Battery capacity requirement question

[Abull]

Hi, apologies if this is a bit basic but I am trying to power a LCD screen using a portable power bank and would like to know what size battery I need to power if for 6 hours.

The screen runs at 130mA at 12V apparently, so 1,560 mW.
This equates to 1.56 Wh
So, 1.56Wh x 12V/1000 = 130 mAh per hour
=> 130 * 6 = 780 mAh

These calculations don't seem right but not sure where I am going wrong. Can any help please?

Thanks!

Alex

 

[Pommie]

Your calculations look correct. However, it's not a good idea to run batteries flat so try to get double the capacity. What type of battery are you going to use? N ote, the battery must be 12V or you need to recalculate.

Mike.

 

[rjenkinsgb]

You are correct, but remember "mAH" only relates to current and time, not voltage. eg. Ten similar cells in series make a battery with the same mAH rating as any one of the individual cells.
If you stay with watt-hours then the voltage is also taken in to consideration.

So, the calculation is OK but assumes you can use a battery that stays at 12V for it's full discharge life, or the screen can stand a significant voltage change from the battery being full to flat.

If you need a constant, regulated voltage, then either the battery must be higher voltage and use a buck regulator, or lower voltage and use a boost regulator.

Using lithium cells, three cells give 12.6V full charge but are down to around 10.5V when flat. If the screen cannot take that range, I'd suggest either two or four cells to allow an appropriate boost or buck regulator to be used.

Loose 18650 cells and battery holders and a separate cell charger are the simplest option as you do not need to worry about cell balance while charging.
If charge lithium cells in series, a protection module that provides under/over voltage protection and cell balance is essential, otherwise cells may burst or catch fire.

Two or four good quality 2.5AH cells should be suitable, with four giving a lot longer runtime.


If you want to use a USB power bank, you need a 5V to 12V boost converter and at an absolute minimum allow for roughly three times the current (and mAH rating) from that, to allow for voltage conversion and inefficiencies.

As the capacity of those is listed on a single cell voltage basis, I'd suggest a 5AH / 5000mAH.

Get a boost module rated at around 2A output, as they are often listed at the short term peak load rating rather than continuous, and it has to stand the switch-on current surge as the PSU in the screen charges its electronics up.