Krio

iTrader: (159)

I agree, for the same motor design at half the KV, a 4s setup will be better than a 2S.

But I'm not following how you are justifying half the C rating in addition to half the capacity. Using your numbers, a 3,000 mah 4S pack at 20C can put out 888 watts (3*20*14.8) without the voltage sagging too low while a 6,000 mah 2S pack at 40C can supply 1,776 watts (6*40*7.4) without the voltage sagging. The 4s setup may be more efficient, but it isn't twice as efficient.

*disclaimer, this assumes that the C rating on the pack is accurate which is an entirely different debate.

billdelong

iTrader: (33)

I'm basing the C rating requirements on my personal experience... 30C is sufficient for 4S packs in my 1/8 cars without any excessive swelling over normal wear and tear in club racing. I have gone with higher C rated packs, but that's only when I need to charge at higher than 1C charge rates. In 4WD SC, anything less than 60C with 2S is asking for trouble, lower C rated packs will swell in no time, but 60C+ packs will at least last 6+ months before the IR starts to go up.

The goal here is to define a cost effective solution... what's the point of running 2S-30C if the pack's IR blows up within a handful of race days

tvih

I haven't managed to swell even low-C 2S with 4WD SCT myself... even the 20/40C 4000mAh pack I had. but I mean, you can't halve both C and capacity, since the overall output of the battery depends on both. If you halve both, you end up with 1/4 maximum draw. As in:
5000mAh 40C = 200A
5000mAh 20C = 100A
2500mAh 40C = 100A
2500mAh 20C = 50A

So even given the 4S being better, you'd be comparing 5000mAh 4S 20C to 5000mAh 2S 40C in terms of perfomance. It's not exact and the 4S will provide longer runtime with same mAh rating, but certainly 2500mAh 4S 20C would in no way match a 5000mAh 2S 40C in current draw or runtime. Assuming accurate C ratings.

But yes, as Krio implied the C ratings - especially higher ones - are a crapshoot, no matter how much someone wants to defend them. For the "defendants" of their relevancy, consider a claimed 6000mAh 2S 100C pack. That translates into 600 amps = 5000 Watts at full voltage. 5000 Watts would be easily enough to power my entire apartment's (discounting RC equipment) electronic appliances and devices (and I have plenty) at once, electric oven included. Yeah, no. It wouldn't be a battery at that point, it'd be a flamin' bomb. And if you think your 1/8 or 1/10 RC car would draw anything near that even for a second without melting pretty much every single cable and component (even if many ESCs boast 500-1000A "burst current"), you are off your rocker. The pack labeled conservatively at something like 30C could actually be capable of giving just as much current safely as that "supposed" 100C. For those interested here is an example thread with a lot of data about testing "true C" ratings. Most packs rated 40+C don't exceed 20C. These don't seem to be car packs mostly, but if you think it's any different there........

Aaand that escalated into a rant, but there it is.

JatoTheRipper

iTrader: (11)

We're not comparing motors. We are comparing LiPo batteries. I gave you as close to an apples to apples comparison as possible. 4S is more expensive.

billdelong

iTrader: (33)

You can't run a 4000kV motor on 4S, get for real dude.

You also can't compare the same capacity because the amount of energy stored in a 3000mAh on 4S is the same amount of energy as 6000mAh on 2S, just look at the weight for each respective pack to see what I mean, both packs should weigh about the same to get closer to an apples to apples comparison.

As far as C ratings go, it's a matter of what is practical... running 2S in a heavy 1/8 based platform is gonna require a higher C rated pack (ideally 60c+) Now some of the lighter basher grade SCT's like the Traxxas Slash can undoubtedly handle a lower C rating and that's because they weigh significantly less. As far as race grade goes, 4S wins every which way.

