MGM controller do I go for amperage or cell count??

[Finnster]

Yeah, basically I say 14 A123 cells is approaching a practical limit on 1/8ths is due to weight and space. Ea cell weighs ~72g, so a 14c pack is 1kg (2.2lbs). A pig by lipo stds, however a NIMH cell weighs 65-70g as well, and no one has failed using 14 cell packs, esp for a basher. 14 A123s are quite big tho, so it can be a trick to fit them (if you have the cells you can see what will fit.) I can only fit 10 to 12 cells on my Revo w/o having to redo the layout, so depends what will fit. I don't think I would go over 16cells tho as a basher.

Ok, so we have a range of cells we have to work in, question is then how to utilize them. For your beefy Savvy, I think a target peak power of 2000W would make a nice and controllable basher (40 +/- as you said.) Now I'm basing this on exp w/ my Revos which is ~10lbs, and all the radar and ET data I have on them. It is a single speed tho, so as Brian said, a 3spd may not require so much, but lets aim for that anyway.

To get 2000W, you need alot of amps or volts, or a nice amt of both. Since A123s are limted to the M1 cell, you can do either 1p or 2P. 60A/ per cell discharge (120A in 2p) gives me 2.8V/cell on my ET. They are spec'd at 70A max, and I'll I've heard about high amp draws on 1P is that it lacks punch, so this limit seems reasonable. So, to get 2000W, you need either ~33V loaded in 1P (@60A) or ~16.5 loaded @120A. That's 12 cells either way, you might be able to get away w/ 10 cells @ 70A, but that's pushing them. 10S would be the lower limit then of a 1P config. Now w/ 14 cells, g/l finding a controller to do that in 1P (50V max) and charging would be a pita, but 7s2p would be real nice. You could get 2000W at only 100A peak, or crank it up to 120A for big power.

Now, the question is how to select the motor you want for the amp draw you are aiming for?.... that's pretty tricky, but the RPM estimates are helpful. Otherwise you hope someone has ET data of that config. To do the RPM calc I use the nominal voltage (3.3V), as that's seems convention and I've lined up my est.s based on that. No hard rules tho. For the 1500kv motor, that gives 35K. For a 1700kv; 39K. What I see from there is that either would be a good chioce, as the 1700 being on the powerful side and a bit hotter, and the 1500 a bit more sedate, but still strong. This is where the hard choices come in, and thoughts about what you are reasonably trying to acheive.

As a very rough guide, doing the same w/ my 8XL and 5S lipo and 4S lipo setups (as linked in sig, don't worry about the wheelies tho, diff chassis, notice top spds): 5S is ~38K and 4S is ~30K using rpm rule (cell @ 3.7V.)
So..... The Neu is much more powerful and eff than an XL, so even more power to the ground. Your truck is heavier, but does have a multi-spd, so they way wash.

Therefore, by my theories, the 1700 would be a pretty balistic motor on 7S2P A123 in the Savvy. It may over draw the A123s as well (my 120A was measured on 8XL/ 5S A123 &/or 4S lipo.) For ~40 mph, the 1500 seems like the best choice, and may quite likely put you over 40mph. If going 6S2P A123, the 1700 would be on order. 5S2P A123 may be hard pressed to give you all the power you need, but its on the edge. There's not a ~1900 kv motor either. 1700 would be kinda slow, and the 1Y (2200 kv) gives good spd, but I'm not sure how the batts would like it, and may run a touch hot. Perhaps someone has direct exp.

Rinse and repeat for other batt and motor combos. This is all theory (based on data measurments and practical experience tho,) so no guarantees until you actually try it. Holy crap that was a disertation...

[Finnster]

One other thing. To get #'s that match my radar, I typically use ~7" on the tires. The ballooning def has an effect, and kv values are not exact. As you go faster, more balloon is needed.

Otherwise I use 3.5v/cell lipo, or 2.8V/cell A123. These are known, measured values under load. If they are quite alot higher or lower, you prolly don't have a properly sized batt for your app.

EX:
Differential Ratio: 2.8461538461538462
Transmission Ratio: 2.2941176470588233
Other Ratio: 1
Spur Tooth Count: 40
Pinion Tooth Count: 16
Total Voltage: 14 (either 5S A123 or 4S lipo, calcs the same)
Motor KV: 2084
Tire Diameter (inches): 7
Spur/Pinion Ratio: 2.5 : 1
Total Ratio: 16.32353 : 1
Tire Circumference (inches): 21.99 inches (558.58mm)
Total Motor Speed: 29384.4 RPM
Vehicle Speed: 37.49 MPH (60.22km/h)
Effective Speed Rate: 2.66mph/V (4.27kmh/V)
Effective KV Value: 2084

Before I taped the tires I got 40-41mph reliably (more balloon) now its actually a bit slower @ 38mph on radar, but is much more controllable. IMO, less than 6" for normal maxx tires will under est the top spd. These are w/ mashers, so YMMV.

