Plea for cheap battery PSU for Rpi Robot


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by simplesi » Wed Jan 02, 2013 10:44 pm
I'm using a Pebble Smartstick as my RPi 5V supply for my Magician Chassis Robots (I bought 2 on special offer at £12.99 each) but I now want to reduce the cost of making further RPi robots for use in my schools. (And for young padawans wanting to build one using their pocket money :) )

Each robot currently needs 2 battery packs - one for the motor and any cheap and cheerful Poundland 4xAA or 4xAAA does that job at at cost of ... £1 :)

But the RPi needs a stabilised 5V with enough current output to power the RPi with a nano wi-fi dongle.

Now I've looked into trying to use another Poundland 6V battery pack and bolt a voltage convertor but the 6V batteries are just about the worst voltage to try and convert to a stabislised 5V.

Weight is also an issue.

Any ideas please?

regards

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by W. H. Heydt » Thu Jan 03, 2013 1:23 am
I think you're going to see a conflict between light and cheap.

Cheap batteries (alkaline, lead acid) aren't light and light batteries (LiPo and such) aren't cheap.

You will also have an issue as the better your voltage converter is, the more it will cost.
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by W. H. Heydt » Thu Jan 03, 2013 1:55 am
I thought about the problems you presented, as have others, and it occurred to me that, while probably not "cheap" by any measure, the following link would provide an inexpensive to operate power supply rated at 7.8v at 1.5A....
http://www.horizonfuelcell.com/fuel_cell_stacks.htm

I wonder if interest in a portable Pi power supply might attract their attention for its educational purposes...
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by MrBunsy » Thu Jan 03, 2013 10:15 am
Why use two batteries? Why not one and just make sure the 5v is sufficiently regulated and decoupled? (throw capacitors at it).

You should be able to regulate 6V down to 5v with a cheapish low dropout linear regulator. Linear regulation isn't efficient at all, but at a drop of only a volt you won't be wasting oodles of power. For regulating 12v you're going to want either switched mode or a huge heatsink - and lower battery life as a result. I have linearly regulated 12 down to 5 (2A load), but the metal box I used as a heatsink does get rather warm.

For my robot I used a completely overspecced 12v lead acid and DC-DC converter: https://uk.rs-online.com/web/p/switchin ... s/7398290/ to power the Pi. The battery weights nearly 3kg, but it can also run the pi continuously for over a day.

You can get relatively cheap (£5-10) lead acids. Some of the lower capacities aren't too heavy, for example 6V, 1.2Ah, 0.28kg - https://uk.rs-online.com/web/p/lead-aci ... s/7270388/

So in summary, if I was after cheap and light, I'd go for a lowish capacity 6V lead acid and a low dropout linear regulator for the Pi. I'd power the motors straight from the battery and hope that decoupling would be enough to keep the Pi happy on the 5v side of the regulator. The robot I did build was neither cheap nor light so I went the other route of huge heavy battery and DC-DC conversion for the 5v rail.

/end of idea dump.
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by simplesi » Thu Jan 03, 2013 10:41 am
Why use two batteries? Why not one and just make sure the 5v is sufficiently regulated and decoupled? (throw capacitors at it).


The motors can easily stall and demand LOTS of current that would proabably cause any smallish capacity battery to dip the volts causing RPi failure. I need reliability as well (just to last for a 1 hour lesson though before needing recharging or new ones if using cheap AA/AAA.

You should be able to regulate 6V down to 5v with a cheapish low dropout linear regulator.

If I could get a battery that stayed at 6V for an hour of 1A discharge then this would work but 4xAA don't cut the mustard

So in summary, if I was after cheap and light, I'd go for a lowish capacity 6V lead acid and a low dropout linear regulator for the Pi.


mm - that might work out and I might even be able to get away with using a simple diode to drop volts down to RPi level

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by malakai » Thu Jan 03, 2013 10:59 am
What about the USB phone battery backup kits. I got a cheap Kodak 5v 700mA true it isn't 1A it will power the pi and cheap keyboard for an hour. But once the Model 'A' comes out it becomes a lot more attractive for something that you may do. One they are cheap and lightweight and the second option they make for great emergency kit for your cell phone.

I know it probably isn't the cheapest option but they do come up now and then on clearance and such. And for the kids easier to get the parents to flip the bill if they are into the idea of having a spare battery for their phone for a third to half the cost of a retail cell phone battery.
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by MrBunsy » Thu Jan 03, 2013 11:55 am
simplesi wrote:The motors can easily stall and demand LOTS of current that would proabably cause any smallish capacity battery to dip the volts causing RPi failure. I need reliability as well (just to last for a 1 hour lesson though before needing recharging or new ones if using cheap AA/AAA.


