Help! Strange Cutout issues on Speed Build!

[dhbleacher]

I am relatively new to speed builds for electric rc cars. The fastest I have gone is 62mph with a tenth scale truck. Anyway I am working on an 82mph (calculated) speed build with my A959B 1/18th scale car. I have done a complete aluminum, carbon fiber, and foam tire rebuild and upgraded the original brushed electronics over to a Traxxas VXL-3s esc, 3500kv brushes motor, and a 3s, 120C discharge, 1500mah battery to run it. However, when I actually go to drive the car it will cut out in 2 minutes and won't even reach anywhere near its full speed. When I test rev it in the air it seems to work fine, but for whatever reason when I go to drive it has those weird issues. My guess is either the battery discharge or mah isn't sufficient enough, or the vxl3s esc isn't capable of what I'm trying to put it through. Any feedback would be very much appreciated. Thanks! 🏎

 

[Doom!]

Are you over heating it?

 

[dhbleacher]

Are you over heating it?

Are you over heating it?

I don't think so, I checked the temp with my hand and it seemed fine.

 

[Tunedfrog]

Not sure about this esc but I have one or two that will shut off if I don't have it set for Lipo and is in NiMH mode.

 

[dhbleacher]

Not sure about this esc but I have one or two that will shut off if I don't have it set for Lipo and is in NiMH mode.

I double checked to make sure its in lipo mode. I think it was but I reaffirmed it. I'll charge my batteries up and take it out for a run just to double check.

 

[Greywolf74]

Buy a better ESC and motor

 

[dhbleacher]

Buy a better ESC and motor

Yea its probably the esc. I know my batteries are fine, they work in other relatively fast rc vehicles (62mph) and I checked with a multimeter. I'm going to up my motor kv to 5900. Any recommendations for a GOOD budget friendlyish esc that can handle whatever torture current draw I put it under (shooting for 80-95mphish).

 

[Jerold]

Sorry for the long explanation. Cutting to the hase, I see two things. ESC and Battery. I would start with the battery first, because it will be a limiting factor even with a better ESC.

Starting with the free changes first, check your ESC Low Voltage Cut off (what @Tunedpipe said). It could be the LVC kicking in. Under load the battery voltage will temporarily sag and it can be pretty dramatic depending on the setup.

The log below is from an 8S run (33.6V) and CNHL 9500 batteries. The Voltage is in red and has a minimum of 22.4V, which would be 2.8V/cell and would definitely kick in the LVC. That's why a lot of speed runners don't use LVC.

The data log from the ESC sampled at 5S intervals. Given that there is a fair margin of error in the system and low sample rate, it could be a bit higher or lower but it's not good either way.

-> Turn off the LVC

Next is the battery, which I would definitely replace with a larger capacity. You will need to do this anyway, because it's way too small for what you're doing. If you upgrade the ESC it will only get worse.

The C rating is a arbitrary number, for the most part, that is stamped on there by the manufactures. They are all self tested, self regulated and the tests are not generally published. There are a lot of posts about this. It's like cheap audio equipment - Do you really believe this is actually 8,000 W amplifier for $120? Sure, and Santa Claus and the Easter Bunny are coming over to dance with me.

This really comes down to current (amps). The ability to provide current is proportional to the capacity of the battery. So one thing you can do is use a known brand and get the largest capacity battery you can fit. For example a 3,000 mah, 50C battery can provide 150A. But a 6,000 mah 25C battery can also provide 150A.

According to Traxxass "MOSFET design provides high-current output (320 amp peak)". They don't state the continuous Amps; however, let's say is 1/4 of that. (purely a guess) which means to run this under average load you need about 80 Amps or about 55C. In theory your battery "should work" but that is average, not max. There is no way you will ever get 320A out of it for maximum performance from the ESC, even if the manufacture is dead on with their C rating.

1. Turn off LVC or, if that isn't possible, set it to NiMH. You need to check your battery manually after each run.
2. Get a better battery. CHNL, SMC, Onyx speed run batteries have all shown good results. if you can't do that double up the 3S you have an run them in parallel. This will double your capacity and double your ability to deliver current.

