Motor/Battery advice needed, my ST-RR conversion is Phat and needs a diet :(
 

Hello all,

The time has finally come for me to make a decision on what to do with my Kyosho ST-RR brushless conversion. I have a nitro and an electric, the nitro with a tank of fuel is about 120g lighter than the electric. Unfortunately, the change in performance, and the different weight distribution make the electric no match for the nitro on the track. The electric is slightly slower on the straights, but makes up for it in its ability to make decent jumps with minimal corrections and huge torque. The nitro appears to be more agile through the technical sections however, and I believe this is due to more of the weight being up front. My lap times with the nitro are about 3-6sec faster than with the electric.

Anyway, there are a couple of ideas I've been throwing around, and I would like some advice from you guys with which way to go.

Current setup is:
ESC: MMM (no BEC at the moment, but I have a CCBEC I can use if
I decide to go with more voltage batts)
Motor: Feigao 8XL 2084kv 30,000RPM max
Batts: 4s2p A123. 13.2v, 4600mAh, 640g :(
Servos: HS-5955TG for steering, MG-995 (ebay servo :)) for brakes

A123 Battery Packs:
4s 2
3s 2

With the current setup, using a 50T spur and 15T pinion, I'm pulling 85A max spikes, with an average draw of around 15A. The voltage drop is ok at this amp draw, as it's split between the 2 packs.

Problem:
I need to be able to run at least 11 minutes of race time before I change batteries. Currently, I get just over 13min from the 4s2p packs. As I said above, the 4s2p packs are too heavy. Also, I only have 2 of them, so on a 20 minute race, I can't finish :(.

The main thing obviously, is that I will need a new motor. The Feigao is ok, but it gets very hot, is very inefficient, and has led a very hard and abused life. The motor choice of course, depends on what batteries I go with.

Option 1 - 7s1p A123
The easiest solution I can think of is to run a 4s and 3s pack in series, to get a 7s1p 2300mAh battery.

4s2p = 13.2v, 4600mAh
7s1p = 23.1v, 2300mAh

So the 7s battery is almost double the voltage, but half the capacity. With my limited knowledge of how electrics work, shouldn't this mean that provided I get a lower kv motor, and gear for the same top speed, I should pull half the amps, and get the same run time as before, correct? The main thing is that the max amp draw needs to drop down to around 40-50A, otherwise the A123's will drop lots of voltage under load, and the top speed, as well as runtime will be much reduced.

That seems to be the best option, as I'm only out for a new motor. I was thinking either the 1200kv or the 1600kv 70mm Medusa.

4s2p on 8XL - 13.2v x 2084kv = 27508.8 unloaded RPM
7s1p on 1200 Medusa - 23.1v x 1200kv = 27720 unloaded RPM
7s1p on 1600 Medusa - 23.1v x 1600kv = 36960 unloaded RPM

Just going on the above figures, the 1200kv Medusa on 7s looks like it would give almost identical performance. Unfortunately, as I don't know the amp draw, I would imagine that the voltage would drop quite a few volts under load. That's why I was also considering the 1600kv, to compensate for that. Obviously, the higher kv motor will have a slightly larger amp draw, I would imagine.

Option 2 - 4s1p LiPo
I purchase 2 dreaded 4s LiPos, as well as a new motor. I need 2 LiPos so that I can have a spare pack to race with, while the other is charging. A Rhino 4s 4900mAh LiPo from HobbyCity.com is 498g, so slightly larger capacity and voltage than my current 4s2p setup, for 2/3 the weight.

4s2p A123 = 13.2v, 4600mAh, 640g
4s1p Rhino LiPo = 14.8v, 4900mah, 498g

For this one, I'd have to get the 70mm 2000kv motor. It'll have slightly more top speed due to the extra voltage, but performance would be the same, maybe slightly higher amp draw (because of extra voltage, but if the motor is more efficient, then maybe not).
They're rated for 20C sustained discharge, and 30C burst. 20C = 98A. I'd have to be pretty careful as my current setup is almost there. I'd be much happier with 65A or 70A max draw however. Still its under the 20C rating so should be fine.

