acexxxoasis

My Feedback: (4)

Hey guys!
So I was running my pig truck yesterday. It was going great truck was running fine and when it started slowing down I pulled it over shut it off when I got inside I took the body off and I see my battery puffed up BAD like a balloon so I took it outside and put it in my bucket of sand so I need a new batt. It was a blue lipo 2s 5000 Mah 40c it's an scte 4000kv geared 15/40 temp was 155 on the motor so is this too low of a C rating? That would be 200 amp burst right? If I went to a 50c battery would that help out?
If I went with a 3 cell would I need a higher c rating or would lower be ok? My understanding of electricity is higher V = lower A draw to do the same work. Still trying to work the bugs out of this thing. Thanks ahead of time.

t9dragon

My Feedback: (11)

The battery you were using sounds like it was fine, but you need to set the LVC on your ESC for 3.4v per cell. Otherwise you are going to ruin another lipo.

cumquat

+5 exactly what i was going to say.

acexxxoasis

My Feedback: (4)

-10 I HAVE the cut off at 3.4v/c and I stop driving as soon as it drops in performance. And I averaged 18-20 minutes runtimes every pack the only time it was ever set below 3.4v/c is when I was trying to figure something out with it weeks ago I then set it back to 3.4v/c.
This truck is a pain in my you know what. But I guess it's par for the course... Motor went out servo went out diff went out whats next oh yeah battery :-/
My question still wasn't really answered.
If I were to grab a 3 cell I could run a lower C rating correct?

(still remembers when 2400 nicd was the best battery you could get and people would spend hours cycling and zapping and matching cells to get 5 minutes runtime that all seems simple now :-/)

t9dragon

My Feedback: (11)

What truck do you have?

Eman77

This is not true, as C rating is directly related to the capacity of the battery, not the voltage. So a 2s 5000 mAh pack with a C rating of 40 gives the same amperage burst as a 3s 5000 mAh pack with a C rating of 40.

That's because Amps = C rating x Capacity

Amps = 40C x 5 Ah > Amps = 200

So both packs (3s and 2s) with a 40C rating and 5000 mAh capacity can discharge at 200 amps.

(also, for example, a 2500 mAh pack would need a C rating of 80C to match the discharge rate of a 40C 5000 mAh pack). It's all about capacity.


Here is a nice writeup on LiPo batteries, there's another page on the site for picking the right charger (as an FYI):
http://www.rchelicopterfun.com/rc-lipo-batteries.html

acexxxoasis

My Feedback: (4)

@t9dragon it's a losi scte
@eman77 thank you that's what I was going for.

As I have learned electricity it's always been v/a•r and
If voltage went up and resistance stayed the same amperage went down
But C rating is not a direct correlation to amperage which is where I was mistaken.

cumquat

actually, he is sort of right. some motors pull a set wattage rather than current. with these motors a higher voltage would allow a lower c rating and even increase run times by a small margin. at 200A, a 2s has a peak continuous wattage of 1480 whereas a 3s has a peak of 2220. so if the motor requires 1400 watts, you can run a 30c with little issue.

Eman77

Right, glad I could help.

Since C rating is something the battery mfr "measures", and it's just a made up number related to capacity, that's why the direct correlation is made. (Or so I think...if I'm thinking about this right).

Now, voltage may well play into what a pack can or can't do discharge-wise, to get that C rating (I have no idea) - for example, is it easier to create batteries with higher C ratings when the voltage is higher? I don't know. All I know is what the number means to a consumer like me, when I'm trying to figure out the discharge rate, etc based on what they already measured.

Would it be easier if they just listed the discharge rate in terms of Amps (instead of a fictional C number)? Probably.

Eman77

Good point, since a higher voltage pack runs things more efficiently.

I guess I was just trying to answer the basic question "does a 3s 40C pack have a higher discharge rate than a 2s 40C pack?" - it does not.

But what your system NEEDS, as you pointed out, is a different question. Which might be more what the OP wants, come to think of it.

Eh, just read the link I posted, and learn all that stuff from there.

cumquat

wait a minute, i was right?! THE END IS NEAR!!!! GET YOU'RE TIN HATS AND WIFFLE BALL BATS! hay that rhymes!

Ttowntoolman

Never ever saw a lipo puff from over discharging to date. I have had a lipo puff up after an extreme impact......and I have had a lipo puff up from charging them to max capacity inside the house, and then letting them sit out in the sun in my pit area. I have heard of a lipo puffing after a bare spot in the wires caused a short. I have had a lipo get so hot that the deans conector melted right off the wires while I was racing, but no damage to the cells. Let us know what you find out if you find out anything for sure.

Link119

My Feedback: (1)

No motor runs a fixed wattage, its a wattage limit that the motor can handle. If there is a wattage rating on the motor, the rating is top wattage pulled. Every motor has a fixed amount of resistance in the windings which determines the top wattage. My raising the voltage to 3s you need a higher C lipo.

Eman77

How so? I don't understand why you need a higher C rating for a 3s compared to a 2s LiPo, if they have equal capacities. If they have the same C rating, same capacity, then they have the same Amperage they can deliver.

