how many leds can you tie off a battery

Keplar240B

how many leds can you tie off a battery

So if I have for example with a 9v PSU 200mA I can run 10 leds in parallel assuming each led branch is drawing 20mA.

But how many can I put on a 9V battery, Does a battery have a current limit.

Capt'n Midnight

It depends on the battery.

It depends on the LED. you can run twice as many red LED's as violet by putting pairs in series simply because the voltage drop is less.

The nearly extinct , except for power tools, NiCAD batterys had very little internal resistance. Shorting a 1.5V AA would give you enough current to make a paperclip glow. Running NiCAD's in reverse was the surest way of killing them. Using one cell on it's own was pretty bullet proof. Letting a lithium cell drop too far is terminal. Don't use lithium in anything unless you have some way of preventing it drain all the way down.

9V batteries are a very expensive way to power LEDs they just don't have the grunt to keep going, multiply the mAh by the voltage to get the bang for your buck. Probably cheaper to get a step up PSU
http://www.buyincoins.com/item/120161.html#.Wu2bsZe0XDc for a pair of AA's to
you can get other ones that go from 1.5V to 5V and I'm sure she sells C-Cells by the sea shore. - you can adjust the voltage to suit.

Rechargable batterys have a mAh rating. So a pair of 2000mAh AA's can produce 100 times 20mA for an hour , so about 100 RED LEDs if you get the current limiting right. But you'll need more AA's to get Violet LED's running.

LEDs are better in series because in parallel the ones with a slightly lower forward voltage (diff batch, different local temp etc) will mean more current, which means they get warmer and you are in a thermal feedback loop where one LED gets a lot brighter, best in series , with each chain having it's own resistor or controller.

You can limit current in lots of ways, one is with a FET just tie the gate and depending on which one you use you'll get about 20mA regardless of supply voltage , but IIRC maybe 3V drop across it. so use more than 9V supply or step up for efficiency http://schematics.dapj.com/2007/02/fet-current-source.html

ED E

Its worth looking at the discharge curves for different C values. Say you have a 2000mAh 3.7v 18650, a 2A load is 1C, a 1A load is 0.5C etc.



Good quality cells like the above see it less but you can "eek" out a little bit more capacity by under driving the cell.