comatsu
Posts: 6
Joined: Mon Nov 11, 2013 8:36 pm

Powering Pi + Screen from AA batteries

Tue Oct 10, 2017 9:42 pm

I'm a total newbie when it comes to electronics, but I've decided I'm going to try to make a pi zero handheld, a learn along the way.

I'm currently using a 3.5 hdmi screen (of this type https://www.aliexpress.com/item/Raspber ... 62038.html ) which works very well. This screen connect to the HDMI port of the pi zero, and also to the pi's usb port for power.

My main issue is that I am trying to power this pi/lcd combination with AA batteries. I know people usually suggest other options, but I would like to understand if I can make this work, and if not, the reason why.

I've bought a 5v boost circuit ( https://www.banggood.com/PFM-Control-DC ... 87949.html ) which is supposed to boost to 5v any voltage starting from 0.9v. However I suspect that it's efficiency and specifications vary quite a lot from those stated, since it has a 'power' led and this won't turn on with 1 aa battery, but needs at least 2 in series (which as I understand it means its not working with 1.5v, but works with 3v).

I first tried 2 aa batteries in series (along with 5v booster) and with these the pi zero turns on, but no sign of life from the lcd. I assumed I just needed more batteries, and tried with a configuration of 2 parallel aa in series with 2 parallel aa ( something like this: https://cdn.instructables.com/F4R/FSSV/ ... MEDIUM.jpg ). Now the pi turns on, and lcd backlight turns on and then turns back off again.

I cannot seem to find the power draw of the lcd anywhere online, and in any case, do not know what formula to use to calculate if this can even be powered with a small number of AA batteries as I've been trying to do.

Would appreciate any comments or assistance...

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Imperf3kt
Posts: 220
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Location: Australia

Re: Powering Pi + Screen from AA batteries

Tue Oct 10, 2017 11:34 pm

AA batteries won't last very long, even with a Pi 0.
Make sure you have a way to safely power down the Pi if the battery voltage drops too low (to avoid SD Card corruption)

I'd personally use at least 4 AA batteries and a DC-DC buck converter to drop the voltage, instead of boost it, to 5v
Raspberry Pi - spend all your time troubleshooting, not learning.

comatsu
Posts: 6
Joined: Mon Nov 11, 2013 8:36 pm

Re: Powering Pi + Screen from AA batteries

Wed Oct 11, 2017 2:50 pm

Imperf3kt wrote: AA batteries won't last very long, even with a Pi 0.
Make sure you have a way to safely power down the Pi if the battery voltage drops too low (to avoid SD Card corruption)

I'd personally use at least 4 AA batteries and a DC-DC buck converter to drop the voltage, instead of boost it, to 5v
Thanks for the reply :)

Will stepping the voltage down instead of up help with the fact that there does not seem to be enough current to power both the pi and the lcd?

MarkR
Posts: 106
Joined: Fri Jan 25, 2013 1:55 pm

Re: Powering Pi + Screen from AA batteries

Wed Oct 11, 2017 3:32 pm

Stepping the voltage down would mean you need less current. Stepping up always needs more current.

But 4x AA is nominally only 6v, but it doesn't take much drop to go below 5v, which can't use a buck converter, and there is also a dropout voltage. So you'd need a boost-buck circuit.

Alternatively, maybe better, use some lithium cells but you need to be more careful and have a special charger. It's possible to just use a normal "power bank" pack, maybe disassemble it but keep the charging circuit etc.

comatsu
Posts: 6
Joined: Mon Nov 11, 2013 8:36 pm

Re: Powering Pi + Screen from AA batteries

Thu Oct 12, 2017 8:53 pm

MarkR wrote:
Wed Oct 11, 2017 3:32 pm
Stepping the voltage down would mean you need less current. Stepping up always needs more current.

But 4x AA is nominally only 6v, but it doesn't take much drop to go below 5v, which can't use a buck converter, and there is also a dropout voltage. So you'd need a boost-buck circuit.

Alternatively, maybe better, use some lithium cells but you need to be more careful and have a special charger. It's possible to just use a normal "power bank" pack, maybe disassemble it but keep the charging circuit etc.
Thanks - guess using aa to power is just not an option

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Imperf3kt
Posts: 220
Joined: Tue Jun 20, 2017 12:16 am
Location: Australia

Re: Powering Pi + Screen from AA batteries

Fri Oct 13, 2017 12:02 am

It is fine if done right. Recently there was a Pi powered RC monster truck thing called a monsterborg which is powered by 10AA batteries and runs 4 high powered motors for several hours!
https://www.kickstarter.com/projects/fr ... ster-robot

Also check out the 3 horse power, li-po powered doodleborg.
https://www.piborg.org/doodleborg
Raspberry Pi - spend all your time troubleshooting, not learning.

asandford
Posts: 1663
Joined: Mon Dec 31, 2012 12:54 pm
Location: Ealing

Re: Powering Pi + Screen from AA batteries

Fri Oct 13, 2017 12:27 am

Imperf3kt wrote:
Tue Oct 10, 2017 11:34 pm
Make sure you have a way to safely power down the Pi if the battery voltage drops too low (to avoid SD Card corruption)
This is the first thing you need to consider - a way to measure the battery state and take action before it drops to a level that would disrupt the Pi.

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