Coburn
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Re: Coburns take on powering the Raspberry Pi via batteries

Sat Sep 10, 2011 6:19 am

Firstly, I understand that there's a number of forum threads out there that ask how to run the Raspberry Pi off a battery; but this is my own take on the subject.

Being a geek that I am, I have limited electronics skills, but I learn quickly thanks to Google and also websites explaining the basics of electronics. I've been thinking of running the Raspberry Pi off a battery source, but the problem that remains is the battery source itself - what do we use?

I was searching eBay for some rechargable batteries, and I found NiMh D-Sized batteries that have a whopping 10,000mAh capacity. I have seen batteries in AA size that offer 3000mAh, but D Sized batteries give you more oomph for the price. Now, NiMh batteries are usually around 1.2V. A quick calculation reveals that 4 D batteries in a series would not suffice, that's ~4.8 volts (roughly). Bump that up to 6 D sized batteries, that's 7.2V. That falls inside the 6V - 24V (?) specification that the Raspberry Pi unit requires.

According to google, 1 Watt is 1000mA. That means, at full load, you'd get 7 hours to battery life - on par with some other Android tablets. This is the calculator I used to work out the battery life, which gives you a ballpark figure. Let's say the Raspberry Pi idles at 250mA, punch that into the calculator, and that's 28 hours of battery life!

So, running the Raspberry Pi off D Sized batteries is a possible solution. Forget AA Batteries if you're looking for long-lasting battery life - rechargable D side batteries exist and at 10,000mAh, they are a tank. One would connect the batteries in a series, allowing you to get 7.2V with capacity of 10,000mAh, maybe attach a LED to indicate the battery status (ie. Green/Red) and then connect the output of the batteries to the Raspberry Pi's board. Maybe you could solder them onto the DC jack's inputs?

Could one of the Raspberry Pi devs please give a ballpark figure of a vanilla Raspberry Pi's (either model A or B, I'm planning to get model B) current draw when idle?

Cheers,
Coburn. ;)

Johannes
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Re: Coburns take on powering the Raspberry Pi via batteries

Sat Sep 10, 2011 6:43 am

7.2V*10000mAh=7.2V*10Ah=72Wh. At 1W, that would give you 72Wh/1W=72 hours of battery life. At 0.25W almost two weeks (72Wh/0.25W=288h).

Coburn
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Re: Coburns take on powering the Raspberry Pi via batteries

Sat Sep 10, 2011 6:53 am

Quote from Johannes on September 10, 2011, 07:43
7.2V*10000mAh=7.2V*10Ah=72Wh. At 1W, that would give you 72Wh/1W=72 hours of battery life. At 0.25W almost two weeks (72Wh/0.25W=288h).

That's roughly 3 days of non-stop battery life, which makes D Sized batteries a excellent choice! I also did see 3000mAh AA batteries if people want to go down that path; those are waaaaaaaay cheaper than D size batteries. I saw a 8 pack of them on eBay for ~$10 AUD, 8 x 1.2V = 9.6V. That would roughly give you 28.8 hours (1 day, 8.8 hrs) of battery life on full blast, 4.8 days on idle.

Of course, this is a ballpark figure, it would also depend on what's plugged into the 'pi.

jamesh
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Re: Coburns take on powering the Raspberry Pi via batteries

Sat Sep 10, 2011 9:06 am

In the office we use 7.2v radio control car battery packs like this

http://www.ebay.co.uk/itm/1-pc.....500wt_1156

Probably easy to deal with than multiple AA or D's. (although that's probably what they are inside!)
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doobedoobedo
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Re: Coburns take on powering the Raspberry Pi via batteries

Sun Sep 11, 2011 2:17 am

That ebay link is a pack of 6 sub-C size cells. They're NiMh so the claim of no memory effect is an outright lie (NiCd were worse for memory effect though).

