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Thread: 8 Cells Nimh > 2s Lipo?

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  1. Arrow 8 Cells Nimh > 2s Lipo? 
    #1
    I've ordered the Mamba Max 7700 for my TC4. In their specs, they say to use either an 8 cell nimh or 2s lipo.

    But from what I gather,

    8 cell nimh = 9.6V
    2s lipo = 7.4v

    Other than lipos being lighter and smaller, would it not be better to go with the more powerful 8 cell nimh? Or is there something else I'm not aware of?
     

  2.  
    #2
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    The NiMH is higher voltage when you are not drawing any current, but they may not be when you are drawing 50+ amps. Most descent lipo packs are capable of much high current output than NiMH cells. If your intent is purely top speed for drag races up and down the street, then the 8-NiMH might be faster.
     

  3.  
    #3
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    I forgot to mention that the real reason that the specs may be stated this way is the motor can handle 8 cells, but there is no Lipo equivalent to 8 cells so you can only use a 2S lipo. 3S is too much for that motor so it can't be used.
     

  4.  
    #4
    Just curious then... what about a lower C rated 3s lipo? I have extra one I used on my airplane. It's a 3s 20C 1100mah. I know I won't get much run time out of it, but if I can squeeze 10 mins of run time out of it that'd be cool.

    20C x 1.1a = 22amps 'but' it's also 11.1v instead of 7.4v...... Does that compensate at all?
     

  5.  
    #5
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    Using a lower amp lipo will blow your battery up very quickly. The voltage drops through the floor and the battery gets HOT very quickly.

    DO NOT use that pack.

    Also, I used a 2s lipo on my Mamba Max 6900 and an 8s GP3700 NiMH pack with my Mamba Max 6900. The LiPo was a LOT stronger off the line and therefore felt faster. I think the NiMH had a slight advantage at top end, but not much.

    Also, you've bought the wrong motor. The 5700 is hard enough to gear on a 2wd truck. The 7700 is just going to wind up overheating regardless of what you do. I doubt you can really gear it right unless you're just doing high speed runs with LOTS of cool-down time. I learned this the hard way on my 6900 motor. Wound up buying a Hacker to replace it, and a 4600 is going on everything else. Then a VXL 3500 I still need to pick up for my Slash.

    -Eric
     

  6.  
    #6
    I have to disagree about the wrong motor. I have the 7700 in a TC3 and it is great. A little hard to control if I gear it for too much speed though. But for normal running I can gear it to outrun most nitro cars and it never overheats.

    If you can get a good lipo I would go that route. It will accelerate harder, handle better, and last longer. 8 cell nimh will just stress everything more and likely overheat your equipment, all for just a little more top end. Just make sure its a fairly high amp rated battery, no 20C 1100's. You run that with your 7700 and you will see fire!! I think an 80 amp capable battery is a safe place to start.
     

  7.  
    #7
    Thanks guys. Yeah I won't bother with that 3s lipo. Know any place for some decent lipos at a good price? I used to buy from XUS Hobby but their website says they're closed for maintenance... for the last while now.
     

  8.  
    #8
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    Quote Originally Posted by burnineyes View Post
    I have to disagree about the wrong motor. I have the 7700 in a TC3 and it is great. A little hard to control if I gear it for too much speed though. But for normal running I can gear it to outrun most nitro cars and it never overheats.

    If you can get a good lipo I would go that route. It will accelerate harder, handle better, and last longer. 8 cell nimh will just stress everything more and likely overheat your equipment, all for just a little more top end. Just make sure its a fairly high amp rated battery, no 20C 1100's. You run that with your 7700 and you will see fire!! I think an 80 amp capable battery is a safe place to start.

    A T4 and a TC3 are 2 different animals. The TC3, depending on surface, can probably spin the tires quite nicely. The T4 is bound to hook up about anywhere with even halfway decent tires, not to mention the extra rotating mass.

    We ran a 7700 in a Stampede on 8 NiMH cells for a while and the thing always ran too hot, even with a 13 or 14t pinion (whatever the smallest thing was we could get on there). He ended up with a 5700 + 2s lipo on that thing and it worked a lot better.

    The system will probably run cooler on the 8 cell NiMH batteries because of their higher internal resistance than a 2s lipo. Heat doesn't come from voltage. It comes from current. You could also run an E-Maxx on 7.2v, but for the same performance you'd have to draw twice as much current (since there is half the voltage) and it would literally run twice as hot, if not more.

    -Eric
     

  9.  
    #9
    I totally agree that a T4 and TC3 are different beasts. I can barely use the 7700 in my T4. They dont make a spur big enough to make it useful on track. But I think he said TC4 not T4, in which case I think the 7700 on 2s or 6 cell nimh is perfect, as long as you can drive well.

    The part about current causing heat I agree as well. My thoughts on that is; with the lighter weight and better voltage holding of the lipo, it will feel faster. Lipo would likely only lose a race in a VERY long straight line. To make the nimh feel the same it would be pulling more current.

