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Anonymous Poster #1

Charging a Nickel-Cadmium Battery

02/05/2024 4:24 PM

Hi,

What will happen if a 7.2 V (6 cells in series ), 800 mAh battery is connected to a 5.0 V, 2,000 mA source (Cellphone charger)?

Thanks,

Job

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#1

Re: Charging ni-cad battery

02/05/2024 5:17 PM

You need a proper designed-for-purpose charging system for batteries...

$12 cheap

https://batteryuniversity.com/article/bu-407-charging-nickel-cadmium

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#3
In reply to #1

Re: Charging ni-cad battery

02/05/2024 8:39 PM

Thank you Guru;

Regards,

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#2

Re: Charging ni-cad battery

02/05/2024 5:24 PM

It will charge to <...5V...> and no further, in about <...800...>/<...2000...> = 25 minutes or so.

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#4
In reply to #2

Re: Charging ni-cad battery

02/05/2024 8:40 PM

Thank you Guru;

Regards,

Job

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#6
In reply to #2

Re: Charging ni-cad battery

02/06/2024 5:09 AM

Either that or it will go BLAT! depending on which way round it is connected.

Please make sure all other CR4 readers are out of the room before so doing.

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#8
In reply to #6

Re: Charging ni-cad battery

02/06/2024 7:27 AM

See #7⇓

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#5

Re: Charging ni-cad battery

02/05/2024 11:37 PM

If your thread title is correct, you shouldn't bother charging it at all! Ni-Cad batteries don't hold a charge well, and to my knowledge haven't been used in new equipment for a decade or two. Recycle it properly!

About 12 or 13 years ago, I investigated getting a replacement Ni-Cad battery for a very nice Makita hand drill. It was cheaper to buy a new drill with a more modern battery than to buy a replacement Ni-Cad battery. The batteries (Li-ion) of the M12 Milwaukee that I bought as a replacement still hold a good charge and do an amazing amount of work.

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#7

Re: Charging a Nickel-Cadmium Battery

02/06/2024 7:25 AM

A NiCad is considered to be fully discharged at 1.1V, so the no load voltage of the battery of 6 cells will be greater than 6.6V. So, nothing will happen.

I'm assuming that the charger won't force discharge the battery down to 5V.

Why are you asking?

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#10
In reply to #7

Re: Charging a Nickel-Cadmium Battery

02/06/2024 7:32 AM

<...Why...asking?...>

Perhaps the temptation to play and discover the wonders of the universe has been overtaken by the desire to involve CR4 readers instead; who knows (rhetorical question - NNTR)?

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#9

Re: Charging a Nickel-Cadmium Battery

02/06/2024 7:30 AM

If the voltage at the terminals of the <...7.2 V...800mAh battery...> exceeds the <...5.0 V...> at the <...Cellphone charger...> then nothing will happen other than the development of a learning exercise.

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#11

Re: Charging a Nickel-Cadmium Battery

02/06/2024 7:33 AM

Is there a prize for the first correct answer?

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#12

Re: Charging a Nickel-Cadmium Battery

02/06/2024 7:49 AM

These posts which describe replacing a 7.2 V NiCad battery with two Lithium Ion cells might be relevant.

https://cr4.globalspec.com/comment/1304174/Re-The-Future-of-the-Battery

https://cr4.globalspec.com/comment/1304187/Re-The-Future-of-the-Battery

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#13

Re: Charging a Nickel-Cadmium Battery

02/06/2024 11:51 PM

If the Ni-Cd pack is deeply over-discharged so that it's series voltage is below 5v then they would at least come up to 5v then stop charging. However, if monitored carefully for voltage and heating, you would be able to charge 3 cells at a time using your 5v supply. You would simply stop charging each set of 3 when they reach 3.75v or begin to get quite warm to the touch. Of course, don't leave them charging unattended. A better plan would be to charge the 3-packs through a 1 ohm 1 watt resistor to lower to current to the 1C rate and protect them better.

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#14
In reply to #13

Re: Charging a Nickel-Cadmium Battery

02/07/2024 3:15 AM

GA

How about two blocks of three using two diodes and a resistor in series. That way you could dispense with the monitoring.

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#15

Re: Charging a Nickel-Cadmium Battery

02/17/2024 7:06 AM

NiCd batteries have a nominal discharge voltage per cell of 1.4, 6 cells series is 8.4V.

If you connect a 5V/ 2 amp cellphone charger, that is probably a 5V constant regulated source with an electronic 5 amp overcurrent limit.

If the battery is charged, it will discharge to 5V when connected to 5V charger, 1.0 volts/cell is discharged for NiCd.

The small, torch battery sized, NiCd do not keep charge well. However, being part or fully discharged for long time does not damage them like lead-acid.

5 volts on 3 cells is ~1.66 v/cell @ 25 Celsius. Once 80% charged, cells will gas at that voltage, with 2 amps available they will overpressure, blow the seals and leak out corrosive potassium hydroxide. No more than 0.1C current when gassing is recommended. Hot cell has lower gassing voltage, a positive feedback to damage.

5V is too low to charge 3 cells safely - the 1 ohm series resistor is too low to limit gassing current. 10 volt charger with 68 ohm series resistor would keep below 80 mA - charging time would be 16 hours.

5 V open circuit with 80 mA current limit would work for 3 cells, but if you want fast you would need a complicated current limit.

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