Simple 6xAAA NiMH or NiCad charger? - Electronic Circuits Projects Diagrams Free

William At MyBlueRoom · 15th November 2006, 06:38 PM

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 · 15th November 2006, 07:37 PM

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 · 15th November 2006, 08:26 PM

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 · 16th November 2006, 03:08 AM

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

januar1983 · 16th November 2006, 06:02 AM

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

k7elp60 · 16th November 2006, 11:55 AM

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 · 17th November 2006, 02:33 AM

220 sounds right to me.

kchriste · 17th November 2006, 04:57 AM

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.

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15th November 2006, 06:38 PM
William At MyBlueRoom Experienced Member	Simple 6xAAA NiMH or NiCad charger? 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? __________________ Bill Home of the Firefly PIC Tutor Inchworm ICD2 http://www.blueroomelectronics.com

15th November 2006, 07:37 PM
Hero999 Experienced Member	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. __________________ What's so bad about Microsoft? Get Opera it's simply a superb browser.

15th November 2006, 08:26 PM
justDIY Experienced Member	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! __________________ If you don't have a planet, what good are gold bars? want to contact me directly? gmail gordonthree check out my project website: http://projects.dimension-x.net Favorite numbers: 09 F9 11 02 9D 74 E3 5B D8 41 56 C5 63 56 88 C0

16th November 2006, 03:08 AM
William At MyBlueRoom Experienced Member	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 __________________ Bill Home of the Firefly PIC Tutor Inchworm ICD2 http://www.blueroomelectronics.com

16th November 2006, 06:02 AM
januar1983 New Member	i suggested to use max 712 from maxim semiconductor you can get the free sample of it www.maxim-ic.com

16th November 2006, 11:55 AM
k7elp60 Experienced Member	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 __________________ The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best. Show me a different way. I have an open mind.

17th November 2006, 02:33 AM
William At MyBlueRoom Experienced Member	220 sounds right to me. __________________ Bill Home of the Firefly PIC Tutor Inchworm ICD2 http://www.blueroomelectronics.com

17th November 2006, 04:57 AM
kchriste Experienced Member	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. __________________ --- The days of the digital watch are numbered. ---