I need help choosing a lipo battery
 

Right now I have a ecx circuit with a viper rc coper head in it right now I am using a 7cell 4200mah Nimh battery but later I will get a 3s lipo for speed runs and I was wondering what should the c rating be?

What is your motor/ESC combo (what is the exact model/specs)? The amps that will want to draw will dictate what your battery will need to supply.

It is a 3900 kv motor and a 100 amp esc I got it for 100 bucks new

540 size

At a glance, I can't find the wattage that motor wants (or amps), so let's assume you need a battery that can supply the 100 amps your ESC can handle.

In that case, C rating x amp-hours gives you amps. (so yes, capacity of the pack comes into play).

For example, a 4000 mAh pack is 4 amp hours. If that 4000 mAh pack had a 25 c rating, then it's 25 x 4 = 100 amps the battery can supply (in theory).

So for you, make sure that your C rating is high enough so that when you multiply by the capacity, you are well over 100. Maybe shoot for 140+ or so to be safe? I know no one wants to over spend, but you don't want your battery working all that hard.

Does that help?

Hopefully others will chime in.

Yes thanks

So a 5000 mah 35c battery would be over kill

In a sense, I suppose, but there's nothing wrong with that! C ratings have gone up a bit lately and become affordable, so I say go for it.

I understand your math, Eman, but I think the word "theory" is the salient point. Theory is good, experience is better. In my experience, 20-25C rated packs tend to get hot, even in my "little" 35A EZrun system. Now I suppose it's possible that Gens Ace over-rates their batteries, but I have found that 20C or even 30C 2S LiPos get well over 110F, where 40C or 50C packs stay below 95F. Based on that, I would suggest the OP get higher C-rated packs for his 100A ESC.

FOXY, what do you say?

I went to a hobby store today they said a 35c 3300mah 3s would be good

You have a fair point - for example, my 25C 5000 mAh pack gets a bit too warm in my castle 5700 setup.

I guess maybe the motto is "overkill is better". You don't need a 90C pack or anything, but a 35c 3300 pack seems a bit low. I'm sorry I don't have a solid answer to the question, but maybe we could suggest something where the math puts him at 160+ amps? (example, a 40C 4000 mAh pack)?

Fortuantely, there are plenty of mid-level C rated packs that are inexpensive to choose from. That's why it doesn't make sense to try and save $5 and get a much lesser pack, for instance.

Sorry my hobby store doesn't have that the next thing up is a 5000mah battery 25c or a 2200mah 60c

I do you think I would be fine with the 3300mah because that was what they recommend

Because it say it can put out 115 amps which is more than enough than I need

Cool mate. And it'll probably be fine. All I'm sayin' is be aware of it. Pick up an Infra-red thermometer (Duratrax has one for $25) and monitor battery, motor and ESC temps. I've had that Castle system. Monitor temps. Try an keep the battery under 100 (measured after a hard run), the motor under 180 (IMO under 170 for safety-margin) and the ESC under 125 or so. If your motor can be opened, first be careful not to loose any shims on the rotor, but inspect it once in a while, blow it out with canned air and clean and lube the rotor bearings.

That willbe hard where I live I get into the 100s in summer

My friend has a infrared thermometer I can uuse I think I will get that battery big jump from 7cell nimh to 3s lipo

Is I a viper system 3900 kv 4 pole

Yeah, I know, it gets bloody hot here too. Generally, I don't run on days where it's over 100. Hell, I don't even go outside!!
3S is going to be a BIG jump from 7-cell NiMh!!! Hope you have a good bumper on the thing and you've got some wide open space.
Shoot, you're in Texas! It's all ABOUT wide open space! So have fun! :cool:

I have this exact same motor and ran it for over months with an 1800mah 2s 25c just fine in my 1/10 2wd stadium truck. Run times were about 5-8min depend on how hard I drove it.

That is exactly what I have but I went to the hobby store they said a 3s would destroy my transmission so I got a 2s

Will a 3s 1800 25c be fine for small speed runs

I dont know what vehicle you are using so I have no way of telling.

It is a electrix circuit