Choosing best battery with lowest price for Raspberry Pi

[Joel Karjalainen]

Hello!

I'm not sure if I'm in the right place to ask, but here it goes anyway.
So, basically I'm making little handheld project with Raspberry Pi model B, so now I need a battery for it.
I have Raspberry Pi model B with USB device, and screen to use with it. Raspberry Pi uses +5V (if you didn't know it) and screen also uses +5V. They are seperate devices, so battery should have two +5V outputs.
I'v read that Raspberry Pi uses about 700mA, but I'm not sure if the USB device counts. I don't know about screen, but I do know that it uses "less than 4W".

Battery must have
- High mAh rating, so I could use atleast 6 hours my handheld device. The more the better.
- Price should be affordable, not too pricey.
- Two voltage outputs, maybe even USB outputs.
- Small size, height: around 2cm, lenght and width: max 10cm.
- Battery can be more than 5V, if I can safely use step-down regulator with it.
- Battery should have long not-in-use life, so I don't have empty battery if the device has been just lying around for couple of weeks.

Thanks in advance. Sorry if there are spelling mistakes, I'm from Finland.

 

[kam1787]

a lithium-ion battery might be best

no need for 2 separate power sources - use one to power both Pi and the screen

you are drawing a significant amount of power; close to 7 or 8 watts plus whatever USB device you attach

 

[Joel Karjalainen]

Thanks for the answer.
So, basically you are saying that it is safe to make this:


In picture, black outline is the regular output, and red is made by opening power cable's wire and connecting screen to it.
Also, I don't think that power consumption is 7W, it sounds a little bit too high.

 

[tpetar]

For 5V voltage supply from Li-Ion/Po battery you should use voltage booster or two batteries in serie. For capacity you mention 700mA and work of 6 hours, to support this needs you should have several batteries in parallel.

Look for Li-Po capacity around 2000mAh, price is around 10eur. Battery like for mobile phone.

Polymer Lithium Ion Battery - 2000mAh (You can find this for 10eur)
https://www.sparkfun.com/products/8483

Best regards,
Peter

 

[Joel Karjalainen]

That seems pretty good option.

I found these Recyko rechargeable batteries: **broken link removed**

Those rechargeable batteries are about 1.2V with 2100mAh (let's round it to 2000mAh) so if I had 4x them, I'd have 4.8V and 8000mAh, right? So Wh = 4.8V * 8000mAh / 1000 = 38,4Wh.
Even though it would require a 1-5V DC to 5V DC converter, does it sound good option? If I'd be able to power up RasPi and screen with them I'd be fine with 4-5 hours of usage.

I could also use 8 of them, doubling everything but having to use step-down converter.

 

[tpetar]

Yes, you can use NiMh batteries, they are also good and for sure can do the job.

Buy NiMh and other batteries from good checked local shop (company), dont buy this and other electronic parts over Ebay. On Ebay you can buy Ferrari with Yugo engine. Ok, something is worth to risk, but I personaly do not like this way. I know many cases where people get cheap batteries declared even for 3500mAh and real capacity after test they shows 300-400mAh. The same story is with Li-Ion 18650, in China they recycle batteries make new suit and sell at low price as new, with declared capacity 3000-4000mAh, and after tests they shows 300mAh.


Best regards,
Peter

 

[kam1787]

Thanks for the answer.
So, basically you are saying that it is safe to make this:
View attachment 96419

In picture, black outline is the regular output, and red is made by opening power cable's wire and connecting screen to it.
Also, I don't think that power consumption is 7W, it sounds a little bit too high.

I dont know the characteristics of your 'screen' but that is what I would do first.

If the Pi is averaging 700mA and your screen is < 4W [we have to assume it is close to 4W!] then that is 5+0.7 + 4 = 7.5W

- - - Updated - - -

Those [Recyko ] rechargeable batteries are about 1.2V with 2100mAh (let's round it to 2000mAh) so if I had 4x them, I'd have 4.8V and 8000mAh, right? So Wh = 4.8V * 8000mAh / 1000 = 38,4Wh. .

4.8V is good enough, though you might want to go to 5x[series] to get 6V.


No need to have a DC-to-DC converter unless you put the batteries in parallel - but with a converter you will have a conversion loss; > 5 to 20%

 

[Mattylad]

And how do you propose to charge the battery?