Hmm, I hadn't thought of that. I'd be tempted to try it anyway - if you're going for low power I assume your motors are quite small? If memory serves lead acids have a very low internal resistance, so they might be able to supply the stall current without too much worry.
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by simplesi » Thu Jan 03, 2013 12:31 pm
I got a cheap Kodak 5v 700mA

How cheap and where from please? :)

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by malakai » Thu Jan 03, 2013 12:42 pm
Unfortunately the battery is no longer made. Shea Silver got one at Walmart in the states for $6 I believe so I went to the Kodak store and got it on clearance for $6 order total shipped $14 but the Bay has a ton that should work http://www.ebay.com/itm/New-2600mAh-USB-Power-Bank-External-Battery-Charger-for-Mobile-Phones-Blue-/251205015133?pt=PDA_Accessories&hash=item3a7cfc565d

Not the one I got but there are a ton of products out there in that range and reported voltages.

One I really want to try with the model A is a solar enabled and range about $12

http://www.ebay.com/itm/2600mAh-Solar-Power-Charger-for-PDA-Cell-Mobile-Phone-MP3-MP4-Adapter-USB-Cable-/271025706405?pt=Battery_Chargers&hash=item3f1a6415a5


The other use I have for mine is I have a split y cable when I need to move my Pi I plug the battery into an open end on the cable unplug from the wall I have about 1 hour to move it and plug it back into the wall and remove the battery it is so nice having that thing. Not having to shut down and reboot.
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by malakai » Thu Jan 03, 2013 1:07 pm
This is what I got http://www.amazon.com/Kodak-Essential-KP1000-Power-Pack/dp/B004GKLW4S

A lot of chargers out there seem to have 18650 lithium battery batteries in them they are cheap about $2 or $3 each from China
Protected 18650 2400 mAh rechargeable 3.6 volt battery they use 4 of them in the cases you could reverse engineer the circuitry these battery packs use to get 5v 1a
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by patooo » Thu Jan 03, 2013 3:02 pm
I think cheapest widely available 18650 battery that can be considered as really good is sanyo 2600mAh (about $5.50)... "$2 or $3" 18650 cells are almost all very bad cells, they have small capacity and degrade quickly.
I have mentioned few solutions that use 18650 cell(s) here
http://www.reddit.com/r/raspberry_pi/co ... er_source/
and here http://www.reddit.com/r/raspberry_pi/co ... batteries/


I think this setup is bad idea. Better try something that is verified, but cheapest solution that can be considered as "good" would be about twice as expensive.
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by Ravenous » Thu Jan 03, 2013 3:20 pm
If you want really cheap, not sure how good an idea rechargeables are. They need chargers, they can suffer if drained too far, some of them can deliver high currents if the little darlings short them (Lead acid or NiCd).

Above MrBunsy suggested a DC-DC converter - I bet that would work well and quite efficiently with eight disposable cells. (I'm using a 500mA one though at 12V and not running a raspi) Not sure what voltage your motors need though.

The Model A when it's out will probably use less power, even better.

And for programming, maybe use a tether wire a few metres long, switch to batteries for testing and competition.

I think I understand how cheap this stuff has to be for schools. (I'm a skinflint myself.)
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by richardp » Thu Jan 03, 2013 3:35 pm
For a Power supply, why not just use a 6V battery and a silicon diode for a 0.7V drop.... effectively giving 5.3V. (add a Shottky for another 0.2V drop)

My question is to you really need a regulator when running from a battery?

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by mahjongg » Thu Jan 03, 2013 3:53 pm
A source of very cheap 12V to 5V @1,5A switching step down converters with a reasonably efficiency, and an easy to USB USB type A connector are so called "cigarette plug USB chargers" like this one:
Image

You can get such adapters for a few pounds/dollars/euro's at most street-markets and in "pound/dollar/euro-shops", they are usually easy to dismantle (try unscrewing the tip), and whats inside is a tiny PCB with a small 8 pins IC, (typically the MC34063) a coil, a diode, and a few resistors and capacitors. Note that the wire going to the springy tip of the adapter carries the +12V signal, and the "side wings" are connected to GND.
Its easy to wire these to a typical barrel connector receptacle, like this:
Image
Wire the +12V lead to the back pin (connected to the internal pin) and the GND to the pin on the middle of the underside, the pin on the side stays unconnected.
which means this is also a good adapter if you insist in powering the PUI from a PSU with a barrel connector, especially as these converters typically start working with input voltages as low as 8V.

An adapter like this is guaranteed to be a good power source for a PI.
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by MrBunsy » Thu Jan 03, 2013 5:50 pm
richardp wrote:My question is to you really need a regulator when running from a battery?


Considering how many stability issues are resolved using a better quality power supply, I'd suspect that without good regulation you're just asking for trouble. But, I suppose it's worth a try.
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by Pete6 » Thu Jan 03, 2013 8:54 pm
MrBunsy wrote:
richardp wrote:My question is to you really need a regulator when running from a battery?


Considering how many stability issues are resolved using a better quality power supply, I'd suspect that without good regulation you're just asking for trouble. But, I suppose it's worth a try.


