What is recommended charger output voltage for a 3V rechargeable AA cell?

[patkim]

Hi,
I may be asking a very basic question, but I am somewhat confused about the principle here.
If I have a rechargeable cell AA NiCad or so of 3 V or may be two 1.5 V in series, what is the recommended output voltage of the charger device to charge cell of 3V?

Is it same as 3 V or just marginally more than 3 or can even bit higher range would do? Thanks.

 

[Alec_t]

Welcome to EP!
The charging profile for cells depends on the cell chemistry. Ideally the voltage is not constant throughout a charge cycle. Have a look at Battery University

 

[Audioguru]

Ni-Cad battery cells are extremely toxic (Cadmium is very poisonous) and are obsolete. They have been replaced by Ni-MH long ago. There is no such thing as a Ni-Cad 3V cell. A cell is 1.4V to 1.5V when fully charged but a discharge averages 1.2V.
You must limit the charging current. You also must limit the maximum charged voltage to 1.4V or 1.5V per cell.
When a Ni-Cad battery that has several cells in series becomes discharged too low then the weakest cell drops to 0V then becomes charged backwards by the other cells. Then that cell becomes shorted.

 

[dave9]

How much charging current is available and how quickly does it "need" to be charged? The one thing in particular that you should not do is try to charge them based on voltage control alone.

At a minimum you need a charge voltage above 1.45V per cell. Depending on the use pattern, if you have a lot of time you can wait for them to charge yet wouldn't leave them charging indefinitely, you could select a suitable series resistor to a trickle charge level, to drop current to approximately (mAh rated Capacity) C/15. Some will say even C/10, or even lower like C/30+ if it will sit charging for days at a time. This is cheap and easy, but also one of the major causes for complaints among owners of NiCd powered products in past years, that charging either took too long or it cooked the batteries, shortening their lifespan too much because the only charge termination is when the user unplugs it.

Along came dumb timer based circuits, a better approach but one with no consideration for how charged the battery was when the charge period started, so could easily still overcharge cells or fail to fully charge them.

Otherwise for safe faster charging you'll want a smart IC based charge control circuit and that will have its own inherent spec for forward voltage drop or minimum required input voltage to achieve 1.45 or higher output per cell in series. For example the control circuit might need 1.5V overhead so could need at least (1.5 + 2 * 1.45 = ) 4.4V input to charge two NiCd cells in series. While making such a circuit from scratch used to be a significant project, today you can probably pick up a bare board off eBay to do this for around $3 delivered from China.

I wouldn't worry too much about discharging too low and then reverse charging the weakest cell on a circuit that only has two *identical* NiCd cells in series. A purely resistive circuit could harm the weaker of the two cells but most circuits would cease to draw enough current to matter if they worked at all once one cell reached 0V and the other was getting close to that too.

 

[WHONOES]

One point that is often forgotten in these items when discussing the charging of NiCad and NiMh batteries is that the charge process is only 60% efficient and must be considered when calculating charge currents. Example, if charging a 1Ah cell on the 14hour cycle you would set the charge current to 100mA.

 

[Bluejets]

You can make yourself a nicad charger with any power supply ( say 12v dc) and use a simple circuit with an LM317 regulator set up as constant current. Then you can add any number of cells ( up to 8 in series) and monitor and adjust output current to say 100 mA with a pot.

Used one here for 20 years.
See if I can find the circuit for you.

 

[WHONOES]

Have a look at the attachment. It has 2 variations on a simple constant current generator.
They are both essentially the same with Example A using a PNP Darlington whilst Example B uses a P Mosfet.
The current is determined by the Vbe of Q1 and Q3 in conjunction with R1 and R4. R3 and R6 represent the battery load.
You will need to allow about 1.7V per cell for the charger though that is not absolute and may be somewhat higher but at the cost of increased dissipation in Q2 and Q5.
As shown in the schematic, current is about 60mA.
You could if you so desired turn the circuit on its head and use NPN and N Channel devices instead.