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Battery Power and USB WiFi?

Posted: Sat Feb 09, 2013 7:43 pm
by hendrixjl
I can get my raspberry pi to come up under battery power - for instance, I'm currently using a 6volt "lantern" battery (the big one).

One thing I can't get to work is the USB WiFi under battery power. I'm running the 6 volts through a 7508A 5-volt voltage regulator (which should support up to 1Amp).

Is 1 Amp enough? Are there USB WiFis that only sip a little power (maybe at a cost of range)?

Re: Battery Power and USB WiFi?

Posted: Sat Feb 09, 2013 7:50 pm
by redhawk
I've never heard of 7508A but if it's a linear regulator similar to the 7805 then your Pi maybe running on less than 5v.
If you have a digital voltmeter check the voltage output at TP1/TP2 with the wifi dongle and without.

What values do you get??

Richard S.

Re: Battery Power and USB WiFi?

Posted: Sat Feb 09, 2013 7:53 pm
by cyrano
hendrixjl wrote:One thing I can't get to work is the USB WiFi under battery power. I'm running the 6 volts through a 7508A 5-volt voltage regulator (which should support up to 1Amp).
I'm assuming a 7805A?

6V is not enough to drive the 7805 reliably. It needs about 7,5 V to function.
Is 1 Amp enough? Are there USB WiFis that only sip a little power (maybe at a cost of range)?
It should be enough if the PSU is a good quality one.

Re: Battery Power and USB WiFi?

Posted: Sat Feb 09, 2013 8:35 pm
by hendrixjl
Yea, it was 7805A. You were right, of course; I'm not getting 5. My voltmeter is wobbly (analog), but I seem to be around 3.5v.
The 7805 is pretty warm.... could I get better voltage if I put two 7805's in parallel so that that they have to support less amperage?

As you might have guessed, I'm not exactly an electrical engineer.

Thanks.

Re: Battery Power and USB WiFi?

Posted: Sat Feb 09, 2013 8:39 pm
by PS1981
The 7805 that I know can take from 7.5 up to about 35v, but the higher the more it gets hot. If I remember correctly you should put a resistor infront, calculated with R=(SupplyVoltage-5Volts)/Current, so if you have for example 24v input that is (24v-5v)/1A = 19ohms. If you don't need all the power then you can use a slightly larger resistor, this helps very well to reduce overheating. Also, screw a heat sink on (or a simple piece of aluminium plate), that helps too.

EDIT: Don't forget though, you may need a 5W resistor though, not just the 0.25 watt standard ones. If the resistor is too large, you won't get enough power through. It sometimes requires a bit of experimenting.

Re: Battery Power and USB WiFi?

Posted: Sat Feb 09, 2013 9:30 pm
by hendrixjl
Thanks for the info. I didn't know anything about the resistor "in front". What does "in front" mean?

Re: Battery Power and USB WiFi?

Posted: Sat Feb 09, 2013 9:33 pm
by PS1981
I mean between the powersupply and the + input terminal of the regulator. Sorry, it's a slightly sloppy way of putting it.

Re: Battery Power and USB WiFi?

Posted: Sat Feb 09, 2013 9:38 pm
by dasimpson
under load the battery voltage will drop the problem you have with the wifi card is it now pulls the volatge lower then just the pi on it's own the regulator wont run at this voltage and cuts out 7805 need minimum of 7.2 volts any lower under heavy load and it wont work

Re: Battery Power and USB WiFi?

Posted: Sat Feb 09, 2013 9:39 pm
by dasimpson
and the 7805 dosent need a reister commen use it has 2 capaciters to regulate the voltage cleaner

Re: Battery Power and USB WiFi?

Posted: Sat Feb 09, 2013 9:40 pm
by dasimpson
this is my diy battery pack for anything that needs usb power


charging circuit http://www.ebay.co.uk/itm/281057379475? ... 1423.l2649
battery's old laptop battery can be found cheap on ebay as little as £3.99
protection circuit http://www.ebay.co.uk/itm/Replacement-C ... 5aebdfedac
inverter http://www.ebay.co.uk/itm/DC-DC-3V-to-5 ... 43aa2867f9
total cost if you get a battery for £3.99 is £11 possible capacity 17600mah

the protection circuit is needed befor the inverter as the inverter will pull the voltage down below what is classed a safe discharge voltage of 2.8 volt
but the battery charge circuit does not need to be connected to the protection circuit but can be if wonted

i feel this will more then hold up to what you wont and can be made larger with more batterys

Re: Battery Power and USB WiFi?

