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Sep 27, 2005, 08:10 PM
Nothing like a good WOODIE!
cooper998's Avatar
Thread OP

nimh discharge voltage


I have searched the net and this forum and still havent found a reasonable answer. What voltage should I use as a cutoff when discharging nimh batteries?
I have seen voltages as low as .2 volts per cell which doesnt sound right to me, and the most common is .9 through 1.0 volts per cell. My speed control is set to cut the motor power at 4.8v to keep the receiver going, but is that too low a voltage? I am worried the batteries may be damaged. I just picked up a Multiplex 5014 which has an adjustable discharge voltage and I would like to set the low voltage to a level that I can use to cycle the batteries and need the proper voltage level to set in the charger.
The battery is an IB3600 7 cell nimh

cooper998
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Sep 27, 2005, 08:18 PM
Registered User
Hello Cooper,

My Schulze charger uses 1.0 volts per cell as a cut off. I think that is very safe. You can go to 0.8 volts if you don't mind living "on the edge."

Tom
Sep 27, 2005, 08:37 PM
Just one more plane!
JWarren's Avatar
0.9 volts per cell.
Sep 27, 2005, 10:21 PM
CamLight Systems
I recommend 0.9V/cell. That 0.2V/cell value you mentioned was probably for a single cell (not part of a pack) where you can take the cell all the way down to zero volts without damage. But, when you have a pack you can't risk bringing the cells down that low in case one cell is discharging faster and reaches zero volts before the others. If you keep discharging, that cell will essentailly be reversed in polarity and will be damaged.

For a very well balanced pack you can go lower than 0.9V/cell, but there's almost no benefit to that since the pack is essentially discharged at approximately 1.0V/cell anyway.
Sep 27, 2005, 11:59 PM
Just one more plane!
JWarren's Avatar
Whether in a pack or a single cell you really should not take the voltage lower than 0.9V / with NiMH cells because it damages the electrolyte between the anode and cathode of the cell. For a 7 cell, 8.4V pack (1.2V x 7 = 8.4V) the cutoff voltage for that pack should be 6.3V (0.9V x 7 = 6.3V). 4.8V is a little low (4.8 / 7 = 0.68V per cell). This low voltage will not allow your pack to last very long.

John
Sep 28, 2005, 06:32 AM
All your base are belong to us
Miami Mike's Avatar
There's no harm in discharging an individual NiMh cell completely. I've done it myself to several cells as an experiment, and then built packs from those cells. They gave me normal service with full capacity for several years.

Problems from excessive discharging can occur when discharging assembled packs of cells though. The commonly seen stopping figures of 0.8 to 1.0 volts per cell are intended to ensure that slightly weaker cells in the pack don't reach complete discharge first and then become charged in reverse by the stronger cells. That's a condition that actually can damage NiMh cells.

I suspect that the advent of Lipo cells, which can be damaged by discharging below a certain voltage, has caused this relatively recent myth about NiMh cells to arise.




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Last edited by Miami Mike; Sep 28, 2005 at 06:40 AM.
Sep 28, 2005, 07:11 AM
Nothing like a good WOODIE!
cooper998's Avatar
Thread OP
Thanks everyone for your input, I will set my charger to discharge to.9 volts per cell. Since I am fairly new to electric flight there is still alot I need to learn but with the help I am getting on this forum it should be easier,

Thanks again,

cooper998
Sep 28, 2005, 08:53 AM
Make sure that one cell in the pack don`t go to zero during discharge. That cell will have very difficult to "catch up" during charging. If you charge with high current, charge with C/10 an hour or two after that and check each cell so they show the same voltage when the charger is connected. Otherwise you get problems. Electrics is fun.//K


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