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

Thu Sep 29, 2011 1:07 am

I'm hoping to get my friend to return an RC car I gave him a while back because I had moved and didn't think I would ever use it... Well, it had a charger and 2 3800(?) mA 7.2v batteries that would be perfect for this.

I just hope he didn't lose it. :s

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

Thu Sep 29, 2011 1:13 am

I guess NiMh is something to look into, i was under the impression that there were a few flaws, i thought that they needed to be drained close to completely before recharging, and then the issue of possible polarity shifting in the individual cells, although not nearly as bad as nickel cadmium. the reason i am partial to lithium is that a single cell can output 3.6 volts or something ballpark to that, whereas the other cells output 1.4 ish. although if you want to get a lithium battery pack with two cells you need a specialized charger (theres a ton of single cell chargers out there though).

Super Caps anyone?

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

Thu Sep 29, 2011 2:32 am

Quote from ricksl on September 29, 2011, 02:13
I guess NiMh is something to look into, i was under the impression that there were a few flaws, i thought that they needed to be drained close to completely before recharging, and then the issue of possible polarity shifting in the individual cells, although not nearly as bad as nickel cadmium. the reason i am partial to lithium is that a single cell can output 3.6 volts or something ballpark to that, whereas the other cells output 1.4 ish. although if you want to get a lithium battery pack with two cells you need a specialized charger (theres a ton of single cell chargers out there though).

Super Caps anyone?

Speaking of capacitors, I was wondering today if capacitors can hold a steady charge after a battery runs out of juice.

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

Thu Sep 29, 2011 11:59 am

Physics time: The voltage over a capacitor is linearly dependent on the charge and reciprocal on the capacitance, or the other way around, Q=V*C, charge equals voltage times capacitance. Charge is measured in coulombs, capacitance is measured in farads. A 1F capacitor charged to 1 coulomb will measure 1V. From this you can see how much energy is stored in a given capacitor at a given voltage.

Let's say you can use voltages between 3V and 5V to power your circuit and you're using a 10F capacitor (or ten 1F capacitors) charged to 5V to store energy. At 5V, there's a charge of 50C, at 3V the remaining charge is 30C. The average voltage is 4V and you got Q equals 20 coulombs out of the capacitor. You know that power is voltage times current, P=V*I. Work (or energy) is power times duration, W=P*deltat. Thus W=V*I*deltat. The current is charge divided by duration, so W=V*Q. In our example, you got 4V*20C=80J out of the capacitor. One joule is one Ws (wattsecond), so if the circuit consumes 2W, then you can power it this way for 40 seconds.

The energy stored in a capacitor is usually given as W=0.5*C*V^2. To see that our calculation is correct, set the low voltage to 0V: Charge 50C, average 2.5V, so W=2.5V*50C=125J. That's the same as W=0.5*10F*(5V)^2=125J. In practice you can't use voltages below a certain threshold.

Let's say you buy two 2600F 2.5V supercaps and put them in series to get to 5V. Again, let's say you can use 3V to 5V. That's W=0.5*(5V+3V)*(5V-3V)*2600F=20800J. At 2W, that would be 20800Ws/2W=10400s or about 2 hours and 50 minutes, if the capacitor could hold a charge that long. Of course you'd have to pay more than twice the price of a Raspberry Pi Model B just for the capacitors. Four rechargeable AA batteries store about twice as much energy.

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

Thu Sep 29, 2011 1:09 pm

I am not an expert in electronics and correct me if I am wrong but I think that one of the main advantages of super capacitors would be the charging time. You could charge them for 10 seconds and have them powering your Raspberry Pi for almost 3 hours. If you loose the charge you just charge it again for 10 seconds and not for some hours like rechargeable batteries.
Another advantage of capacitors would be the capacity of delivering big current in case you need it, for example to power up a motor or another device that needs a big amount of current for a short period of time.
As you say, the problem with super capacitors would be price.

