Battery running time - confirmation needed

I am wanting to run my arduino as well as some external 12V (5W) LEDs.

I have found a cheap DC 12V 1800MAH Super Capacity 1800MAH Rechargeable Li-ion Battery.

Ignoring the Arduino for now, based on V=P/I, the current of a single LED will be 5/12 = 0.42A

Does this mean that the above battery could power the LED for 1.8/0.42 = 4.32 hours

Assuming the arduino draws 60mA=0.06A, both together should draw 0.48A. The battery should therefore run provide power for 3.75 hours.

As my project only needs to run for 2 hours the above battery should be fine; or am I looking at this all wrong?

Just realised that I would probably need to add a resistor for the LED. This will change my current being drawn.

Hi,

I have found a cheap DC 12V 1800MAH Super Capacity 1800MAH Rechargeable Li-ion Battery.

How cheap?
If it is on ebay, be very very very careful about capacity claims.
If it s this one.


I would be very careful, there are no specification listed in the actual sales page.

Banggood have a better spec sheet.
Benggood review for the product is worth reading, one guy complained it was only 300mA and they offered to replace it.

Tom... :slight_smile:

Hi thanks for that. I will take your advice and go for a more reputable, local supplier. Regarding my calculations about (excluding any possible calculation errors) is my approach and assumptions correct?

abasel:
Just realised that I would probably need to add a resistor for the LED. This will change my current being drawn.

With high power leds, a resistor is not suitable. You need constant current drivers. These driver circuits will consume a little power themselves, but not much. So your calculation is ok.

But I would echo the warnings about power packs purchased on eBay and other sites. I once purchased some 18650 size Li-ion batteries with a quoted capacity of 9,900 mAh. When I measured them, they were 1/20th of that.

Sometimes sellers have "problem" with calculating mAhs:
When they have 3 batteries with 600mAh and 4.2V connecting them together they get one 12.6V 1800mAh battery. Sadly most people get either 4.2V 1800 mAh OR 12.6V 600mAh battery.
In fact it is said even "reputable" manufacturers have this problem: capacity of power banks is often quoted as capacity of the included battery, i.e. 10 000mAh power bank in fact includes 10 000mAh battery so it is not able to provide 10 000 mAh @ 5 V.