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berlin
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Hi

I have 4 transistors pwm'ing 5 LEDs each, so all in all 20 LED. I want to run it off a rechargeable 9V block and I am trying understand how long the battery will last.

Can I assume that 20 led at ≈ 20mA (mixed 2 types and have no specs) with 220Ω R is 400mA and how does that relate to the
200maH that the 9V block has? I am considering using several batteries in parallel but how many would get my through the day. Its an art installation and I could have the batteries swapped every morning.

The arduino needs some juice too, its a pro micro and I don't know its requirements. http://www.sparkfun.com/products/10998

Most thankful!

Fubbi
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The 200mAh basically means that, if you draw 200mA then it will last 1 hour.

You need to know the total current draw.  I would suggest placing an ammeter in between the battery and the power input (in series in the positive line) and measure the actual current requirements.

Then you can calculate how long 1 battery should last.
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Manchester (England England)
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how does that relate to the 200maH that the 9V block has?
It says the absolute maximum time is half an hour, in practice it is likely to be half this.
 
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I am considering using several batteries in parallel
Don't do it, you will get all sorts of problems with the batteries cross charging each other, even with non rechargeable ones.

Those small 9V blocks are very poor at current, you are best off using some AA NiMh rechargeable ones.
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berlin
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I thought 20 LED would be easy to run for a day... What would you use? I really want to avoid using a power cable for aesthetic reasons

I am "in the field" and the little pocketmeter i brought can't measure A smiley-sad

thank you
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If you're "in the field", then connect it via a cow.  The louder it moos, the more current you are drawing.

I would probably use a lithium based power source for that kind of life time.

It's hard to advise without knowing actual current draw.
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I would look into something like this http://www.ebay.com/itm/Flight-Max-4000-mah-2S-7-4V-Lipo-Battery-20C-Fits-Traxxas-Rustler-VXL-New-Hard-/310403171828?pt=Radio_Control_Parts_Accessories&hash=item48457885f4. Without know the current draw it's hard to tell, but I'd questimate the battery I linked to would get around 10 hours of run time at full brightness. You can also run the leds at a lower brightness with PWM and it can sometimes reduce the power consumption without drastically reducing the brightness.
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If you're "in the field", then connect it via a cow.  The louder it moos, the more current you are drawing.

I would probably use a lithium based power source for that kind of life time.

It's hard to advise without knowing actual current draw.

Make sure you use a low-impedance cow.
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I thought 20 LED would be easy to run for a day
I don't know what made you think that?
Not knowing the spec of the LEDs doesn't help but I make it probably closer to 320mA rather than 400mA, but that is still a lot for a whole day.

That should say:-
Make sure you use a low-impedance cow - bull
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Is it an outdoor showing by any chance? Maybe a solar panel?
also using say AA's or even some d batterys will last a whole lot longer, maybe use an ldo regulator to keep the battery voltage as close to 5v as you can, or go into the 3.3v range and draw less, while still being enough for the leds and wasting less power across the resistor
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I am "in the field" and the little pocketmeter i brought can't measure A smiley-sad

I would -love- to see a picture of this meter; unless this thing is about 30-40 years old, I can't believe that it would only measure resistance and voltage, but not current (as if you can do those two, you can kinda get the third almost "free"). I mean, I can go down to Harbor Freight and purchase a full multimeter (Cen-Tech piece-o-junk that it is, good enough for most purposes, though) with all functions, including transistor and diode check features for $1.99 USD with a coupon; so I would really like to see what someone sold you. You probably got ripped off, if it is as you describe, unfortunately.
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Alternatively, get a small resistor (say 1Ω), put that in your positive battery line, and measure the voltage drop across it.

The current is the measured voltage drop divided by the resistance.
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With 9V, you can connect LEDs in series.

4 LEDs in series will drop approximately 6-8 Volts (depending on exactly which LEDs you're using).
Add a resistor to limit it to 20ma. (If your LEDs use 6.8 V, the remainder is 2.2V that the resistor has to drop. 2.2V/0.02A = 110 ohms.)
Make 5 strings like this for a total of 20 LEDs.

Total current draw is 5 *20ma = 100ma.
 
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maybe use an ldo regulator to keep the battery voltage as close to 5v as you can

Eek, no!  LDO (linear) regulator + battery = flattery.

If you want your battery to last, you need a switched mode regulator circuit.

Instead of the, say, 25% efficiency of an LDO, you'd have nearer 80% or more with a SMPS.

You could easily use a car cigarette lighter -> USB adaptor.  That will give you a good 5v with little power wastage.  Most of the chips they use handle between 7.5v and around 70v as an input.
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berlin
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I am "in the field" and the little pocketmeter i brought can't measure A smiley-sad

I would -love- to see a picture of this meter;


here it is:


How about this:
http://www.clasohlson.com/se/Blyackumulator/Pr363019000

Or better yet (less bulky) this, can I have a few of these in parallel?:
http://www.clasohlson.com/se/Laddningsbara-batterier-Sanyo/Pr363020000

thanks
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berlin
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With 9V, you can connect LEDs in series.

4 LEDs in series will drop approximately 6-8 Volts (depending on exactly which LEDs you're using).
Add a resistor to limit it to 20ma. (If your LEDs use 6.8 V, the remainder is 2.2V that the resistor has to drop. 2.2V/0.02A = 110 ohms.)
Make 5 strings like this for a total of 20 LEDs.

Total current draw is 5 *20ma = 100ma.
 

sounds good, I have already built the circuit with 4 transistors so I need 5 led on each... Also I have soldered 220Ω resistors to all my leds already...

I am off to the electronics store now, any last second advice? lead battery?

thanks
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