I agree. Whilst the Pi itself might tolerate a slowly decaying battery voltage until it finally gives up, the robot motors will be causing power surges that will bring about sudden voltage variations - mostly downwards but with an upsurge when the motor stops.

I am about to use a 7.2V racing pack and a 8705 voltage regulator. The 7805 is a linear regulator and has an approximate 2V drop from input to output. This is nearly an ideal situation for your project since you can draw up to 1Amp at 5Volts which will be stabilized buy the 7805 and there is 7.2V available at high-ish current for your motors.

These racing packs are designed to run motors and have very low internal resistance and so will be a lot more stable than most other battery types. Since they are rechargeable, their relatively high cost will be offset by the fact that you can use it over and over again.
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by simplesi » Thu Jan 03, 2013 9:03 pm
Not going to use a battery that costs as much as an RPi! :)

The thing is, the Foundation has come up with a £20 computer that has all lots of bits on it - its made me be very parsimonious when it comes to spending money on peripherals :)

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by Leo Rest » Thu Jan 03, 2013 11:10 pm
How about one of these...
http://powerplusbatteries.co.uk/index.p ... duct_id=80
You'll need an rc bec as used to power a receiver, this will give you 5v and also manage the battery Voltage

I would spend more to get a greater capacity but if your hung up on keeping the price low then you're limited by that choice.
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by gritz » Thu Jan 03, 2013 11:34 pm
simplesi wrote:Not going to use a battery that costs as much as an RPi! :)

The thing is, the Foundation has come up with a £20 computer that has all lots of bits on it - its made me be very parsimonious when it comes to spending money on peripherals :)

Simon


It's tricky. Reliability would suggest one supply for the Pi and one for the motive power. The Pi really needs to be shut down in an orderly fashion 'cos a reboot caused by even a momentary outage may scramble the operating system, requiring the SD card be reflashed. You could have one stiff power supply, with battery monitoring (to prevent over discharge and Pi shutdown), but if you want cheap then The Law Of Exponential Ballache may dominate you with it's upward sweep. I think that the Pi takes a bit less than 1/2 an Amp, plus whatever the wifi eats.

The trouble is that peripherals aren't expensive per se, it's just that the Pi is not a lot of money at all, so it tends to make everything else look spendy. I'm working on something at the moment that's built around a $9 DSP chip. There's not that much else to the project, but the all-up cost will be in the region of £200 (plus whatever I haven't written down). That's how projects can be if someone isn't mass producing the exact thinger that you need.
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by mc349 » Thu Jan 03, 2013 11:39 pm
What about something like this? You use your cheap AAs to power it and then let the module regulate down to the voltage required:
http://www.ebay.co.uk/itm/390513655150
Or search for LM2577 on eBay.
I have one which I'm using to power the PI from a 12V source and hope to eventually put it in the car.
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by richardp » Fri Jan 04, 2013 6:55 am
MrBunsy wrote:
richardp wrote:My question is to you really need a regulator when running from a battery?


Considering how many stability issues are resolved using a better quality power supply, I'd suspect that without good regulation you're just asking for trouble. But, I suppose it's worth a try.


You are right... the RasPi does not have a 5V regulator, but it does have a 3v3/other ones... so the USB ports are completely at the mercy of the input voltage. Very little input voltage protection on the device.

I would use a DC-DC power supply for its larger input range.

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by simplesi » Fri Jan 04, 2013 11:32 pm
Well I've had some initial sucess with one of these
http://www.ebay.co.uk/itm/DC-DC-3V-to-5V-1A-Voltage-Step-up-Power-Regulator-Transformer-Module-500KHz-NEW-/290703100274

(I forgot I'd ordered one a couple of months ago - found it in one of my little plastic boxes :) )

I've wired it up to a 3xAA battery holder and stuck 3 pound shop alkalines into it and its currently powering my Robot RPi (directly via pins 2 and 6) whic is a Model B with just a Nano WiFi in it (I talk to the RPI using VNC).

Its initially worked fine:)

I'm going to just leave it on for an hour and see if it lasts :)

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by simplesi » Sat Jan 05, 2013 12:16 am
Well its been 40 mins and its still up and running and responding to my VNC session

Just checked the battery voltage and its at 3.2V.

I did measure it off load at the beginning at 4.7 but forgot to measure it onload at the beginning - schoolboy error :9

When these go flat - I'll stick a new set in and measure it then :)


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by simplesi » Sat Jan 05, 2013 12:43 am
Well it died about 2 mins ago (lasted an hour) with battery output volts down to 2.6V

So could be OK for an hours lesson - I'll need to try it out on some AA rechargeables and see how they last :)


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by itimpi » Sat Jan 05, 2013 6:55 am
simplesi wrote:Well it died about 2 mins ago (lasted an hour) with battery output volts down to 2.6V

So could be OK for an hours lesson - I'll need to try it out on some AA rechargeables and see how they last :)

Rechargeables can vary enormously in the charge they hold. I have found http://www.vapextech.co.uk/ to be a good source of high capacity rechargeables at reasonable prices.
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