draning nicd batterys

[buickgnx]

does anybody know of a  device to drain my nicd battery with out having to run the motor. its 7.2 v if that helps

[802Chives]

A resistor, a big one....hahaha. seriously tho you could use a resistor, but you need to pay attention to power rating. Let us run thro an example:

Say you wanted to use a 10 ohm resistor. Ohms Law: V(volts)=I(current)*R(resistance) rearange to solve for current as you know the resistance and volts: I=V/R ~ I = 7.2/(10) ~ I = 0.720A

Once you know current and voltage you can caulculate power: P(watts)=I(amps)*V(volts) ~ P=0.720*(7.2) ~ P = 5.2W

Therefore if you use a 10ohm you need a 5W resistor and thats huge.  The same calculations for a 100ohm resistor would mean you need a 0.52W resistor, but will take longer to discharge. Always make sure you resistor is rated for larger then you calculations spec out. I would make sure you got a 3/4W 100ohm if that is what you are going to use. If you just meet or dont exceed the power rating the resistor it will get so hot it will burn up.

So a straight resistor from positive to negative will do the job, just make sure to calculate your power so you dont start a fire.  You could probably do it any number of ways too, maybe search the RC forums for ideas too, those guys are top notch  

Let me know if you need more help.

[802Chives]

double post but is ok cuz im admin 

I read up a bit on NiCd batteries and I wanted to know why you want to discharge NiCd batteries.  The reason I say is that you can damage those batteries by discharging them too much.  There is also an issue with certain battery packs that dont have balanced cells, that one cell will discharge more than the others.

They recommend not discharging below 0.8V per cell, and in your case I think you have a 4 cell battery pack, so never discharge it below 3.2V.

I think you are worried about or have become victim to the "memory effect" of these types of batteries.  If you are worried about it then make sure to buy/build a charger that discharges before it charges.  If you think you have cells that are victim to the memory effect then you may want to read up on it some more as mostly memory effect is a myth, however there is some truth to it.  Also you dont have to discharge them everytime you use them, rather just once in a while.

please explain you situation a bit better and I will try to help you come up with the best way to deal with it.

[buickgnx]

well the guy from my hobby shop said to discharge them for storage so i dont want the memory. im trying to be as causes as i can because i had to spend my money on this and its my pride and joy.

[Skullmaster]

for our batteries me and my dad made this thing that hooked up to the battery that had 5 white automotive lights. We would just plug it in and wait till the lights went off

[802Chives]

ah ok for storage.  I am not sure what the best way is for a battery pack like that, I have seen a lot of conflicting things about how far to discharge them.  Most sources say that 0.8v per cell is the best, however it sounds like you have a sealed pack of cells so there really isnt a good way to see each cell voltage or discharge each cell equally.

I would suggest looking for discharge circuits that people have made for similar battery packs, there are tons of discharge and balancing circuits you can find through google. Or if you use light bulbs or a resistor just make sure to count you cells and not discharge past (#of cells) * 0.8V = max discharge voltage.

In the future I would suggest making your own battery pack using individual cells, however I think NiCD batteries are kind of old technology and lithium or NiMh may be the way to go if you do that.

[buickgnx]

ill get lipo when i can moiter the pack voltage.