LED&Battery

[watzmann]

Hi All ,

i have a huge amount of Blue LEDs , i intend to sue them but i don't know the current consumption of this Blue LED

secondly , suppose that this blue led consume 20 mA , and i have a 12 volt battery 7A, how long it will takes if i used four of this leds on this battery ,

i know it will be a total of 80 mA , but don't know how to know the time it wil take to discharge my battery .

thank you

 

[Sceadwian]

Blue LED's consume about 20ma's at 3.1 volts. But any voltage over that will quickly cause it to draw more current and self destruct. You would have to wire four blue LED's in series to have a chance of not blowing them up upon connecting them to a 12v battery. Even then the nominal voltage of a 12 volt battery is 13.8 volts which means each LED will be getting about .3volts too much on a full charge, which is probably too much. You'd need to drop about 1.4 volts total (.3 for each LED), which you could do using a 70ohm resistor in series with 4 blue LED's. This would protect the LED's from even a fully charged battery. If you want more than 4 LED's then create more strings of 4 LED's and a single resistor and run them in parallel to create as many light as you want. If you're using enough LED's you may be able to get away with not use a resistor as the voltage of a 12V battery will sag when it's put under load, you would have to determine how much the voltage sags under the load you would be presenting it and test the battery with a dummy load and measure it's voltage drop directly. If you mention exactly how many LED's you have (or at least want to use) it'd be easier to calculate it out.

 

[Krumlink]

Could you also put a couple diodes in series and have the voltage drop across them?

 

[Voltboy]

The battery must say somewhere its amperes-hours or miliamps-hours, which means it can supply that amount of amperes for one hour before running out of energy.

 

[k7elp60]

I assume the battery is lead-acid. Lead-acid batteries have a 20 hour rate. this means normal discharge for a 7A would be 7/20 or .35 amps. If it is discharged at a higher rate it will not last 20 hours to the cut off of about 11 volts. If it is discharged at its 7A rate it will only last about 30 minutes.
If the charger is removed and the battery sits without a load or a charger for 24 hours it will only show a voltage of about 12.7 volts. This voltage can vary with ambient temperature and with the actual capacity of the battery.

 

[watzmann]

i know i how to wire my LEDs with battery , but my problem was calculating the time ,

thank you all

 

[Sceadwian]

watzmann, apparently you don't, because if you use 4 20ma LED's in series with the proper voltage the current draw will be 20ma's not 80. Current requirement doesn't go up with LED's in series just the voltage required to get that current.

 

[watzmann]

but i'll connect them in parrallel

 

[audioguru]

Connecting 3.1VLEDs in parallel when you have a 12V battery wastes nearly 3/4 the battery's power. Connect three in series then in series with a current-limiting resistor for much less wasted current.

 

[Diver300]

but i'll connect them in parrallel

You shouldn't parallel LEDs. There will be differences in the voltages, so some will be brighter than others. Each parallel path of LED circuits should have its own current control, often a resitor.

 

[Sceadwian]

LED's work very well in series, and allow you to use a lower value current limit resistor.

 

[Hero999]

What's your battery's minimum and maximum voltage?

Typically the minimum will be about 10V and the maximum about 13.8V for a lead acid which will dim the LED quite a lot when empty and blow it at full charge, if you're using a resistor value designed for 12V.

If your 12V supply varied alot then you could use a low dropout constant current source which will keep the LEDs at full brightness between 10V and 13.8V.