Started working on this build again. Pretty much narrowed it down to either a 12s8p build or a 18s4p build. Forgoing the HW ESCs for now.
If going with 12s pushing 20A/cell, it gives me just under 7KW (dv6s)
If going 18s pushing 28A/cell, it gives me about 7500W (dv100s)
With those cells and settings, it’s about a 600W difference. Not a huge amount with some pros and cons to each configuration.
Motors would be pushing 130A on each setup. 205kv for 12s, 151kv for 18s.
What I’m failing to understand is how the motors react between these 2 setups, and wouldn’t it make sense to gear down at low voltage high amps? Then gear up at high voltage low amps?
i’d go 173kv on 18s should you go that route. that’s what i’m running. doesn’t get too hot and i’ve been pushing 130a motor 60 battery iirc into em just fine. (to be fair, it’s still pretty cool out) was running 11/67 gearing on 160mm slicks, super punchy but need more top speed.
why more A/cell on the 18s ? different cells? or esc amp limits at 12s?
What I’m failing to understand is how the motors react between these 2 setups, and wouldn’t it make sense to gear down at low voltage high amps? Then gear up at high voltage low amps?
so. gear down usually means the motor spins more per wheel revolution. ie more torque less top speed.
gear up, the opposite. lower kv has a simillar effect to gearing down. more torque less top speed.
all things being equal if you gear for the same top speed, then more batteries = more power.
12s8p = 96 * 4.2 * A/cell = W.
18s4p = 72 * 4.2 * A/cell = W
so 96/72 = 30% more power available in the battery for the 12s8p. because it’s 30% more of the same cell.
but usually something like the ESC is the limit and has a sweet spot where it can deliver the most watts. which is it’s max voltage x it’s max amps. other bottlenecks are possible such as motor max amps or voltage, wiring max amps, … etc.
usually as kv goes up amps that it can handle goes up.
so if you’re picking the same limit for two different kvs suggests your creating an artificial limitation on one.
for example. reacher specs, note how as kv goes up max amps goes up.
173kv not too high for 18s? I’ll be running 160mm slicks as well, and aiming for a top speed of ~70. I’m only ~110lb (125lb with gear), so can sacrifice some tourqe for top speed.
Esc limits. Dv100s has 60A as continuous (I normally set the continuous reccomend amps just under the max for makerx stuff).
Dv6s has 80A.
I’ll be going with 42a this time instead of 45b, so want to keep it under 30A/cell as well to reduce voltage sag.
This is also part of what’s also throwing me off. The 12s8p pack has more power, but the esc is the bottleneck (with my limitations for max set to reccomend continuous). The dv100s can fully utilize the pack, and saturates the reccomend 60A. That still just gives me 700W more.
And voltage increases top speed while reducing torque? And a higher voltage means a lower KV, so some of that torque comes back, however less battery amps can be pushed, so even less tourqe?
What about voltage and motor amps? If I’m pushing 130A motor on each, at 18s that will be more power to the motors than 12s? I think that’s the main thing I’m missing.
Admittedly I’m still learning more about it myself, but I thought higher kv motors on higher voltage setups can produce too much heat, and spin too fast for the bearings.
Haha, ya kph not mph.
And good to hear I can gear around 4.5. I’m actually thinking 4.3 (for 18s), and hopefully I can push enough amps + throttle curve to give it enough kick.
I didn’t notice you were chosing between two different ESCs. so that makes sense they’d be different bottlenecks.
more voltage increases rpm at the same kv.
lowering the kv lowers rpm and increases torque.
or you can gear down which decreases top speed and increase torque.
high kv high voltage blah
gearing down gets the higher amp limit of the higher kv. so the sweet spot is the highest kv your ESC can support that’s below the magic iron losses zone of the motor. (too high a rpm x pole pairs). … idk what that is most of the time without testing.
and this is where you take suggestions from the crew here who’ve used different motors voltage combos.
often you also can’t gear the resultant rpm from high kv high rpm down enough which drives us to lower kv in the first place.
roughly yes. but noting that you’ll never get more power than battery voltage * battery amps.
Are you talking about the currently available D100S? I run my D100S at 90A battery 200A motor per side, so 180A battery 400A motor total at 21S. I have a log pulling 11.7kW and there was still a little bit of battery amps left that I just didn’t have the courage to pull. So it can definitely handle much more than 60A. Although if you want to go this high on motor amps you definitely need an external heatsink. With an external heatsink my temps are in control in the current weather. Fair warning though, if you run 45A per cell from P42A like I do it sags super hard. If you don’t want a lot of sag then 30A per cell is about right.
Also I am running 175KV 6395s at 21S. Although I am geared so high that I can’t come close to my top speed. I’ll make a 1:8 gear drive eventually so that I can run a reasonable top speed on the 9" tires so I can actually get closer to the iron loss territory.
@zero_ads is running 21S on 205KV motors though, and he is not having overheating issues as far as I know. So you can run reachers pretty fast.
Also regarding the KV. If you go up in KV you can run proportionally more amps. That’s why I went for the 175KV instead of the 145KV. I just upped the amps for the 175KV to the D100S’s limit, so I gained back the torque from the higher KV, and I gained top speed that I can in the future gear down for more torque. The only tradeoff was more heat in the ESC, but with an external heatsink the ESC is doing fine for now.
So while you can’t just create extra power out of nothing, I thought motor amps are independent from battery amps, and the ESC uses some “magic”(duty cycle) to allow more watts to the motors? Thinking this over now, I’m questioning my understanding of duty cycle and motor amps.
a motor tries to turn at an rpm specified by duty_cycle * voltage * kv.
if it can’t do that it ends up drawing more current, which becomes torque, to get to that rpm.
let’s say we command 40% duty_cycle.
let’s say motor current demand is 100A
that’s 100A 40% of the time, so the actual battery current gets averaged out to 100A * 40% duty_cycle = 40 battery amps.
inversely if we have a 40A max battery setting. then at 40% duty cycle, motor_current could get to 40A/40% = 100A motor before battery amps is maxed out at 40A
now, at 5% duty cycle, 40A / 5% = 800 motor A !!!
but… likely we’d have moved towards the target rpm before the motor demanded all that torque/amps.
in RC cars they used to say "don’t gear too tall "… this was how they limited the motor current demand.
Lucky us we have ESCs that can monitor motor current and back off the duty_cycle. “limit motor current” if needed.
that’s the magic, the ESC controls duty_cycle. it must watch current to limit it. and when it shoots over the limit (battery or motor) it will back off the duty_cycle to reduce the target rpm therefor reducing current demand from the motor.
if motor current shoots past motor current max before the vesc can react, then we hitt abs over current threshold and throw a fault .
if you can figure this out you can play with the params and compare two boards relative power curves.
and you can also see that the battery amps / duty_cycle limit is like 1/x graph, with motor current limit setting a flat limit at lower duty_cycles.
