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> 9v Battery, 9v battery problems
chaos51
Posted: December 14, 2011 03:55 pm
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Hi,

This is the first time I post here, so I hope I selected the right forum.

I am building a homemade little robot based on an AVR microcontroller,
and I have some issues with my battery.

It's a standard "square-ish" battery.
It is 9v. One of those you find in the supermarket.

My problem is that after operating only a short amount of time, the power has decreased pretty much. The battery is hot.

I cannot really see why. Everything seems to work ok. I have a motor controller based on the L293. I have put capacitors in place to get rid of the motor noise. I am using pwm to drive the motor, and several tests indicated this to be working pretty ok. (I can regulate the motor speed, I can swap direction, this all works)

It's just when I put the "robot" on the floor, the speed is not really controlled by the pwm, but more by the state of the battery.

-- Am I supposed to put something in series with the motors? (Inductive, or restsitive)? If it's inductive, will it not kill my PWM control?

-- Could it be that the gearbox is geared too much for speed, and too less for power

-- Could it be, I need a stronger battery?


My Robot is approx 1/3 kg or so, and size after a lunchbox.


Any tips?

Thanks
DaC
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Ice-Tea
Posted: December 14, 2011 04:25 pm
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How much current are you drawing?
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CWB
Posted: December 14, 2011 04:26 pm
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"1/3 kg or so"
to move a certain amount of mass at any velocity takes a certain amount of power .
"no free lunches"

yep ... a power source capable of more mAH or WH is needed .
those 9 volt batteries are on the puny side for the job at hand .


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Ice-Tea
Posted: December 14, 2011 07:39 pm
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9V batteries aren't made of magic. Inside you'll find 6 miniature 'standard' cylindrical cells. Think AAA but only a lot smaller.

So a 9V battery will do a lot worse than a bank of 6 AA or AAA cells. Think a 9V cell has an interna resistance of 1-2 ohms or so...
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MacFromOK
Posted: December 14, 2011 08:17 pm
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Wouldn't hurt to post the circuit either, just in case there's a design issue. beer.gif


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Sch3mat1c
Posted: December 14, 2011 09:15 pm
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When we did the same thing at school, we had a 4 x AA battery pack, and even that ran down fairly quickly (~10 hours run time on a charge?).

There isn't much you can do. Get a smaller, faster motor and gear it down -- now the whole thing moves really slow. Get a bigger battery (stack a few 9V's in parallel?), now you can push the motors, but it's a lot heavier, and the L293 might get dangerously warm under all that load.

If you changed from L293 to a more efficient driver (something with MOSFETs that doesn't drop 2V to nowhere), you'd save maybe 30% on battery charge, but that's not nearly enough to keep it from sucking down a poor 9V.

Tim


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chaos51
Posted: December 16, 2011 08:44 pm
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Ok, it looks like I did some thing wrong here.

-The motor I am using, is designed for 3 volt, not for 9 volt
-The motor draws alot of current (1A or so), when it stalls
-The motor stall quite alot, since I used a gear ratio, chosen for speed, and not torgue (38.2:1)

I think these combinations make the L293D warm up alot. And overheating protection stops it from operating.

Stop, wait for cool down, and start seems to work. Which is as I would expect, if the overheating protection kicks in.

I am using this motor / gearbox combo http://www.pololu.com/catalog/product/114/specs

So, what I did (before I found out my motor facts), was to pigy bag another L293D to avoid warming up. It helps, but not enough....

So Now I am looking at
-Gearing down the motor a bit
-Making the voltage lower (Can you just usae a voltage regulator, for a motor load?, and any suggestions, which)
-Reading up on the replacement motor, that runs ok up to 12 volts, and hasa smaller stall current.


I don't know the amps it is pulling yet, because my amp meter is out of functions currently.

Regarding the circuit, I'l post it, after I got around to drawing it... I only have partial circuit drawings so far....


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Ice-Tea
Posted: December 16, 2011 09:46 pm
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What's forcing you to use a 9V cell? 3V engine -> 2AA cells.
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Sch3mat1c
Posted: December 17, 2011 02:32 am
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Well there's your problem...

A 3V motor rated for 1A will try to draw 3A under the same conditions (probably a moderate load), which will quite gladly short out your 9V battery. And overheat the L293, rated for 800mA IIRC.

If you actually power the L293 from 3V, you'll hardly have anything left for the motor (check the datasheet, L293 drops 1-2V under normal conditions). You could run it from 5V, but this doesn't help you anyway because it'll be less than 65% efficient, just terrible.

If you can find a 9V motor with the right gear drive, you'll really do a Good Thing here. Your other option is replacing the L293 with something more efficient and supplying it from a switching regulator (not linear), which gets more complicated than you'd like.

Tim


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CWB
Posted: December 17, 2011 03:24 pm
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they have a 6V (nominal)/130 size replacement motor available :
http://www.pololu.com/catalog/product/1117

this will improve the situation ... it will allow you to use a lower voltage rechargeable battery pack that is more commonly available (ie : 7.2 volt NiMH) but has a much greater mAH capacity than a set of AA cells .
for experimentation ... you can use AA alkaline cells but this will soon become very expensive .
it would be better to invest in a NiMH pack and simple wall wart charger sooner or later .

the question is : will the rest of the circuitry run off of 7.2 volts ?


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chaos51
Posted: December 17, 2011 06:09 pm
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Hi,


3A !!! That explains alot.

Also, would a (power)fet motor driver get higher efficiencies then the L293D?

Further, I was indeed considering this motor at
http://www.pololu.com/catalog/product/114/specs

There is one reason why I am am a little in doubt though.

- It's 6v, not 9v, so it will improve, ideal it wont be, if I keep to 9 volts.
I probably will go for it, as it seems the best choice I have just now....

Regarding power:

-My main voltage requirement is 5 volt, for the rest of the circuit (though 6/7 may do, I need to read up on my AVR 644P specs for that

-Also my circuit is build with a 7805 voltage regulator. I am not so sure, if it will still work at 7.2 volts (again I need to read up on that)

Thanks for the helpfull answers so far ;o)

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Sch3mat1c
Posted: December 17, 2011 06:35 pm
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Stall current 800mA means about 1.2A stall at 9V; the 9V battery will most likely drop to about 6V in the process, so this would be a fine solution.

A MOSFET type driver, operating at a high enough PWM frequency (10kHz maybe) will get much higher efficiency than the L293. If you program it to stay below 100% duty cycle, you can limit the peak current draw.

7805 will run from 7.2V okay, less and it will begin to drop out. I would guess 6.5V is the absolute lowest it will go before 5V stops being 5V -- if you have only the microcontroller on this supply, it will continue to operate down to 2.7V depending on clock frequency and fuse settings (brownout detect). Its output signals will be similarly reduced, so if you're driving other logic (motor driver input, RS232 driver, etc.) it may not work that low.



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tekwiz
Posted: December 17, 2011 09:14 pm
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Use a bigger battery in any case. 6 AA batteries will have close to 3X as much energy as a 9V, which is usually nothing but 6 AAAA cells in one case.
If you MUST use the 9V, go with a rechargable unit. NiMh rechargables are a bit less voltage, but they will supply considerably more current because their internal resistance is much lower than the disposable type. It's the internal resistance of the battery that makes it get hot under heavy loads. wink.gif


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