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

[Purplefade]

BrianG, point taken on the 1515, I too think probably a wise choice, not to mention more cost effective than the 1521s and probably as well a better fit for the chassis.

AAngel, you right, the Fiegaos are pretty much all I've experienced in an XL. I've ran a few Ls, still Fiegaos, but certainly no comparison to the XLs and I've run a few Novaks, again no comparison to an XL. So on that note, your right, I probably have no idea what I'm in for from a quality motor, but I sure looking forward to finding out Any idea what your 8ight T weighs in at?

aqwut, interesting thought and does make some since. Hum, wonder if that's true or if it really just comes down to quality of the windings, mags, etc???

purp

[Purplefade]

BrianG, point taken on the 1515, I too think probably a wise choice, not to mention more cost effective than the 1521s and probably as well a better fit for the chassis.

AAngel, you right, the Fiegaos are pretty much all I've experienced in an XL. I've ran a few Ls, still Fiegaos, but certainly no comparison to the XLs and I've run a few Novaks, again no comparison to an XL. So on that note, your right, I probably have no idea what I'm in for from a quality motor, but I sure looking forward to finding out Any idea what your 8ight T weighs in at?

aqwut, interesting thought and does make some since. Hum, wonder if that's true or if it really just comes down to quality of the windings, mags, etc???

purp

[Finnster]

A123s can be tricky to use as they just come in one cell type. With lipos you can just pick a voltage and then decide what size lipos to get from there. A123s is the other way around. Example, my new little toy that came today:



Its an MGM 9032. I wanted to keep my A123s, but needed a reasonable cell config to do it. I'm switching my 5S2Ps to a 10S1P setup, and am looking at getting a 1515/2Y (1100kv) to go with it. I could have kept it at 5s2p and gotten a 1y with a 16018, but I wanted to go HV and super eff and cool. Any more than 10 cells it starts getting heavy and hard to fit. This controller will also allow me to run upto 2 xtra cells for some insane action @ 40V, but keeping amps below 60A hopefully. Its "only" a 1700W system, but should be more powerful than a 2000W+ XL system due to the eff increases.

A 6S lipo equiv is hard to do, as that is 14 cells A123, so really the next node seems to be 10S so you don't have to have a paralleled cell.

[Purplefade]

Originally Posted by Finnster

A123s can be tricky to use as they just come in one cell type. With lipos you can just pick a voltage and then decide what size lipos to get from there. A123s is the other way around. Example, my new little toy that came today:



Its an MGM 9032. I wanted to keep my A123s, but needed a reasonable cell config to do it. I'm switching my 5S2Ps to a 10S1P setup, and am looking at getting a 1515/2Y (1100kv) to go with it. I could have kept it at 5s2p and gotten a 1y with a 16018, but I wanted to go HV and super eff and cool. Any more than 10 cells it starts getting heavy and hard to fit. This controller will also allow me to run upto 2 xtra cells for some insane action @ 40V, but keeping amps below 60A hopefully. Its "only" a 1700W system, but should be more powerful than a 2000W+ XL system due to the eff increases.

A 6S lipo equiv is hard to do, as that is 14 cells A123, so really the next node seems to be 10S so you don't have to have a paralleled cell.

Fnnster, when you say 14 cells is the practical limit, I assume your talking power to weight vs the high end li-pos? I've got 30 A123s (3 DeWalt packs) that I've broken out and originally had setup up as 6 - 5S1P packs and was thinking of going to 4 - 7S1P packs for the higher voltage and increased power to go with the higher end motor. That said, I'm on the fence with controller, the 16024 seems to give me head room, but at the cost of cell weight if I stay with the A123s.

Is the logic that the higher the voltage the general rule is the lower the aperage, weight to weight and power to power, hence your choice of the 9032 on 10S? What's your thought on 7S2P (dual 7S1P packs) on the 1515 1.5y, 1500 kv at 25.5v = 38225 rpm with 13 pound "hanging off of it"? Would I be better suited to dump the A123s and roll back to li-pos.. Thinking out load I used to run li-pos, Maxamps and it got real expensive, real fast, all I do is bash and it seemed every other week, even protected, I had a puffed cell and another dead li-po...

