I have been looking at the Traxxas LiPo batteries and see that the specs show a C rating of 25. Looking around I see that the C rating represents the current draw as a ratio to the capacity of the battery pack.
Can someone tell me how long and at what voltage these packs can maintain that C rating?
For instance, a 11.1 v 5000mAh pack with a C rating of 25 should produce 5A*25 or 125Amps. If I were to put a constant .088 ohm load on one of these packs, how long could I expect this pack to maintain 11.1 volts?
What would be a more typical extended load? I'm guessing very short bursts of less than 50 Amps with more regular bursts in the 30 Amp range with WOT on a flat surface being around 10Amps.
I am comparing these numbers to the typical laptop LiIon cell which is rated at 2200Ah and .4C. I can put a 4amp load on these for 30minutes but 30 Amps won't last more than a few seconds before the voltage drops to 2V.
I started off looking to build an electric go cart built from used laptop batteries and treadmill motors and came across Traxxas in my research. Now my son and I must own a couple of rustlers. Don't think we can swing the VXLs but I'm trying to scrape up the money for a couple of the XL5s.
The question you ask demonstrates a problem with C-rating. There is no industry standard. While some of the cheaper overseas brands obviously use something around 70% of the packs rated capacity as a benchmark, other companies use 80% or even more. From every report I've seen or hear (not owning any Traxxas packs yet) the 25C is very conservative. Meaning (this is a guess) other less scrupulous companies may rate these as 30C or 40C and back that up by pointing to a 70% limit. Unfortunately, I haven't seen ANY company voluntarily step forward and divulge their standards.
I'm not going to try to answer the load questions - that's "above my pay grade" but I will ask one question: you want the voltage to be 11.1V (3.7V/cell) or greater, right? A 3S fully charged is 12.6V (4.2V/cell.)
Again, no expert here, but electric go-carts are typically much higher voltage with rather low kv motors.
Last edited by ksb51rl; 11-24-2011 at 10:03 PM.
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If the cells in a LiPo pack are accurately rated (and I believe Traxxas cells are) , the cells should hold the full C-rated load from full capacity down to 3.0 Volts, without a horseshoe-type drop and rise in the discharge curve and without the cells temps exceeding 140°F. Also, the cell should give up a minimum of 80% of its stated capacity during this full discharge test.
KSB, on the go kart question. I have seen carts mostly in the 24 volt range with motors rated 500 to 1000 Watts. The motor I am working with is rated at 2.8HP@130V/2089Watts and 1.75HP Continuous at 100V or 1300 Watts. I have a source of free used LiIon cells and got the motor for free. That means I have to find something to do with them.
Jakey, thanks for the info. I think 3.0v is reasonable. I guess my question is, how long does it take to get there from full charge. My guess is it is somewhere more than 5 seconds as I think I saw a separate C rating somewhere from Traxxas for a 5 second burst.
From a local racetrack's website:
Go kart motor
Speeds up to 45 MPH
EK20 high performance electric motor rated at approximately 18 HP (Comparable gas-powered karts rated at approximately 6 HP)
High-torque output for rapid acceleration
Electronically adjustable power output to ensure evenly matched karts.
Forward and Reverse switch for full maneuverability
Radio controlled speed and automatic pit-speed reduction
Engine automatically decelerates when brake and gas pedals are applied at the same time
Go kart power supply - batteries.
4 12-volt batteries
Rapid charge capabilities
Rapid Recharge: 6-12 minutes
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I've seen these carts. Much faster than what I had in mind. They use LA batteries and usually charge between runs.