sdp30

Was looking at a few lipos and some said 20c 25c and others said 12c which one is best

guver

My Feedback: (325)

25C is best ^ is best.

kistner

My Feedback: (10)

if I might add to the question, what is the minimum acceptable for our uses? will 12c work, or 15c, or . . .

shaunmichaeljohnson

there is no "best"

the C rating is relative to how many amps you can draw from your pack.
so for exampel my rustler VXL motor can draw 65A continuous (any more and it'll fry).

to get the maximum continous amp draw you devide the capacity (in mah) by 1000, then multiply the number by the C rating to give you the amp draw.

eg, 4000mah pack = 4000 \ 1000 x 12c = 48 Amps

so if you have a current draw on average of say 60+ amps then you're gonna want somthing like this...

8000mah pack = 8000 \ 1000 x 12c = 96A
or
4000mah pack = 4000 \ 1000 x 25c = 100.

so it's all about capacity to amp draw.
if you have a low capacity pack then you'll be chasing a high C rating,
if you want a big pack (8k or more) then you can get away with a lower C rating in smaller cars (rustler, slash etc)
and if you have a big car (revo, E-maxx or E-8ight) then a high C rating AND capacity is gonna be needed.

hope this makes sence
shaun

kistner

My Feedback: (10)

so what your are saying is you would never go higher than a 15c 4333mah (if there were such a battery) battery for the vxl motor? or a 10c 6500mah? etc.

guver

My Feedback: (325)

The figure is a max for batteries , so is a 15C or higher.

I would say "never go lower than"

Snaut Rocket

hey i got a question.
i looking at a 8000 mah with 80 amps contious and 120 burst.
the mamba max says it can handle 100 continous.
How much can the mamba max take in burst?

Snaut Rocket

and another ?
if i have 2 batterys with the same c rating and rum parralel does that double the c rating.

guver

My Feedback: (325)

It seems many people get this backwards. The battery needs/can be sized with a higher current capability than the motor (can pull)

The motor pulls current, the battery doesn't push current.

guver

My Feedback: (325)

Yes.

kistner

My Feedback: (10)

so I am now more confused than when I started reading this thread.
talk to me like I am stupid because I am just not getting it -
is there a minimum c rating that would be considered adequate for our uses - yes or no?
were the nimh batteries in the 10c range? yes or no?
is there a max battery size per configuration in regards to c rating and MAH(which would vary depending on your parts) - yes or no?
MAXIMUM battery size doesn't matter to the extent of the motor pulls the current - yes or no?
not trying to be a smart *****, I want to understand

guver

My Feedback: (325)

The rating must be combined with the capacity of the pack since "C" stands for capacity (see Shaunmichael's response)

The capacity multiplied by the "c" number gives the total so you need a combo that equals your max current of your specific set-up. I suppose that's a no?

NI batts can range widely 2C to 30 C probably. I guess that's a yes.

guver

My Feedback: (325)

Maybe I can make it simpler:

Find out max and average current of set-up with desire voltage.

Select largest battery that will fit with desired voltage making sure it has at least the current capability by multiplying the capacity by the actual number that is before "c"

It's a multiplication problem 20C , 10C ect. The c is the capacity and your product needs to be larger than your current of your vehicle.

Maybe I made it harder ,lol.?

Chris_RC

Im not battery expert but I dont see how this can happen. I would think you would get twice as much capacity but not C rating. If that was true, wouldnt racers run 2 batteries in parallel because that is about the same weight as nimh battery but it will have 60C? (30CX2)

Read this thread (there is also a page 1) and I think you will understand. [link]http://www.rcuniverse.com/forum/m_7898973/mpage_2/tm.htm[/link]

I bet the Orion SHO 5300 nimh cells get 35-40C. They are extremely powerful and get great runtime. Only thing is that they are around $120 for a assembled battery or $85 for 6 loose cells!!!

Access

2 batteries in parallel means twice the capacity, yes. It doesn't change the C rating per se. But with two batteries, you can safely supply twice the current than one of that same size normally could.

ie. a 5000mAH, 20C battery supplies 100A. 2x 5000mAH, 20C batteries in parallel would be a 10000mAH, 20C battery, it would supply 200A.

Most NIMH are good for 10C and some of the high-end ones 15C or so.

The "C" thing is explained well enough above. "C" = capacity. "20C" means 20 times capacity.

And if you want to get into the fine details you actually lose a little bit of C rating when you put two batteries in parallel. Especially if the batteries were treated differently in the past, or are at different parts of their life cycle. With two batteries in parallel, there is no gaurantee the internal resistance is 100% equal, so depending on the difference you may only be able to safely draw 1.9x or 1.8x the current or so.

Chris_RC

So the C rating wont increase, just teh amps. got it. About getting cells balanced how about 4 cell lipo balancer?

Snaut Rocket

so then i shouldn't run 2 2s 8000 mah lipos in parallel if there continuous amp rating is 80+ with the mamba max, I just want to know so i know if i need the mmm or not.