I think the ideal power supply would be a mixed battery/super capacitor solution that would be able to store a lot of power and provide big amount of current if needed. I think that a step up converter or many super caps in parallel would also be necessary in order to power the Raspberry.

Sorry for my english but I am a spanish native speaker.

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

Thu Sep 29, 2011 1:27 pm

Unless there's a regenerative braking USB accessory that I don't know of, where are you going to get the current to charge a supercapacitor much faster than a set a of AA batteries?

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

Thu Sep 29, 2011 1:38 pm

@Johannes

You need to read up on supercapacitors. They can be charged in seconds, not hours as batteries. Most PSUs can deliver far more power than batteries can absorp, they have to throttle power not to fry the electrodes or boil the electrolyte.

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

Thu Sep 29, 2011 2:26 pm

Apologies for the repost, but does anyone know if you could use a couple of 18650's (3.7v, 2600mAh) to power the Pi?

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

Thu Sep 29, 2011 3:26 pm

@kme

You can charge a supercapacitor in seconds, but you need a power source which can deliver the necessary current to the capacitor. If you want to charge a capacitor with the power to run a 2W device for 3 hours and you only want to take 60 seconds to do so, then you need a 2W*10800s/60s=360W power source, and you can't exceed the maximum voltage of the capacitor. That's serious current. Hence the reference to the regenerative braking USB accessory. I question the usefulness for Raspberry Pi applications.

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

Thu Sep 29, 2011 3:32 pm

Wire them in series... and sure 7.4V would be sufficient to power the r-pi.
Dear forum: Play nice ;-)

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

Thu Sep 29, 2011 3:34 pm

Quote from abishur on September 29, 2011, 16:32
Wire them in series... and sure 7.4V would be sufficient to power the r-pi.

Roughly how long would they last for between charges? I fail at electronics.

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

Thu Sep 29, 2011 3:59 pm

7.4V*2.6Ah=18.72Wh. Divide by the average power consumption to get the running time in hours. (This assumes that the onboard regulator is a switching regulator, not a linear regulator.)

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

Fri Sep 30, 2011 1:58 am

What I'll do for starters is run the Pi off AA Batteries. I'm getting a 6 AA Battery Clip from eBay.

Also, what do you think about this? It's a 4300mAh 7.2V battery pack. Check the user's ebay store, there's heaps of battery packs in there.

Nickel Zinc Rechargeable batteries, anyone?

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

Fri Sep 30, 2011 8:14 am

Quote from Johannes on September 29, 2011, 16:59
7.4V*2.6Ah=18.72Wh. Divide by the average power consumption to get the running time in hours. (This assumes that the onboard regulator is a switching regulator, not a linear regulator.)

So did we ascertain that the R-Pi uses 1W at full whack? If so then that should be about 18h runtime off two AA sized batteries.

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

Fri Sep 30, 2011 9:13 am

Quote from crundy on September 30, 2011, 09:14
Quote from Johannes on September 29, 2011, 16:59
7.4V*2.6Ah=18.72Wh. Divide by the average power consumption to get the running time in hours. (This assumes that the onboard regulator is a switching regulator, not a linear regulator.)

So did we ascertain that the R-Pi uses 1W at full whack? If so then that should be about 18h runtime off two AA sized batteries.

Also factor in USB usage (ie. 100mA to 300mA for a WiFi adapter), and optionally bluetooth. USB Touchscreens will take up 500mA if not more.

Note: my WiFi adapter according to Windows consumes 450mA. That's a little power hungry.

I would personally look for something above 2000mAh. That way, I know I'm going to get ~2-3 hours at least.

Bulky-ness is also a worry.

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

Fri Sep 30, 2011 10:23 pm

sorry about all the trouble with the super caps suggestion and such, it was a joke mainly cause we were shooting around ideas of different power storage things, true they might have a few useful applications, one such would be in parallel to a solar cell, were say it is cloudy that day and the voltage of the solar cell could drop if a cloud passes over and shutoff your raspi (oh no!). But a super cap could prevent shutoffs like that from occurring by providing extra charge when the supply dips. Or even acting as a filter for noisy signals from cheap power supplies, either way super caps might have a use, but it will take some work on our part to figure out how to integrate them.

as far as the cost ive seen 2600 2.5 volt capacitors on sale at electronics goldmine for 10 dollars, i have bought ones that are a twentieth of that capacity for more.

