Can 12V 2500 mah NiMH batteries release around 150A?

[jonnyk]

@turtlemeister
Example of an rc car with 5000 mah lipo battery pack with 25C discharge rate(25X5=125A):
http://www.youtube.com/watch?v=Vn1zNAvZp-8&feature=related

 

[cf8]

I'm surprised no one has pointed this out yet:

Actually i know a guy who started his car with an RC plane battery pack:
http://www.youtube.com/watch?v=1w_K08fwZnA&feature=channel_page

This is a perfect example of why you can't trust everything you see on the Internet. For curiosity's sake, what do you mean when you say that you "know" him? Have you actually met him? Did you do this yourself? The current required to cold-start a car is insane; that's why they use cables as big around as your thumb, and not 24 AWG wire like the guy in the video did.

 

[TurtleMeister]

Example of an rc car with 5000 mah lipo battery pack with 25C discharge rate(25X5=125A):http://www.youtube.com/watch?v=Vn1zNAvZp-8&feature=related

So we're talking about Li Poly batteries now? I checked and there is a battery of this type rated as specified. The 7.4 volt version weighs 247 grams and costs $125. And according to my calculations it would have to have an internal resistance of much less than 0.1 ohms. I did not know there were batteries on the market with such low internal resistance. And I'm still skeptical because I don't believe everything I read. :) But my answer to your original question is still NO.

 

[jonnyk]

@turtlemeister
Take a look at the Power on this table:
http://en.wikipedia.org/wiki/Rechargeable_battery#Table_of_rechargeable_battery_technologies

As you can see an NiMH battery has btw 250-1000 W/kg output AT 1.2 V which is abt 1.5 times less than the Lipo at the same voltage and weight but much more than the earlier NiCD. Hence at 12V an NiMH will have a 2500-12000 W/kg output OR a 250-1000 W/100g which is a feasible pack for rc cars.

 

[TurtleMeister]

Stacking cells in series does not increase the W/kg. If you double the voltage (power) by adding another cell you've also doubled the mass, so W/kg stays the same.

 

[vk6kro]

I weighed a single AA NiMH cell. It weighed 30 grams.
So, you would get about 3 of them in 100 grams.

They would give you 3.6 volts with an internal resistance of 0.3 ohms (if the previous figures for internal resistance are correct) so they would have a short circuit current of 12 amps. (3.6 /0.3 ). But short circuit means you don't get any voltage out.
Maximum power out of 10.8 watts would be available with a load of 0.3 ohms.
But at that power, the voltage would only be 1.8 volts. So the current would be 6 amps.

Just for fun, if you did somehow pass 150 amps through one of these AA cells, the power would be 150*150*0.1 or 2250 watts. I've never seen one of these explode but, if it was well sealed, it should be spectacular.

 

[Phrak]

This is a perfect example of why you can't trust everything you see on the Internet.

No you can't. But that video is plausible. Try this one.

http://www.youtube.com/watch?v=a2JkMCzjTVE"

Lithium Poly's are in a class by themselves.

 

[cf8]

No you can't. But that video is plausible. Try this one.

http://www.youtube.com/watch?v=a2JkMCzjTVE"

Lithium Poly's are in a class by themselves.

I re-watched the video, and I see your point. However, if I understood correctly, jonnyk was under the impression that the LiIon pack alone could cold-start the car - in the video, though, it was mentioned that the car battery did have some life left.

Also, I liked the video you just posted, that's amazing, however I did hear:

...powered by nine hundred ninety lithium battery cells...

Either way, I want a car made by them!

 

[Phrak]

I re-watched the video, and I see your point. However, if I understood correctly, jonnyk was under the impression that the LiIon pack alone could cold-start the car - in the video, though, it was mentioned that the car battery did have some life left.

Also, I liked the video you just posted, that's amazing, however I did hear:

Either way, I want a car made by them!

Well done. I didn't catch the part about the car battery still having life in it. lithium batteries have their drawbacks in terms of safety. Gasoline still stores energy better. Charging batteries burns coal. If mischarged they can burn real nice, and start a fire. But for peak power output in an electric vehicle, there's no comparison between NiMH and LithumPoly. I think the cost per Joule is about twice that of NiMH.

The series resistance of lithum polymere batteries seems to continue to decline. I'd have to check my notes to see what I was using 2 years ago to be sure (deit: I used 25 mOhms per cell). It may be to the point that the motor resistance is the predominant limit on output torque.