Posted: Sat Feb 09, 2013 9:41 pm
by dasimpson
other option is 2 lantern batterys in series for 12 volt this should also stop the voltage drop problem

Re: Battery Power and USB WiFi?

Posted: Sat Feb 09, 2013 9:51 pm
by hendrixjl
Thanks for all of the replies and all of the information.

I have another goal of keeping weight down. I guess I need to regroup on this one.

Let me ask, Does anyone think 6 AAs in series would be able to run the Pi + WiFi?

I'm also interested in a lower-power digital rf link for tty.

Re: Battery Power and USB WiFi?

Posted: Sat Feb 09, 2013 9:51 pm
by PS1981
The 7805 regulator only gives off 1A of current, putting a (small) resistor in prevents overheating without too much loss of power. Believe me, it's tried and tested, and I'm not the only person to suggest it. Try to pull more power and you're going to make things get hot anyway. I'm not wanting to argue about this, but I must let you know that I'm not inventing things here.

Re: Battery Power and USB WiFi?

Posted: Sat Feb 09, 2013 10:06 pm
by dasimpson
the resister is not going to help with the lantern battery the problem is the voltage of the battery is to low and putting a resister inline with the reg is a new one on me thats all

Re: Battery Power and USB WiFi?

Posted: Sat Feb 09, 2013 10:10 pm
by dasimpson
i think the diy battery i posted is the best option for been light wait it is how my gameboy rpi mod is done it runs the rpi the screen and iff i need to an external 2.5" hdd
hendrixjl wrote:Thanks for all of the replies and all of the information.

I have another goal of keeping weight down. I guess I need to regroup on this one.

Let me ask, Does anyone think 6 AAs in series would be able to run the Pi + WiFi?

I'm also interested in a lower-power digital rf link for tty.

Re: Battery Power and USB WiFi?

Posted: Sat Feb 09, 2013 10:21 pm
by MattHawkinsUK
Yesterday I ran my Pi from 6 AAs using a UBEC from eBay. I put the 5V straight into the GPIO. I had a Bluetooth dongle plugged in.

It ran for 5 hours before I shut it down.

I'll give it a try with a WiFi dongle.

Re: Battery Power and USB WiFi?

Posted: Sat Feb 09, 2013 10:26 pm
by dasimpson
thats not bad what was the mah rate of the batterys and at what voltage were they at when you finished

Re: Battery Power and USB WiFi?

Posted: Sun Feb 10, 2013 7:07 am
by PS1981
dasimpson wrote:the resister is not going to help with the lantern battery the problem is the voltage of the battery is to low and putting a resister inline with the reg is a new one on me thats all
Yes the resisor will not help with the lantern battery, but in any case the 7805 will also not help with the lantern battery. I'm trying to explain how to use a 7805 regulator properly!

Re: Battery Power and USB WiFi?

Posted: Sun Feb 10, 2013 9:32 am
by SteveDee
The main problems with dropping voltage through a linear voltage regulator (like the 7805) are: the power wasted and the heat generated.

The power you waste is equal to the voltage drop times the current.
So, if you want 5V for your Pi and the Pi draws 700mA, using a 12V supply to feed a linear regulator, means you waste: (12V-5V) x 0.7A = 4.9Watts
(Note that the Pi is only using 5V x 0.7A = 3.5W, and total power consumption from your battery would be 12V x 0.7A = 8.4W).

The wasted 4.9W is mostly turned into heat, and it doesn't matter whether you use a series resistor in the input to your 7805 regulator, the heat generated by this power loss is the same. Incidentally, the correct way is not to use a series resistor, but to fit the 7805 to an adequate heatsink.

For battery operation, its generally not a good idea to waste power. So it would be better to use a more efficient switched-mode regulator, rather than a linear regulator.

Maybe one like this: http://uk.rs-online.com/web/p/products/672-7155/

Re: Battery Power and USB WiFi?

Posted: Sun Feb 10, 2013 10:05 am
by rew
PS1981 wrote: If I remember correctly you should put a resistor infront, calculated with R=(SupplyVoltage-5Volts)/Current, so if you have for example 24v input that is (24v-5v)/1A = 19ohms.
This is not a good idea. First, at 1A the 7805 will need the full 7.5V or 8V. But now you're dropping to 5V at the input.

