I made a Ni-CD 7.2V 4000mAh battery. What power adapter do I need to charge it?

How many volts and how many amperes should the adapter output?

Each battery cell is 1.2 volts. I have a 9V 500mA power adapter.Is this adapter suitable for charging my battery and how long it will take?The adapter will automatically detect if the battery is full to stop charging it?

  • \$\begingroup\$ "Power adapter" and "battery charger" are two different things. \$\endgroup\$ – Chupacabras Jun 26 '19 at 6:31
  • \$\begingroup\$ So where is the difference?A battery charger is a power adapter. It transforms AC into DC \$\endgroup\$ – Stelios Liakopoulos Jun 26 '19 at 7:52
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    \$\begingroup\$ Battery charger monitors state of the battery and controls voltage and current that goes to the battery. Power adapter is just a dumb source of voltage (or current). \$\endgroup\$ – Chupacabras Jun 26 '19 at 8:28
  • \$\begingroup\$ my power adapter has a led that is red when i connect a drained battery and when the battery is full it turns green.This means that the power adapter(or battery charger) will automatically detect if my custom battery is full? \$\endgroup\$ – Stelios Liakopoulos Jun 26 '19 at 9:03
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    \$\begingroup\$ Read the manual or datasheet for that charger. \$\endgroup\$ – Chupacabras Jun 26 '19 at 11:59

You made the battery, shouldn't you be telling us what we need in order to charge it?

Typically I would say you need a 400mA constant current with a voltage of 1.41V * number of cells. You would then charge it for 16 hours with some sort of timer. At that rate it would be fairly safe regardless of discharge level, but you still don't want to leave it charging for longer than the 16 hours.

If you wanted to charge it faster (and you know what the maximum rate of your battery is) you would need to monitor the voltage and/or temperature. But I think this is probably beyond the scope of your question.

In regards to your edit:

9V and 500mA are both slightly too high. The main concern is the 9V, since you should be using 8.5V. The 500mA isn't too much higher, but you would need to reduce the charge time.

Now if you use that to charge your battery, it will charge. But the over voltage will be damaging it. You could probably solve this problem with a low loss diode.

I'm not clear if what you're using is a purpose built battery charger or just a random power pack? In either case I'm going to say it probably wont stop charging, since it seems like it would expect a different battery if it's outputting 9V and wouldn't be calibrated correctly to switch off for yours when it's charged.

  • \$\begingroup\$ i edited the question \$\endgroup\$ – Stelios Liakopoulos Jun 25 '19 at 14:30


If you want to charge it slowly, as stated in @hekete's answer, charge at a rate of C/10 (means 4000mA/10=400mA) for 16 hours and the charger's output voltage should be 1.4V per cell, which equals to (7.2V/1.2V) x 1.4V = 8.4VDC. But with this method, you need a mechanism (i.e. MCU, timer, etc.) to stop charging so that it does not past 16 hours.

If you want faster charging, C/3 for 5 hours or C/5 for 8 hours (with the same charging voltage) can be acceptable. But please note that you need to check battery voltage and temperature as well. Also, to ensure any problems not to occur, you need to discharge the battery down to at least 1V per cell then start charging.

  • \$\begingroup\$ i edited the question again... \$\endgroup\$ – Stelios Liakopoulos Jun 25 '19 at 15:21
  • \$\begingroup\$ also i don't care for fast charging \$\endgroup\$ – Stelios Liakopoulos Jun 25 '19 at 15:28

Back in the days when people used NiCd batteries, you could buy off-the-shelf chargers that would charge them. I still have a 7.2V "racing pack" charger specifically designed for such a job.

A 9V adaptor would over-charge the pack, quickly damaging either the adaptor or the battery. You would need to limit the current to no more than 400mA (C/10). Without knowing the specification of the adaptor, it's hard to tell how sophisticated that current limiter would need to be. NiCd cells can be trickle charged at rates of up to C/10 for long periods safely.

NiCd batteries are largely obsolete now, as:

  • NiMH ones have a larger capacity.
  • Cadmium is toxic.
  • \$\begingroup\$ But i have a battery that is 7.2V 1800mAh and it came with a 9V 500mA charger and the battery is being charged without any problems \$\endgroup\$ – Stelios Liakopoulos Jun 25 '19 at 15:49
  • \$\begingroup\$ @ΣτελιοςΛιακοπουλος if it is voltage limited and current limited, and was intended for charging batteries, then it should work fine. But if it is just some random 9V power supply that was laying around, then it's impossible to say. 500mA is only just over C/10 \$\endgroup\$ – Simon B Jun 25 '19 at 15:55
  • \$\begingroup\$ What do you mean Voltage limited and current limited? I said that it outputs 500mA and 9V.So this is its limit right? \$\endgroup\$ – Stelios Liakopoulos Jun 25 '19 at 16:02
  • \$\begingroup\$ @ΣτελιοςΛιακοπουλος Maybe. It could mean that the power supply normally puts out 9V, but if you attempt to draw more than 500mA, then voltage will drip until the current is 500mA. Or it could mean that the power supply puts out 9V, but if you take more than 500mA from it, then it will burn out. Or it could be an unregulated supply that puts out roughly 9V at 500mA, but it varies with the amount of current you take. \$\endgroup\$ – Simon B Jun 25 '19 at 16:09
  • \$\begingroup\$ And how long it will take to charge the battery? \$\endgroup\$ – Stelios Liakopoulos Jun 25 '19 at 16:30

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