Simple 6xAAA NiMH or NiCad charger?

William At MyBlueRoom

New Member

• Nov 15, 2006

• #1

I have 6 AAA nimh / nicad batteries I'd like to trickle charge. Can I simply use a resistor and a 9v battery? If so what is the maximum trickle charge I should use C/20?

 

Hero999

Banned

• Nov 15, 2006

• #2

Yes you can, but discharging a 9V battery into some cells seems pretty pointless. The 9V battery will be flat before the NiCads even reach half charge.

 

justDIY

Active Member

• Nov 15, 2006

• #3

just jack the 9v, assuming its a alkaline straight into your nimh pack, maybe a blocking diode to prevent back charging... no resistor required... 9v's have such a low mah rating and high internal resistance, your external resistor would just be redundant.

but like Hero points out, it is pointless, since a 9v doesn't contain enough energy to recharge those larger cells.

figure 9v * 300 mAh = 2.7 w/h ... and the aaa pack might be 7.2v * 650 mAh = 4.68 w/h - not even close!

 

William At MyBlueRoom

New Member

• Nov 15, 2006

• #4

It's not a 9V battery, it's a 9V wall wart. I've seen simple current limiters using a LM317 but I was hoping to simply use a small resistor. Not too worried about the NiCads as they are only 50c each at the dollar store. Using 6 of them to power my ICD2 clone & trainer. 300mah @ 1.25v

 

J

januar1983

New Member

• Nov 15, 2006

• #5

i suggested to use max 712 from maxim semiconductor you can get the free sample of it www.maxim-ic.com

 

k7elp60

Active Member

• Nov 16, 2006

• #6

There is one variable that may cause problems. The typical 9V wall wart is unregulated so the the voltage output may be quite a bit higher than 9V with a small load.
I have had excellent results of charging Ni-Cad and Ni-Mh batteries with a charge current of 0.095C or less. In your case 300 x .095 = 28.5 Ma.
If you figure the wall wart may be at 12V and the batteries are near 1V when discharged you could use a 240 ohm resistor. (12-6)/.025 = 240

 

William At MyBlueRoom

New Member

• Nov 16, 2006

• #7

220 sounds right to me.

 

K

kchriste

New Member

Forum Supporter

• Nov 16, 2006

• #8

Nicads are pretty tolerant of overcharge but NIMH are not. You will get better life out of your NIMHs if you charge them at a rate of apx 0.5-1C and terminate on the negative slope of cell voltage change. You can also terminate on cell temperature rise. NIMHs do not take a slow charge very well (Nicads are OK at 0.1C) and their lifespan will be shorter as a result.