reconfiguring NiMH 9volts

Hi Folks,

Though I've been staring at glowing things in the dark with fascination my whole life, I've only just recently discovered this terrific site! I've really enjoyed reading here and have gotten little else done in the last few days

I've been experimenting with some unconventional LED lights built into things like cassette cases Right now they are just hot glue and plastic, but fun! I want to make some even smaller lights. I recently took apart an older 9v NiMH battery. I can't remember the brand and the case is now gone, but I was curious because it had a plastic case with a lot of give to it and didn't seem to be full at the bottom. After opening it up I found these:



They are completely self contained little cells. All in series like that at 7.5v they are perfect for running 2 white led's in series with a very small resistor. Calculated only about 9ma lost in the resistor, so rather more efficient than other things I've experimented with.

They are spot welded together but could easily be cut apart to make a long flat battery, or even reconfigured all together to give just enough voltage for 1 led and a couple of cells, or added together to make strange and custom cell configurations for odd and interesting lights.

The problem after doing that is how to you charge it? If I don't reconfigure it than I can just put a 9v clip in the light and plug that into my NiMH charger, so thats OK. But if I change the number of cells then I can no longer do that right?

I've been doing low level hardware hacking for a long time, but have never messed with trying to recharge batteries before. If I were to connect 2 sets of these cells in parallel to get more runtime, but keep the same voltage could a regular 9v NiMH charger still be used? If I reduced the cell count to 2 or 3 for an even smaller light then I would need a custom charging solution correct?

Any pointers to charging solutions greatly appreciated!

Thanks,
James

"Calculated only about 9ma lost in the resistor, "

Just curious, but, what did you mean by that?

I was using this led resistor calculator and it has a neat readout of how much power you're loosing in the resistor. The closer your supply and desired voltages are the lower it is. With a supply voltage of 7.7 volts (measured off that torn up 9 volt) and a diode forward voltage of 7.2 volts (from running 2 3.6v ones in series) it shows only 9ma lost in the resistor.

I'll probably overdrive them a little, towards 30ma so that will up my loss in the resister to 15 ma or so.

actually, now that I am playing with that battery a bit more I think that I must have cooked it as it doesn't output anywhere near the current that it should and the voltage drops off quickly under load.

Right now it's just on the breadboard, so the power supply isn't that important. I'm experimenting with dimmer circuits before I decide on what to built it into.

Thanks,
James

I see, it's actually power thats being dissipated in the resistor as heat. If you have 9mA through the resistor, and it's inseries with the load, then you have 9mA of current through the load.
You can't 'lose' current.

I lost some current around here somewhere. The flashlight will be brighter when I find it again

More info on that battery, it was an energizer 9v and I cooked it by letting the energizer recharger trickle charge it for too long. Right now allelectronics.com has a nicd version of the same thing with individual cells that could also be reconfigured. I just got 2 of them in the mail so I'll be playing with that here shortly too.

While I'm chatting about them, do these cells need to "breathe" while charging or discharging? (meaning do they outgas during normal cycling) I'm thinking of potting the whole assembly in some clear casting resin. I know it's heavier than it would need to be, but it's just an experiment.

I've seen one mod around here somewhere that was potted in a big block of polyester resin. Would that act as a heat sink, helping to conduct the heat away, or would it just insulate the LED's and make them actually hotter?

You don't want to pot these in clear cast or any thing else. They do vent some gas when being overcharged or when being discharged at a high rate. You might get away with it with these because of the small size and capacity involved. You might be able to leave a vent open some how when you pot them? A better way may be to just heat shrink them together.
To calculate power loss in a led circuit with a series resistor; measure the voltage across the series resistor. This voltage will be the supply voltage minus the led forward voltage. Take the series resistor voltage times the current in amps and you will get how many watts you are wasting in the resistor. No matter what you do the voltage drop across this resistor is always going to be; voltage of power supply minus forward drop of led. The only way with this method of driving leds to increase efficiency is to match the driving voltage as close as possible to the needs of the led and by doing that decrease the amount of voltage the resistor needs to drop across it.