how to reduce the current without changing voltage for charging a 9 v battery??

Discussion in 'Power Electronics' started by Debjoy Bhowal, Aug 21, 2016.

1. Debjoy Bhowal

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Aug 21, 2016
I have some rechargeable ni-mh 9v batteries (all 200mah) laying around and a 9v adapter with current output of 210ma... I know that 9 v batteries require 1/10th of its capacity to be charged (ie 20ma)...So ,is there a way to reduce the current output without reducing the voltage (maybe with some kind of resistors in series) so that i can charge these batteries safely??

2. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
A proper charger is best.

3. Debjoy Bhowal

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Aug 21, 2016
Thank you for your response..I agree with you..but it will be great if I can use my existing adapter so that I don't have to spent any extra money

Thanks again

4. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010

What voltage is marked on those rechargeable batteries? 8.4V?

You can charge those batteries with just a resistor to limit current but you risk damaging them.

5. Debjoy Bhowal

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Aug 21, 2016
It is about 9.2 to 9.5 v in open circuit

Only 9v is written on the body ...in some it is written "standard charge: 15hrs at 16ma"

6. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Try a 68 ohm resistor and measure the current through the resistor AND the voltage across the battery (do you have 2 multimeters?).

Alternatively, measure the voltage across the battery and then across the resistor and use ohms law to determine the current.

Monitor this and let me know what the voltage across the battery and the current into it are after about an hour.

If the battery gets hot, stop charging it immediately.

7. Debjoy Bhowal

4
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Aug 21, 2016
Thanks for the reply I'll try that...cause I don't have the 68 ohm resistor now. And I have only one multimeter.... BTW I have tried to make a circuit by myself on paper assuming that the internal resistance of the battery will be 1ohm to 2ohm
Will this work?
Here is the pic of the circuit

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8. duke37

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Jan 9, 2011
Do not use the circuit you have shown, you will discharge whatever is in the battery and kill the power supply.
Do what (*steve*) says.

I suggest that you learn the difference between voltage and current.

Arouse1973 likes this.

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10. (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Yeah do what I said and measure a across the resistor with your multimeter on a voltage setting.

11. Bluejets

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Oct 5, 2014
With the 317 a minor adj at the pot will cover any number of cells from 1 to 6