    A good example is my TC3. I dont own any lipos so for the longest time I was just using cheap chinese 3300 cells. With 8 on my 7700, I ran thru a radar at 56, then 58, then 54MPH completely sideways. I had about 150 feet run up on the radar. After 4-5 passes it was pretty hot, but thats geared REALLY high. Finally I got some good name brand matched 4200s. I ran 7 of those cells for a few speed runs and I got a pretty consistent 54MPH. The car felt much faster though, and I got a few more passes in before the temp monster creeped up. For the last test I found a friend willing to let me use his Reedy 5000 7.4 lipo. The car rocketed out of the hole and felt like a new machine. I hit 51MPH over and over, and made a good ten passes before having to let it cool. The best part was it was going straight after about 75 feet, with the nimh it was hard to keep it going straight. Since the better batteries hold their voltage much better, there is not as much current needed to go fast.
     

  10.  
    #10
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    Sorry, I misread the original post.

    You make a good point about the voltage making power instead of the current, which is right on. If you're bashing an offroad vehicle, the full power tends to come quite often, in which case the lipos will run warmer. If you're actually trying to make a controlled acceleration though, the LiPo will run cooler because you don't need as many amps to make a given amount of power.

    -Eric
     

  11.  
    #11
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    Well I just ordered a F103GT Courage and I am going to be running that with a Mamba Max 7700 and a 6cell nimh or a lipo. What kind of speed should I expect.
     

  12.  
    #12
    I dont really know that car very well but I think its a pan car, right? If so it will be a handful. I would guess you could gear that car to go just about any speed you want if you have good batteries. Anywhere between 35 or 40 on up to 60 plus if you want.
     

  13.  
    #13
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    Haha, we actually ran a 7700 with 8s nimh in a pan car.

    I don't know if we ever actually topped it out or not. Spektrum didn't have enough radio range for our setup. It was probably good for 60+ miles an hour but I doubt it ever really broke 50.

    Be ready to add a big wing to the back. It'll look like a high school student's Honda Civic when you're done but it ought to stay on the ground at least.

    -Eric
     

  14.  
    #14
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    I actually do have a 8 cell that I bought a couple of months ago maybe I will try it out.
     

  15.  
    #15
    I say good luck with that. I have an old Bolink Legends car i put my MM7700 in with 7 cells and its just ridiculous. I had to baby as lightly as I could for at least 200 feet. Then I would roll on the power VERY slowly and it would fishtail down the street as far as I could see it. That was only a few lucky passes though, usually I would give too much power too soon and it would end up in the ditch. Watching a R/C fishtail at 50 plus is lots of fun though.
     

  16.  
    #16
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    You guys are going to wind up making me toss my Mamba Max 6900 into my BoLink Digger.

    -Eric
     

  17.  
    #17
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    Or you can get the new 9000kv and try that one.
    Actually LRP makes a motor which is over 11,000KV.
    And on their website it only costs $45.
     

  18.  
    #18
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    It's hard enough to gear a 6900kV motor let alone something that nasty.

    I just don't understand... I guess it's a sales number... If it's a bigger number it must be better.

    -Eric
     

  19.  
    #19
    Yeah those high kv motors are def not for racing or normal driving in any car. They are fun to do high speed passes but thats about it.
     

  20.  
    #20
    Hey guys, thanks for all the replies! These Mamba 7700 motors are rated at 100 amps.

    So would a 2s lipo rated at 100 amps continuous be the absolute minimum? Or should I go higher for good measure? I ordered a 130amp continuous one and am seeing lots of cheap 100amp ones.
     

  21.  
    #21
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    Your motor will not draw 100 amps continuous.

    Doing some simple math, we found out that our average 1/8 buggy brushless conversion was only averaging about 12-14A continuous. You will probably see peaks around 80-100 amps though. That's when you generate the heat; under acceleration.

    A 20C 5,000mAh battery will be fine. That gives you 100A continuous and probably something like 150-200A burst.

    The more amps it claims it can give will mean lower internal resistance, so the pack will run cooler and hold voltage better since less power is wasted as heat.

    -Eric
     

  22.  
    #22
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    Would the motor pull less amps if it is in a lighter car? I am going to use a battery that is 80 amp contusion with a mamba max. IT should be ok shouldn't it.
     

  23.  
    #23
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    fishymamba;2394472]Would the motor pull less amps if it is in a lighter car? I am going to use a battery that is 80 amp continuous with a mamba max. It should be ok shouldn't it.
     

  24.  
    #24
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    You should be fine. Just watch the temperature.

    LiPos should NEVER be above 130F at any point, so give it 120 on the outside for some safety. If you're warmer than that, the pack will start to swell due to internal breakdown. Things don't end well if you keep overstressing it like that.

    -Eric
     

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