The A123s being "bullet proof" caught my attention real quick and they have decent power for what they are. Man, all in all brushless isn't that complicated when it comes right down to it, but when it comes down to spending 275\375 for an ESC and another 280\300 for a nice motor, wow, one really hates to make a mistake, lol...

It also appears, using the speed calc, that 16\47 gearing with 2nd gear of my 3 spd trans and "maxx" tires, 4.25 inch or so, only nets me 30 ish +\- mph, I'm not looking for 100 mph, but 700 out of pocket for 30 mph seems steep...

Savage diffs - 13\47
Savage 3 spd trans figured in second gear
7S2P A123 - factored at 3.6v
Nue 1515 1.5y
"maxx" tires 4.25 inch - I may be off on this, should have measured before I calculated and posted, apologizes if I am.
16\47 pinion\spur

Thanks!
purp

[BrianG]

I'm thinking he means 14 cells is the limit because the battery pack starts becoming quite heavy.

Higher voltage creates higher current on a given motor. The idea is to select a motor with a higher wind (therefore higher resistance) making for less current. As you can see though, higher voltage means heavier packs even though they may be just 1p. When you start getting really high voltage (over 6s) regular lipos start becoming more and more appealing because of their reduced weight. I was hesitant to get lipos at first (opted for emolis at the time), but now I have them, I certainly wouldn't go back to NiMH. Safety isn't the forefront of my thoughts as long as they are charged/discharged correctly, and reasonable attempts at protecting/securing them are implemented (no tape or zip-ties for me!).

No, BL isn't really complicated, but the initial cash outlay can be steep. That is why we are all here; to help people make the best decision based on their budget and their needs/desires.

Maxx tires are closer to 5.6", which can make a HUGE impact on overall speed. And don't forget to account for ballooning! That too can make a substantial difference. Using 5.65" for the tires, I get 25.34mph in 1st, 31.74mph in 2nd, and 39.12mph in 3rd. And since you are using the 3 speed tranny, you can gear the spur/pinion more aggressively since first gear is so low. I don't know what spur/pinion options are available for the savage, but I would gear up a bit. You should easily be able to attain 45-50mph with that Neu motor.

[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 BrianG

I'm thinking he means 14 cells is the limit because the battery pack starts becoming quite heavy.

Higher voltage creates higher current on a given motor. The idea is to select a motor with a higher wind (therefore higher resistance) making for less current. As you can see though, higher voltage means heavier packs even though they may be just 1p. When you start getting really high voltage (over 6s) regular lipos start becoming more and more appealing because of their reduced weight. I was hesitant to get lipos at first (opted for emolis at the time), but now I have them, I certainly wouldn't go back to NiMH. Safety isn't the forefront of my thoughts as long as they are charged/discharged correctly, and reasonable attempts at protecting/securing them are implemented (no tape or zip-ties for me!).

No, BL isn't really complicated, but the initial cash outlay can be steep. That is why we are all here; to help people make the best decision based on their budget and their needs/desires.

Maxx tires are closer to 5.6", which can make a HUGE impact on overall speed. And don't forget to account for ballooning! That too can make a substantial difference. Using 5.65" for the tires, I get 25.34mph in 1st, 31.74mph in 2nd, and 39.12mph in 3rd. And since you are using the 3 speed tranny, you can gear the spur/pinion more aggressively since first gear is so low. I don't know what spur/pinion options are available for the savage, but I would gear up a bit. You should easily be able to attain 45-50mph with that Neu motor.

LoL, I see your point, I went back and measured my tires and you are in fact correct, not that I ever had any doubt. When I made the change, the numbers quickly fell into place.