The problem with this plan is the cost of the batteries. $80USx2 is heaps cheap, but thanks to our lovely Aussie Dollar, $160+25US for the batts plus shipping will cost me near $300AU :( Terrible exchange rate lol. The motor will be the same issue, but that's being purchased for me, so no worries there hehe.

Option 3 - 6s1p LiPo (or 2 x 3s1p)
Lastly, I could go for a 6s LiPo. With this option, I don't want to get a 6s LiPo with huge capacity because they're big, heavy beasts. I'd more than likely go with 2 smaller capacity 3s packs wired in series to save weight.
A Rhino 2350mAh 3s LiPo is 205g in weight, so 410g for 2.
Obvious problem there is the discharge. 20C for that pack is only 47A. I'd have to be really careful with the setup to make sure its pulling only 35A-40A but that would result in a grandma style car performance wise.

Again, either the 1600kv, or more probably the 1200kv Medusa for this setup, but I don't think this is a serious contender. Also, the fact that I'd have to buy 4 x 3s packs would be an issue, ie more packs to charge etc.

Anyway, any advice would be greatly appreciated, and also, does anyone have any new information on the shipping date for the 70mm Medusas? They still say "preorder" on the site.

Thanks
-Sid

Buy the 36-70-2000 medusa and run on 4s ~5000mah.

No votes for the A123s? :(

Or buy a 5s 4k...same energy as a 4s 5k and w/the 5s set up it will be more efficient and should run cooler

The higher the # of cells...4s, 5s, 6s, etc...the lower the kv motor you need. As stated above, a 2000kv Medusa will be good on 4s, but w/5s, go down on kv.

Either way, upgrade to a Medusa or a Neu motor...they are much better that the xl motors and much more efficient.

So having pre-ordered an 80mm 2000kv motor, I now need to decide which batteries O_o

I'm currently running 4s2p A123, but at 640g, they're pretty fat. I just weighed my cars again and my nitro ST-RR is about 400g lighter with fuel in, than my electric ST-RR.

In the first post, I said the weight difference was 150g. The only change I've done since I weighed the cars back then was to remove my home brew aluminum battery tray and replace it with the custom made RCM tray that finally arrived (thanks Mike :)) I remember thinking that the bottom plate and tray were quite heavy, but 250g is pretty huge lol.

I will remove the battery tray and weigh it separately tonight to make sure, but I might have to make up a better aluminum tray to save some weight.

The whole point of this exercise was to make my setup as light as possible while maintaining a battery/motor combo that can make at least 15-20min of driving time.

The batteries I am looking at are the Zippy packs from Hobby city. I was originally going to go with 4s 5000mAh packs as they were slightly more capacity than what I have now, and slightly higher voltage so I figured I'd get around the same run time (13 min) maybe slightly more with a better motor. There should also be much less voltage sag under load, and so my top speed should be slightly increased to what I have now.

Magman mentioned going up to a 5s 4000mAh setup, and gearing down to maintain the same speed, but to go down to the 1600kv motor. Unfortunately, as I've already ordered the 2000kv, would I be able to get away with a 5s pack with the 2000kv motor and gear down with a 16t or 17t pinion (running 15t atm)?

I also might have a lead on some 5s TP 5000mah packs which are only slightly used, but are the same size as the Zippy ones.

Is that the right use of the terminology by the way? Gearing down = gearing to reduce top speed with bigger pinion, gearing up being the opposite?

Anyway, the 5s 5000mAh packs are 659g, so essentially, the same weight as what I have now....

A 5s 4000mAh pack is 581g, or 60g lighter than what I have now.

They have a 3300mAh 5s pack which is 443g and much lighter than what I have now, but what sort of run time would I get with only 3300mAh?

thanks for all the help/advice so far guys :D Much appreciated