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Postby i_soul » Sun Jan 14, 2018 6:46 pm

Hey guys and girls!

I need a portable power source for a RPi 3 for a small robot I am building. The motor controller and motors will have a separate power source. Now, I though of using a portable mobile charger (power bank) but most of those that supplies ~2.5A are quite large to fit on the robot chassis and quite expensive.

Next, I though of using an UBEC and some rechargeable batteries since these can be fitted more easily in the, somewhat, smaller spaces of the robot chassis. I guess this would do the trick.

Still I got some questions regarding whats the suitable voltage and batteries I should use...

1) I got a spare couple of 18650 type 3.7V Li-Ion batteries, which means these could provide 7.4V. Would these be enough?
2) Should I get a rechargeable 9V PP3 battery. Would this be any better?
3) Would you recommend something else?

Is it possible attach a micro USB end to the UBEC wires in order to power the Pi from its micro USB in order to leave the GPIO free for the usage of a HAT (was thinking of Picon Zero)?

Keep in mind that I don't really much care for the batteries to last too long. My primary concern is to power the RPi 3 using portable power source for at least a couple of hours. If this works I can use to RPi 3 based robot to get it programmed by kids using Scratch2/ScratchGPIO!

Thanks in advance!

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Postby Dave » Mon Jan 15, 2018 9:21 am

As long as you provide at least 6.5V to the UBEC you should be fine.

As for attaching a micro USB connector, yeah this is dooable, something like this will make it a bit easier to solder the wires on to - https://www.modmypi.com/adafruit-indust ... icro%20usb

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Postby i_soul » Mon Jan 15, 2018 3:49 pm

You made my day Dave as I was wondering if I can easily find a couple of these DIY micro USB connectors! One last question, which is somewhat irrelevant to powering a RPi 3 but it has to do with powering stuff using an UBEC.

UBEC's output is at minimum 5V 3A. Could this be used to power stuff like a RPi Zero or pHATs that take external power (like motor controlling pHATs)? Could the 3A output potentially damage a RPi Zero?

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Postby Dave » Tue Jan 16, 2018 9:30 am

i_soul wrote:You made my day Dave as I was wondering if I can easily find a couple of these DIY micro USB connectors! One last question, which is somewhat irrelevant to powering a RPi 3 but it has to do with powering stuff using an UBEC.

UBEC's output is at minimum 5V 3A. Could this be used to power stuff like a RPi Zero or pHATs that take external power (like motor controlling pHATs)? Could the 3A output potentially damage a RPi Zero?


The UBEC can supply up to 5A peak (not continuously) but can supply 3A continuously - Current is "requested" by the device being powered. For example if the Pi "wants" 1A then the UBEC will give it 1A, if the Pi then wants 1.5A the UBEC will give it a bit more etc... Unlike voltage, that is "given" by the power source. (This is just a very basic explanation)

You can use the UBEC to power any 5v device you want that won't draw more than 3A continuously. You will need to make sure that the power input to the UBEC can handle the power output from the UBEC though.

Power = Current x Voltage

Max Output Power = 3 x 5 = 15W

The UBEC has about a 90% efficiency rating so you'll need an input power of about 16.67W

These are just rough values and the reality is you will get a bit of a voltage drop (depending on the device you are powering), so you will want to up the input power a bit.

Hope that makes a bit of sense :)

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Postby i_soul » Tue Jan 16, 2018 2:21 pm

It made sense perfectly! Thanks for the info!!!

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