 

[TurtleMeister]

I wonder if anyone has ever considered using an ultracapacitor to enhance the performance of their RC? They have extremely low internal resistance and an amazing specific power of around 15000 W/Kg. Seems to me if you connected one of these across the input of the ESC it would help make up for some of the shortcomings of the battery.

 

[hean8209]

NIMH as used in RC cars have a discharge rate of 5-10C (C = times capacity).
So, with 3000mah nimh cells of good quality (10C) you would get 30 amps.
To get 1000 watts of power output, you would need a voltage of 36 volts,
which means 30 cells.. that would weight 2.5-3.0 kilograms approx, more than most 1:8 cars weigh.
Which translates to.. not very fast.

Good LIPO batterys have a 35C rating, and are available at 8000mah,
weighing only 0.3 kg if even that.
You could pull 3080 watts of power out of that battery alone,
with only 3 cells at 11.1 volt.
It would probably cost 200 USD for only one battery though.

Besides, with NIMH brushless is not on par with nitro in any way.. that's just hype.
you need LIPO to go fast, and you need expensive lipos to go really fast.
To expect anything else is to fall for the exaggerated hype.. an electric motor is only as good as its power supply, nothing more, nothing less.

Another thing on "power output".. a nitro engine is NOT 80-90% efficient,
so a lot of power dissipates as heat.. not drive.. and you can't expect a nitro engine to do
70 000 rpm (possible with brushless, but not safe, and the motor won't last long.)
Electric motors put out more efficient torque, and have a wider power band.. that is why
they accelerate better than a narrow band nitro engine (A lot of power at the last 2-3k of rpm)
All this translates to: higher final gear ratio possible for the same speed,
which means faster acceleration.
To equal 1000 watts on a nitro, at 30k rpm,
you would need twice the gearing, and twice the rpm, at 500 watts.
No big deal for an electric motor.
This, of course is if they weigh exactly the same, which they dont.

So 400 watts electric rc is more powerful than 1000 watts combustion rc,
hands down.
1 kw would equal more than 2.2 bhp, found in common .28 nitro´s in 1:8 cars.

Since the electric LIPO machines are lighter, you can gear them lower to get more speed,
and still accellerate like crazy.
That is why they are faster.. the factor of all things combined, not anyone part in itself.

Less weight + good powerplant + correct gearing = one fast mean car.

At the higher end (read: VERY EXPENSIVE 3 cell LIPO), sure brushless is faster, but costs quite a bit.
At the low end (read: VERY CHEAP 3000mah NIMH), brushless can't even compare, and nitro is the winner in performance no matter how many cells you put in (NIMH is heavy and low power).

I have tried BL and NIMH, and its really fun to drive with insane torque.. just don't buy a
too "hot" motor, or you will explode your poor nimh batteries.. 10 turn or higher works well with NIMH, anything lower REQUIRES LIPO.. don't even consider NIMH for those motors.

To sum up: The power in an electric RC car lies within the battery,
not so much in the motor. A poor motor with a good battery will perform much better than a good motor with a poor battery.

 

[uart]

Just for amusement here's another data point. The NiMH batteries in a Toyota prius are only 6.5AH yet they deliver peak current at around 100 Amps during acceleration. This corresponding to about 15C, though normally (for battery longevity) they deliver/receive considerably less than that amount.

 

[famousken]

Actually I have jump-started a car with RC car batteries. They were two 6 cell packs made of sub-cs, 1700mah, so 14.4V total. It worked, but one of the packs was no good after that. I think it is incredible how much current these batteries can supply, and they CAN supply upwards of 50A in short bursts, I have had shorts on the leads coming out of the batteries (AWG12) and it destroyed the silicon insulation on them, so there was way more than 20A flowing.

 

[The Electrician]

A brand new, just charged, Energizer 2300 mAH NiMH AA cell measures .027 ohms internal resistance (measured with a Wayne-Kerr LCR meter, .001 ohms resolution). Shorted into a 100 amp shunt, I get 35 amps. After 10 seconds of this, it gets hot; no surprise there.

 

[Altrepair]

I am surprised no one mention this so I will. A DC motor controller is like a buck converter in that varying the speed by voltage is done with variable square waves and the motor is like the inductor that stores the energy. One-hundred fifty amps could be supplied to load from a buck converter while pulling less current from the power source such as a battery. But this is an indirect way and the voltage will have to be less on the output. I doubt a direct connection to this battery would allow 150 amps to be pulled from it. If it could then the battery will most likely never function again. Lithium Ion batteries seem to be a totally different animal all together.