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Offline aruna1Topic starter

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lipo info
« on: December 21, 2010, 08:41:46 AM »
hi
how much C rating lipo is suitable for normal robot? like line follower,fire fighter normal stuff
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Offline macdad-

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Re: lipo info
« Reply #1 on: December 21, 2010, 09:24:18 AM »
Li-Poly batteries are excellent for robots due to their high efficiency(~90%), their able to be recharged and relatively high Amp-Hours to their actually physical size. But they are relatively expensive(and overkill) if you were going with a simple to moderate robot. They are also expensive as far as the rechargers for them.

Nickel Metal Hydride(NiMH) batteries are cheaper than Li-Poly batteries(Their chargers are also cheaper), but have lower efficiency(About 70%), and lower Amp-hours.

But Li-Poly batteries are excellent if you have several motors or other high current devices. Just as an example, if you had a LiPo battery about the size of a 9V Battery, it would have atleast twice the Amp-Hours versus a NiMH of equal size.

Sources:
http://www.maxamps.com/Lipo-430-74-Pack.htm
http://www.all-battery.com/9vsizenimhrechargeablebatteries.aspx

Hope this helps
,Nick(macdad-)

Offline aruna1Topic starter

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Re: lipo info
« Reply #2 on: December 21, 2010, 09:37:23 AM »
so
tell me if i buy a 1500mAh lipo will it last longer(more drive time for robot) than 1500mAh NiMH?
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Offline waltr

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Re: lipo info
« Reply #3 on: December 21, 2010, 10:09:03 AM »
In theory they would run for the same amount of time.
However it would depend of what the usable minimum required voltage for your bot is compared to the minimum voltage of the battery.
For a LiPo this is about 3.0V before the voltage really drops fast. The absolute minimum voltage spec is 2.5V for most LiPo.
Can you circuit utilize a voltage this low? If they need 3.3V then the circuits are under voltage with over half the battery capacity unused. A DC-DC buck-boost voltage regulator can use all of the battery's capacity.

The biggest advantage of LiPo over NiMH is the weight and a very low self discharge rate. Another is that LiPo's are rectangular so they can take up less space. Read the short write-up on using LiPo instead of NiMH:
http://www.xheli.com/lipobatteries.html

Offline hardmouse

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Re: lipo info
« Reply #4 on: December 21, 2010, 04:00:30 PM »
I am always using lipo, too. But one thing to be aware of thou. Protect it well and don't even make a scratch or any tiny hole on it or it will become a small bomb.

Offline aruna1Topic starter

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Re: lipo info
« Reply #5 on: December 22, 2010, 08:42:45 AM »
In theory they would run for the same amount of time.
However it would depend of what the usable minimum required voltage for your bot is compared to the minimum voltage of the battery.
For a LiPo this is about 3.0V before the voltage really drops fast. The absolute minimum voltage spec is 2.5V for most LiPo.
Can you circuit utilize a voltage this low? If they need 3.3V then the circuits are under voltage with over half the battery capacity unused. A DC-DC buck-boost voltage regulator can use all of the battery's capacity.

The biggest advantage of LiPo over NiMH is the weight and a very low self discharge rate. Another is that LiPo's are rectangular so they can take up less space. Read the short write-up on using LiPo instead of NiMH:
http://www.xheli.com/lipobatteries.html



well even we use buck boost converter. we cant use full capacity of the battery right? because we cant drain a battery to zero.
I intend to buy 3S1P battery.main power usage is for my 6v motors and servos. power requirment for other circuits are small
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Offline waltr

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Re: lipo info
« Reply #6 on: December 22, 2010, 10:16:50 AM »
Quote
well even we use buck boost converter. we cant use full capacity of the battery right? because we cant drain a battery to zero.

Wrong.  The full capacity rating of the battery is to the Minimum voltage, about 3.0 Volt for a LiPo. A good buck-boost converter will operate below the 3.0 minimum battery voltage but you should never take a LiPo below this voltage. I have been using the LTC3440 to power a PIC and a display from an 600mAh LiPo with excellent results.
Never, never take any battery to zero volts unless you want to replace the battery.

Read more on the technical aspects of batteries.
Here is one:
http://forums.all-battery.com/index.php?showtopic=279
Look at the Tenergy 3.7V 2000mAh Single Cell Discharge Curve (about half way down the page).
That shows the mA-Hr capacity at different discharge rates. All the curves stop at a battery voltage of 3.0V. With a 2C discharge rate the full 2000mA-hr was measured before the battery dropped to 3.0V.

