# C ratings for go-kart + CALB/Thundersky Lipos



## EVfun (Mar 14, 2010)

I keep beating up my 60 amp hour Thunder Sky cells for 7C on demand (420 amps) and they don't seem to mind. The voltage is down to about 2.7 vpc at that point. Nothing seems to get warm. I drive the Buggy like a go-cart.


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## lostinspacebar (Mar 4, 2011)

Yeah, it looks like the 60AH would be a safe choice. But so much more expensive 

Trying to see if I can get away with the 40AH. Sorry if I wasn't clear.


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## EVfun (Mar 14, 2010)

It is likely you can pull the same C rate with 40 amp hour Thunder Sky cells. Usually, the smaller capacity cells can dish out current at the same to slightly higher C rate. If a 60 amp hour TS can pull 7C (420 amps) then a 40 amp hour TS cell should be able to pull 7C too (280 amps.) 

I haven't used CALB cells, but their spec sheets show higher peak rates for the smaller cells compared to the large units. By most reports CALB cells have similar sag under load to the TS cells. With either maker, 10C (400 amps from a 40 AH) is likely to pull the cells down to 2.4 or 2.5 volts each. 

A 72 volt pack at 280 amps, counting voltage sag and motor efficiency, should be about 20 horsepower. I was being a little pessimistic with those numbers, 24 cells at 2.7 volts each and a motor efficiency of 82%. That should be a pretty quick go-cart. 

I would recommend looking for suggestions from the owners of Lithium powered motorcycle conversions as they are far more likely to use 40 amp hour cells.


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## Duncan (Dec 8, 2008)

Hi Lost

What about Headway? - 10c continuous 15C burst and some people say much more


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## lostinspacebar (Mar 4, 2011)

@EVFun : Thanks for the info. I'm relieved to hear the smaller cells can also hit the same-ish C ratings. Since it's a go-kart, I will probably be on an autocross track most of the time. Aka lots of corners, so I will probably have lots of braking time too. Are you looking at discharge curves like these - http://www.electricmotordepot.com/content/LiFeP04/CALB/40Ah.pdf? The X-axis is in AH, which makes me think I need to find something else that actually shows voltage vs A instead of AH.
And yeah, I'll go see if I can find me some bikers 

@Duncan : The headway cells do have higher C ratings, but they end up being about the same cost as 60ah. But that was from a quick search. Maybe I can look around more. I initially didn't look at those since I would have to put the cells together to make a pack and it looked more involved than using the TS/CALB cells. Do you have any experience with these headway cells?


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## MN Driver (Sep 29, 2009)

The spec sheet from the official CALB site en.calb.cn says 10C for the 70Ah and smaller cells for 10 seconds. They don't factor in repeated draws like that over and over though. If you think you'll be drained in less than 20 minutes with the amount of power you are using the batteries are likely not going to handle it for a very long time, especially if you draw 400 amps when they are 30% state of charge or lower which is when they'll get really hot. I haven't tested this but its as far as I'd go if I did what you are doing. 40Ah cells will also give you a lighter pack. If I needed more power and less current draw I'd stick with the same cell size but add more cells if the controller could handle the voltage and could put a cap on max motor volts.


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## lostinspacebar (Mar 4, 2011)

MN Driver: I might just limit the motor controller to maybe 300A max. Even then I won't really be doing the peak draw for than maybe 2-3 seconds before I hit speeds that I won't want to exceed ... But do I basically stop drawing much from the batteries when they hit 30%? I am guessing after that they go into deep discharge? I am basically looking for these batteries to last me maybe 1.5-2 years at least.

Thanks,
Aditya


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## MN Driver (Sep 29, 2009)

Peak draw of 400 amps for 3 seconds shouldn't be too bad for them from the sound of how you'll be using the cart.

The voltage sags more(which generates more heat), especially towards the bottom 20% of their capacity but if you want to be safe with them you would need to either be a more gentle at 30% approaching 20% because Jack Rickard did a continuous draw at 4C and the temperature left my own 'comfort zone' past 140f at 30% and they got hot enough to boil the electrolyte and vented just below 20%.

I don't think you'd have any trouble getting 2 years out of them even if you abused them fairly hard.


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## lostinspacebar (Mar 4, 2011)

Cool. I guess I'll take the shorter run-time per charge and just stop using the kart once it gets to 30%. Right now I am assuming I can use all of the battery storage (72V x 40AH) and get close to 20 minutes of usage. But I guess with losses and sag etc, I am probably looking at 10-15 mins of actual run-time. Still good enough for me I think.


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## MN Driver (Sep 29, 2009)

I'd be sure you have the range you need to do what you want, I'm not sure what the usage stats are on a go-kart like you are using and with the driving style you are going to being thrashing around with. You'll have to use some form of Ah counter to determine the point where you should stop if you are trying to stop at 70 or 80% depth of discharge. Might also be a good idea to get a temperature measuring device to thermocouple to a cell terminal(hottest measurable spot) to get a ballpark of how hot the cells are, if they are cool you'll be more comfortable thrashing them around with higher amp rates.


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## lostinspacebar (Mar 4, 2011)

Yeah. Definitely going to setup temperature and current draw monitoring. I have all the parts except for the batteries, so going to try and setup the wiring/monitoring before I add in the batteries and BMS. Hopefully I won't be setting things on fire... 

Thanks for the all the info guys.


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