Your maths answer is in the right ball-park (even though you seem to be confusing power with Ampere-hours) but are you really putting 6 cells in series (I assume '9 battery stack' was a typo) and connecting that directly to your RPi's 5 volt supply input?Wisar wrote:This hardware stuff confuses me The wimpiest of AA batteries, a nicad, should produce between 600 and 1000mAh of power. So a nine battery stack should provide between 4.5 and 7.5 watt hours (volts times amps or .6 amps * 6 batteries * 1.25v). If the RPi uses 2 watts then should it not run for 2.25 to 3.75 hours...and even more on alkaline batteries? Granted this is not what is happening for me in reality but I want the math to match reality and I know I am screwing something up!
What lion battery are you using,Wisar wrote:aaa801: I have one of those chinese imports you pointed to in an earlier post on the way along with a Li-ion battery! My current solution can wait until then though it will take a while.......
Actually I think my guestimate was actually way off, and I also was thinking that maybe more modern NiMH rechargeables, would be better, they routinely do at least 2000mA/H at about 1.2 Volt.Wisar wrote:This hardware stuff confuses me The wimpiest of AA batteries, a nicad, should produce between 600 and 1000mAh of power. So a nine battery stack should provide between 4.5 and 7.5 watt hours (volts times amps or .6 amps * 6 batteries * 1.25v). If the RPi uses 2 watts then should it not run for 2.25 to 3.75 hours...and even more on alkaline batteries? Granted this is not what is happening for me in reality but I want the math to match reality and I know I am screwing something up!
Your guess would be wrong! A meter cannot 'cap' the current through it. Excessive current could blow a fuse if present or burn out PCB tracks or damage precision resistors.poing wrote:My cheap multimeters have a max of 200mA for measuring Amperage. One of them has a 200mA fused input and a 10A unfused one. My guess is yours works likewise and the current is capped.
Such rebooting frequently signifies lack of power to the RPi. As more bits of it 'come online' as it boots the current demand increases. If that cannot be supplied the voltage drops and and the RPi reboots.Wisar wrote:Am now trying to assess what is actually happening on the reality side of the above equations and have another issue. My power for the RPi and for the USB hub both come through a power regulator. I have that regulator connected to a wall dongle so it can get regular steady power. Everything is bread boarded so it is easy to stick a meter into the mix.
When I try to measure the current draw I get what looks like an average draw of 200mAh or so...the problem being that the RPi does not seem to boot (network does not start and logs show no entries). Unfortunately I have killed the serial port on the RPi so I can not connect a console but when I connect it to a TV it shows that it is just looping on trying to boot. Shows colored screen, flashes some console messages, back to colored screen. In this test the RPi is connected to the powered USB hub but has nothing else attached that is drawing power.
On the peripherals front...the USB hub, with a wireless dongle, webcam, and Arduino connected, routinely draws between 200 and 230 mAh depending on how active the network connection is and whether a picture is being taken.
What I don't understand is why have a digital multimeter in the circuit to measure current draw from the RPi would have such a profound impact on it? Could it be dropping the voltage, or impacting the current available, just enough to cause a boot problem? I think the bottom line is that my cheap power regulator is just that.
I just happen to have a DT830B (and two more that look very similar). I used another DMM to measure the resistance of the DT830B plus leads when switched to 200mA. A touch under 2 ohms.Wisar wrote:Re what multi-meter...
Errr....one that you can get for 6 quid on eBay? A DT830B to be specific. Any drop in power available is a problem given my current under power situation combined with a cheap power regulator. Better batteries and another regulator are on the way.....
http://compare.ebay.co.uk/like/23077834 ... adtype=pla
Thanks for the help,
What did you use to connect from the battery pack to the usb input of the Pi ???penguintutor wrote:I've successfully run a Pi off 4 x AA batteries, but you'll only get a couple of hours or so.
I just used 4 x rechargeable 2100mAh batteries in series. This gives about 5.2v fully charged. Note using normal non-rechargeable batteries will result in a higher voltage - I don't know how the Pi will react to that.
Connected this to the Pi (nothing else connected) and used a crontab to record the uptime every minute. Voltage dropped to about 5v when powered up and slowly dropped. The Pi was still running at less than 4.25V - although I expect it would not have been able to power most USB devices before that point. Unfortunately my 1st multimeter has died (after 20years service) and my third is not accurate enough, so I wasn't able to measure voltage and current at the same time. That ran for about 3 hours 30 minutes.
I then tried with Ethernet connection (still no monitor / keyboard - after all if you can power a monitor you probably have a better power source than AA batteries). I also added a cron job on another machine to probe the Pi at 1 minute intervals (simulate remote monitoring application). I measured the current this time which peaked at about 430mA, ran normally at between 360mA and 400mA.
The network connection remained active for about 2 hours 15 minutes and the Pi continued to work without networking for a further 25 minutes.
If you are in a situation where you need to run off batteries then chances are going to be no Ethernet. With the model A the lack of Ethernet will save some power, but adding wireless dongle will probably take a little more than the wired Ethernet. The bigger problem with wireless is that I guess the dongle will stop working at a higher voltage than the Pi.
So AA batteries can be used if it's only for a short period of time. I'm guessing some kind of on-board computer for a robot or model aircraft etc. Obviously any additional peripherals will take more power from the batteries. Looking for anything longer and you are going to need bigger or more specialist batteries.