# Powering a PC monitor with batteries

So, this is where I am at currently( no pun intended). I am assuming that with an inverter for both components and 3 - 4-18650 cells in series - in parallel rated at 4800mAh would be good enough? Ill have to look at the controller again but I think the data and power are delivered in the same ribbon.

• You've calculated Wh, not Ah. Divide your Wh by the battery voltage. – brhans Sep 26 '18 at 20:12
• Watt is not Ampere – Eugene Sh. Sep 26 '18 at 20:13
• Beyond the capacity of the battery, you'll also have the issue of providing the monitor with the right voltage, and having a battery that can supply the right amount of current. – jcaron Sep 26 '18 at 21:28
• That seems logical, looking at it again the units do not work out correctly in the google supplied equation. Thank you. – Alex Gasper Sep 27 '18 at 1:48
• I was also going to potentially use a power bank with a outlet and use a DC-DC adapter for the monitor or something similar. From what Ive determined making a battery pack would require a board to ensure battery wear and charge leveling and protection features. – Alex Gasper Sep 27 '18 at 1:50

24 W for 8 hours is 8 x 24 = 200 Wh (watt-hours). (Don't worry about the calculation error. It is nothing compared with the ill-defined battery specification.)

Since $$\ P = VI \$$ we can calculate the amp-hours required for a particular battery voltage. With a 12 V battery $$\ Ah = \frac {P}{V} = \frac {200}{12} = 17 \ \text {Ah} \$$.

Ah is a popular measurement but it is only useful when comparing systems with the same voltage as, for example, in auto batteries. Wh is a true measure of the energy stored in the battery and allows much simpler comparison.

First I have a monitor that has uses 12 V and a micro controller that uses 5 V. I am assuming since these are in parallel they will equate to 17 V draw on the source.

No. Imagine you have two 12 V lamps in your car. Does that "equate to a 24 V draw" on the 12 V battery? Obviously not. Parallel connection increases current, not the voltage.

The panel is drawing ~24 W and the controller 4.5 - 5.2 W under load (~32 W max for system). If I made cells consisting of 5 - 4.2 V 18650 batteries in series equaling 21 V cell, would this give me 256 Whr / 21 V = 12.2 Ah?

• You need a 12 V supply for the monitor.
• You need a 12 V to 5 V converter for the controller.
• The monitor and converter are wired in parallel.
• You need 200 Wh for the monitor.
• You need 5 W x 8h = 40 Wh for the controller.

Where you use a voltage converter you need to include a factor for efficiency. If the 12 V to 5 V converter has an efficiency of 80% then the input power required = $$\ \frac{P}{eff} = \frac {5}{0.8} = 6.25 \ \text A \$$.

• @Transistor- I've reviewed this info and I have come to a conclusion and would like to ask you if what I have it right. First I have a monitor that has uses 12V and a micro controller that uses 5V. I am assuming since these are in parallel they will equate to 17V draw on the source. If I am assuming wrong please let me know. – Alex Gasper Sep 27 '18 at 2:41
• The panel is drawing ~24W and the controller 4.5-5.2W under load ( ~32W max for system). If i made cells consisting of 5-4.2V 18650 batteries in series equaling 21V cell, would this give me 256Whr/21V=12.2Ah? – Alex Gasper Sep 27 '18 at 2:41
• See the update. – Transistor Sep 27 '18 at 6:28

for an 8 hour run time, I would need 20*8-24*8 Ah power source.

You are calculating watt-hours not amp-hours. To get amp-hours you need to divide by your battery voltage. However you should also allow some margin for inefficiency. So if you are using a 12V battery I would guess you want to be somewhere around 30AH.

how do laptops power the same screen

laptop screens tend to be both smaller, dimmer and have narrower viewing angles than standalone monitors. Plus the power systems in laptops are likely to be better optimised than something you can cobble together yourself.

And while 8 hours on a laptop is certainly not impossible it tends to require running the laptop with a relatively dim screen and without any heavy GPU/CPU activity.

• This would technically be a laptop panel and a controller board. – Alex Gasper Sep 27 '18 at 2:54

Marginal as to on-topic-ness, but:

Don't guess, measure - connect a voltmeter and ammeter to the circuit under load and see what is actually happening - be sure to check what the load looks like as it turns on, as well as while running. This may require cutting a cord, or getting a spare cord you can cut, or some such means (clever clipping, but be careful not to short things) to allow measuring.

As pointed out in comments, your calculation does not deal with voltage at all, and is thus watt-hours, not amp-hours (well, unless you are using a one volt battery, which has its own challenges for most applications) and will look more reasonable when you have done that part. As an aside, a Laptop display may not be identical to a similar flat-panel display intended for plug-in use (where the designer does not care that much about power consumption.)

• @Ecnerwal- I appreciate your input, its a very valid point and I will certainly do this. – Alex Gasper Sep 27 '18 at 2:40