mfs

I have a stock Slash that I have happily been running on 7.2 and 8.4v NiMH's. I have been running the truck in a part of my yard that is dirt and some low grass as well. After each run, my motor was slightly warm and my batteries were pretty hot. This was running 90/19 and 90/21. I clean the motor after each day of use using motor spray and then lube the bushings with bearing oil.

I attended a race two weeks ago and experienced the same results. I ran the truck at 90/21 using 6 cell NiMH (DTX 5000's). The batteries were warm to hot and the motor was merely warm and the ESC stayed COOL. The track is loose and dusty.

I decided that my hot battery problem might be a result of the ESC/Motor overdrawing the battery, so I decided to invest in a couple of Venom 5000mAH 20C batteries a new charger and balancer. I figured if the motor is outdrawing the battery, then their may be some untapped performance left in the stock Titan 12T. The LiPO's will be for racing only, so I figured 10 minute mains would be fine with the batteries with no LVC.

I went to the track over the weekend to test out the new LiPO's and work on my setup. I set a timer for 10 minutes and ran around the track. The truck definitely has more speed and punch. After the 10 minute run the LiPO only discharged to 3.8v. After the run, the LiPO was COOL to the touch but the motor was HOT. I let the motor cool and dropped the pinion down to 19t and did another run. Same result, cool battery and HOTTTT motor.

So I guess my question is this: Do I really need to gear WAY down to use LiPO in a stock Slash? I did not have a smaller pinion handy or I would have tested it. It seems strange to me that 8.4v (peaked) LiPO would run so much hotter than the 9v (peaked) 7-cell or 8.4v (peaked) 6-cell's. Am I correct in assuming that the discharge rate of the LiPO is so much greater that it is making the motor way hotter?

Duster_360

iTrader: (21)

Its normal for NiMhs to get hot - thats the way they work. Lipo is much better at holding voltage than NiMh where voltage starts falling as soon as you remove it from the charger. There is no comparison in the discharge rate between NiMh and lipo. A good lipo will discharge far more amps than all but the most carefully prepped, matched, quality NiMh cells.

You need to continue to gear down and see if you can find a reasonable gearing that will run at acceptable temps.

mfs

Interesting. All of the setup was identical except for the switch to lipo and the motor was running hotter. Same gearing. So I assumed that the fact that a good nimh can discharge at 30-40 amps vs a lipo (rated) at 100 amps could have made the difference.

Xmod_Racer

iTrader: (11)

for my slash on a big track, 19/86 on lipo seems the way to go. For short tracks like indoor carpet, i go all the way down to 16/86 with good results. Motor at decent temps, but my truck is blazing quick

mfs

Well there ya go, this track is fairly big. Built for Nitrogen buggy and truggy. I would think that 90/19 would run quite cool.

Maybe my Titan is just reaching the end of its lifecycle? 20 or so packs.

Xmod_Racer

iTrader: (11)

i doubt it. My titan has atleast 40 and i know of titans that can get over 100 packs easy and still are quick

BBRoadRacer

iTrader: (9)

Batteries get hotter when you draw power nearer to their output limit. The Titan motor will never pull the 30-40 amps that the NIHMs can put out, but the NIMHs get warm because that is the nature of NIMHs. You will never get anywhere near the 100 amp output of your LiPo, and the battery will never get hot unless you over discharge it.

When you compare the dicharge curves of LiPo vs NIMH, the NIMH drops below the 8.4 volts of the LiPo very quickly. The LiPos voltage drops off very slowly. Having the higher voltage longer is what gets the motor hotter.

Also with the Titan, look at the brushes. They start with ridges on the faces of the brushes. Once they are worn to a full face, they start running hotter.

mfs

Even with a good maintenance schedule it seems to be getting a bit slower already. See below.

So that makes perfect sense. I was thinking increased amp draw, but sustained voltage makes a lot more sense. Thanks.

I raced again this weekend and stuck with the 21T and was getting smoked on the straight by stock Blitz's. This isn't why I didn't win, they were much better drivers . Not sure how they were geared, but I do know that they were running stock Blitz motors and 2s lipos. I am thinking that my motor may be on its last legs.

I wasn't really having bad heat issues running the 21T this time around. I really don't know what the difference was from the last practice. I was actually thinking of gearing up to 90/23 for the main.. and probably should have since the track hooked up nicely during our main.

Thanks to everyone for the replies!