Maj_Overdrive

My Feedback: (11)

Back to ratings for a second though. Yeah there needs to a standard for rating batteries. Whether it's holding a certain voltage under a certain load, temperature or whatever. Then you could compare apples to apples instead of varying degrees of bs.

I dont think Kv is as suspect. Kv is always rated unloaded. Kv will vary slightly but this is due mostly to manufacturing tolerances, variances in how the motor is wound, exact magnet strength, etc. Now power ratings, peak wattage, etc that is more suspect. What's acceptable? Sure the windings can handle 2000watts going through them but the motor will be 250f in 20 seconds killing the magnets. Even Castle admits that the 90% efficiency it has for some of its motors is only at a power level the motor cannot reliably sustain continuously.

Which brings us to why lipos (and esc's) survive even though they shouldn't. Google some Castle esc datalogs and it becomes clear. Motor draw is nowhere near constant with only peaks approaching or meeting the rating of most packs. Obviously these peaks don't last long and average draw is much lower. An esc and battery may be 150amp rated but neither would survive long trying to sustain that level of current. But they can take it when the peak amp draw is 150amps while average amp draw is more like 80amps. But even that average draw of 80amps is just an average of a single application of throttle that lasts what, a few seconds? Then you release the throttle for a turn or stop the vehicle completely.

So so you mash the throttle from a stop, amps rise fast, hit a peak of 150amps for a split second to get the vehicle moving. Once it's moving amps drop down to around 100amps, fluctuating as the vehicle finds, loses then finds traction again, until tapering off little more. Then you release throttle or brake before doing it again. And taking off from a stop cause the highest spikes in amp draw, if you're already moving amp draw isn't as high. So while on the throttle average amp draw is more like 80amps which the esc can dissipate and the battery is more able to handle. Over the course of a whole pack though average amp draw is even lower, maybe 30amps, due to the breaks in acceleration while turning and braking. This is why boats and planes are harder on lipos, they have a more constant draw that's also higher due to the amount of drag on them.

Testing. Constant amp draw like you mentioned is key. The Big Squid video I mentioned was a simple test but amp draw was not constant. It was 2 automotive battery testers, the kind with the coils and the sweeping analog gauge, a multimeter to measure voltage and another to measure current by inductance. I forget the exact number of amps it drew but I believe it was 60amps and was definitely right at the packs constant c rating. Either way at the start of the test amp draw was 60amps, but amp draw wasn't constant and dropped as the packs voltage dropped. The load needs to vary to keep amp draw constant. The load center required to maintain a constant amp draw is a specialty item and not a cheap one, and why I used the term "laboratory spec".