization tray which utlizes a Resistor/Diode series combination is used, then the cells shoudl discharge between 0.7Vdc and 0.5 Vdc per cell. I am assuming this is a Silicon Diode used.

Will this single cell discharge method hurt a NiMH? Do any of the Equalization trays on the market work sufficiently for NiMH's?

Thanks

[highroller]

I tried several methods since the first generation were introduced but use one method that works for me. And the dead shorting was tried on fully charged 1200 packs years back. But the pack was dropped in ice water until it discharged (not suggest you do this).

The single cell method is best, as all cells may not discharge to the same voltage. Discharging each allows them to charge close to the same voltage levels. I use the Trinity Real Time 2, and Integy Octane2. I first discharge the pack at 30 amp down to .90 or 5.00 volts 6cell, 3.40 (4cell), place in tray and remove when lights go out. The pack is stored discharged or allowed to cool at room temperature 55-60 degrees before being charged again.
NiMh do tend to false peak if inactive for 3weeks or more, putting a cycle thru them helps remedy this and keep them from seeming flat.

[sosidge]

I've noticed no improvements from discharging NiMH's to 0v, in fact I've had one pack go bad shortly after doing so, so I personally wouldn't recommend it.

[fReShJiVe]

Originally posted by highroller
I tried several methods since the first generation were introduced but use one method that works for me. And the dead shorting was tried on fully charged 1200 packs years back. But the pack was dropped in ice water until it discharged (not suggest you do this).

The single cell method is best, as all cells may not discharge to the same voltage. Discharging each allows them to charge close to the same voltage levels. I use the Trinity Real Time 2, and Integy Octane2. I first discharge the pack at 30 amp down to .90 or 5.00 volts 6cell, 3.40 (4cell), place in tray and remove when lights go out. The pack is stored discharged or allowed to cool at room temperature 55-60 degrees before being charged again.
NiMh do tend to false peak if inactive for 3weeks or more, putting a cycle thru them helps remedy this and keep them from seeming flat.

highroller - wouldnt putting it on the octane2 till the lights go off drop the volatge for each cell lower than 0.8v per cell???that's what my multimeter shows when i used it till the lights go off....now what i do is just put it on the tray till the lights dim and take it off....please do tell me what you think about this as i may have a faulty multimeter with me

thanks

[Rinkrat99]

I think we are looking at the wrong issue. Technically, a NiMH should not be discharged below 0.9V per cell. By running the packs down on my discharger (homemade), I have set the autoshut-off at approx 5.6V (0.9V x 6 cells). If we use this same method on a NiMH it should be fine but to equalize the pack it is best to bring them all down to the same value per cell.

The question is, if a diode/resistor combo is used per cell, the diode characteristics should shutdown at 0.7V per cell. Will this damage a NiMH cell?

Wouldn't it better if a 0.9V per cell equalizer tray be designed?

It seems to me that the trays out there are made for NiCd's and not NiMH.

[born2rc]

I use an Indi Booster2 for mine. It hasn't screwed up yet

[highroller]

My older NiMH and all others are done the same way discharged to 5.40 or 5 volts and trayed until the light goes out or they reach 0 voltage. Some have been left on longer(busy and forget or on the track). The older first generation NiMh and even the Panasonic SMH have lost as much as 40-60 seconds of runtime from their original numbers while the Sanyo HV 3000, 3300 and Gp3300 having been staying staple with about 10-15 seconds of lost runtime. Leaving them in the Equalizing tray longer could possibly cause problems, the Discharge Trays will definitely take them down lower if you don't monitor each cell individually. Sometimes the bulbs go out on one cell before the next or others I just unscrew the contacts. Keeping the voltage at 0 (like dead shorting) is what harms them, once the load or they are removed from the Equalizing try residual voltage builds back up.

With my charger the cutoff is set depending on the number of cells but only uses a 20amp load, I like the 30+ amp load for my type of use, with the homemade discharger the cutoff devices (MPH Tracker & Deans) are shutting off around 5 volts for 6cells and 3.40 or 3.38 volts for 4cells. When I'm cycling packs for runtime then I'll use the .90 cutoff.

[Rinkrat99]

Hi all

I found out some interesting technical facts today about NiMH cells.

Cells which are discharged to a level of 0.7V should be fine. There should be no degradation of performance.

Now the manufacturers of the cells specify that 0.5V is a safe level to discharge to but this is not realy true. At 0.5 V a cell may not be able to be charged fully and the cell may actually see a cell reversal.

So based on this, a equalizing tray for NiMH which goes to 0.7 V per cell is acceptable for NiMH cells.