Secondly, the raspberry pi is very sensitive to short drops in the powersupply. Think of it this way: If you drop the power for a milisecond, the CPU will have had to execute over 50 thousand instructions! It will be able to do a few of them on the on-board capacitors, but soon those run out.

So when something draws 0.7A nominal it is quite likely to draw say twice as much during short periods. The 7805 can easily handle those peaks of up to 1.5A. But with the resistor, you'll be dropping the input of the 7805 well below 5V, and the raspberry crashes.

A $1.50 LM2596 from ebay will work better. I haven't measured the dropout. PM me if you need to know.

Re: Battery Power and USB WiFi?

Posted: Sun Feb 10, 2013 10:09 am
by PS1981
For one thing we're talking on different wavelengths. I'm only suggesting that the resistor can work to help reduce overheating of the regulator, and I'm not talking about bringing the voltage levels down, and I'm not saying that it should be done when supplying from a battery and we've already said that the voltage regulator won't work when the power supply is too low!
Just forget it!

Re: Battery Power and USB WiFi?

Posted: Sun Feb 10, 2013 11:00 am
by redhawk
If you want to reduce overheating then you shouldn't be using linear regulators a step-down buck converter would be better since they switch power on/off rather than use a resistive output.
Unfortunately even step-down buck converters need the input voltage higher than their output so a 6v battery would still be unsuitable.
Either you increase the battery voltage output or alternative use a step-up and step-down buck converter i.e. 6v -> 9v, 9v ->5v at the cost of power efficiency.

Richard S.

Re: Battery Power and USB WiFi?

Posted: Sun Feb 10, 2013 11:57 am
by SteveDee
Two more observations.

1) In principle, you can make switching regulators/switch mode power supplies to suit any input voltage and any output voltage. Given that 6V is a common battery size, and USB is a common interface, I'd be very surprised if someone out there (probably in China) isn't already making a 6V to 5V switching regulator. (if not, lets get together and design one!)

2) A quick look at the Pi schematics and I think I'm right in saying that the only device that connects directly to 5V is the BCM2835 (A13-A15). I haven't seen a datasheet on this, just general (2nd hand) comments that Pi needs 4.75V to 5.25V. But if the over-voltage limit is higher than this (say, up to 5.5V), you MIGHT be able to use a 6V volt battery via a 3Amp silicon diode (which should drop almost 0.7V). BUT DON'T TRY THIS THEN COME CRYING TO ME IF IT BURNS! Do the research (then tell me I'm wrong). Check the BCM2835 spec, check your USB devices specs, and check the loaded 6V battery + diode output voltage.

Re: Battery Power and USB WiFi?

Posted: Mon Feb 11, 2013 8:11 am
by cyrano
hendrixjl wrote:Thanks for all of the replies and all of the information.

I have another goal of keeping weight down. I guess I need to regroup on this one.

Let me ask, Does anyone think 6 AAs in series would be able to run the Pi + WiFi?

I'm also interested in a lower-power digital rf link for tty.

6 rechargeable AA cells in series will give you 7,2 V. Might work with a 7805.
6 non-rechargeable AA cells will give 9 V, which is perfect.

If weight is a consideration, have a look at 9,6V RC car batteries. Look at ebay for affordable ones.

The main problem with batteries is that you should also have a "battery empty" detection to shut down your RPi and avoid deep-discharging the batteries. Several complete solutions are being offered on this forum.

Re: Battery Power and USB WiFi?

Posted: Mon Feb 11, 2013 10:17 am
by Gustavo_Murta
Low drop out Regulators :

Use low drop out regulators. The output can be 5V or 3,3V if you need. The output current can be 500mA to up 3A, depending of the regulator.
These regulators are frequently used in cars. Some manufacturers poduce it like Infineon, Linear, National, etc.
Some examples:

IN = 6V out = 5V current 1,5A => LT1086
http://www.linear.com/product/LT1086

IN = 5,5V, out = 5V current 3A => LT1529
http://www.linear.com/product/LT1529

IN = 6V out = 5V current 1A => LM2940
http://www.ti.com/lit/ds/symlink/lm2940-n.pdf

Gustavo Murta (Brazil)