NiCd Battery Powered LED Lamp

[xtp]

Bright white LEDs must be supplied with min. 2.9V (@about 0.5mA

 

[audioguru2]

Bright white LEDs must be supplied with min. 2.9V (@about 0.5mA  ;D)

They certainly won't be bright at only 0.5mA! 20mA is usually used.
When you stepup the voltage then you also stepup the battery current.
The voltage stepup converter gets warm which wastes power that must also be supplied by the battery.

Ante recently posted these circuits. The 1st one regulates the stepped-up voltage.
http://elm-chan.org/works/led2/report.html
http://elm-chan.org/works/led1/report_e.html

 

[xtp]

I tested the

 

[audioguru2]

So how many LEDs do you need? ???
What is their total current for enough brightness? ???

 

[xtp]

I'll use 2 to 4 LEDs, each consuming about 2mA@3V.

Any type of convertor will do (except the ones that include ICs) as long as it won't use more power than the LEDs :-\.

 

[audioguru2]

Stepping up the voltage increases the battery current due to the Law of Energy.
The two-cell Ni-Cad battery will be 2.5V over most of its life. If the LEDs need 8mA at 3V then they consume a power of 24mW, plus power for the step-up circuit.
24mW from the 2.5V battery will cause its current to be 24/2.5 = 9.6mA plus current for the step-up circuit. The current for the step-up circuit might be an additional 3.2mA for a total drain on the battery of 12.8mA. If the circuit is on continuously, 800mA/hr cells must be recharged every 2 days.

Use 2500mA/hr Ni-MH AA cells and the thingy will operate for more than a week before needing a recharge. Or you can increase the LED current for more brightness and recharge more often.

 

[xtp]

But if the circuit is used only six hours a day then the batteries will have to be recharged only after a week ;D.
The step up circuit should be simple and compact, so the lamp will easily fit in a pocket.

 

[audioguru2]

But if the circuit is used only six hours a day then the batteries will have to be recharged only after a week ;D.
The step up circuit should be simple and compact, so the lamp will easily fit in a pocket.

Good, then you can use tiny 850mA/hr AAA Ni-MH cells. They are much smaller and lighter than AA Ni-Cad cells. ;D
Ni-Cads are obsolete and toxic. Energizer doesn't make them anymore. :'(

 

[Alun]

I believe they still do, but they're probably no longer available to the general public. Nicad does have its advantages like being more abuse tollerant, longer life and higher discharge capcity.

 

[audioguru2]

Most of my old Ni-Cads leaked or shorted so I've been using Ni-MH cells instead for a few years. The capacity of Ni-MH cells is much higher than Ni-Cads.

 

[Alun]

Most of my old Ni-Cads leaked or shorted so I've been using Ni-MH cells instead for a few years.

A metal crystal has probably grown accros the electrodes, you could possibly revive them by giving them a few >10A 1 second pulses to vapourise the crystal.

This won't happen with modern reflex battery chargers anyway as

The capacity of Ni-MH cells is much higher than Ni-Cads.

About double if I remember correctly, but this and their non toxicity are a the only advantages NiMH cells have.

 

[audioguru2]

I'm tired of zapping my shorted Ni-Cads. I don't think Ni-MH cells develop those crystals.

It's funny that the Ni-Cads I take good care of, by removing them from the charger when they are fully charged, short and leak. But the Ni-Cads in drills and things have been over-charging for years and are fine.

 

[audioguru2]

[The difference is they use the industrial grade batteries which have better seals and have more ruggedized construction to take on more abuse.

 

[xtp]

Good, then you can use tiny 850mA/hr AAA Ni-MH cells.

I have a lot of new NiCd batteries and I plan to use them.
NiMH batteries are smaller ;D, but they require a more complicated charger than the NiCd batteries.

 

[audioguru2]

I have a lot of new NiCd batteries and I plan to use them.
NiMH batteries are smaller ;D, but they require a more complicated charger than the NiCd batteries.

You don't need a complicated charger unless you leave them charging for weeks, or need fast-charging.
Just charge them through a current-limiting resistor at their capacity/10 overnight, from a voltage source of 1.45V per cell in series.

 

[xtp]

OK...
But I'm still going to use my NiCd batteries...
You suggested two circuits in your first reply. How can I modify them to to get a lower/higher voltage at the output?

 

[audioguru2]

Hi Xtp,
Getting a higher voltage or current from those DC-DC coverters requires modifying or changing them to get a higher current in their inductor.
Look in our Articles section for the good article about DC-DC converters:
http://www1.electusdistribution.com.au/images_uploaded/dcdcconv.pdf