 

[dhbleacher]

Ok. Thank you so much! I can definitely confirm that it is LVC kicking in. Within about a few seconds-ish, I'm given the LVC error light and my throttle is limited to 40%. I plan to upgrade my car to a 5900kv motor instead of a the 3500kv because the speed calculations i used were theoretical and I forgot to take into consideration that the motors rpm is never actually continously at max rpm on a 3s lol. Plus all the 80-90mph speed runs I've seen online with my car were with around 6000kv motors. A really challenging thing with my car is I can hardly fit the one tiny 3s under the shell (120C 1500mah) because its 1/18 scale. I'm most likely going to have to make some modifications to extend my chasis dimensions and stick a different shell on it. Do you think my current esc (Traxxas VXL-3S) and 3s battery/batteries will work with the 5900kv if I try nimh mode and runniny my batteries in parallel like you said?

 

[Jerold]

Ok. Thank you so much! I can definitely confirm that it is LVC kicking in. Within about a few seconds-ish, I'm given the LVC error light and my throttle is limited to 40%. I plan to upgrade my car to a 5900kv motor instead of a the 3500kv because the speed calculations i used were theoretical and I forgot to take into consideration that the motors rpm is never actually continously at max rpm on a 3s lol. Plus all the 80-90mph speed runs I've seen online with my car were with around 6000kv motors. A really challenging thing with my car is I can hardly fit the one tiny 3s under the shell (120C 1500mah) because its 1/18 scale. I'm most likely going to have to make some modifications to extend my chasis dimensions and stick a different shell on it. Do you think my current esc (Traxxas VXL-3S) and 3s battery/batteries will work with the 5900kv if I try nimh mode and runniny my batteries in parallel like you said?

Traxxas says there is "no limit"

https://traxxas.com/products/parts/escs/vxl3s

So it "should" work with the motor, but you may not get more speed from it.

Changing the motor and or ESC may not be the answer. The amount of energy to propel you car to what every speed is the same regardless of your KV and gears. That energy comes from the batteries and it's and it's about current. If you pull too much current the battery voltage will droop and drop your RPM. At that point the ESC and motor are along for the ride.

Theoretically more KV equates to more RPM and leads to speed. But gearing counts. The higher KV motors have less torque and if you keep the same gears it may never get there because your are still overtaxing the system. In that case you would be better running lower gears and higher RPM. Which which puts you back in the same place.

If you are using the factory battery with the Deans connector, I would bet it's an issue.

You might look at China Hobby or SMC. They seem to have good energy density. Or get two smaller batteries, stack them up and run them in parallel.
https://chinahobbyline.com/collections/cnhl-voltage-11-1v-3s-lipo-batteries
https://www.smc-racing.com/index.php?route=product/category&path=67_99

But before you do that, let's try some much cheaper alternatives.
1. Use a good connector. Deans is not designed for high current application. Traxxas claims the ESC "provides high-current output (320 amp peak)". There is no continous current quoted, but I would bet it's at least 80-100 AMP. I would suggest using at minimum an XT60/EC3. I run XT60 in my Slash on 3S and haven't had any melted connectors, but I'm not speed running it and I'm using the stock gearing.
2. See if you can fit two batteries in parallel. This doubles your current (Amps) capacity. Stack them up and fit your wires.
3. Keep an eye on your temps. More speed/current means more heat.
4. Modify your driving style. This might just fix your problem. Ease into the throttle and build up the speed slowly. Punching it leads to a massive current spike. If you ease into it, the system has time to build up the RPM. You will need a lot of room.

To give you an idea of what that looks like, look at the data log on the left. Notice the current spike (green) coincides with the quick throttle ramp (Black) and the massive voltage droop (red). The one on the right is what I would like.

 

[dhbleacher]

Traxxas says there is "no limit"
View attachment 164006
https://traxxas.com/products/parts/escs/vxl3s

So it "should" work with the motor, but you may not get more speed from it.

Changing the motor and or ESC may not be the answer. The amount of energy to propel you car to what every speed is the same regardless of your KV and gears. That energy comes from the batteries and it's and it's about current. If you pull too much current the battery voltage will droop and drop your RPM. At that point the ESC and motor are along for the ride.

