C rate battery ?

[rol243]

Guys, is a 30c to 70c burst lipo better than a 60c only battery pack ?

[CraigP]

It's just gamesmenship with words for marketing. They want to put a big number on their battery to attract customers. Normally a battery that has a single rating is better, because it states the amp draw you can run at until the discharge point. "Surge Ratings" can be vague, and don't give the user a good idea for how long the battery can maintain that rate. I think the car guys might have more need for a dual rated battery. Boats pull power, all the time!

[rol243]

thanks CraigP, i have been looking for some 2s packs to make up 1 x 4s sized pack eg; 2 x 4000 mah - 4s = 1 x 8000 mah 4s pack. most of the 2 s packs i have seen have rather low c rates other than the others in question that state something like 30 to 70 c burst. i would like a 60c / 4000 mah / 2s pack.x 2 of.

[CraigP]

Well, 60c on a 4000mah battery is 240 amps... the question is: how many amps do you need? Also, basic runtime calculations:

4000mah=4ah that means you can pull 4 amps for one hour, but that completely depletes the battery 100%. You can't do that. So it needs to be pro-rated by 25%, so 4(.75)=3. So you actually pull 3 amps for one hour, and leave a 25% reserve. One hour is 60 minutes. So the runtime calculation is: 3(60)/240= 0.75 minutes, or 45 seconds! So my question is, how much current do you really need? That's a really short run time!

[rol243]

CraigP, i am only getting no more than 2 minutes from my 5500 and 5800 mah / 65c / 4s packs so i was hoping if i went to 2 x 2s packs of say 4000 mah each and a good c rate size i would get more run time and not over heat the packs. i don,t want to much run times just enough for a good race plus mill time . these packs will be pushing something like a 4060 - 2450 kv in a rigger.

[CraigP]

Ok, let's figure what you know...

Let's figure the middle of those two packs, or 5600mah. So that's 5.6AH. Now solve for the current, based on the known 2-minute runtime. 5.6*0.75= 4.2ah, (save 25%) so 4.2(60)/2=126A

This is a good method to determine your real current draw, in absence of a data logger. So 126A means your current battery is running a C-Rate of 126/4.2=30, or 30C.

The higher C-Rate can be because better conductors are used inside the battery or the battery chemistry has a lower Internal Resistance. Now if you want the battery to come in cooler, then that's more a factor of IR. Let me show you the math for that... Let's assume an IR of 0.0015 Ohm, or 1.5milliohm. The formula is: W=I^R So, 126*126*0.0015=19.85W, which will get a battery warm! Look what it is at 2.5milliohm: 126*126*0.0025=39.7W!

So as you can see, IR, is the dominant factor, although higher C-Rate may play a factor, but hard to quantify. But looking at the numbers, anything over 60C is probably money not well spent. I hope that helps you!

[CraigP]

Note: This is where fellow boaters reporting on their paks's IR can really help to decide which battery to get. They don't spec this from the battery manufacturers so it's up to us to spread the word on the batteries with the best IR.