i want to discharge a battery pack using a 10 ohms resistor the pack is a 3200mAh at 9.6V how do i find out what the wattage of my 10ohm resistor needs to be? and can i calculate the capacity of the battery using this method of constant resistance discharge? or am I just wrong ?
Your 10 ohms resistor will dissipate 9.2W. A 10W resistor would get extremely hot.
A 9.6V Ni-Cad or Ni-MH battery has 8 cells. Don't discharge lower than 1V per cell which is a total of 8V, or the weakest cell will be damaged.
A 3200mA/hr battery will supply 640mA for 5 hours. It might supply the 960mA into your 10 ohm resistor for 3.2 to 3.3 hours if it is good and is fully charged.
I'd use a 2X10ohm/10W resistor fo discharging (with good vent ). A lot of NiCd&NiMh can be discharged at high rates (sometimes 2-5C) but not knowing the tech used for making them u'l beter use a lower discharging curent like .1...0.2C. For example (in NiCd cells) sintered plates tolerate higher curents then sponge/foam plates and so on.
DO NOT discharge them under 1v/cell (OK...maybe once in 6 months for reducing the memory efect&voltage depresion ).
Better make an automatic discharger with a opamp or comparator wich stops discharging the bat at certain voltage (I use 1.05V/cell).
thanks for the responses, i have one more question what's the comparison of the two kinds of method of discharging if i use the 10ohm resistor to verify it's capacity against discharging it on cadex 7400 analyzer?
The Cadex will discharge the battery at a constant rate. Your resistor method will see the discharge current decrease as the battery discharges making it harder to calculate the actual mAh of the battery. Not to mention that the Cadex does all the math for you anyway......
www.energizer.com have datasheets for their battery cells. Their AA Ni-MH cell is rated at 2500mA/hrs at a current of 500mA.
It is also 2500mA/hrs at currents of 250mA, 500mA, 1250mA and 2500mA. At a current of 5000mA its capacity is a little less.
Divide the resistance of your 10 ohm resistor by 8 to 1.25 ohms per cell then insert the 1.25 ohms into the graphs.
EDIT: The voltage will average at about 1.2V and therefore the current will average about 1.2v/1.25 ohms= 0.96A. It will reach 1.0V in about 2.4Hrs. That is 2300mA/hrs. A little inaccurate.