### Author Topic: Choosing the correct battery for robot  (Read 2125 times)

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#### Abdulla M.A.

• Jr. Member
• Posts: 30
##### Choosing the correct battery for robot
« on: September 15, 2010, 12:02:28 PM »
Hi, just check my calculation, I did the following calculations to choose the correct battery for my robot.
1-LM324--> if the supply voltage from 7805-----> P1=0.8mA x 5=4mW
2-Two Sensor (QRE1113)-----> P2=2x(5x10mA)=100mW
3-L298-----> P3=5x60mA=300mA
4-Motors(http://www.sparkfun.com/commerce/product_info.php?products_id=319)--->P4=2x(5x230mA)=2300mW
5-7414--->P5=1uAx5=5uA

so the total power about 2705mW, if the battery voltage is 9.6v , so the current should be  541mA, is this correct???
grateful for any help

Abdulla

Note: I do not want to use the calculator of the web.
"A scientist can discover a new star, but he cannot make one. He would have to ask an engineer to do that."
"For an optimist the glass is half full, for a pessimist it's half empty, and for an engin

#### Soeren

• Supreme Robot
• Posts: 4,672
##### Re: Choosing the correct battery for robot
« Reply #1 on: September 16, 2010, 10:50:57 PM »
Hi, just check my calculation, I did the following calculations to choose the correct battery for my robot.
1-LM324--> if the supply voltage from 7805-----> P1=0.8mA x 5=4mW
2-Two Sensor (QRE1113)-----> P2=2x(5x10mA)=100mW
3-L298-----> P3=5x60mA=300mA
4-Motors(http://www.sparkfun.com/commerce/product_info.php?products_id=319)--->P4=2x(5x230mA)=2300mW
5-7414--->P5=1uAx5=5uA

so the total power about 2705mW, if the battery voltage is 9.6v , so the current should be  541mA, is this correct???

You're assuming that the LM7805 doesn't count and you're assuming a steady load, which it will never be, especially not with the motors. The load on the L298 should be the motors, so you gotta view it a bit more holistic.
Nevertheless... Since the motors will be the main load on the battery, just size the battery for that (plus perhaps 10..20% extra for the rest) - battery selection is ballpark-figures anyway.
Part of the selection is choosing how long the battery should drive the load, a bit you seem to ignore?

Note: I do not want to use the calculator of the web.

Why not? Easier to get others to do the job?

It's very comforting to know that you claim to be an engineer and are teaching students... A first year engineering student should be able to seize up a battery just looking at what the loads will be.
Regards,
Søren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Engineering is based on numbers - not adjectives

#### billhowl

• Supreme Robot
• Posts: 376
##### Re: Choosing the correct battery for robot
« Reply #2 on: September 17, 2010, 12:15:13 AM »
Hi, just check my calculation, I did the following calculations to choose the correct battery for my robot.
1-LM324--> if the supply voltage from 7805-----> P1=0.8mA x 5=4mW
2-Two Sensor (QRE1113)-----> P2=2x(5x10mA)=100mW
3-L298-----> P3=5x60mA=300mA

The 7805 need to source out 81.6mA, your supply is 9.6V so this power by 7805 will (9.6V-5V)x81.6mA = 372.6mW or when the battery is full chage (11.6V-5V)x81.6mA = 534.6mW

4-Motors(http://www.sparkfun.com/commerce/product_info.php?products_id=319)--->P4=2x(5x230mA)=2300mW
5-7414--->P5=1uAx5=5uA

so the total power about 2705mW, if the battery voltage is 9.6v , so the current should be  541mA, is this correct???

This Motor having the stall current of 800mA ---> P4=2x(5x800mA)= 8000mW
So all in will 8839.6mW so the current should be 920mA or more.
This assuming your motors is power by the battery directly.

#### Abdulla M.A.

• Jr. Member
• Posts: 30
##### Re: Choosing the correct battery for robot
« Reply #3 on: September 17, 2010, 09:51:46 AM »
Sorry I forget 7805

@Soeren
I know how to choose the correct battery, I built a robot in the past year you can check it on my web, second I would
like to do very accurate calculations so I asked for that because I do not have that big experiance in robots.
BTW, I do not need to claim. OK!!! we are humans, so everybody can do a mistake. I saw if you answer my question in good
form will be better from these comments. You feel as a perfect engineer, here !!!!! but in
Electronics Forum, you are nothing
so stop "The Claim Engineer"

@billhowl
thank you so much, just I have few questions
1- why 7805 need to source out 81.6mA?
2- did you have the data sheet of this motor??

Abdulla
« Last Edit: September 17, 2010, 09:58:48 AM by Abdulla M.A. »
"A scientist can discover a new star, but he cannot make one. He would have to ask an engineer to do that."
"For an optimist the glass is half full, for a pessimist it's half empty, and for an engin

#### billhowl

• Supreme Robot
• Posts: 376
##### Re: Choosing the correct battery for robot
« Reply #4 on: September 17, 2010, 10:06:09 AM »
LM324 --> 2 x 0.8mA = 1.6mA
Two sensors 2 x 10mA = 20mA
L298 --->  60mA
Total 81.6mA
You have not include current for Microcontroller.

#### Abdulla M.A.

• Jr. Member
• Posts: 30
##### Re: Choosing the correct battery for robot
« Reply #5 on: September 17, 2010, 10:43:36 AM »
Can I use L293D if each channel can only provide 600mA, in Sparkfun web they said it's possible, but how is this possible
if the motor draw 800mA at stand still??
thankx

Abdulla
"A scientist can discover a new star, but he cannot make one. He would have to ask an engineer to do that."
"For an optimist the glass is half full, for a pessimist it's half empty, and for an engin

#### billhowl

• Supreme Robot
• Posts: 376
##### Re: Choosing the correct battery for robot
« Reply #6 on: September 18, 2010, 07:28:21 AM »
Two important mistakes for newbies to avoid:  low-voltage motors very often draw more current than the L293D can safely supply.  And they usually draw more current than the onboard regulator 7805 can comfortably supply.  The results are burnt-out L293Ds and/or unreliable operation because the motors drag down the 5V supply when they're running.

It's always better to run the motor(s) off a separate power supply if you can.  Next best is to run them off the unregulated input to the MCU (but make sure it has the needed current capacity).

Tamiya 980112M Mabuchi FA-130 Motor
Mabuchi FA-130 Motor

motors draw a bit too much current though, piggy back two SN754410 or L293D, and it shouldn't cause any harm.

#### chelmi

• Supreme Robot
• Posts: 496
##### Re: Choosing the correct battery for robot
« Reply #7 on: September 18, 2010, 09:22:06 AM »
I use this gearbox in my current project (I need to update my blog about it...), but changed the default motor to a higher voltage, lower current one.
http://www.pololu.com/catalog/product/1117

This way I can used "standard" h-bridge and I don't have to worry to much about stall current, battery, etc.
And apparently, 6V is not good for these motors:
http://www.pololu.com/docs/0J11

#### Abdulla M.A.

• Jr. Member
• Posts: 30
##### Re: Choosing the correct battery for robot
« Reply #8 on: September 18, 2010, 10:05:59 AM »
Thank you alot guys for help.

Abdulla
"A scientist can discover a new star, but he cannot make one. He would have to ask an engineer to do that."
"For an optimist the glass is half full, for a pessimist it's half empty, and for an engin