[NSRCA-discussion] Going Electric - Battery Questions

[Keith Hoard]

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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