CHEAPO Lipo's - Questions about Amps and C rating

[Demon-TC3]

Hey all,
i was brousing the Bay and came across the cheapest Lipo's ive ever seen - 4000mah 4s Lipo (10 - 12c) for about $25.00 shipped.

so, i bought one without really thinking (had a 20$ eBay voucher that was about to expsire so, oh well).


Now, im planning to run this in my truggy, but looking at it, 10 - 12C means that it can put out 40 - 50 Amps and the Feigao 9XL pulls a max of 74A (according to Feigao).


So, does this mean the cells will explode / swell up under use or that it just wont perform as well as a higher C rating? i mean, at this price i will just run 2 in paralell and gain 8000Mah of run time and 80A discharge...


take care all,
Demon

[GSMnow]

From my little LiPo experience, you will lose a little performance due to the higher internal resistance of weaker cells. The real danger is if you demand more than 12C from them, the internal temps will climb fast and the cells could swell up and possibly fail. The cells I have seen with a 10C/12C rating means 10C constant, and just 12C maximum for like 5 seconds. My Max Amps 2S 3000HV packs are rated 20C/30C/50C 60 Amps constant, 90 amps for 30 seconds, and 150 amps for 5 seconds. And with a Mamba Max 5700 I still manage to get them warm in a normal run and after 2 cycles in a row from 8.4 volts to cutoff, driving hard, it got a bit hotter than I would like.

The Feigao 9XL 74 amp rating is not the maximum it will ever pull, it is the maximum it can handle long term without overheating. Acceleration bursts, climibing a hill, overgearing etc. can all make it pull much more current, it is just a matter of how long it is exposed to these increased currents. On a Mamba Max ESC at 4S it could easilly hit 150 amps for acceleration bursts.

[Demon-TC3]

Oh dear :p

Well, i will get the pack, see how it goes, if it is a bit warm after a run i will consider getting 2 packs in paralell or something, that should really help things out right?


take care,
Demon

[GSMnow]

Two pack in parallel will cut the resistance in half as far as the ESC is concerned, so it will certainly help out any current deficite. And as far as each pack is concerned, they will only see about 1/2 the current. How well they share is dependant on how well matched they are. I would expect the same temps out of the weaker of the packs at about 1.8 x the current. So for safety, your 10C 8000 2P with non matched cells should be fine to 64 amps. 80 would be in if they perfectly shared the load. Once you get the voltages matched and then tie them in parallel, LiPos will stay voltage matched very nicely. But if one cell has less capacity, it's voltage will internally fall a little faster, and that will make the stronger cell supply more of the current to keep the voltages balanced.

As always, monitor your temps and if they are okay (under 120 F on a LiPo pack) you should be fine for a long time. Even 40 amps is alot of power. I only average 20 amps on the race track. Sure, I peak at 80 or so, but since it runs over 10 minutes on just 3000 mah, it has to be less than 20 amps average.

[Demon-TC3]

well this was only for my basher, i still need to pickup a balancer and monitor etc.


the only thing i dont have at the moment is a LVC device, i was planning to put in a 4cell lipo monitor (lights go red as soon as a cell is reaching low voltage point / is un matched) and was sticking one of those into the balance port.

for now all i have is a charger capable of charging Lipo's everything else needs to be ordered.

wont use the pack untill its all here tho...allong with a liposack!


take care all,
Demon

[GSMnow]

I certainly have to recomend having a low volt cut, I guess the lights will work, as long as you can see them clearly. The performance of my LiPo does not fall off at all, it still pulls wheelies, then I get just a half lap of weak acceleration and it stops. If the low volt cut of the Mamba Max was not there, it would not have the sluggish half lap, it would just keep pulling strong as the cells went well under 3.0 volts per cell.

I went with the Method Balancer from Max Amps when I bought the packs, and us it for virtually every charge. It is simple and just plain works. The real nice part on the Method, is that the charger goes through it so it can actually open and stop the charge if the voltage of any cell goes too high. This makes it a great safety device. My packs run so cool while charging, I am not concerned about a LiPo sack. I do have my packs on a 12x12 ceramic tile, just incase something happens. The pit tables at the track are seiously dried out wood. Even when I get a pack warm in the car, I just lay it flat on the ceramic tile, and it cools off pretty fast.

I also believe that balancing at full charge is more important that what they do at low voltage, as long as the lowest cell does not go below 3.0 you should be fine. I set the cutoff on my car at 6.2 volts, so the cells could be at 3.0 and 3.2 and still nt hurt anything. I have never seen them even get close to that far off. They are usually within 0.05 volts of each other at any charge state. And if the capacities were off, by even 20%, balancing at full charge is the best. At low volt cut, one cell will be dead at 3.0 and the other will still have a 20% charge at 3.2 or so. Charge from there, and they will still hit full charge together. If you balanced them at end of run, the stronger cell would be pulled down to match at 3.0 volts, and then when you charged up, the weaker cell would top out first, and the balancer would have to pull current on the weak cell to keep it from overcharging, while the stronger cell tops up. I have to recomend NOT balancing after a run, only when charging.

