Charging NiCad batteries

[Studio1]

Does anyone know whether it is possible to charge a 13.2v NiCad power pack with a 14.4v NiCad power pack charger? Or should I use my Variac to reduce the voltage out of the 14.4v to 13.2v?
Any comments ??
Michael

[McMurdo]

If it's a rapid charger then it will have a built in charging supervisory circuit so turning the mains voltage down wont work.

NiCd chargers work in various ways from a simple resistor to limit charge current to full switching constant voltage/constant current circuits with voltage ramp monitoring and timers.

[Herald1360]

It really depends on what sort of charger (see above). IF it's a simple constant current charger with a current setting suitable for your pack, then yes. Otherwise, probably no. You can always charge a NiCd pack at C/10 (a charge rate of 1/10 in amps of the AH capacity of the pack). Under those conditions a full charge will take about 14 hours and overcharging within reason (ie a day or so not weeks) will not hurt. This is easy with a reasonable bench PSU (just set the output voltage to about 2V per cell and the current limit to C/10).

Not too difficult as a bodge job if you subtract the nominal battery voltage from whatever dc voltage supply you have available and put a resistor in series to drop that voltage difference at the charge current you want it'll be near enough.

[DAVEHALL]

I had a neat little circuit using a 78xx as a constant current source for charging NICADS,from an electronics book .Can find it and post it up if anyone interested

[Herald1360]

I think we've been here before:

https://www.vintage-radio.net/forum/...highlight=nicd

see posts 6 & 7

Ignore the LED driving references, a constant current source is a constant current source

If you do use a 78XX, the series resistor will need to drop XX volts at the current required.

[Studio1]

I would appreciate a basic interpretation of: 'You can always charge a NiCd pack at C/10 (a charge rate of 1/10 in amps of the AH capacity of the pack)' C/10 seems to be a common factor in NiCad charging terminology but I don't know what C/10 means or how it's applied. Sorry to show my ignorance!
Michael

[Bob_Moss]

An example may be the best way to explain.
If you have an AA Ni-Cd rated at 2000mAh, then C would equal 2000mA.

Charging at C/10 (usual rated continuous charge current) would be at a current of 2000/10 = 200mA.

This would take approx. 13hours to charge from flat because of the inefficiencies in charging. This would not need to be monitored.

Charging at C/2 would be a current of 2000/2 = 1000mA, would take 2.5 hours approx but would have to be carefully controlled i.e. by timer (risky), temperature monitoring (awkward) or -ve dV/dt which looks for a drop in charge voltage at full charge (most commonly used in commercial "intelligent" chargers.
This is my brain dump, plenty more info on t'interweb.

Bob.

[CambridgeWorks]

Michael, C/10 is Capacity of battery (in Amp hours or milliamp hours) divided by 10. eg 4amp hour nicad (some "D" cells), divide by 10 = 0.4 amp hour (or 400mAH, same), then use this figure to charge for 14 hours. Hope this helps. Robin

[Studio1]

Originally Posted by DAVEHALL

I had a neat little circuit using a 78xx as a constant current source for charging NICADS,from an electronics book .Can find it and post it up if anyone interested

Yes, Dave, I would appreciate this.
Michael

[richrussell]

I'm about to buy a new battery for my radio controlled helicopter. Just goes to show the difference between 'vintage' technology like Ni-Cd and more modern technology like Li-Polymer in terms of safe charge and discharge currents.

This Li-Po 11.1V, 2200mAh battery can be charged at 2C (approx 4.5 Amps), and can be discharged continuously at 35C (77 Amps) or for short bursts of a few seconds at 60C (132 Amps). That's a serious amount of power density in a battery that measures 12cm x 3.5cm x 2cm.

[Leon Crampin]

I have a 22V helicopter Li-poly battery in my Hacker PR18. It was given to me as a dud battery so I found the defective cell and shorted it out - 19V is perfect for the job.

These batteries have a very impressive performance with minimal self-discharge.

For charging NiCd batteries I always wire a low voltage incandescent lamp in series with a simple power supply. This approximates well enough to constant current charging (as the lamp cools with reducing voltage, its resistance decreases) and also provides visual indication of charge state.

With a well chosen lamp in the circuit, overcharging is avoided.

Leon.

[Herald1360]

7805 charger circuit shown (but LM317 is better- less dissipation since it's effectively a "7801.25")

[MichaelR]

Originally Posted by Leon Crampin

For charging NiCd batteries I always wire a low voltage incandescent lamp in series with a simple power supply. This approximates well enough to constant current charging (as the lamp cools with reducing voltage, its resistance decreases) and also provides visual indication of charge state.

With a well chosen lamp in the circuit, overcharging is avoided.

Leon.

If you are charging a single cell that statement about overcharging may be true , but possibly not if you have a number of cells in series. If the cells are imbalanced ( not all at the same charge state) then you could overcharge a cell that has a higher charge state .

As for Lithium Polymer I would remove the cell that is faulty not short it out. Lithium Polymer cells as you have noted store a massive amount of energy , mistreat them and you will get a fire. I would certainly advise that you do not use anything other than a purpose designed charger for lithium as it is unforgiving when things go wrong.

Mike