I've been thinking of ebaying my A123s and going back to Li-Pos, but I think I'll give them a fair shot in this new truck before I take that route. Not to mention my wife would have me skinned and hung out back if I dumped them and spent more money having just picked them up, lol... I was initially using a simple "L" frame for my packs, hung from each side of my Savage using a plate similar to Serums, looks like I'll have to rebend myself some "U" frames and start protecting my investment!!

[BrianG]

Lol, I have been wrong a number of times, so don't think I'm infallible.

Don't get me wrong, I'm not suggesting that you sell the A123s and get lipos because the M1 cells ARE nice. It's just that those cells get a little heavy once you get to the high cell counts.

If you decide you want really high speeds, ballooning can be as high as 0.5" to 1" depending on the tire. You plug that into the calc and watch the top speed get up there fast!

[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

[Finnster]

Well I'm always looking for more info so I can make better predictions, but they are always just that.

Check out ZPB's Revo thread in here. He is using a 1700kv Neu/7S A123, albiet a smaller 1512. He's was only pulling a max of 80A, but on a lighter Maxx. In on 1P, the lighter Revo is getting a bit more v-drop. If you stick to 2P, you should be good upto ~140A, so I think the 1700kv 1515 would be a monster, but very doable, esp on a 160A ESC (I'm upgrading my prediction ;) ).

I have no idea if the tranny will handle it (not my dept.) but the motor and batts should give you a kickass setup.

[Purplefade]

Originally Posted by Finnster

Check out ZPB's Revo thread in here. He is using a 1700kv Neu/7S A123, albiet a smaller 1512.

If you stick to 2P, you should be good upto ~140A, so I think the 1700kv 1515 would be a monster, but very doable, esp on a 160A ESC (I'm upgrading my prediction ;) ).

Finnster, couldn't find ZPBs Revo, but I did find his insane twin motor, twin MM Maxx, that thing is nuts! The good news though, I think we've solved my "problem" :-) I've pretty much decided on the MGM16024, and the Neu 1515 2.5d/F running on 7S2P A123s. Worst case, if I have to I'll back her down to 6S2P, but as you mentioned, with the headroom of the MGM and a solid motor choice, my packs will most likely be my limiting factor and I think they'll be OK.

Now all I have to do is worry about that tranny, lol... Fingers crossed it holds up long enough for me to get things situated, tested and have at least some fun with it before it goes south on me. Worst case, it looks like I'll be scowering the net for steel\alloy gears for that three speed, or looking for some other creative way to slide a "slipperential" solution in there.

Just thinking out loud back to what you mentioned, with the three speed and first gear being so low, I should have more pinion options making me think the 1700Kv should be fine as I can gear up or down based on heat, seem reasonable?

Now all I have to do is wait for Mike to get the MGMs and my 2.5 back in stock

I think the last thing I need to iron out and I may give Serum a ping for this one, is a creative motor mount. I originally dropped my stock motor plate down by about 3/16 of an inch or so, maybe slightly more and used a simple "clamp ring" setup to attach the motor to the mount using the stock adjuster holes. It worked VERY well, but is a challenge to adjust the mesh on due to its very close proximity to the tvps... Loved Serums adjustable setup, but as of now it appears that it may have been a one off solution, I'm surprised Mike doesn't yet have an offering for the Savy, but I guess we have to work off of demand :-(

GREATLY appreciate all of yours, BrianGs and the gangs info and input on my project and please, please don't hesitate to "smack me in the head" if you see me wondering off course

If I ever get this finished (my wife just hates me spending money on RC, but hey, you have to pay to play right) I'll certainly post pics!

[Purplefade]

Originally Posted by Purplefade

Finnster, couldn't find ZPBs Revo, but I did find his insane twin motor, twin MM Maxx, that thing is nuts!

DOH.. No sooner did I post this and I found the thread. Looks like an excellent project and I'll certainly be keeping my eye on how things go, not to mention tossing in my two cents when I find out how my 7S2P packs hold up.