Offline aruna1Topic starter

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Re: lipo info
« Reply #7 on: December 22, 2010, 10:29:32 AM »
Quote
well even we use buck boost converter. we cant use full capacity of the battery right? because we cant drain a battery to zero.

Wrong.  The full capacity rating of the battery is to the Minimum voltage, about 3.0 Volt for a LiPo. A good buck-boost converter will operate below the 3.0 minimum battery voltage but you should never take a LiPo below this voltage. I have been using the LTC3440 to power a PIC and a display from an 600mAh LiPo with excellent results.
Never, never take any battery to zero volts unless you want to replace the battery.

Read more on the technical aspects of batteries.
Here is one:
http://forums.all-battery.com/index.php?showtopic=279
Look at the Tenergy 3.7V 2000mAh Single Cell Discharge Curve (about half way down the page).
That shows the mA-Hr capacity at different discharge rates. All the curves stop at a battery voltage of 3.0V. With a 2C discharge rate the full 2000mA-hr was measured before the battery dropped to 3.0V.


well I will be powering motors and servos with this battery.that means it may need couple of amperes in maximum.any suggestions?
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Offline aruna1Topic starter

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New question
« Reply #8 on: December 25, 2010, 10:56:54 PM »
is there any capacity difference between 11.1V 2200mAh lipo and 7.4V 2200mAh lipo? what I meant is since my motors are 6V and circuits are run with 5V and I'm going to use a buck converter to reduce battery power to power up circuits and motors is there any advantage of using 11.1V 2200mAh lipo instead of 7.4v 2200mAh lipo? will they have different discharge times? one will work more time than other?
thank you and sorry for the noob question. I havent use any lipo before so want to know everything about what I'm going to buy.
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Offline billhowl

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Re: lipo info
« Reply #9 on: December 26, 2010, 11:44:24 AM »
11.1V 2200mAh lipo  is 11.1 V * 2200mAh = 24420 mWh = 24.42 Wh
and 7.4V 2200mAh lipo is 7.4V * 2200mAh = 16280 mWh = 16.28 Wh
The capacity difference between 11.1V 2200mAh lipo is higher and 7.4V 2200mAh lipo is lower.

Offline aruna1Topic starter

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Re: lipo info
« Reply #10 on: December 26, 2010, 11:47:32 AM »
thank you billhowl.
there is something I dont get.
in simple way are you saying 11.1v 2200mAh battery will give more running time than 7.4V 2200mAh battery?
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Offline Joker94

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Re: lipo info
« Reply #11 on: December 26, 2010, 09:44:02 PM »
generally you look at what you want to power. if you only want to power the micro controller and some servos there is no point going for an 11.1v battery as you will be wasting a lot of power when you regulate it to 5v and 6v. there for you would probably go for a 7.4v battery.

but if you have motors that you want to run at 11.1v then you would go for the 3 cell lipo.

so if you are concerned about the total run time, increase the capacity. capacity being the mAh rating. so instead go for a 3300mAh or higher rather than the 2200mAh battery.

as for the 2 digit C rating provided, most batteries will be 20C or higher. and if you are only powering servos and the micro controller a 20C should be enough. but if you plan to use lots of servos , e.g. more than say 6 i would go for around a 25C battery. and if you were to have continuous running motors i would go for probably a 30C.

it all depends on the size of the motors and servos but it is always better to add in a safety factor(a factor allowing for expansion or unaccounted for power drain) and not having to buy another battery when you want to add more servos or add weight etc.

hope this was clear and helps!

Offline aruna1Topic starter

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Re: lipo info
« Reply #12 on: December 26, 2010, 09:47:41 PM »
generally you look at what you want to power. if you only want to power the micro controller and some servos there is no point going for an 11.1v battery as you will be wasting a lot of power when you regulate it to 5v and 6v. there for you would probably go for a 7.4v battery.

but if you have motors that you want to run at 11.1v then you would go for the 3 cell lipo.

so if you are concerned about the total run time, increase the capacity. capacity being the mAh rating. so instead go for a 3300mAh or higher rather than the 2200mAh battery.

