recharging NiCad batteries
Mike W
Posts: 105
I have a quick question about recharging NiCad batteries. I have read that it is recommened to chatge at a rate of .1C· C being the·total capacity.·for example·(1000 mA/h battery should be charged at 100·mA/h)
My question is, at what voltage
I have read that a 1.2V NiCad should be charged at·1.45 volts·but I am not sure.
I am charging a·9.6V battery pack do I need to increase the voltage above 9.6V and if so how much?
Thanks for any help or advice
Mike W
·
My question is, at what voltage
I have read that a 1.2V NiCad should be charged at·1.45 volts·but I am not sure.
I am charging a·9.6V battery pack do I need to increase the voltage above 9.6V and if so how much?
Thanks for any help or advice
Mike W
·
Comments
The Art of Electronics recomments about 1/10th amps/hour and that it should not be on the charger for longer than 14 hours. So, their ball park is a 12 to 14 charge period.
Overvoltage can be controlled by a current limiting circuit. Generally, I would say that you don't need more that 115% of the battery's rated voltage. But with a current limiting circuit, you may see a high supply being used because it is merely convienient.
Obviously, you might want to use a 12volt DC source and that would be above the 115% by about a volt. I suspect that as long as a current limiting circuit is properly in place, the extra voltage would merely be dumped at heat from the transistor or voltage regulator that is being used to control the current.
Still, you can overcharge with such a circuit by leaving the device on the charger for more than 14 hours [noparse][[/noparse]unless you have a timed shut-off]. A trickle charge over longer periods of time should be far less.
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Post Edited (Kramer) : 4/13/2006 4:08:44 PM GMT
Kramer you are right about this if you over charge NiCad batteries they will
not last very long
A trickle charge over longer periods of time should be far less.
I use this set up alot and works very well and get 2 or 3 years life out of a set of batterys
You can charge NiCad batteries with a trickle charger
Charge them at a rate of 30 m amp using a LM317 reg in the current limiting mode and you can use any 12 volt power supply with this set up
You can leave the batterys on charge until you need them and the battery never get warm the only thing is that if you let the batterys run down very low they take a long time to charge back up
For this i put a switch that is for 30 m amp // or 100 // or 200 i do not go any higher then 1/20th amps/hour
When you do it at 200 m amp mode that is for 7 hours
You can overcharge with such a circuit by leaving the device on the charger for more than 14 hours [noparse][[/noparse]unless you have a timed shut-off]. A trickle charge over longer periods of time should be far less.
thanks for your responce You really helped with my understanding of charging Ni Cad's
however I am still confused on the voltage that needs to be set for the output of the charger. Is it the same as the voltage of the battery pack or is it at a higher voltage.
9.6V charge at 9.6
or 9.6 charge at 11.2
thanks again
Mike W
Mike,
For the classic el-cheapo slow charger you need a DC supply a couple of volts higher than the battery voltage and a resistor to limit the current. If you have a 12V supply, 9.6V battery and want 50mA then you would choose a resistor = V/I = R = (12-9.6)Volts/.05Amp = 48 Ohms.
!!! Be sure to check the actual output voltage of the power supply!!! Wall transformers are usually not well regulated and can run several volts above their rated voltage. It's not uncommon to get 16V from a 12V supply, so in the example above you'd need a 128 Ohm resistor.
Lead acid batteries require a regulated voltage to avoid gassing, so this simple charger isn't appropriate, and don't even think about charging Lithium chemistry batteries like this.
BTW, Chips like the MAX712 , or microcontrollers can look for the slight drop in voltage that NiCd and Nimh batteries display as they reach full charge. This is usually called deltaV or peak charging and is preferred for fast charging. For a description and graphs of the process see: http://pdfserv.maxim-ic.com/en/ds/MAX712-MAX713.pdf
edit - you can find pretty cheap smart chargers > http://batteryspace.com/index.asp?PageAction=VIEWPROD&ProdID=2378
Post Edited (Russ Miller) : 4/14/2006 2:12:10 AM GMT