# Determining Battery Requirement for connecting multiple devices

Discussion in 'Power Electronics' started by jakekusters, Dec 3, 2013.

1. ### jakekusters

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Dec 2, 2013
Howdy,

I'm working on a project at the moment (not in that section as this is more generic info) and it requires that I power two devices off a single battery (a choice really, I want to make something portable). So for this and future projects I would love to know how to calculate my battery needs.

As for my current battery needs I am attempting to power a Raspberry PI (700 mA (3.5 W)) and a laptop screen I am using a 12V 5A power adapter to power the LCD controller board.

Any help would be awesome.

Cheers

Jake

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
The 12V 5A power supply makes the LCD monitor sound like the power hog (potentially up to 60W.

Let's assume the total power consumption is 60W, a 12V 7AH battery would last for just over an hour.

You would probably profit from finding a monitor that requires less power.

Practically, I'd probably power the device from a power source that is always above 14V, that allows you to use a switchmode buck regulator to maintain the 12V. Another regulator could be used to provide the 5V for the rpi.

If I was using lead acid batteries (not great due to their weight), I might use two, thus giving me a 21V to 28V DC rail.

3. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,418
2,788
Jan 21, 2010
Jake, I combined your two threads before splitting them again. At first I thought you were discussing the same thing, but on closer examination, it does appear the topics are significantly different.

4. ### jakekusters

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Dec 2, 2013
Cheers Steve. This thread is more related to me understanding the relationship between power needs and batteries. Regarding the information you gave how did you work out that it would last just over an hour? (Mostly new to this game haha)

1,114
158
Aug 13, 2011
Steve used basic Ohms law and power law calculations.

5A X 12V = 60W, 7Ah X 12V = 84Wh, etc.

Convert each of your load voltage and current requirements to a wattage figure. Do the same for any proposed power source. This allows you to make quick estimations for loads and sources of dissimilar voltage. Then apply a derating factor for each conversion such as buck regulation or inherent discharge curve for a battery type, etc.

By the way, there are off-the-shelf products available to do what you want. This is what I use for similar purposes, but the power requirement for the monitor might be too much for it:

http://www.powerstream.com/PST-MP3500.htm

Last edited: Dec 3, 2013