Are you saying you need to provide more amps from the battery when running a higher voltage, because the system will demand more amps at that higher voltage?

2walla

My Feedback: (10)

The C rating of a pack is its ability to HOLD RATED VOLTAGE UNDER A LOAD. IT HAS NOTHING TO DO WITH THE BATTERIES ABILITY TO STAY COOL AND NOT PUFF... If you don't give your battery adequate cooling it will puff even under low load conditions. IE a 10C battery 1 cell 1000Mah pack will hold rated 1cell voltage under a 10 amp load under test conditions.. Say for example you are running a 10C pack and everything is fine. Truck runs ok.. Now you buy a great 40C pack from Hobbyking cause you can afford one now and put it in your truck... Now all of a sudden your truck melts down.. Why?? because the 10C packs voltage was sagging under load and everything was happy.. With the High C rated pack the voltage doesn't sag and the battery keeps giving the motor the current it wants at rated voltage... Now she melts down.. Remember our motors really mimic an AC motor, and will try to turn at the unloaded speed defined by the frequency of the speed control and the applied voltage. They will continue to draw as much current as they can get to meet the load you put on them to try to reach that speed... If you add a cell to the pack, the motor will want to spin faster- that why you usually need to gear down when you add cells. A great setup on a 3s may be bad on 4s unless you regear.. Always check a setup with a watts-up type meter when you change cell count or even go to a newer high c battery.. .The molten pile of melted motor/speed controller you may save may be your own.

Link119

My Feedback: (1)

The motor would be pulling more amps with 3s than 2s because it is a higher voltage and the same motor.


As for the C rating, a sagging voltage means an overload of the battery's ability to pump voltage. A higher C rating means the battery voltage wont sag under a higher demand from the motor. When the voltage sags, the battery gets hot.

SyCo_VeNoM

huh?
you inhale lipo smoke when writing that?
voltage sagging is bad it will make the battery work harder, and potentially make it expand like what happened with the OP
as for the molten ESC/motor scenario if the truck is properly geared that won't happen. If anything a voltage spike from a lipo thats voltage is jumping up and down will fry the ESC faster (specially CC's due to spikes )
Higher C-rating is for constant amp draw... an ESC/motor won't take more than it needs which is why higher C-rated than you need is recommended. I've used 5000mah 35C lipos on a 35A setup a lot, and have yet to smoke one cause its geared properly, which according to your logic(from how I read it) should have burst into flames due to the batteries voltage not dropping...

As for the OP's issue lets face it crap happens. Battery could have shorted internally, got damaged from jumping, or something who knows.

a65l

My Feedback: (17)

That's not quite true. To make your truck go, the motor must create work. Work is measured in Watts. It takes a given amount of watts to make your truck go. Just for arguments sake, lets say that it takes 1000 watts to make your truck tool along at top speed. Watts are voltage * amperage. To make 1000 watts with a 2s lipo, nominal 7.4 volts, the battery will have to be able to deliver 135 amps continuous. If we up the lipo to a 3s battery, nominal 11.1 volts, the battery will have to be able to deliver 90 amps continuous.

It's simple math....

acexxxoasis

My Feedback: (4)

great responses! Thank you all for your input!
Right now I'm waiting for lipo funds to become available...
As I stated prior I never really had any issues with this set up until this last run really and the battery was definetly puffed (almost 2x the size) its sitting in a bucket of salt water right now :-/ but I did notice after looking at the battery tonight when transferring to the water from sand where the positive wire comes out it did have a little insulation missing did not appear to be able to short against anything however I'm not going to say it did not.

Now the motor is stated to be a 2000 watt motor and 120 amp Esc so at 11.1V and the 120A Esc the motor would see a maximum continuous wattage of 1332 and 888 at 7.4V so a watt meter would be good to check what the motor is ACTUALLY pulling.
But still basing my maths off these numbers
To get 2000 watts of power from the motor at 7.4V it would take 270 amps which is in excess of the 200A of a 40c
5000mAh (40c*5Ah=200) so a 55c battery "would be ideal" (55c*5Ah=275A) to "properly" keep from overloading the pack.
And a 65c 5000mAh pack would allow enough leeway in 2s configuration to have enough overhead to account for things that unloaded tests can't call for i.e. being stuck somehow.
For 3s if math serves right it would be 180A to get 2000w at which a 40C-45C 5000mAh 3s should be fine correct (40*5=200 45*5=225)
So with proper regearing a 45C 3s would be equivalent amperage to deal with the draw where as a 55C would be for 2s.
But either way it's going to be a minute before I can get a new pack especially if a higher C is needed C goes up Un proportionally to price :-/

Edit

Btw basing all math off ohms law and watts law
V/a=r
V/r=a
A*r=v

W=a*v
A=w/v
V=w/a

acexxxoasis

My Feedback: (4)

Anyone use the turnigy nano-tech hi C lipos? They seem cheap but at $ I can afford hard to pass over [link=http://www.hobbyking.com/hobbyking/store/%5F%5F17269%5F%5FTurnigy%5Fnano%5Ftech%5F5300mah%5 F2S2P%5F50%5F100C%5FHardcase%5FLipo%5FPack.html]This 2s2p[/link] very well priced, but if the pack is garbage don't wanna spend $ on it :-/

