Stupid newb question
 

So, I am running a 3 cell lipo pack and under the heaviest load the battery hits 6.5v. LVC isn't quick enough to catch the dip. 3 cell lipo. When I am done with the run the pack isn't warm because the average draw was only 2-3C, but the bursts are in the 40-50C range. Is the pack damaged? Since the pack isn't warm can I say the performance didn't skip a beat?

Good point; is the eagletree showing these drops?

6.5V on a 3S pack is pretty deep; what is the capacity of these cells? you are mentioning 40-50C, but what current are we looking at?

It's not good for a pack to drop bellow 3-3.2V per cell; it shortens the lifespan, but so does overheating the packs. When the burst are very small, the pack simple hasn't got the time to heat up.

It's obvious that the cells are not up the task.

Yes, eagletree logger.

Here is the discussion, it will shed more light on the situation. http://www.rccrawler.com/forum/showthread.php?t=103639

I would like further opinions on the subject. I am well versed in lithium battery construction and handling, and as most in this hobby I have preconceived notions about how a pack should respond to use/abuse. I have one opinion, others see it differently.

what capacity are these cells you used?

As you also know; dropping bellow 6.5V on a 3S pack is simple killing batteries. You should consider an upgrade in available power.

If it were my pack, I'd continue running it this way until signs of damage presented themselves: puffing, weak cell(s), decreased capacity, increased resistance, etc. But of course that means the pack will need to be monitored pretty carefully while charging, to ensure safety.

Thats pretty low. As already asked, I would like to know he specs of the pack (capacity, current rating, etc.).

I agree with Serum with the whole "upgrading available power" thing. Time for some FlightPowers?

3S 1550 MA cells

Low Volts: 6.33

High watts: 696

High Amps: 101.62


mamba max ESC, crawlmaster sport motor, stalling conditions enough to make the motor puff a wisp of smoke.

Thats almost a 65c burst for that pack. No wonder the voltage drop is so low. I dont know of any lipo that can withstand a 65c load. It would seem the batteries are significantly under sized.

Double the pack capacity IMO. That way the spikes are only 30-35C

And yet when I suggest the batteries are undersized I am accused of attacking the test results with a closed mind- 'because it is the motors fault'. Impressive that they will burst the amount, until the voltage is taken into account. Maybe they should be labeled 65C cells now?

Jeeze, and your tranny/axles/tires can take that much torque? That crawler must be god-like.

One way to tell is to double the capacity of the battery and rerun the test.

I noticed in the test that with the 840mah (I think that was the capacity) the amp spikes were lower than with the 1550mah pack. It looks like something is happening to cause the system to draw as much as the battery can possibly put out. If you double the pack capacity again and the amp draw continues to rise I would say there is something else worng besides the battery. Something in the system is causing the battery to basically dead short and just dump everything it can.

The ax10 is pretty stout.

Keep in mind these are stalling tests, performed by Jason @ MA

My thoughts exactly. The data indicates that more capacity is needed. The testing is basically turning into "dead short the battery" testing at this point.

I'm just now reading through the thread. Strange.

Strange indeed. I don't know what I did to piss him off, but I figured I should try to drop my preconceived notions about battery usage and see what other savvy users think.

Can someone link me to the thread?

I find it funny that most of us users (Serum especially) have been blasting MaxAmps for discharged graphs like no other, yet they just wont put out.

Reason probably is that they don't want to make it shown that their packs are grossly overrated.

Anyways, one suggestion for the current draw is because the motor is trying to get X watts. As it draws the current for it, voltage drops, which then requires further current to get X watts. However, this further current FURTHER drops voltage, requiring more current, and it goes round and round until the pack voltage is down like no other and the pack is dishing out tons of current.

Either way, in the end, sounds like a need for a more capable pack.

Post three, I link the thread.

lol...it DID sound like Jam was getting pissed off...

Ask him to do the test again with a 5000mah pack or something. Something that will undoubtedly provide PLENTY of current.

Good idea, but as you know these motors have resistance, and they work off of that. So if voltage drops, current will also drop. In the case of a stall, it's the same thing: as voltage drops while the motor is stalled, current also drops, and the torque output drops ('cuz torque output is proportional to kt and current).

at a certain point the pack can't deliver more current, due to limitations of the internal resistance. This internal resistance also heats up the pack during discharging.

My main concern is the micro trauma that will happen inside of a pack on such large bursts. Sure, the pack may not be warm after a 60C burst, but that doesn't mean that small burns didn't happen on the thinner spots of the lithium layers. On top of that, every user runs a different rig, a different terrain, and a different driving style. While most will know when to let up, some kids may not let off the throttle when the rig gets bound up and end up smoking, puffing, or flaming a pack that is fine for other people to use.