[Purplefade]

Originally Posted by Finnster

Yeah, basically I say 14 A123 cells is approaching a practical limit on 1/8ths is due to weight and space. Ea cell weighs ~72g, so a 14c pack is 1kg (2.2lbs). A pig by lipo stds, however a NIMH cell weighs 65-70g as well, and no one has failed using 14 cell packs, esp for a basher. 14 A123s are quite big tho, so it can be a trick to fit them (if you have the cells you can see what will fit.) I can only fit 10 to 12 cells on my Revo w/o having to redo the layout, so depends what will fit. I don't think I would go over 16cells tho as a basher.

Ok, so we have a range of cells we have to work in, question is then how to utilize them. For your beefy Savvy, I think a target peak power of 2000W would make a nice and controllable basher (40 +/- as you said.) Now I'm basing this on exp w/ my Revos which is ~10lbs, and all the radar and ET data I have on them. It is a single speed tho, so as Brian said, a 3spd may not require so much, but lets aim for that anyway.

To get 2000W, you need alot of amps or volts, or a nice amt of both. Since A123s are limted to the M1 cell, you can do either 1p or 2P. 60A/ per cell discharge (120A in 2p) gives me 2.8V/cell on my ET. They are spec'd at 70A max, and I'll I've heard about high amp draws on 1P is that it lacks punch, so this limit seems reasonable. So, to get 2000W, you need either ~33V loaded in 1P (@60A) or ~16.5 loaded @120A. That's 12 cells either way, you might be able to get away w/ 10 cells @ 70A, but that's pushing them. 10S would be the lower limit then of a 1P config. Now w/ 14 cells, g/l finding a controller to do that in 1P (50V max) and charging would be a pita, but 7s2p would be real nice. You could get 2000W at only 100A peak, or crank it up to 120A for big power.

Now, the question is how to select the motor you want for the amp draw you are aiming for?.... that's pretty tricky, but the RPM estimates are helpful. Otherwise you hope someone has ET data of that config. To do the RPM calc I use the nominal voltage (3.3V), as that's seems convention and I've lined up my est.s based on that. No hard rules tho. For the 1500kv motor, that gives 35K. For a 1700kv; 39K. What I see from there is that either would be a good chioce, as the 1700 being on the powerful side and a bit hotter, and the 1500 a bit more sedate, but still strong. This is where the hard choices come in, and thoughts about what you are reasonably trying to acheive.

As a very rough guide, doing the same w/ my 8XL and 5S lipo and 4S lipo setups (as linked in sig, don't worry about the wheelies tho, diff chassis, notice top spds): 5S is ~38K and 4S is ~30K using rpm rule (cell @ 3.7V.)
So..... The Neu is much more powerful and eff than an XL, so even more power to the ground. Your truck is heavier, but does have a multi-spd, so they way wash.

Therefore, by my theories, the 1700 would be a pretty balistic motor on 7S2P A123 in the Savvy. It may over draw the A123s as well (my 120A was measured on 8XL/ 5S A123 &/or 4S lipo.) For ~40 mph, the 1500 seems like the best choice, and may quite likely put you over 40mph. If going 6S2P A123, the 1700 would be on order. 5S2P A123 may be hard pressed to give you all the power you need, but its on the edge. There's not a ~1900 kv motor either. 1700 would be kinda slow, and the 1Y (2200 kv) gives good spd, but I'm not sure how the batts would like it, and may run a touch hot. Perhaps someone has direct exp.

Rinse and repeat for other batt and motor combos. This is all theory (based on data measurments and practical experience tho,) so no guarantees until you actually try it. Holy crap that was a disertation...

OUTSTANDING and thank you for the excellent detail! Hated to Quote the entire post, but you made some good points that cuaght my attention. I'm leaning towards\have decided to go with 7S2P, 7 cells on either side of the chassis and would actually still have some room to spare even using the standard length Savage chassis, that said, as you pointed out, going to 16 cells just really pushes the weight factor up there, not that 14 is light or anything... I'll be using the FLM chassis so no issue with plenty of mounting points to choose from and should be plenty durable for "hanging" them off the sides.

Now it's just the motor, I'm leaning towards the 1500 Kv Neu for the safety factor, but haven't totally ruled out the 1700 Kv either. To your point, using the 3 speed should give me some flexibilty in gearing and take some of the initial load off the 1700 Kv should I go with it, or at least that's my thinking anyway. Thougts on that?

Thanks for the great info,
purp