It'd be worth your while looking into LiPo packs too. 2S are 7.4V and 3S are 11.1V they're much lighter than NiMh. That Rc car pack should be pretty heavy at that capacity.

NiMh heavier = more capacity
LiPo larger physical size = more capacity

The downside of LiPo is that they shouldn't be over-discharged.

iAreNewb
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Re: Coburns take on powering the Raspberry Pi via batteries

Sun Sep 11, 2011 4:20 am

I'd probably go with LiPo for my use cases since 1) in robotics applications, or with a relatively large touchscreen (5-10"), physical dimensions in L/W/H are relatively insignificant and 2) weight is a serious turn-off in both robotics & portable electronics.

Coburn
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Re: Coburns take on powering the Raspberry Pi via batteries

Sun Sep 11, 2011 6:30 am

Hm, this is giving me some interesting ideas to chew on.

Another thing is to power your USB touchscreen via a USB Power Bank. I have two that are 2000mAh and 5000mAh respectively. One would plug one cord (ie. Power) into the power bank, and the other (ie. data) into the 'Pi. The only downside to this is two things to charge, and most likely the USB power bank will die before the raspberry Pi does. Mine do have LEDs that tell me how much life is remaining (ie. 1 flashing led = almost dead, 1 led = 5% to 25%, 2 led = 25% to 75%, 3 led = 75% to 100%, and so on).

I most likely will use a combo of two power packs until we can run the 'Pi off 5V.

Blars
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Re: Coburns take on powering the Raspberry Pi via batteries

Mon Sep 12, 2011 2:38 am

Warning about Chineese batteries: many are marked with inflated ratings.

Coburn
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Re: Coburns take on powering the Raspberry Pi via batteries

Mon Sep 12, 2011 7:28 am

Thanks for the warning! :)

Also, here's some that look interesting:
7.2V Ni-MH 3300mAh Battery Pack
6V 1600mAh Battery Pack

I did see a Li-Po battery pack here but it looks expensive - $100 AUD! That hurts the wallet.
Both link to Jaycar, which has UK/US/Europe/Australia stores. These are by "Powertech" which seem to be a genuine battery - I'm sure Jaycar wouldn't sell something with a inflated rating!

Although, I have yet to see a socket that fits the "Tamiya" type plug.

doobedoobedo
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Re: Coburns take on powering the Raspberry Pi via batteries

Mon Sep 12, 2011 9:45 am

For LiPos at a reasonable price you need to check out the RC scene discounters (try Hobbyking).

If you had an RC car (especially one made by Tamiya ;)) then you'd be familiar with the Tamiya plug, it's been a standard RC car plug since the late 1970s. But even then you'd want to cut it off and replace it with a different connector, as they have a tendency to melt with modern batteries and motors...

scologic
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Re: Coburns take on powering the Raspberry Pi via batteries

Mon Sep 12, 2011 10:12 am

LiFepo also has the need for recharge within 2 months or sooner as a general rule depending on spec. Once a LiFePO dies it's dead.

You also need to look at possible charge in use, and that then needs a decent charger and management circuit.

You can also get F sized cells from the US with larger run times. Jut remember adding a USB device adds Wattage and reduces time. IE a USB display + board is 5v 2A generally with Arm = 10 Watt.

Have fun but be safe.

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crundy
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Re: Coburns take on powering the Raspberry Pi via batteries

Mon Sep 12, 2011 10:19 am

Could you use a couple of 14500's (3.7v, 900mAh) to power an R-Pi? They're about the same size as AAs so you could bung two of them into a 2xAA hobby case like this.

