# Calculate how long a battery will feed a project

#### camilozk

Apr 20, 2014
122
Hi!

I have a project with 32 meters of Led strip, and I would like to try to feed it with a battery. I have been using a "Wattage and Current Meter" to figure out that the configuration that I currently have "consums" (is this the right word?) 0.6KW/h in 24 hours.

If I would use a battery which specifications are:

Nominal voltage 12V
Nominal capacity (20 hour rate) 28Ah

How can I calculate how long will I have my project working on this battery?

thanks!

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
13,720
0.6KW/h in 24 hours.
kW/h is not a meaningful unit. Energy is measured in kWh (that's kW*hours).
Let's assume you mean 0.6 kWh consumed within 24 hours.

Your battery has a theoretical energy content of 12 V*28Ah = 336 Wh = 0.336 kWh. A battery can only be zsued to ~80% of the theoretical capacity (give or take a few percent depending on battery chemistry, temperature etc.). This leaves you with ~0.27 kWh useable energy.
Taken in relation to the measured energy consumption the battery can sustain approx. 24h*0,27/0.6 ~11 h of operation.

Another route is this:
0.6 kWh over 24 hours amounbts to a power of P=0.6 kWh/24h = 25 W.
25 W from a 12 V source means I=25 W/12 V = 2.08 A.
2.08 A from a 28 Ah source means Tideal=28 Ah/2.08 A = 13,5 h.
Taking again into acount 80 % useable energy, this time reduces to Treal = Tideal*0.8 ~ 11h.

#### camilozk

Apr 20, 2014
122
Thank you very much Harald for your support.

Another question I have about this project is that, since I am using 5V led strips, and the controllers are also working on 5V, which would be the best way to bring the 12V down to 5V?

#### BobK

Jan 5, 2010
7,682
For that amount of power, nothing but a switch mode power supply will do. Otherwise you would be wasting 7/12 or 58% of the battery power.

Also, show us how you got 0.6KWH per day. I calculate that typical strips would use nearly an order of magnitude more than that.

Bob

#### camilozk

Apr 20, 2014
122

Could you please link me to an example os switch mode power supply that would work for this project?

I didnt calculate 0.6kwh. I got this number from an electronic Wattage and Current Meter. I have got 896 pixels in my project, and each of them consums 60mA at full brightness, which would equal 54A. But the programs I use are far away from full brightness so the consumption is far lower.

#### Audioguru

Sep 24, 2016
3,656
Please provide a link to the 5V very high current LEDs strips you are planning to use. Most LED strips operate from 12V and have tiny LEDs in series that use low current.

#### Cirkit

Oct 28, 2015
155
camilozk it seems that the wattage meter you are using is measuring the power consumption on the primary (mains voltage) side of the power supply that is powering your project/LED strips.

You really need to monitor the power consumption on the secondary (low voltage) side to determine what capacity of battery you will require.

Apr 20, 2014
122

#### Cirkit

Oct 28, 2015
155
What you need to do is measure the current with a multmeter in series with your WS2812B LED strip and circuit (microcontroller etc.) on the low voltage side. So in-between the output from your power supply and into your circuit/LED strip.

If you can program the output from your circuit to all WS2812B modules displaying white, that should give you the worst-case current figure as all RGB LEDs will be lit.

#### Audioguru

Sep 24, 2016
3,656
The datasheet is a horrible translation from Chinese and does not make sense. It says 12V but shows a maximum of 5.3V and 5.5V. The amount of current used by the LED is not mentioned. It does not say how many LEDs are on the LED strip. It might be a datasheet of one LED IC. You need a datasheet of the entire LED strip.

#### BobK

Jan 5, 2010
7,682
WD2812B is a chip with a single RGB led and PWM to allow 256 levels of brightness for each color. It draws 20 mA per color at the max setting. Count the number of them and multiply by 60mA to get the max current needed.

And they do run at 5V, not 12.

Bob

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