Subj : Battery Discharger To : Neall Mercado From : Peter Harle Date : Mon Jan 08 2001 03:28 am -=> Quoting Neall Mercado to All <=- NM> @MSGID: 6:751/321 298878e0 NM> * Crossposted from: ELECTRONICS NM> Hello there! NM> I only want to know how does a battery discharger works. NM> Is it just like putting a resistor across the terminals of the Ni-Cad NM> batteries? If it works just like that, what is the recommended NM> resistance for safe and fast discharging of a 1.2V 700mAh Nickel NM> Cadmium battery? NM> NEALL MERCADO G'Day Neall, For Ni-Cad cell dischargers to function without damaging the cells some form of "discharged state monitor" is usually incorporated. That is, once the voltage drops below say 1.1 volt per cell, the discharging should cease. That could be achieved with suitable electronics such as a comparator, its output driving a constant current source used to discharge the battery. The discharge rate can be anything you like as long as it's within the designed capability of the cell(s), and your time constraints. For your application, you could discharge the cell in less than an hour if the discharge current was around 700 mA. Or, proportionally, 30 minutes at 1400 mA, 15 minutes at 2800 mA etc. I wouldn't take it further i.e. 7.5 mins at 5600 mA. Internal cell heating at that current density may damage the cell. A suitable resistor for discharging at 700mA per hour is: Volts/Current=Resistance, 1.2V/0.7A = 1.71 ohms rated at 1W. (P=IxR=Watts). Two 3.3 ohm 0.5W resistors in parallel will result in an equivalent 1.65 ohm resistor. Two such units (4 resistor) in parallel will discharge the cell at around 1.4 amps in around 30 minutes, provided the cells were fully charged to begin with. If you wish to discharge the cell in less than 15 minutes, eight 3.3 ohm 0.5W resistor in parallel should do it too. Hope this helps. Peter H. --- Blue Wave/386 v2.30 * Origin: COMM Port OS/2 juge.com 204.89.247.1 (281) 980-9671 (1:106/2000) .