Druss

wow, there is a lot of info flying around.

The "c rating" is a function of the battery but not the capacity. The mah is the capacity (C), the "c rating" is just a way for the manufacturers to tell you how quickly the battery can "give up" it's charge. That's why it's important to get well matched cells. A battery made up of 3 cells with different "c ratings" would discharge at different rates and therefore would become unbalanced.

So the C of a battery is the capacity in mah (milliamp hour), the c rating (factor by which you can increase the output) is different and let's not confuse them. A 5000mah 20c battery can output 5000ma in 1 hour but can increase the discharge rate by a factor of 20. So the amps the battery can output is equal to the C x c rating / 1000. So a 5000mah 20c battery can put out 5000 x 20 / 1000 = 100amps. If you're wondering why we divide by 1000 it's to convert milliamps into amps.

When you put two batteries in parallel with the same c rating you double the C (capacity) but not the c rating. So 2 2S batteries with a capacity of 5000mah and a c rating of 20 can produce 10,000mah at 20c which is 200amps.

It was correct when it was stated that the batteries don't push, the motor pulls. The motor has an amp rating, that is how much the motor can pull. You need both an ESC and a battery that can meet it's needs. So a motor that can pull 100amps needs an ESC capable of handling 100amps of continuous current and a battery capable of outputing 100amps continuous. If your battery can put out 200amps that's fine but it will only be asked for 100amps.

shaunmichaeljohnson

a C rating is just a number for the formula.
it's just that, a number, you cant increase it or decrease it, it's like part of an algerbraic equasion.

pretty simple when you understand.

Access

No, if the cells are in series, there is ONLY ONE PATH for the current to pass through. As the whole battery is discharged, each individual cell must discharge at the exact same rate. If the C ratings of a series group of cells are different, that only determines things like temperature increase and voltage drop. A series group with different C ratings is far from optimal as you can only safely run the battery at the lowest C rating of any given cell. But it's more important to match capacity itself than the actual C rating (if the cells are to be discharged down to 3V).

Where do these myths come from. You can't change the laws of physics. Basic electronics theory; series = one current path (only) and voltages add; parallel = multiple current paths but voltages are the same.

The Amp rating of a motor is how much current that motor can safely pull. It's ultimately the resistance and impedience that determines how much current it is pulling at any given moment. And motors don't draw continuos current b'cos of that impedience. I know they call it DC brushless or DC brushed but in reality it is an AC system. Any system with a motor is really an AC system once that motor starts spinning.

If you want to design it safely, you have to look at both RMS currents and peak currents. Not continuous currents. Those are for DC only. You do the peak current like a DC system (straight resistance) and RMS current based on the manufacturer ratings and such. But if you neglect peak current, you do something like run your vehicle into a wall at full throttle by accident and if the wheels happen to stop spinning before you let up on the throttle, the ESC will go poof and die. Thermal shutdown probably won't save it at that point.

Dawman

My Feedback: (33)

Boy am I confused now . I always thought it was like Druss stated . But what Access states makes sense also .

happywing

Sure takes a lot to say more "C's" is better. Bottom line.

Chris_RC

Same here. [&:]

Access

Okay let me post this example to try to clear it up.
Say you have a Neu 1506/1Y motor 0.0030 ohm resistance; a mamba max ESC, 0.0003 ohm resistance, about 2 feet worth of 12awg copper wire (0.003 ohm). Total resistance 0.0063 ohms.

The battery is an 11.1V (3s), 4000mAH battery with a 5-to-15-second "burst" C rating 40C. Meaning it's gauranteed to stay above 9V for 5 seconds even if you pull 160A out of it. Approx. internal resistance for a voltage drop ~2V @160A = 2 / 160 = 0.0125 ohm

So the peak current we need to account for would be 11.1V / (0.0125 + 0.0063) = 590A.

Now look at something like the VXL ESC:
http://www3.towerhobbies.com/cgi-bin...&I=LXSDB8&P=ML
Peak current stated by manufacturer 320A, meaning it shouldn't be used for this setup. Even though the on-resistance is double that of the mamba max, that alone isn't going to drop that much.
It's fine for use with the blue "VXL" motor (which is going to saturate at a couple hundred amps anyways) but not with something like the Neu 1506.

The only electronics / physics formula I used above is Ohm's law, E = IR or sometimes also stated V = RI; (voltage) = (resistance) * (current).

One more thing, take a look at the new Losi ESC as another example
From the horizon site, Operating current 100A continuos, 540A peak, and on-resistance 0.0004 only slightly higher than the mamba max. 540A would be closer to the 590A needed and hence probably good enough (at some point even the Neu motor will saturate).

Druss

sorry, access is right in that the current flow through the circuit is the same but also consider that the c rating and capacity together determine the available continuous current and max current. The issue here then is that one cell may have to work harder, and therefore get hotter, than the other cells in order to provide the same current as the system. This is the reason that usually the one cell puffs and not all together. The weakest link theory.