windoge89
Posts: 10
Joined: Thu Jul 17, 2014 7:08 pm

Small high capacity battery

Sun Aug 10, 2014 5:45 am

Hi. I am currently working on a raspberry pi laptop and need a good battery power supply. The problem is that I can't seem to find a high enough capacity battery to power both the Pi and the screen that is small enough to fit in the enclosure (I have exactly 4 x 10 cm to work with).

I know the Pi takes somewhere near an amp at the absolute max, and the screen takes about 500mAh, so does anyone know of any solutions? I know that you can disassemble battery banks to make them smaller, but I only want to do so if I know doing so will get it to the size I need. The size can be slightly bigger than specified.

Do you think running of of rechargeable batteries with the proper voltage regulators is a good idea space wise?
Thanks!

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MattHawkinsUK
Posts: 538
Joined: Tue Jan 10, 2012 8:48 pm
Location: UK
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Re: Small high capacity battery

Sun Aug 10, 2014 12:23 pm

I would find a USB power bank that fits the space. They often have 1 and 2A outputs. I've had a few review products from RAVPower and Lumsing. Beware of unbranded devices making false claims about capacity. In general they all use the same cells internally so capacity is proportional to size.

Over time the capacity will drop with age and that is where the unbranded eBay specials start to look a bit rubbish.
My Raspberry Pi blog and home of the BerryClip Add-on board : http://www.raspberrypi-spy.co.uk/
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Duane Degn
Posts: 55
Joined: Thu Jul 10, 2014 6:30 am
Location: Idaho, USA

Re: Small high capacity battery

Sun Aug 10, 2014 1:51 pm

windoge89 wrote:Hi. I am currently working on a raspberry pi laptop and need a good battery power supply. The problem is that I can't seem to find a high enough capacity battery to power both the Pi and the screen that is small enough to fit in the enclosure (I have exactly 4 x 10 cm to work with).
How deep is this 4 x 10 cm area?
windoge89 wrote: I know the Pi takes somewhere near an amp at the absolute max,
Is this a measured value or are you quoting a value from some other source? I haven't measured this myself but I thought the Pi drew around 500mA of current. I suppose a keyboard and mouse/touchpad could bring this up to an amp.
windoge89 wrote:and the screen takes about 500mAh,
You mean 500mA right?
windoge89 wrote:so does anyone know of any solutions? I know that you can disassemble battery banks to make them smaller, but I only want to do so if I know doing so will get it to the size I need.
I think for a flat rectangular area a Lithium Polymer (LiPo) would give you the most capacity for the size. I measured the volume of a LiPo pack I have and I come up with 76mAh per cm cubed. If your battery area (4x10cm) were 1 cm deep, you could probably fit a 3Ah cell in the space. This 3Ah cell would give two hours of 1.5A current but you'd need to boost the voltage from the pack to 5V so 1.5A at 5V would require a higher current from the battery. If we assume the average voltage from the battery is 3.8V and the boost converter has 90% efficency, then I think to get 1.5A at 5V would require 1.5*(5/3.8)/0.9 = 2.19A from the battery. So your 3Ah cell would only last 1.37 hours (if I did the math right).
windoge89 wrote:Do you think running of of rechargeable batteries with the proper voltage regulators is a good idea space wise?
Is there another option?

There are places which sell LiPo cells in all sorts of shapes and sizes. You may be able to find one to fit the available area without wasting much space.

You'll need some sort of changing and boost circuit. You could take apart a module like Matt was talking about and use the charging boost circuit in your computer or you could probably find the appropriate circuit for sale some place. I don't know of a circuit for sale capable of the current you need but this circuit from ElectroDragon takes care of the charging and voltage boosting for a system requiring about 400mA. I'm sure there are 1.5A versions for sale somewhere.

You'll need to worry about dissipating the heat generated by the charge/boost circuit. If you take apart a module like the ones Matt suggested, you could see how they take care of the heat issue.

You might want to stick with a ready made module as Matt suggests and use it as an external supply. Using LiPo cells incorrectly can cause fire/explosions and a ready made module is probably safer than trying to make your own charging/boost system.

If you do try to make your own system, make sure to get a second opinion about the design by posting pictures and a description of your system here or to some other electronic forum. Others may see a problem you miss.

windoge89
Posts: 10
Joined: Thu Jul 17, 2014 7:08 pm

Re: Small high capacity battery

Sun Aug 10, 2014 4:48 pm

I have about 2.5 cm to work with vertically. Maybe I do need to use a prebuilt one. I have managed to change the layout to make more battery space and eliminate a potential ribbon cable issue. I am going to use a Patriot portable power pack to power it. I shall show you the design once I am done drawing and modeling it (I am using a 3D printer to make the case).

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