[NSRCA-discussion] Going Electric - Battery Questions

[Patrick Harris]

Well there you go. There's your answer.

On Fri, Dec 7, 2012 at 9:04 PM, Keith Hoard <khoard at gmail.com> wrote:

> The ESC isn't a variable resistance. It is a switch that turns on and
> off thousands of times per second. So each "on" sends full voltage and
> current to the motor.
>
> If you use higher C batts, then they will hold voltage better during
> each ON cycle, thus delivering more amperage and wattage to the spinny
> thing up front.
>
> Sent from my iPhone
>
> On Dec 7, 2012, at 22:57, Larry Diamond <webmaster at diamondrc.com> wrote:
>
> > "The higher C packs will have lower internal resistance, which means
> they'll maintain a higher voltage under load."
> >
> > Agree...
> >
> > "Higher voltage means the same motor with the same prop will draw more
> current.  More voltage * more current = a lot more power (W = V * I)."
> >
> > Not sure I buy what is being implied... Electronic theory is correct...
> but for one simple reason I would not agree...
> >
> > The ESC controls the voltage to the Motor, not the battery.
> > The motor is rated xxxKv or volts per RPM.
> >
> > Current draw of the motor is a function of the motor performance and can
> be graphed as Voltage vs Current. This graph will remain constant except
> for variables like current surge (Increase of Throttle) and internal motor
> resistance as a function of wear and tear. Another variable is the actual
> prop being used creating a function of mechanical resistance resulting in
> more or less current drain for a given voltage.
> >
> > Ohm's Law Power Formula is Watts = Voltage * Current (as you stated)
> >
> > The Propulsion Power is fixed at a given voltage supplied by the ESC.
> Except for current spikes related to surges due to throttle increases. (I
> suppose this gets complicated just like chasing throttle response in a
> nitro engine)
> >
> > The Motor will not see a higher voltage resulting in the lower battery
> resistance. However, the ESC will...
> >
> > Ohm's Law: E = I * R; or, Voltage = Amps * Resistance (Ohms)
> >
> > So, a decrease in internal battery resistance will actually result in a
> decrease of current draw from the sum of the system to maintain a constant
> supplied voltage by the ESC.
> >
> > Current is pulled from the system or supplied based on demand. Kinda
> like the vacuum of the carb sucking the fuel from the tank.
> >
> > None the less, I was told off line by a reliable source flying FAI, that
> you can feel the difference between a 40C and a 20C Discharge Rate. I don't
> believe this is related to the internal battery resistance resulting in
> system power (watts) though. Perhaps more system power available than a 20C
> pack on the backend of the discharge cycle or during throttle increases
> (current surges/throttle response), but all controlled by the ESC.
> >
> > I really was only asking if my math was right for discharge rate and
> battery capacity vs flight time. Both have been answered...
> >
> > This is almost as entertaining as a discussion on Snaps, which I haven't
> seen in a while... :)
> >
> > If I'm off base, please let me know...
> >
> > LD
> >
> > -----Original Message-----
> > From: Duane Beck
> > Sent: Friday, December 07, 2012 6:40 PM
> > To: Larry Diamond ; General pattern discussion
> > Subject: Re: [NSRCA-discussion] Going Electric - Battery Questions
> >
> > The higher C packs will have lower internal resistance, which means
> they'll maintain a higher voltage under load.  Higher voltage means the
> same motor with the same prop will draw more current.  More voltage * more
> current = a lot more power (W = V * I).
> >
> > Duane
> >
> > ----- Original Message -----
> >> From: "Larry Diamond" <ldiamond at diamondrc.com>
> >> To: "General pattern discussion" <nsrca-discussion at lists.nsrca.org>
> >> Sent: Friday, December 7, 2012 6:17:47 PM
> >> Subject: Re: [NSRCA-discussion] Going Electric - Battery Questions
> >> Interesting point... I don't understand how changing from 20C to 40C
> >> pack makes a difference using the same Motor and ESC combo. If it's
> >> battery heat, my electronics background would tell me the pack is
> >> probably being dicharged too deep. Not good for the life of a battery.
> >>
> >> I suspect the result of not changing the Motor/ESC and only changing
> >> the battery will reult in a shorter battery life; and, over loading
> >> the power plant which will probably result in damaging the ESC and/or
> >> the Motor.
> >>
> >> I could be wrong, cause I really have no experience with electric
> >> power set ups in planes. I just don't understand the logic going from
> >> 20C to 40C and the result being "too hot for intermediate".
> >>
> >> It wouldn't surprise me to see Masters or FAI having a higher power
> >> set up, like 3,500 to 4,000+ watts. Then depending on the max current
> >> draw,  40C may make sense.
> >>
> >> My set up will be about 2,700 to 3,000 watts based on specs... It may
> >> not be sufficient for Masters or FAI.
> >>
> >> Perhaps this will turn out to be a good constructive discussion for a
> >> number of people like me, but I'm curious as to other opinions who
> >> have practicle knowledge.
> >>
> >> Larry Diamond
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