Theoretically more KV equates to more RPM and leads to speed. But gearing counts. The higher KV motors have less torque and if you keep the same gears it may never get there because your are still overtaxing the system. In that case you would be better running lower gears and higher RPM. Which which puts you back in the same place.

If you are using the factory battery with the Deans connector, I would bet it's an issue.

You might look at China Hobby or SMC. They seem to have good energy density. Or get two smaller batteries, stack them up and run them in parallel.
https://chinahobbyline.com/collections/cnhl-voltage-11-1v-3s-lipo-batteries
https://www.smc-racing.com/index.php?route=product/category&path=67_99

But before you do that, let's try some much cheaper alternatives.
1. Use a good connector. Deans is not designed for high current application. Traxxas claims the ESC "provides high-current output (320 amp peak)". There is no continous current quoted, but I would bet it's at least 80-100 AMP. I would suggest using at minimum an XT60/EC3. I run XT60 in my Slash on 3S and haven't had any melted connectors, but I'm not speed running it and I'm using the stock gearing.
2. See if you can fit two batteries in parallel. This doubles your current (Amps) capacity. Stack them up and fit your wires.
3. Keep an eye on your temps. More speed/current means more heat.
4. Modify your driving style. This might just fix your problem. Ease into the throttle and build up the speed slowly. Punching it leads to a massive current spike. If you ease into it, the system has time to build up the RPM. You will need a lot of room.

To give you an idea of what that looks like, look at the data log on the left. Notice the current spike (green) coincides with the quick throttle ramp (Black) and the massive voltage droop (red). The one on the right is what I would like.


View attachment 164022View attachment 164023

Thanks again! I did go out and test it again on fully charged batteries. I did a slower acceleration (since i was on a longer road) and it definitely went faster and it didn't cut out for a while (turned LVC off too). Its definitely not going to hit 80mph on this current setup though (I realized the calculations I was using aren't super accurate because they don't account for drag and other factors, they are just theoretical). Its looking like its maybe hitting about 55ish. Ideally I would like to hit about 92-100mph but idk how I would go about doing that. I've seen people hit over 100 with this car, so I know its possible. Someone did it with a rly expensive mamba esc and 5900kv motor, but I would preferably like to not have to spend 120 bucks for an esc. Anyways, any suggestions on how I could go about doing this and be affordable? Would you recommend downing my pinion size? Maybe I could down it from the current 27T pinion with 42T Drive Gear to the 17T pinion with the 38T Drive and up my kv to 5900? Any incite would be nice, I'm trying to beat my friend in a speed war lol. Also, any recommendations for a cheap gps (preferably around the $50 mark)?

 

[Bogda89]

Ok. Thank you so much! I can definitely confirm that it is LVC kicking in. Within about a few seconds-ish, I'm given the LVC error light and my throttle is limited to 40%. I plan to upgrade my car to a 5900kv motor instead of a the 3500kv because the speed calculations i used were theoretical and I forgot to take into consideration that the motors rpm is never actually continously at max rpm on a 3s lol. Plus all the 80-90mph speed runs I've seen online with my car were with around 6000kv motors. A really challenging thing with my car is I can hardly fit the one tiny 3s under the shell (120C 1500mah) because its 1/18 scale. I'm most likely going to have to make some modifications to extend my chasis dimensions and stick a different shell on it. Do you think my current esc (Traxxas VXL-3S) and 3s battery/batteries will work with the 5900kv if I try nimh mode and runniny my batteries in parallel like you said?

That motor and ESC is more than adequate, you need bigger battery. There is one guy on YouTube, so far he made Traxxas bandit to go 88mph with stock vilenion

 

[Jerold]

That motor and ESC is more than adequate, you need bigger battery. There is one guy on YouTube, so far he made Traxxas bandit to go 88mph with stock vilenion

Double Ditto. If the slower throttle ramp helped but still cutting out, it sounds like the battery is the weak spot. It can't supply the current and the voltage is dropping. You wont' see this with a battery checker because the droop happens when it under load. When you release the throttle the voltage comes a back up and I'm willing to bet it's 4+V when you check it.

If you upgrade your ESC/Motor it will only get worse.

 

[tntpoof]

its the battery .go up in mah. the batterie's mah is issue motor wants more amps than battery can deliver