[got2bqik]

GSM your going to need a lipo sack at a ROAR event if you chose to run Lipo.It's in the rules.
Tim

[GSMnow]

Is someone a little too paranoid??

First they require packs that can be dropped on concrete without damage.
Then they must hadle a 0.1 ohm discharge til dead without rupture.
And they must withstand overcharging to 12 volts 3.8 over max without rupture.
And you still need to charge it in a LiPo Sack????

Does all of the nitro fuel need to be stored in bomb proof bunkers?
Do you neeed an SFI fire suit to fuel the cars in the pits?

P.S. I would rather see a cell start to puff than have it hidden in a sack

[SS Pede]

I'm inclined to agree with you there GSMnow. I wouldn't be surprised if a NiMH pack detonated when charged to 12 volts!

[got2bqik]

It's as if roar and lipo manufactures are in cohoots. Diggin deeper and deeper into our pockets.
Tim

[anthonyho]

Hi GSMnow,

I hope I didn't break the rule in sidetracking a bit here for questions not directly following the topic. Firstly, I noted that people are talking about 5 cells these days which I can't quite follow. As 5 cells gives only 6 volts, less than 7.2 volts that 6 cells produce, the power should be less, right? Does it mean that the touring cars running on the tracks are actually overpowered so that they are going for down powering in exchange for lighter weight?

Secondly, I am running an ESC with max output of 120A matched with a 3.5R sensored brushless motor, and have two kinds of Lipo pack in stock, one being 4000mah 15C and the other 5000mah 25C. My understanding is that the 5000mah 25C pack should give the maximum power my ESC needs but as it doesn't fit (a bit too big for the battery compartment) into my car very well, I wonder if I can use the 4000mah 15C instead. Apart from the lower current (60A versus 125A) it produces which I don't mind, would it be unsafe to use it for my current configuration, e.g. the Lipo may get overheated? If yes, would it help if I reduce the pinion ratio, i.e. lower gearing?

Your enlightenment is much appreciated.

[GSMnow]

In an ideal world, you should run a data logger on it to see how much current you really are pulling. I am probably going to buy the Eagle Tree system, it is going for just $65 at Tower. Worth every penny. The important thing is the max instant peak needs to be under what the battery can handle. The constant average current should not be a problem at all, since 15C means running the pack dead in just 4 minutes. My 20C means just 3 minutes. I get over 12 so I am not worried about long term current, only the peaks. Do you have a spec for the peak current on the 4000 15C pack? The 15C rating should be a constant, which matches my 20C 3000's at 60 amps, but my sustained is 90 amps and burst is 150 amps. I trust running my packs pretty hard, and just have to watch the max temp. I may go down a tooth on pinion to be safe, as I did get it pretty warm with back to back 12 minute runs.

The 5 cell rule is pretty silly. But I did run my Mamba Max 5700 on 5 cell NiMh and it still made gobs of power. I went up from a 16 to a 17 pinion, and it was plenty fast. I am running a 15 now with the LiPo. On our local oval track, they used to run 27 turn stock motors on 4 cell. For the new brushless world, they decided on 21.5 turn motors on 2S LiPo to get about the same performance. In fact they are going faster. Man they run huge pinions in those pan cars.

[anthonyho]

Thanks GSMnow for your reply. I don't have the spec for the peak current other than the 4000 capacity and 15C rating. The reason I ask is because I used to have it powered up a 6.5R motor and the pack is dead cold after the run. But when I recently changed to a 3.5R motor (with the same ESC), the pack becomes much warmer (comparatively speaking) when I unloaded it from the car. So, I just want to draw a safety line that I shouldn't attempt to go beyond or simply not using it for that application at all. I used it on my other ESC with 60A output and it is dead cold after use.

For the 5 cell issue, am I right to understand that the better performance or speed comes from the lighter weight by the missing cell?

[GSMnow]

If you are not at minimum weight, the loss of the cell might helpa little, but most modern cars need ballast to make weight, so that is not as much of an issue. Not having the weight in the batery does let you put the weight where you want it, so that can help, and also having only 5X series R vs 6X series R could allow for a little more current at low speeds. This did result in a bit more punch out of slow corners for me. My pretty good 6 cells pack drove okay, but when I put in the 5 cell it came out of the slow corner harder, to the point I pulled a wheelie on just an infield short chute. And with the Mamba Max 5700, the lower max speed, also makes it easier to control on a tight track. I did have to trade off some of it, as I was then being beat on the long straight, so I went up one tooth, but it still had wheelie torque. The increased grunt was certainly not in my mind, I turned some great laps on 5 cells and my motor temp was up due to the higher currents.

Power is voltage x current. Current makes heat. To get the same power to the wheels on a lower voltage, you need to pull more current. So everything ran hotter, and the run time was shorter.

[anthonyho]

Thanks for your sharing from which I learned a lot more about physics than I did in high school! :-)