mAH Question

android · Post by **android** » Thu May 18, 2006 12:07 pm

So, I have a LED array that uses 200mA and I have batteries that have a 2000mAH rated capacity. 4 batteries are in a AA holder in series giving me 4.8 volts.

Should I have approximately 10 hours of life

2000mAH/200mA = 10 hours

or do I have around 40 hours:

(4 * 2000mAH)/200mA = 40 hours

Thanks,
Andy

Sparky · Post by **Sparky** » Thu May 18, 2006 12:26 pm

Strickly speaking you should have 10 hours of operation. Accept that as time goes on the battery voltage will drop, your circuit may cut out, or things will get dim.

At early life of the battery the voltage drop off may be fairly sharp, that-is it will hold close to the rated voltage right up to the 10 hour mark, then cut out.

With only 200 mA of draw though your battery should be sitting happy even at the end of its charge.

Also here's the sereis parallel thing
Items in sereis accumulate their voltage but do not accumulate their current capacity.

Items in parallel accumlated their current capacity but not their votlage.

android · Post by **android** » Thu May 18, 2006 4:40 pm

Thanks for the response. I didn't know which way to calculate this.

I know that as soon as my volts drop below Vf the LEDs will stop, but I think NiMH batteries try to hold voltageas long as possible since they're already .3 low from the 1.5V standard.

Also my LEDs are 3.6Vf, so it's got to drop quite a bit.

Sparky · Post by **Sparky** » Thu May 18, 2006 9:52 pm

Are you regulating the power to the LEDs? driving them hot ie. greater than 3.6 will reduce their life. They will actually get hot. Not sure how you want to work it, a resistor is wastefull, and a regualtor needs a minium votlage difference to work. Probably a current driver will give you the longest run time while protecting hte LED. This is what they use in portable devices where runtime over battery life is highly desired.

android · Post by **android** » Thu May 18, 2006 10:28 pm

I used the LED calculator to come up with the correct resistors. I just have them hooked up directly to a 4xAA battery pack with NiMH batteries. Volts should be around 4.8 when they've got a good charge on. I did the resistor calc for 6V though in case I need to use alkalines.

Should I stick in some additional electronic doodad before hooking up to the LEDs?

Sparky · Post by **Sparky** » Fri May 19, 2006 6:19 am

No other electronics are needed. You should be good to go.