Voltage sag in relation to watts
 

Hi!

I got a question about the voltage sag in relation to watts/amps. I don't know what's wrong with my LiPo's but I've seen a voltage drop to 30V. (10S LiPo, 37V nominal) Anyway... I'm pulling 4000W, 133A and the voltage drops to 30V. But, what if the voltage of my LiPo would only drop to 37V?
Is there any way to calculate how many watts I would pull? It isn't 133 * 37 = 4921Watt, or is it? Because the amps will fall, I think?


Patrick

Under load you are dropping lipo voltage to 3v per cell. That is pretty bad, what brand/spec lipos are you using? I would suggest gearing down, as you are overloading the batteries.

Is this happening early in the run, when the batteries are close to fully charged? Or at the end? You need to set the lvc higher as well, try 3.3v per cell.

Yes, I know... I'm using Turnigy Flightmax 5000mAh 5S 30C LiPo's, 4 of them. Wired in 10S2P. So they should be capable of 300A continous , but they can't even handle 130A peak. All the cells are good. I seriously don't know what's wrong with the LiPo's...

But does anybody know the answer too my question? :intello:

That is way too much voltage drop as the pimp mentioned. You might have a "bad" cell or two in your packs. Check your packs first or try new batteries.

Yeah, either your measurement equipment is off calibration (Eagletree?) Or the somehow the pack is bad.

To answer your question about power, if in fact the voltage sag is correct; then if you had less voltage drop, say down to only 37v, then your power would be exponentially more.

37v/30v = 1.233 * 133amps = 164amps * 37v = ~6000watts peak draw, probably less (this is potential peak draw, there are other factors)

Measured with a Castle Creations ICE HV 160. I've got a 150A Eagletree somewhere lying around, I could measure the voltage drop with it.

The batteries are about 2 years old and I didn't use them much. This is the only reason I can think of that they perform poorly. But I ALWAYS stored them 70% (or was it 80%) charged and made a 5 time discharge <-> charge cycle about every half year. Cells are all good, double checked those.

Thank you ZPB! I was aiming for 7kW, so that comes pretty close.



Another question: When using these LiPo's and applying full throttle as fast as I can, I feel the motor spinning up kind of slow. But then out of nothing I get some kind of turbo boost to full speed. Could this be the Low Voltage Cut-off of the ESC? (I set it to soft cut off) That when I give full throttle the voltage drops to 30V - LVC cuts in, power is low, voltage is increasing again - and then the "turbo" comes? (it's hard to explain with my limited english knowledge, sorry)

Yeah, well I would say those batteries have simply lost performance then from sitting and not doing much... lipo's have a bad time with sitting around and internally corroding. The power draw from the truck is also limited by how much traction it has.

Hmm.. crap. They probably don't perform better after a few cycles, do they? I can just throw them in the bin

I have no experience with the HV esc's, but my best guess would be some sort of soft cut-off like you said, it must cut the power a bit to let you know... instead of the typical temporary power shutdown of most cut-off's. ?

They will always gain a bit of performance after a few cycles of non-use, but when they are that far gone, it will never be enough. I just retired some Hyperion G3's I had since spring 2009, only because they had not been used in a long time. They never really lost much performance, they still have a pack iR of 5-6 mohm vs. 4 mohm when new... they lost capacity mostly, and they lost it unevenly is the main thing (2 3s packs in series for 6s), so I can't use them together; other than that, they have plento power.

Jup, that's soft cut off. It won't give you full power but only 50% or so.


Thanks for all the help ZPB :)

No prob.

Venlo, that's in the far east right? My relatives live around Oudewadering, Leiden, Leiderdorp, and others.

South-east indeed. It's only about 2 hours driving from Leiden. Holland is such a small country :lol: