searching for 1.5 volt led low ma for battery charge ind

[edwardo]

I am using a solar cell for charging a NiMH rechargeable AAA 1.2 volt. battery. Iwould like to use a LED as a battery charge indicator. The solar cell puts out 3 volts at 5ma in full sun.It trickle charges the battery . Has anyone done this or can you tell me what LED I should use. and where I can get it .Thanks Edwardo.

[josmith]

In general high output LED's are brighter at low currents. Since you are working in bright sunlight, use a clear lens LED so that it will be clear when off and red when lighted. With the small amount of current you have to spare, it's still going to be hard to see.
Consider wrapping a few turns of wire around a magnetic compass to make a current indicator or if you can find use a ma. meter.

[jwax]

In general, Edwardo-no.
Typical led's require 2 volts, and more than 5 ma. to light.
You'll need a bigger solar cell.

[josmith]

I missed the 1.5 volt requirement. Most led'd are rated at two volts or higher At rated current. Many will begin to light well below that. Since you don't have more than one or two ma to spare a two volt rated led will probably run on 1.5 at your low current.

[L. Daniel Rosa]

Red. Other colors have higher voltage requirements.

[cato]

Most LEDs are 2 Volt? Things sure have changed since I was a kid.... then most LEDs were 1.2 Volt. The 2 ma variety were a bit priceier than the 20ma kind...but they were readily available....oh yeah and we had to walk to school in the snow in our bare feet......<p>[ February 22, 2004: Message edited by: cato ]</p>

[cato]

Do a Google search for Low Current LEDs. <p>The closest I found to your needs was a Marktech LED that for which the light output was rated at 2ma but for some reason they only rated the forward voltage at 20ma. At 20ma the nominal forward voltage was 1.85V

[Externet]

Hello edwardo.
The Ni-Cd cell needs about 1.4 during charging, a red LED needs about 1.8 V to shine.
1.4 + 1.8 is beyond the 3V supply from your solar panel.
If a particular brand LED works with just 1.6 V; then you will able to tell charge by connecting the cell and the LED in series to the 3V supplied by the solar panel; BUT that will be only at full sun and you may not see the LED under full sun.
The alternative is using a plain galvanometer. It will deflect proportionally to charge rate. Forget about the LED.
Miguel

[Edd]

<<The solar cell puts out 3 volts at 5ma>>
Unless you made a typo or “miss-com-puted” your current value, you need to beg,borrow,buy or “procure” yourself a heftier output solar cell(s) unit. With that rated current value, it wouldn’t even meet/keep up with the internal cell loss/leakage of that cell. Try to get up, at the minumun, above the 50 ma level.
Moreover, ANY series loss or current drain in the power charge loop can have adverse affects at that low of a working power level of a Solar setup in an eficient utilization of the Solar cells.<p>On state of charge:
If it were my situation, I would do what has served me admirably for 50+ yrs in making an exacting /positive/ DYNAMIC evaluation of a SINGLE cell.(No STATIC measurements of voltage while subjected to a proportional load) Take an analog VOM and put it on its 10 amp DC scale and take a RAPID reading of the cell , taking no further time than the pointer takes to peak out. at its max reading. This is typically ~1/2 of a second….However, if you have fast ranging Fluke DVM or one with an analog LCD scale with its hash mark scaling on it, that would work as well. One finds that fresh new alkaline D-C-AA-AAA’s will give a peak current burst of up to 8 amps. Eveready makes one silver cased exotica Low Z cell that I have read a 9 amp peak output from. The worst case seems to have been down at the 6 amp level on some house brands….(which actually may have been relabeled NOS of some major manuf). At later testing, the instant available current then diminishes as the cell is depleted. When a cell is found to be down to the 2-1 amp region it is ready to be chunked as one at that level won’t even reliably run a digital wall clock.
BTW...what work does a single AAA do for U ?<p>73's de Edd
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[jwax]

Here's a typical LED voltage/current plot.
Note it doesn't draw any current until the applied voltage reaches 1.8 volts.
www.solarworld.com has a great collection of solar cells.

[jwax]

http://www.marktechopto.com/index.cfm?a=138
That's the link to the voltage/current plot I tried to picture. Sorry.

[edwardo]

I want to thank all you guys for your help. I have been educated about leds . I found what i was looking for a LED 1.5 volts 2.5 ma. . Thanks to Cato who reminded me of Google After much searching I found a LED at LUMEX Part #SSL-DSP5093UPGC. Thanks for all the comments and help it has been a real education. edwardo.