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crundy
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Re: Coburns take on powering the Raspberry Pi via batteries

Mon Sep 12, 2011 10:29 am

Actually I suppose 18650s (3.7v 2600mAh) would be better:
http://www.mostbuying.com/chea.....l?pro=gsuk

Edit: Whoops, wrong link

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abishur
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Re: Coburns take on powering the Raspberry Pi via batteries

Mon Sep 12, 2011 4:29 pm

I plan on making an e-reader when the r-pis are shipped. I'm looking at using the Maxim DS2715 to do my battery charging. (I'm not affiliated with them I'm just gonna give their chip a shot). I know NiMH have their issues, but I think it will be both a good solution for the money I'll put down on it initially.
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Coburn
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Re: Coburns take on powering the Raspberry Pi via batteries

Mon Sep 12, 2011 10:55 pm

Well, if anyone works out a working charging/battery circuit layout when the RPi's are released for battery power, that would be great. I have a old android tablet that is made in china which has two batteries that provide it with ~7 hours battery life, and the CPU is constantly 800Mhz without any power management.

If we connected those two packs in series, that's 7.4V. Then, you would have to make a setup to do charging from a DC Plug (maybe one of those DC plug packs?). Alternatively, if I used a NiMh power pack, I could just unplug it and recharge it manually...

My question remains though, is the cutoff voltage for the RPi. If it falls below 6V, what happens? Does it continue running or does it power off?

I sure hope the RPi team considers making a battery pack for the RPi after it's launch. ;)

edit: how does one recharge lithium batteries? And what does it mean by "protected"?

Johannes
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Re: Coburns take on powering the Raspberry Pi via batteries

Tue Sep 13, 2011 7:35 am

Charging lithium batteries is simple in principle (Don't exceed the maximum cell voltage, don't exceed the maximum charging current), but if you do it wrong, you can end up with a battery violently catching on fire.

Coburn
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Re: Coburns take on powering the Raspberry Pi via batteries

Tue Sep 13, 2011 11:41 am

According to the wiki:


Preliminary Alpha board power usage measurements. This is with graphics output on the HDMI port, no power from USB plugs.

Input 7.5V, ~180mA Linux running only with prompt.
Input 7.5V, ~300mA heavy graphics running on system.


These are some ballpark figures for prototype boards. So, it looks like the Raspberry Pi will consume around ~200mA at ~7.5V idle. Not too bad. I have found some USB Touchscreen monitors on eBay (if you find any decent cheap ones, let me know - preferably if they can ship to Australia) that have Linux support, which run off one USB cord, and the monitor has a port for a SD Card! Interesting.

Throwing that into the mix, that means that roughly, you're looking at, say, less than 1000 mA (1A) of current draw. Of course, using the Model B version (which I intend to get) is going to draw some more power for it's ethernet and LAN ports...

Coburn
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Re: Coburns take on powering the Raspberry Pi via batteries

Mon Sep 19, 2011 7:23 am

Question: Does anyone know of a good cutoff device (like a watchdog (?)) for NiMh/LiPoly power packs?

I have seen one here but I am unsure if it would work (this looks like it's more for RC toys and such...) Also, thanks for the Hobby King link - I will use them for a NiMh/LiPoly power pack. :)

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Jongoleur
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Re: Coburns take on powering the Raspberry Pi via batteries

Mon Sep 19, 2011 8:43 am

If you don't mind a chunky battery pack, how about going for an old-tech solution?

Sealed lead-acid batteries provide plenty of oomph and are less flakey than NiMh or exotic Li technologies. Maplin ( http://www.maplin.co.uk/sealed.....code=UD07H ) in the UK offer a range with some suitable candidates, like this 6v/20ah unit for 20 quid. Just cobble together a box with flylead, warning LED and switch and you're off! :-)

(Ok, I admit they're not ideal for some applications where battery power is useful...)
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Coburn
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Re: Coburns take on powering the Raspberry Pi via batteries

Mon Sep 19, 2011 11:08 pm

Funny you should mention that, I have a old 12V battery from one of dad's electric fencers that he gave me as an experiments battery. It's 12v 7Ah, but it's bulky. Not really a good idea for carrying around.

I will post the list of stuff that I am thinking of buying (ie. Batteries, voltage watchdog (LED), etc).

