Designing a NI-MH charger

[Gayan Soyza]

I’m planning to design a charger for NI-MH rechargeable batteries due to unavailability of NI-CD batteries. I know it’s not easy to build a charger for NI-MH Bcuz it must be treated well than other types. Some how I must take the risk of this project.

Batteries I’m going to use/charge – (AA) NI-MH 1.2V, 2550mAh, standard charge 16Hrs at 260 mA. I’m going to charge two batteries in series.

As far as I know about rechargeable Nickel based batteries NI-CD & NI-MH are treated almost using the same principles like feeding constant current & voltage……but taken into mind the charge rate for NI-CD is 0.1C & for the NI-MH trickle charge rate is below NI-CD 0.05C-0.1C. Also considering when to stop the charge.

Before building the circuit I decided two methods for NI-MH charges. Listed below.

Method1

Using LM317T as a current source in series with a current limiting resistor applying to the batteries & turn off the supply power within a calculated time.
According to my battery spec, it will be 255 mA charging for 14Hrs & turn off the supply power.

But for half charged battery will it be a problem? They will charge quickly or overcharge?

What will be the result if I change the C rate about 0.090C.I heard that NI-MH must rapid charge rather than slow charge.


Method2

Using a comparator IC to detect the battery voltage if it is come to a certain level the output driver will cut off the supply or adding a PIC chip to this stage deciding whether to do a very small trickle charge or an occasional pulse charge or a Timer cutoff condition.

These are the two methods I can do at home .for detecting temperature, pressure, voltage drop per cell in mV would be consider later. And also I cannot find any battery charge controller IC’s in my place.

So I need some pickup from you guys. Any comments on these methods, what’s the best way? What are the missing points?

Hoping a reply soon.
Thanks

 

[k7elp60]

What will be the result if I change the C rate about 0.090C.I heard that NI-MH must rapid charge rather than slow charge
Thanks

Charging them at this rate 0.09C at room temperature they will never over charge. Granted they will warm up but timing will not be necessary.
I have built many ni-cad and nickle metal hydride chargers and charged the batteries at that rate with great results. As long as the charger has the capability of producing about 1.5V per cell with the cells in series
in a constant current mode like with a LM317T it should work fine.

 

[audioguru]

Energizer recommends a trickle-charge rate that is only 1/40th of the Ni-MH cell's rating for max life.
Panasonic recommends a 1/20th to 1/30th the rating for trickle-charge.

 

[Gayan Soyza]

Oh k7elp60 has already charged the batteries with the rate of 0.09C....nice to hear that they are not over charge at this rate so better alter to this rate.

Some battery manufactures recommends different trickle charge rates...I see...

I'm using GP batteries they recommend the standard charge 260mA ,16hrs....almost 0.1C....

 

[audioguru]

Stopping the charging after 16 hours is not the same as continuously overcharging a battery.

 

[JTECH]

Ni-MHi

Oh k7elp60 has already charged the batteries with the rate of 0.09C....nice to hear that they are not over charge at this rate so better alter to this rate.

Some battery manufactures recommends different trickle charge rates...I see...

I'm using GP batteries they recommend the standard charge 260mA ,16hrs....almost 0.1C....

Whatever charge-rate you use....WATCH THE TEMPERATURE! A small thermal cut-out can prevent disaster.