I read on the Internet that a typical coin cell battery has an ampere hour of 225mAh. I connected up a CR2032 battery to two blue LEDs and left it switched on expecting it to take approximately 5 and 1/2 hours before the battery was exhausted, but 9 and 1/2 hours later the LEDs are still glowing brightly. How long should it take before the battery is dead? Is there a reliable way to calculate this?
Voltage – How Long Will a CR2032 Battery Last with Two 5mm Blue LEDs?
ledvoltage
Related Solutions
8 parallel strands of 2 LEDs each and a 150ohm resistor. All loose anodes should be connected to V+. The loose cathodes should be connected to the collector of a TIP31 NPN transistor. The emitter should be connected to ground. The base should be connected to a 100ohm resistor, and the other side of the resistor should be connected to a digital output. 16 LEDs, 9 resistors, 1 transistor, 1 digital output.
(12V−(3.2V⋅2+1.2V))/30mA = 146.66667 Ω
30mA⋅8 = 240 mA
(5V−1.8V)/(30mA⋅8/10) = 133.33333 Ω
simulate this circuit – Schematic created using CircuitLab
At 12V and 9.6W the strips would consume 0.8A of current.
Divide that current into the battery capacity. 4.8Ah / 0.8A = 6 hours.
This is a very rough calculation, it's probably less than this. Real testing would be needed to verify this.
Best Answer
2 blue leds, so typically 3.2~3.6V forward voltage drop at 20mA forward current, will run at a much lower current when the voltage across them are less than that typical forward voltage.
They are likely to be running at 5mA or less, when the CR2032 is fully charged at 3.2V. From a random blue led data sheet http://descargas.cetronic.es/WW05A3SBQ4-N.pdf:
And a current vs capacity graph of a CR2032 https://www.ept.ca/features/designing-sensor-based-iot-devices-coin-cell-batteries/:
As the available voltage drops, the leds use less current, so they last longer too. It's like a see-saw where the led current and the battery voltage kinda level themselves out nicely. So between you not calculating the correct current drawn, and the curve of both the led and battery, the actual lifetime of the setup will be much longer than a general formula of (Battery Capacity in mAh / current draw) would tell you.
Leds do not have to run at their normal max current. That's just when they are their brightest. But they are still bright at lower voltages/currents.