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

Fri Sep 30, 2011 11:09 pm

Quote from Coburn on September 30, 2011, 02:58
What I'll do for starters is run the Pi off AA Batteries. I'm getting a 6 AA Battery Clip from eBay.

Also, what do you think about this? It's a 4300mAh 7.2V battery pack. Check the user's ebay store, there's heaps of battery packs in there.

Nickel Zinc Rechargeable batteries, anyone?Be very careful of NiMh battery packs from Hong Kong on ebay. If you're lucky, they may even be as much half the advertised capacity, but only if you're lucky...

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

Sat Oct 01, 2011 12:57 am

Battery warning noted.

Also, how would one recharge the batteries like these that crundy pointed out? Although, I like these 3000mAh batteries...

I suppose you could run two sets of these batteries in parallel to double your capacity. Might do that once I get my own Pi.

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

Mon Oct 03, 2011 11:16 pm

Question. My local hobby store has a battery pack that's 3000mAh and it features two plugs, one for charging, and the other one for charging. Now, my question is that is it possible to float charge the batteries while in use?

For example, the connection plug is connected to the Raspberry Pi DC Jack. The other charging plug is plugged into a NiMh balance (?) charger. Say the battery is running low, can one switch on the charger and charge the battery while the Raspberry Pi is switched on (running from the battery in this case)?

The low voltage warner setup would be a buzzer and a LED to warn the user the battery is getting weak.

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

Thu Oct 06, 2011 7:06 pm

I just got a circular in the mail a couple days ago from a hardware store pretty close to where I live (Harbor Frieght Tools in the United States), and it has an advertisement for a $12.99 NiMh and NiCd battery charger(which charges all different battery sizes)http://www.harborfreight.com/b.....47618.html, and an assortment of four NiMh batteries of any type (excluding 9v which only includes one) for $6.99. http://www.harborfreight.com/p.....97864.html

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

Thu Oct 06, 2011 8:56 pm

There are also a good deal of coupons you can use. Currently, I'll be getting $4 off and a free screwdriver set worth $3.49, with a grand total of $23.51 including shipping .

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

Mon Oct 17, 2011 10:22 pm

I just ordered a 6x AA battery clip, in which I aim to use 3000mAh rechargeable batteries to power the Raspberry Pi. Of course, it's only a temporary measure.

Here's a diagram I drew up one night. I was tired, so forgive my messy drawings.


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

Tue Oct 18, 2011 2:19 pm

I didn't look through the whole thread, but the best thing for this would be laptop batteries.
(There are jumbo AA-looking batteries in side each laptop battery)

http://www.dealextreme.com/p/u.....pack-26248
six of these would provide 7V and 7500mah.
(protected version: http://www.dealextreme.com/p/u.....pack-26247)

Since flash lights use these batteries, it's a good idea to checkout the flash light forums for the best batteries.
Example: http://light-reviews.com/forum......php?t=513
A bit out of date though.

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

Wed Oct 19, 2011 1:33 am

The question there would be is how would you charge them then? Would you take a Wall charger, gut the contents out of it, connect battery leads to the charger, etc? Also, what would the input voltage need to be to charge them as if we're running 3 sets of these in parallel (3 sets of these batteries paired together to make 7.2V) do we need to charge them at their native charging rate (5V?) or charge them at something like 8 or 9V?

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

Wed Oct 19, 2011 2:56 am

Sorry to sound spammy (I just posted a link to another Adafruit product in a different thread), but have you guys seen this inline battery charger? https://www.adafruit.com/products/280
I know the battery voltage is lower than what the RasPi uses, but can't we elevate it after the charger?

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