10 replies to this topic

[John West]

My device runs fine when powered by a usb cable connected to my computer. However, when plugged into a usb cable connected to a portable usb battery (which runs everything else just fine), it doesn't work as well. Does this make any sense? On a similar note, if I wire up a nine volt battery, does anyone have any rough guesses as to how long said battery could light up 5-10 leds continuously? My device has that many leds, and the device will be on for 4 or 5 hours at a time. Does a 9 volt have enough capacity for 4-5 hours, or is it more like 40-50 hours? I know that smarter people could calculate wattage per led * number of leds * number of hours or some such and compare to the 9 volt battery, but I don't know the right numbers and formula to do it myself. Thx.

[Stefan W.]

That depends on your LEDs and your wiring. A stock alkaline 9V battery provides 400mAh or something, so for a really simple circuit, with resistors and "standard" 20mA LEDs, a 9v battery will light 10 LEDs for 2 hours. For low-power 2mA LEDs you'd get 20 hours. By pulsing the current you could lengthen the span also ... if you're looking for battery life, use 6 AA batteries instead of one 9v batteries. They last far longer ... And also, yes, it makes sense that a portable usb battery is no good power source - they usually don't deliver a regulated voltage like a computer does, which is usually fine for charging e.g. mobile phones, but the netduino might not like what it gets.

[Mario Vernari]

Consider that a good 9V battery (alcaline, non-rechargeable) is able to give up to 500mAh. It means 500mA for one hour, or 50mA for 10 hours. That's theoretical, but it gives a rough idea on the capacity. The Netduino plus itself needs about 80mA. One led could take 15mA (it depends on the resistor and the voltage applied), so 10 leds is about 150mA. Under these conditions your battery will die in a couple of hours. You won't be happy, but the manufacturer of batteries does! Cheers

[John West]

Bummer! How many mAh do six 1.5volt batteries have? Maybe I'll switch from leds to a 7-segment display. That could work for my scenario, but won't be as aesthetically pleasing.

Consider that a good 9V battery (alcaline, non-rechargeable) is able to give up to 500mAh. It means 500mA for one hour, or 50mA for 10 hours. That's theoretical, but it gives a rough idea on the capacity.
The Netduino plus itself needs about 80mA. One led could take 15mA (it depends on the resistor and the voltage applied), so 10 leds is about 150mA.
Under these conditions your battery will die in a couple of hours.

You won't be happy, but the manufacturer of batteries does!
Cheers

[CW2]

but I don't know the right numbers and formula to do it myself.

As Mario mentioned, you can roughly estimate the duration from the battery capacity. For more reasonable results, you'd need to know battery discharge curve, because the real capacity may vary significantly depending on the current drawn (example). If the size/weight is not the constraint, AA or AAA batteries will give you higher capacity...

[Stefan W.]

Bummer! How many mAh do six 1.5volt batteries have?

Since you put them in series, you want to know how much one has, and you can find that out at http://en.wikipedia....wiki/AA_battery.

[John West]

So, assuming that I use alkalines with 2700-2900mah, having 6 in series will get me 2700

10 leds at 20mah = 200mah
80mah for Netduino plus
Six 1.5v alkaline batteries at 2700mah

So, I'll get around 9.6 hours with six 1.5v alkaline (or 7ish hours with 2000mah eneloops). I realize this isn't an exact science, but is that more or less correct?

Thanks for being patient and giving me the links to wikipedia. Sometimes I forget to go there first!

Since you put them in series, you want to know how much one has, and you can find that out at http://en.wikipedia....wiki/AA_battery.

[Mario Vernari]

Many many years ago we had to create a microcontrolled circuit having those constraints:

• powered by battery (no recharge possible);
• the battery should ensure an average lifetime of about five years;
• ...others...

Well, we chosen to use a pack of 4 alkaline D-type batteries (total 6V, Duracell brand). They have almost 20Ah!
The funny part of the story was about the lifetime. The actual duration was not limited by the circuit itself, but the natural auto-discharge of the battery itself!
Cheers

[ZakieM]

Running into the need of using a device without near-by power source I played with this a bit. There is an option to put the device to sleep (assuming you don't need it to run all the time). This works in some Netduino like devices but NOT on the Netduino itself. I got C# for putting the device to sleep mode, but it did not work. I confirmed that with Microsoft .NETMF developers too. GHI Electronics has support for putting the CPU to sleep and perhaps others do too. Though I got some of GHI boards as a result, I did not verify the sleep functionality.
With my project, I eventually gave up and used Arduino Fio with LiPo battery and the device sleeps for 2seconds, wakes up check status and if nothing to do, goes back to sleep. By that I extended the lifetime of a 2000mA battery by 10 folds at least. With the Fio being extremely battery friendly I completed the project and battery last there for 4 months already without recharging. Still, I felt bad for not doing it with the Netduino (defeat the purpose I called it).

[John West]

Ok, now that my project is complete, been thinking about power more. Since the LEDs only take 3.3v, does that change the calculations since I'm feeding the netduino plus 9 volts? To think more, what if I fed the netduino plus 12 volts? I'm sure it's easy, but I'm hopeful that maybe this will somehow factor into getting more life than mentioned here.

[Mario Vernari]

Ok, now that my project is complete, been thinking about power more. Since the LEDs only take 3.3v, does that change the calculations since I'm feeding the netduino plus 9 volts? To think more, what if I fed the netduino plus 12 volts? I'm sure it's easy, but I'm hopeful that maybe this will somehow factor into getting more life than mentioned here.

You must think in "current" terms, instead of voltage.
For instance, even 3.3V powered, if your led sucks 10mA, then your battery will have to feed 10mA.
This leads a lot of energy wasted, because most of the time the voltage is divided/regulated using simple circuits (which is based on wasting energy).
However, there are sophisticated power supplies that have a high efficiency. Is it worthwhile?
Cheers