as for the 2 digit C rating provided, most batteries will be 20C or higher. and if you are only powering servos and the micro controller a 20C should be enough. but if you plan to use lots of servos , e.g. more than say 6 i would go for around a 25C battery. and if you were to have continuous running motors i would go for probably a 30C.

it all depends on the size of the motors and servos but it is always better to add in a safety factor(a factor allowing for expansion or unaccounted for power drain) and not having to buy another battery when you want to add more servos or add weight etc.

hope this was clear and helps!

well my robot uses 2 6V pololu motors http://www.pololu.com/catalog/product/998
hope 7.4V 2200mAh battery with buck converter can drive these
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Offline Adityav95

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Re: lipo info
« Reply #13 on: December 26, 2010, 10:02:33 PM »
a 7.4V for 2 motors of 6V each is fine but i would recommend u regulate ur voltage to 6V itself using something like a IC7806 etc. U should follow this: http://www.societyofrobots.com/schematics_powerregulation.shtml

Offline aruna1Topic starter

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Re: lipo info
« Reply #14 on: December 26, 2010, 10:31:16 PM »
a 7.4V for 2 motors of 6V each is fine but i would recommend u regulate ur voltage to 6V itself using something like a IC7806 etc. U should follow this: http://www.societyofrobots.com/schematics_powerregulation.shtml

hi Adityav95,
I intend to use a buck converter instead of linear regulator.
Linear regulators have a nasty habit of wasting battery power specially when drawing higher currents
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Offline Adityav95

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Re: lipo info
« Reply #15 on: December 26, 2010, 10:39:41 PM »
I understand linear regulators waste power but only if the voltage is much higher(eg:12V regulated to 6V etc) but current draw doesn't effect it much/at all, but as to an article i read in a mag, i heard even buck converters, zener diodes etc also waste power although the absolute amount may vary. I mostly use linear regulators and it sacrifices less than 10% of ur battery life, which is OK for me. I've really never used a buck converters or zener diodes, so I wont say much about it. :D
« Last Edit: December 26, 2010, 10:41:55 PM by Adityav95 »

Offline waltr

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Re: lipo info
« Reply #16 on: December 27, 2010, 09:41:38 AM »
The Pololu 6V micro & mini gear motors run fine directly from a 7.4V LiPo through a motor driver like the TB6612FNG. This is what I have on a Bot.
http://www.pololu.com/catalog/product/713
http://www.pololu.com/catalog/product/998
Quote
These tiny, high-quality motors are intended for use at 6 V. In general, these kinds of motors can run at voltages above and below this nominal voltage, so they should comfortably operate in the 3 9 V range.
is from the motor data on Pololu's web site. Do read it.

There is no need to regulate the voltage to the motor driver/motor. The PWM of the motor drive control is an efficient method to regulate the motor speed. Also read the specs for the motor driver and controllers. They all can take higher voltages.

Now you only need to regulate the voltage to the processor and other electronics.

Quote
I understand linear regulators waste power but only if the voltage is much higher(eg:12V regulated to 6V etc) but current draw doesn't effect it much/at all

The power loss IS dependent of the current through a linear regulator and the voltage drop across the regulator. The loss is: P(W) = Voltage drop(V) * Current(I). This also goes for any other method that drops the voltage (diode, resistor).

Switching regulators (Buck, Boost, etc) can be more efficient if properly designed and built. Check the data sheets for efficiency curves. The efficiency can vary depending on the input voltage and the load current.



Offline aruna1Topic starter

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Re: lipo info
« Reply #17 on: December 27, 2010, 09:53:03 AM »
The Pololu 6V micro & mini gear motors run fine directly from a 7.4V LiPo through a motor driver like the TB6612FNG. This is what I have on a Bot.
http://www.pololu.com/catalog/product/713
http://www.pololu.com/catalog/product/998
Quote
These tiny, high-quality motors are intended for use at 6 V. In general, these kinds of motors can run at voltages above and below this nominal voltage, so they should comfortably operate in the 3 9 V range.
is from the motor data on Pololu's web site. Do read it.

There is no need to regulate the voltage to the motor driver/motor. The PWM of the motor drive control is an efficient method to regulate the motor speed. Also read the specs for the motor driver and controllers. They all can take higher voltages.

