Calculating what battery I need

[archieroques]

Hi,

I am working on a laptop, which is to be built out of a R-Pi B+, and an HDMIPi Screen. It will be portable, and have a built-in mouse and trackpad. It will also be powered by a Lithium-polymer battery, running at 11.1v, which will feed into the driver board for the HDMIPi, which in turn will power the screen and provide a 5.2v output for the pi.

My main question is this - what size of Li-Po battery do I need? According to the HDMIPi site, the screen will need about 5 watts, whilst a post on these forums says that 5v will be ample for the Pi. I want to determine what size (in mAh) of battery do I need? I don't really mind how long it'll go for, but I'd be hoping for about 2 hrs at least, but I just wanted to know what sort of thing to look for, as there are thousands out there.

I will only be using a couple of low-power USB peripherals (WiFi, Keyboard, Trackpad) and a Pimoroni Display-O-Tron 3000 too.


Thanks in advance,

Archie

[davidcoton]

There are still too many unknowns (to me at least). But here's an approximation you can refine:

According to the HDMIPi site, the screen will need about 5 watts, whilst a post on these forums says that 5v will be ample for the Pi.

OK, so 5W at 11V means ~450mA, just hold that for now.
5V is not only "ample" for the Pi, it is required. (In fact 5.2V is near the top limit). But how much current (without peripherals)? From memory the B+ uses about 700mA, so allowing for conversion inefficiency say 400mA at 11V.

I will only be using a couple of low-power USB peripherals (WiFi, Keyboard, Trackpad) and a Pimoroni Display-O-Tron 3000 too.

Well, that "couple" makes four -- what is their total current consumption? WiFi might not be as low-power as you want. Say (wild guess) 800mA at 5V or around 450mA at 11V.
Now total consumption at 11V is 1.3A. So you need 1.3Ah capacity for each hour run-time. Add margin and reserve to taste.

[archieroques]

Thanks so much - that's really helpful.

It doesn't matter that it's not spot-on, I just really needed a rough estimate. I think I'll go for a 2600mAh battery - this should give me around two hours of runtime. In addition to this, the HDMIPi driver board will run off a 5.2V Micro USB, so this will enable me to use it in a car, or wherever I have a wall plug.

[archieroques]

5V is not only "ample" for the Pi, it is required.

Apologies, I meant 5 watts! I know that I need 5v for the pi.

[archieroques]

Ok,

So I have changed my plan a little, and I now plan to use a Lumsing 6000mAh battery pack, which can be found on Amazon for £15-20.

This has been done here: (http://www.raspberrypi-spy.co.uk/2014/1 ... ent-100939) and he/she says that they can get about 2 ¾ hrs out of it when looping HD Video, with wifi, keyboard and mouse.

Thanks anyway - that was really helpful.

Archie

[Djinny]

You may want to consider getting a nice high efficiency adjustable switching regulator. I don't know if the HDMIPI will run off a flat 5.0v which would be safer for the pi.
But you can get a cheap one on amazon or ebay or something. 3A would be more than enough, and are the norm. Just adjust it to 5.0v and you're set.

WiFi adapters take a lot of power, so my suggestion is to make a short extension with the positive lead to a hardware 2 pole switch. If you don't need wifi, just flip the switch and you're set.

Here's some stuff ill be doing/using to save power.
I plan on using an octopus usb hub, like the one pictured below. Easily pop open the case, and desolder the wires. I have a 7 port one, that uses 2 internal 4 port hub controllers. I opened it, cut the power trace that goes from main usb port, that leads to the 5/7 other ports, and then made 5 of the ports external power only. I then connected the remaining 2 ports to the host's pos voltage line (since i had cut the trace), so 2 ports can be powered by the pi. I then used red nail polish on the plug ends to mark which ones are powered internally.

It works amazingly well. This way when i build my own pi laptop, i can disassemble the usb hub, remove the cables, and make it into a smaller package, i can connect the 5 externally powered ports to an external switching regulator (with a switch). keeping the other 2 connected normally through the pi. So a wireless keyboard receiver will be powered by the pi. And i can flip a switch to cut power to the rest of the ports. or individual ports.

I love switches though, i was going to have a 5 switch dip-switch (for usb ports) and multiple larger switches to turn off everything from the pi, the usb ports, the wifi, the hdmi screen, my little 2x16 lcd screen(for my micro-controller), a switch for the micro-controller board itself, my "xbee", switches for everything.



PS that 6000mAh battery looks amazing! no regulator needed!
I think i'll use that too, just add a usb power port inside the laptop and a little tray that can be accessed externally, just use them as slide in, slide out battery packs.

[archieroques]

Hi Djinny,

I did think about having some external-power-only USBs, but there are a couple of problems:
Firstly, the way I have designed it, the USB&Ethernet side of the Pi will be facing the outside of the laptop (so I can easily plug stuff in) so the wire to the hub would need to loop round out and back into the case again if I did that. Also, there will only be one USB port on the outside, as I don't think I really need any more (I'm using a B+ pi, and 3 ports are for Keyboard/Trackpad/WiFi). It's quite a lot of effort to just get one USB port to go powerless!

I'll also be operating it off a 12V wall adapter when I have power. However, if I do later encounter problems with the supply, I will definitely do what you suggested - it's a great idea!

I will post here when I have finished the project, so that you and others can know how I have got along with the Lumsing battery unit. I'm hoping to get some of the last components for christmas, so I should have it all done before the new year!

Have Fun,

Archie

[Djinny]

I personally considered desoldering one or both of the usb connectors. I have the tools to do it. Just so i can feed those 2 ports inside the system. While still having 2 ports external.
It would give me a bit more control over the +v line too.
Or like you said, you may only need 1 external port, so that hub i linked, could have one of it's cables in place of that 1 port. The hubs i use are 7 port, and look just link the pictured one. just with more cables coming out.

I'm making mine into a case like this. But slightly bigger.
http://www.vaultz.net/medicine-cases/va ... -case.html

With the wireless keyboard external, with gamepad controls on the inside. Since it'll be a robot controller first, a laptop second. with the robot control doubling as gamepad for emulators.

[archieroques]

I'll see how I get on. There's going to be enough cutting and soldering of wires anyway (I'm putting switches on the USB cables so that you can plug it into a USB port and power the charger or power the pi directly, according to which way the switch is set). I also need to shorten the keyboard cable, possibly the trackpad one and maybe some other ones too. I also want to be able to power the system from an external 12v power plug, but I am going make it so that I can't accidentally put 12V and 5V into the HDMIPi driver at the same time and fry the circuits, which I know I would do otherwise!

I'll see how I get on with the setup that I've got, but if I fancy some extra soldering/come across problems then I'll do as you said.

That case looks very cool. I think that I'm going to design and make my own case, but I have no access to a CNC machine/ 3D printer so it would all be cut on my electric scroll saw. I'm trying to think of some cool materials to make it from as well, as the polycarbonate and MDF combination that I've always used in the past is not very good looking, nor is it very strong or light.

I'll post updates when I have the components! My battery is arriving tomorrow

Archie