Thunder__X
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Re: Coburns take on powering the Raspberry Pi via batteries

Wed Sep 28, 2011 9:54 pm

Quote from Coburn on September 20, 2011, 00:08
Funny you should mention that, I have a old 12V battery from one of dad's electric fencers that he gave me as an experiments battery. It's 12v 7Ah, but it's bulky. Not really a good idea for carrying around.

I will post the list of stuff that I am thinking of buying (ie. Batteries, voltage watchdog (LED), etc).

Also funny that you should mention that, I have an 18v ryobi battery pack that my dad gave me, because it wouldn't hold a charge very well. but, I think I want to find a more standard rechargeable battery. Maybe four rechargeable 9v batteries connected in parallel.

ricksl
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Re: Coburns take on powering the Raspberry Pi via batteries

Wed Sep 28, 2011 11:02 pm

i wouldn't suggest 9volts, not for this application at least, 9volts are actually just 6, 1.5volt cells in series. they are actually called AAAAs and the current capacity of each is less than 1Ah and thats for alkaline chemistry cells, when you get into rechargeable you'll be using NiMH or NiCd cells in series with each other which have an even higher internal resistance and an average voltage of 7.2 to 8.4 volts respectively. true you could put them in parallel but you would still have less watt hours than 4 AA batteries. I looked into powering it with two rechargeable lithium 3.6 volt batteries in series, the ones i looked into have less watt hours than a 9volt non rechargeable but more than rechargeable 9 volt rechargeable. the reason i like lithium batteries is because they have a nice voltage curve were even at 50% drained they still have a high standing voltage. Not saying there aren't other ways to power the device but as far as weight and portability would use a lithium cell.

Thunder__X
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Re: Coburns take on powering the Raspberry Pi via batteries

Wed Sep 28, 2011 11:54 pm

Quote from ricksl on September 29, 2011, 00:02
i wouldn't suggest 9volts, not for this application at least, 9volts are actually just 6, 1.5volt cells in series. they are actually called AAAAs and the current capacity of each is less than 1Ah and thats for alkaline chemistry cells, when you get into rechargeable you'll be using NiMH or NiCd cells in series with each other which have an even higher internal resistance and an average voltage of 7.2 to 8.4 volts respectively. true you could put them in parallel but you would still have less watt hours than 4 AA batteries. I looked into powering it with two rechargeable lithium 3.6 volt batteries in series, the ones i looked into have less watt hours than a 9volt non rechargeable but more than rechargeable 9 volt rechargeable. the reason i like lithium batteries is because they have a nice voltage curve were even at 50% drained they still have a high standing voltage. Not saying there aren't other ways to power the device but as far as weight and portability would use a lithium cell.

I would like to use lithium batteries, but I'm not sure how to go about building a charging circuit for them. I'd like to find some sort of easy external way to charge the batteries. We could buy a $14 dollar psp wall charger, some knockoff psp batteries in series, and mod it like this.

Coburn
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Re: Coburns take on powering the Raspberry Pi via batteries

Thu Sep 29, 2011 12:11 am

The only thing would be the issue of making a voltage cut-off circuit or something. I wouldn't want to ruin a PSP battery by running it below it's minimum voltage.

I also heard that some LiPo batteries have got over-discharge protection circuitry built-in, which seems to be a interesting option. I also heard that you can drain Ni-Mh batteries without ill side-effects (this info came from my local hobby store).

Seeing how the Raspberry Pi will shut off if it's under 6V, I suppose we'd be safe - a normal discharged PSP battery is around 2.9V, so that's 2.9V x 2 = 5.8V.

Food for thought!

Thunder__X
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Re: Coburns take on powering the Raspberry Pi via batteries

Thu Sep 29, 2011 12:31 am

We could try, http://www.google.com/products.....CHwQ8wIwAQ

These batteries can be charged externally.

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