SyCo_VeNoM

seen a few people say they loved em on the forums, can't remember seeing anything bad written about them

acexxxoasis

My Feedback: (4)

Ok I'm thinking of ordering some at 30-50$ they are still half any other pack I've seen

2walla

My Feedback: (10)

[quote/]
huh?
you inhale lipo smoke when writing that?
voltage sagging is bad it will make the battery work harder, and potentially make it expand like what happened with the OP
as for the molten ESC/motor scenario if the truck is properly geared that won't happen. If anything a voltage spike from a lipo thats voltage is jumping up and down will fry the ESC faster (specially CC's due to spikes )
Higher C-rating is for constant amp draw... an ESC/motor won't take more than it needs which is why higher C-rated than you need is recommended. I've used 5000mah 35C lipos on a 35A setup a lot, and have yet to smoke one cause its geared properly, which according to your logic(from how I read it) should have burst into flames due to the batteries voltage not dropping...

As for the OP's issue lets face it crap happens. Battery could have shorted internally, got damaged from jumping, or something who knows.
[/quote]



Sycho venom. I suggest you get a bit more education on how electric motors work before you offer advice.. 1)Lipo batteries puff because the get too hot.. either from too much load or not enough cooling or both..
2)An ESC/ motor will take as much as MUCH CURRENT as it can get to try to run at its syncronous speed. If you overgear or overprop it can and will draw excessive amps and melt down the weakest link in the system .. Most newbies to electric find out they are overgeared/ overproped when they get their first high C battery pack and put it in an existing setup and find out when something melts because the voltage now doesn't sag and the motor will keep trying to draw as many amps as it can get to try to get up to RPM.

From what I have seen, the higher C rated packs seem to take more cooling. I have several of the Turnigh Nanotechs and they puffed in the same plane that I have been using the regular 30c ones in with no issues. I have a castle data logger esc in it and the flight that puffed the batteries doesnt look any different than the ones with the old 30c cells.. Friend has seen the same thing too.. The best batteries I have seen so far as far as cost/puffing/ performance are the Sky 40c batteries .. you can abuse the heck out of them and they dont seem to puff..

SyCo_VeNoM

1 learn to misspell my name properly...

2 you are wrong in how electronics work...
It will take as much as needed not as much as it can get... Your house generally has 15-20A breakers in it that does not mean your TV will pull 20A's? No it will take as much as needed and carry on. Same is true for motors/esc's. They over heat due to trying to get more work out of them than they are meant for. To do the extra work they need to pull more current which in turn adds more heat than are designed for leading to them melting.

A higher rated pack will not get as hot as a lower rated one seeing it is doing less work, and hence will require less cooling. The same goes for ESC's, and motors higher rated than the task the less stress is put on the system, and less heat generated.

Also were not talking about a plane which is generally a pile of foam that is an insulator(yes I know there are balsa ones, and others but most common ones I seen use a foam fuselage), and holds in heat... Most ground based RC's have the batteries exposed to where the air flow is so they don't get hot unless they are being overly taxed...

Also we go lipo so the voltage WON'T sag If we wanted sagging voltage we would have stuck with NiMh. Unlike you flyboys we have more complicated ESC's (yup we can go backwards) that current load varies more. So we are not always running at one voltage like how you guys move the throttle up, and don't touch it. Our ESC's stop, brake, and have to deal with backwards EMF (a lot seem to pump that back into the battery) so if the batteries are not up to par they will puff, and take the electronics out with it.
Also if the current sags the ESC will pull MORE current and get hotter, and put more stress on the battery, and ESC to try and keep the motor rotating at high speeds.


Reason I'm assuming your airborn newbs melt their system is cause they went from I'm guessing NiMh which outputs 7.4V'ish and down when under load, and went to lipo which outputs 8.4V's when charged and under load. So the prop would basically be spinning faster, and having it overprop'ed. Similar happens in ground based going from NiMh to Lipo but its no where near as bad seeing we can vary our speed easier and not have to worry about our RC falling out of the sky.
There are a few crappier ground based RC's(talking bottom of the barrel brands) though that are poorly designed that use NiMh's voltage droop as a tool to keep the motor, and ESC cool, and they will fry on a higher rated battery, or Lipo. But for majority of us airborne impaired people the manufacturers taken that into consideration with our way higher priced electronics (just looked a 80A flying ESC is $30 vs our groundbased ones are $90ish for a semi decent brand).


Edit was the last paragraph

domizgood

Ok read all this and I lean towards the bigger/powerful the battery the easier the esc and motor have to work.
What's the best batts for a savage flux? stock wheels, esc and motor but have the 25t pinion (coz the grub screw rounded off in the 20t)
What's the best batts for the 20t pinion?
Both set ups for 6s. I've got 2x3s 6000mah 30c or 35c can't remember right now turnagy nano techs. They seem fine and run well with both pinions. What do you think.