Battery capacity enquiry

Thread Starter


Joined Nov 14, 2021
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?




Joined Sep 22, 2020
If you need to run at 130mA for 6h, the *smallest* battery that you can use is

\[ 130mA \times 6h = 780 mAh \]

If I were you I'd look at the 1200mAh range :)

Audioguru again

Joined Oct 21, 2019
The capacity rating on a battery is when its voltage has dropped WAY down (sometimes half its new voltage) where your device being powered has stopped much sooner. Then the battery must have more capacity than you think.


Joined Sep 24, 2015
Welcome to AAC.

A 12 volt lead acid battery is considered dead when it reaches 80% of its voltage. So that's only allowing 20% useful power. To assuredly get the full time you want I'd multiply 780 by 5. That gives you 3900 mAh. BUT! Different battery types have different profiles. Different batteries will give you different run times; and depending on which battery you choose you may have to consider some sort of battery management system (BMS). A lithium ion type battery typically appears to be fully charged at 3.8V (per cell). It's normally charged to 4.2V and should not be discharged below 2.8V (IF I remember my numbers correctly). Others who know more will clarify or correct if I've misspoken.

Audioguru again

Joined Oct 21, 2019
I doubt that a huge car battery will used to supply 12V at 130mA for 6 hours.
Maybe he might use a little 3-cells Li-PO rated for 1000mAh but then its voltage might drop to 9V.

3.8V is the "storage charge" for a Lithium-ion cell when its amount of charge is about half. They are sold at 3.8V and a label says to charge them before use.
My radio controlled model airplanes use a 2-cells Li-PO battery. When the loaded voltage drops to 3.15V per cell then the main motor pulses as a warning and when the loaded voltage drops to 3.05V per cell the main motor stops but there is enough power for the servos for a controlled glide to a landing (in a tree or on a roof?).

Jon Chandler

Joined Jun 12, 2008
A 12 volt lead acid battery is considered dead when it reaches 80% of its voltage. So that's only allowing 20% useful power....
Ummm. No. This assumes a linear relationship between voltage and percentage of power used from the battery.

The discharge curve for a lead-acid cell is shown below. The cell voltage is just over 2 volts when charged, and is considered fully discharged when the voltage drops to 1.6 volts. The discharge curve is almost linear to that point. But beyond that point, the discharge falls off rapidly. At 1.6 volts per cell, virtually 100% of the energy available from the cell has been drained.

Curves for other battery types will be similar to this – a large area where the voltage stays somewhat constant, followed by a much sharper drop-off when most of the energy has been used.Screenshot_20211115-082907_Edge.jpg


Joined Sep 24, 2015
There are plenty of SLA's (Sealed Lead Acid) that aren't as big as a car battery. I have a few on my workbench right now. One is a 12V 5.1Ah SLA. I also have an old AGM (Absorbed Glass Mat) in the garage it's another 12V battery, I think it's 20Ah, also sealed. Doesn't have to be a car battery. That'd just be overkill. More on an AGM battery if you care <— click.
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