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30A pack build
#1
hello everyone,
I'm building a pack for our robotics competition. for that, we need around 30 A constant current. till now we are using lipo batteries. but they are not good as labeled. we bought 5200 mah lipo battery but it came out only 3300 mah. also, they are costly. we are searching for some other options. we build one 4s1p battery from lifepo4 cells which are rated for 6c discharge . those lifepo4 cells are of 6000mah. it is working fine. but when we increase the load it starts heating. still, those cells are rated for 36A, they get hot when we draw around 30A for 10 min. I spot weld the cells. we need an alternative to this.

we need a battery which will have low weight, high current discharge, also good capacity and voltage of around 12v.

please do help.
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#2
How big and how heavy can the battery be? Also, what do you consider a good capacity? And what do you mean by hot? Cells under high load will get warm or even hot, that's normal and not an issue as such.
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#3
(01-13-2019, 06:03 PM)DarkRaven Wrote: How big and how heavy can the battery be? Also, what do you consider a good capacity? And what do you mean by hot? Cells under high load will get warm or even hot, that's normal and not an issue as such.

battery weight should not be more than 700-800 gms. we looking for around 5000-6000mAh battery. cells are getting hot but also the strips which we use for spot welding they are also getting very hot. 
 we thought that this issue of cells heating is because of Chinese cells so we are thinking to use  Samsung 25R cells, in series-parallel to achieve 40A discharge current. but the main problem is spot welding the strips. those strips cannot handle that much of current. they getting very hot. we choose lifepo4 for 2 reasons, they are cheap and have a large area on the positive side so we can spot weld two strips parallelly. 

and also please ignore my bad English.
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#4
That's correct, the nickel strips usually used to spot weld cells like 18650s aren't suitable for 30A. You should look for thicker strips, 0.3mm, and use several layers.
But anyway, I'd just get a proper quality LiPo battery. It's not that a LiPo wouldn't be able to do everything you ask, it just seems that you got a bad one. 5Ah 3s or 4s LiPo will be less than 500gr.

Everything you build with cylindrical cells won't reach the energy density nor the current output of a LiPo within these weight restrictions. If you desperately want to use cylindrical cells then I'd suggest Samsung INR21700-40T.

LiFePO is also viable and a logical choice, especially for high current draw, but the energy density is probably not high enough to meet your weight restrictions. The smaller cells are usually still 8Ah or 10Ah and the finished battery will be over 1kg. But it would solve the issue of the nickel strips heating up because these cells usually come with screw terminals.
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#5
https://www.imrbatteries.com/samsung-40t...a-battery/ -- 4s2p would be 8000mah 60amp discharge 544grams


https://www.imrbatteries.com/samsung-30t...a-battery/

cheaper option:
https://www.imrbatteries.com/samsung-30t...a-battery/ - 4s2p would be 6000mah 40 amp, 376gram
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#6
All cells get hot when you push the limits... A cell that states max concurret current Will get hot to touch!! 60c easy. 18650 cells easy 80c... Wink

Lipo 4s pack is what i would go for. Look for a turnigy or other from for instance hobbyking. Dont buy knockoffs.
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#7
Yes sir. Lipo is great. But the problem is we cannot import lipo directly to India. We contacted hobbyking for lipo. But they can't ship ship lipo to India due to regulations in India.if anyone knows some different sellers who sells lipo to India please do mention.

(01-13-2019, 07:11 PM)Jason Morris Wrote: https://www.imrbatteries.com/samsung-40t...a-battery/ -- 4s2p would be 8000mah 60amp discharge 544grams


https://www.imrbatteries.com/samsung-30t...a-battery/

cheaper option:
https://www.imrbatteries.com/samsung-30t...a-battery/ - 4s2p would be 6000mah 40 amp, 376gram
They don't ship to India.?
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#8
Well, what do you have available in India? If this is a general problem then it doesn't make much sense to list things you could do.
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