Now you only need to regulate the voltage to the processor and other electronics.



well if you dont regulate power to the motor you will get change in speed ffor same PWM value as the battery voltage drops.this is unless you use encoder feedback system to maintain the speed
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Offline Adityav95

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Re: lipo info
« Reply #18 on: December 27, 2010, 10:31:18 AM »
Quote
The power loss IS dependent of the current through a linear regulator and the voltage drop across the regulator. The loss is: P(W) = Voltage drop(V) * Current(I). This also goes for any other method that drops the voltage (diode, resistor).

but power(W) = Voltage^2/Resistence {i.e. V square/R}, so does'nt that mean current doesnt affect it. I think only voltage affects it as only the voltage is being regulated. Correct me if I'm wrong....

Offline aruna1Topic starter

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Re: lipo info
« Reply #19 on: December 27, 2010, 10:39:06 AM »
Quote
The power loss IS dependent of the current through a linear regulator and the voltage drop across the regulator. The loss is: P(W) = Voltage drop(V) * Current(I). This also goes for any other method that drops the voltage (diode, resistor).

but power(W) = Voltage^2/Resistence {i.e. V square/R}, so does'nt that mean current doesnt affect it. I think only voltage affects it as only the voltage is being regulated. Correct me if I'm wrong....

well Adityav95
hope you seen this equation

V=I*R

now substitute this V to your equation and you get

Power = I^2 * R   {I squre * R}
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Offline Adityav95

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Re: lipo info
« Reply #20 on: December 27, 2010, 10:51:43 AM »
Quote
V=I*R

now substitute this V to your equation and you get

Power = I^2 * R   {I squre * R}

Well yeah I've seen it. Then......maybe u r right. ;)

Offline waltr

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Re: lipo info
« Reply #21 on: December 27, 2010, 12:41:14 PM »
well if you dont regulate power to the motor you will get change in speed ffor same PWM value as the battery voltage drops.this is unless you use encoder feedback system to maintain the speed

That is true but without feed-back the motor speed will vary due to changes in the load. I use feed-back to maintain the bots speed and since the Bot is a two wheel differential drive, maintaining wheel speed is critical if you want the Bot to go straight. Otherwise the Bot will always go in a (large) circle even if the voltage is regulated due to differences in each motor, gearbox and other factors.

Offline waltr

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Re: lipo info
« Reply #22 on: December 27, 2010, 12:52:00 PM »
Quote
V=I*R

now substitute this V to your equation and you get

Power = I^2 * R   {I squre * R}

Well yeah I've seen it. Then......maybe u r right. ;)

The power equation (as I posted above) is: P = E * I
You can then use Ohm's law and solve for either E or I then substitute into the power equation to get power in terms of V & R or I & R as per the two other equations posted.

Which equation you use depends on what quantity is measurable. In the case of the power loss in a linear regulator, the current and voltage drop is measurable (resistance is not) so these are used with a appropriate equation.
For the power loss in a resistor then the resistance and either the voltage drop across the resistor or the current through the resistor is used.

Offline aruna1Topic starter

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Re: lipo info
« Reply #23 on: December 28, 2010, 07:58:13 PM »
well if you dont regulate power to the motor you will get change in speed ffor same PWM value as the battery voltage drops.this is unless you use encoder feedback system to maintain the speed

That is true but without feed-back the motor speed will vary due to changes in the load. I use feed-back to maintain the bots speed and since the Bot is a two wheel differential drive, maintaining wheel speed is critical if you want the Bot to go straight. Otherwise the Bot will always go in a (large) circle even if the voltage is regulated due to differences in each motor, gearbox and other factors.

now that you mentioned it I'm experiencing it with my every robot.they always go on circles  :).
so what are other methods to send robot straight? without encoders?
« Last Edit: December 28, 2010, 08:00:19 PM by aruna1 »
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Offline waltr

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Re: lipo info
« Reply #24 on: December 29, 2010, 08:37:41 AM »
Quote
now that you mentioned it I'm experiencing it with my every robot.they always go on circles  Smiley.
so what are other methods to send robot straight? without encoders?

Some other method to measure the Bots Heading and then feed-back to keep that heading.

Wheel encoders really are the easiest method.


Offline aruna1Topic starter

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Re: lipo info
« Reply #25 on: December 30, 2010, 07:18:28 PM »
Quote
now that you mentioned it I'm experiencing it with my every robot.they always go on circles  Smiley.
so what are other methods to send robot straight? without encoders?

Some other method to measure the Bots Heading and then feed-back to keep that heading.

Wheel encoders really are the easiest method.


like a using a compass to make fix direction?
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