*** EDIT

Ohms Law... W = V x A

2S = 4.2V x 2 = 8.4V
4S = 4.2V x 4 = 16.8V

4S-3000mAh = 50.4W
2S-6000mAh = 50.4W

Both packs store equal power @ 50.4W

JatoTheRipper

iTrader: (11)

I have no idea why you keep mentioning a 4000kv motor. We are talking batteries not motors. The original comment that started this was when platgof stated that 1/8th scale batteries (4S) are cheap compared to 1/10th scale SCT batteries (2S). It's simply not true when you compare apples to apples like I did. You keep comparing theoretical apples to theoretical apples.

billdelong

iTrader: (33)

Which is why I used Ohm's Law to help you understand that we need to look at "Power" draw in terms of "Watts", you are fixated on "Volts" which is only part of the equation, you must factor in "Amps" which is the current pulled from the "Motor" to balance the equation for total "Watts"... if you make any adjustment to V, then you must adjust the A.... this is fundamental physics... smh

Apples to Apples means that both equations must pull the same Power draw in Watts.

What's different is that higher voltage improves efficiency so 4S wins, there is no debate, go take a class in Physics and then you'll finally have an idea of why this is the case

DirkW

iTrader: (1)

You guys are talking about two different things - and both of you are right.

Of course a 4S with the same numbers as a 2S is significantly more expensive (after all it contains twice as many otherwise identical cells). However, going for a 4S with the same overall "power" for the system (i.e. since you doubled the voltage, you theoretically only need half the amps - in capacity and C-rating), you may end up cheaper.

So just decide if you want to talk about same overall power, or same power per cell.

billdelong

iTrader: (33)

Been chewing on this thought all day today and this doesn't work.... I think you mean Overall Power vs Overall Capacity.

If you compare equal power (Watts = Watts) on 4S vs 2S using Ohms Law then 4S wins being more cost effective.

If you compare equal Capacity, then 4S will have double the power, sure the 4S pack will cost double the price up front simply because you're buying double the power, but for argument sake, let's say any sane person using the same 4000kV motor in this theoretical mistake would then be forced to reduce the throttle EPA to 50% so not to burn up the motor on 4S would then find their run time to be roughly 125% longer making 4S most cost effective in terms of run time per dollar spent.

No matter how you slice it, increasing voltage will increase efficiency, this is a given fact:
http://www.atcllc.com/wp-content/upl...osses-2017.pdf

billdelong

iTrader: (33)

No sign of Tim this week, but they had enough entries (6) show up for ST so they broke into a separate class, there was a fair amount of incidental contact, but no worse than any other lower main level class. We had 3 each for beginner and 4WD SCT this week and I think we had less contact than any other class on the track, it was pretty chill, we just had fun clocking some laps and had some close racing with friends. Spec Slash also had 6 entries this week and they were a pinch cleaner with minimal contact than the open wheel classes based on what I observed.

The more popular classes right now are 2WD and 4WD buggy, 19 and 21 entries respectively this week and it's the lower main drivers who have a harder time getting in the clean lines so you see more contact. Perhaps the SCT classes were more popular back in the day so you might've seen more contact where there were a higher percentage of sportsman drivers, I don't think it's the class that determines the skill level of driver.

The club in my area is also doing away with "Sportsman/Expert" divisions too.

Since I finally saw ST break out into it's own class for the first time this week, I went ahead and bought a used XT2 from a local so I'll run whatever class will be more popular between ST and SCT going forward.. fingers crossed 4WD SCT picks back up again!

2stroketim

It's been rain, snow, melted snow, rain, and rain.

My track is too wet to drive.

And it's cold.

That is all.

platgof

iTrader: (4)

Nothing beats an indoor track for sure. I raced this weekend and am giving up SCT 4x4 for now. I really like my buggies, all three of them. Just had a bad night and have had enough. My B64D is all the car I really need for now, but plan on running my B5FL, and might bring out the Losi 3.0 E as well.

tvih

Dammit platgof, stop killing short course

JRSlash

iTrader: (7)

No worries. I am taking his truck out of his hands and race the $#!t out of it on 3S. Only outdoor where 3S is allowed or nobody really cares. I know all the racetime entertainment events are "open" so 3s is a most for the long 10 minute mains. I have done it with my old protek 8500 100c but 3s gives me the peace of mind. Last years psycho, they were dropping like flies after 7 minutes.... 10 or 12 trucks finished from 19... LOL

Malazan

iTrader: (2)

4x4 SCT is our second largest class after Ebuggy