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### Topic: Battery life... (Read 4436 times)previous topic - next topic

#### fubbi

##### May 28, 2012, 05:00 pm

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 220? 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

#### majenko

#1
##### May 28, 2012, 05:06 pm
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.

#### Grumpy_Mike

#2
##### May 28, 2012, 05:07 pm
Quote
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.

Quote
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.

#### fubbi

#3
##### May 28, 2012, 05:55 pm

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

thank you

#### majenko

#4
##### May 28, 2012, 05:58 pm
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.

#### iggykoopa

#5
##### May 28, 2012, 06:09 pm
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.

#### clvrmnky

#6
##### May 28, 2012, 06:10 pm

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.
I yield() for co-routines.

#### Grumpy_Mike

#7
##### May 28, 2012, 06:14 pm
Quote
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

#### winner10920

#8
##### May 28, 2012, 06:37 pm
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

#### keeper63@cox.net

#9
##### May 28, 2012, 08:05 pm

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

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.
I will not respond to Arduino help PM's from random forum users; if you have such a question, start a new topic thread.

#### majenko

#10
##### May 28, 2012, 08:10 pm
Alternatively, get a small resistor (say 1?), 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.

#### John_S

#11
##### May 28, 2012, 08:49 pm
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.

#### majenko

#12
##### May 28, 2012, 09:00 pm
Quote
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.

#### fubbi

#13
##### May 29, 2012, 09:33 am

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

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

here it is:

http://www.clasohlson.com/se/Blyackumulator/Pr363019000

Or better yet (less bulky) this, can I have a few of these in parallel?:

thanks

#### fubbi

#14
##### May 29, 2012, 11:02 am

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 220? 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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