Capacitor For MM 4S

[Arct1k]

Oh god I'm 17 again and thinking about capacitor charge and decay graphs!

RC circuits charge according to 1-e-t/RC

So if I want to reach 63% (1 time cycle) in 3secs on a 2700uf cap

3/2700uf = 1111 ohm resistor correct?

12 secs would get me to mostly charged...

[Arct1k]

....

[BrianG]

Since it takes 5 time constants to get to 100% (or really close). I would pick something like 10 seconds, divide that by 5, and then use the formula to get 740 ohms for your resistor. Anything between 680 and 1k would be good enough since any voltage value on the cap over 0v will help reduce that spark.

well i have a ?

i installed the novak cap i had and also put some holes in the case, and i also run a fan, and my MM has 18 fets, and it runs on 5s (did a quick test and it had a little cogging , didnt change my settings from 4s - which has no cogging at current settings and gearing)

this is in my xt8 with a 9xl

....?....is it worth really running on 5s for the few less amps that this will draw or stay safe and run it on 4s and not blow it up.....

thanks

[Matthew_Armeni]

Just upping the voltage will not change the amount of amps the motor will draw, a 9XL draws the same whether on 4s or 5s (if I'm not mistaken, I'm pretty sure I'm not) it will just perform better on 5s.
Someone please correct me if I'm wrong about this

[sikeston34m]

Originally Posted by Matthew_Armeni

Just upping the voltage will not change the amount of amps the motor will draw, a 9XL draws the same whether on 4s or 5s (if I'm not mistaken, I'm pretty sure I'm not) it will just perform better on 5s.
Someone please correct me if I'm wrong about this

I would like to take a stab at this one.

Watts=amps X Volts

Let's look at two setups both geared for 40mph top speed.

#1 runs on 4S

#2 runs on 6S

Both cars accelerate at equal rates and in a race would be completelyl equal.

The idea that higher voltage setups draw less amps relates back to Watts = Amps X Volts

Since both cars are equal, both motors are putting out the same total watts of power.

#1 is drawing let's say 60 amps at top speed. 4S lipo will be about 14.8 volts loaded. 60amps X 14.8 volts = 888 Watts of motor power.

#2 being the equal to #1 that it is, will be drawing this kind of energy to produce the 888 Watts of power. 22.2 volts under a load on 6 S
888 Watts = 40 amps X 22.2 volts

What can we conclude from this information?

If you noticed, the word "equal" was used alot in that description. If you take #1 setup and just put it on 6S without making any other changes such as gearing, it's going produce more power and draw more amps.

If your goal is to lower the amp draw of a setup, you must change the gearing to lighten the load (resistence) on the motor so it will draw less amps.
This could even involve using a different motor of a lower kv to reach this goal.

thats what i thought, the 9xl is a better 5s motor , but it runs good on 4s

i will have to get some smaller pinions and try it

[Matthew_Armeni]

Thanks! I never really thought about the re-gearing though it should have been ovbious, doh! Now, is there also a point at which increasing voltage and re-gearing will no longer be beneficial? I mean at some point the motor will be spinning too fast and cause overheating, right? Is this where finding the sweet spot of the motor comes in? Around 35K is what I've been reading on here.

But, the 9XL doesn't draw that many amps anyway, entjoles- what are you hoping to get by running 5s? I'm guessing more top end.

[sikeston34m]

Originally Posted by Matthew_Armeni

Thanks! I never really thought about the re-gearing though it should have been ovbious, doh! Now, is there also a point at which increasing voltage and re-gearing will no longer be beneficial? I mean at some point the motor will be spinning too fast and cause overheating, right? Is this where finding the sweet spot of the motor comes in? Around 35K is what I've been reading on here.

But, the 9XL doesn't draw that many amps anyway, entjoles- what are you hoping to get by running 5s? I'm guessing more top end.

Yes, there is a point where just changing the gearing will no longer be beneficial.

Let's go back to the two setups I mentioned that were equal in performance with just a few things different to make them perform the same.

#1 on 4S, the motor spins at 35k but has the same gearing as #2. (2365 kv motor drawing 60 amps at 40mph)

#2 setup on 6S spins the motor at 35k. (1577kv motor drawing 40 amps at 40mph)

With the two setups equal in top speed, what are we hoping to get? Less amp draw usually means less heat build up in components. Heat build up is like an Energy leak. It's like running a battery powered electric heater. We spend that energy to make heat instead of motor power.

#2 setup will run longer on each charge and probably tend to accelerate faster than setup #1 because the lower the kv of a motor, the more torque it makes.

[phildogg]

I'm lost. could I add a cap and not worry too much about the zap? any Radio Shack carry a useful cap or not?
phil

[sikeston34m]

Originally Posted by phildogg

I'm lost. could I add a cap and not worry too much about the zap? any Radio Shack carry a useful cap or not?
phil

I kinda like the zap when I plug in those Lipo's. It tells me the power is going to be flowing like gang busters.

I don't know what all the big fuss is about on "wearing out connectors". I haven't worn one out yet, but if I do, they're pretty cheap ya know?

Out of all the capacitors that Radio Shack carries, NONE of them are low ESR or rated at 105C. They have the really cheap stuff.

well i was thinking of the sweet spot, on 5s and using 3.7 v/p/c = right at 35k rpm , and also a speed of 38 mph with out gearing up on 4s(around 29k rpm)

i was also thinking of whether or not 5s will let the smoke out

[KaztheMinotaur]

What is low ESR and what will happen if the caps are not low ESR?

[JeffEmbracedDC]

Originally Posted by KaztheMinotaur

What is low ESR and what will happen if the caps are not low ESR?

I have been told Low ESR basically means Low Resistance. Just a better performing capacitor. I was told a normal cap would work fine but a Low ESR would perform better and keep temps even cooler.

[JeffEmbracedDC]

Three questions.

1. Is 5600 uF completely unnecessary?:

2. Would you lose any performance (other than 2 seconds of runtime) choosing a 5600uF over a 2700uF cap?

3.Also, do we know if these Novak caps are Low ESR?

http://cgi.ebay.com/Novak-Power-Capa...QQcmdZViewItem

-Jeff