I would like to power a LED off a 1,2 volt rechargeable battery,850mAh,energizer without any resistors.Since the LED has a forward voltage around 2 volts,working fine when connected to a 1,5 volt alakaline AA battery,it's obvious that the voltage isn't a problem.However,I am not so sure about the current.Would the LED be fried if I connected it to this battery?
Powering a LED off a 1,2 volt rechargeable battery
batteriescurrentdcled
Related Solutions
Power
According to the WS2812 datasheet (I can't find a WS2812B datasheet and you didn't provide a link) the integrated LED chip and controllers run on a supply voltage of 6 to 7 V and a current of 20 mA per R, G and B LED. Total all-on power demand given by \$ P = VI \$ will be \$ 2000 \cdot 3 \cdot 6 \cdot 0.02 = 720 W \$. You have underestimated on your power budget.
Current
Max current is simply the sum of all the currents when all LEDs are on. \$ I_{MAX} = 2000 \cdot 3 \cdot 0.02 = 120 A \$.
Battery
Batteries are typically rated in Ah (ampere-hours). The manufacturer will provide an amp-hour rating at a specific discharge rate. If you discharge at a higher rate the amp-hour rating decreases.
Ignoring that for a moment, at 6 V (no voltage conversion) you need 120 A x 16 h = 1920 Ah capacity. Let's round it up to 2000 Ah. We can de-rate, for example, if we assume that on average 50% of the LEDs will be on, to 1000 Ah.
As an example, the 6 V Yuasa battery pictured above has a 12 Ah capacity. You would need \$ \frac {1000~Ah}{12~Ah} = 83 \$ to give 1000 Ah capacity. Weight: 83 x 2.05 kg = 171 kg. Guide price: 83 x €16 = €1,328 from Farnell.
Charging
Charge current for an ideal battery will be \$ \frac {Ah~capacity}{charge~time} \$. For an 8-hour charge time that would work out at \$ \frac {1000}{8} = 125~A \$.
Comments
These are high currents at low voltages. A 100 to 200 A, 6 V lead acid battery charger will be difficult to find.
- You could split the power circuits into smaller independent battery circuits with individual chargers for each.
- All negatives could be connected together along with the data GND but the individual battery circuits should not share a common positive.
- Limit the number of LEDs in each chain to a number that the wiring can support.
- Derate everything in my calculations by some further safety factor.
You can only rely on the (Amp Hour) the battery has. Or for small batteries the mAh (milli amp hour). Anyway all these figures are estimates anyway. The lifetimes depends e.g. on temperature.
I don't know what you need the LED for but you can get LEDs which are a lot more efficient: 1 mA would give you 20x as long.
As K H said: you are wasting energy in a series resistor: Try to find a switching current source. A simple PWM circuit with an inductor comes to mind.
Best Answer
Depends on two factors. The Battery's Internal Equivalent Series Resistance, and the IV Curve of the LED. The 2V stated Forward Voltage of the LEDis likely at 20mA, the typical Forward Current for the stated expected life of the LED.
At 1.2V fully charged, the LED likely won't be damaged, but the brightness will be considerably dimmer than it would be at a voltage and current close to 2V @ 20mA.
For example, super cheap led flashlights are composed of an Led (Lets say Blue at 3.4V) and a 3V Coin Cell Battery (CR2032). The Battery's lower voltage and high ESR (about 20 Ohms) allow the LED to be nice and bright without needing a resistor.
Frankly, just sacrifice an LED in the name of scientific experimentation, and try it. If it blows, you know you need a resistor. If it doesn't, you are all good*. A quick test of a red LED and a brand new 1.5V AA Alkaline battery shows it working. How long the LED would last like would take long term experimentation to know.