[NSRCA-discussion] Going Electric - Battery Questions

[Larry Diamond]

Duane,

Been thinking about this today, with the new understand of how an ESC
actually works...

This makes a lot of sense as to what many are saying about having a higher
discharge rated pack... Assuming I understand it correctly...

I'm going to make some comments to see if I have the correct understanding.
Some electric newbies may find this enlightening... Please correct any
misunderstandings or statements.

Is it fair to say...?

1. All things equal in the system, except the battery discharge rate:
	a. The same battery voltage under load (40C or 20C battery) at a
given steady (constant) throttle setting, the RPM is the same.
	b. Assumes different points of time in the discharge curve, only
comparing the same voltage under constant load.
	
2. The voltage curve for battery discharge is slightly shallower for a 40C
over a 20C battery.
	a. Resulting in a slightly higher RPM for a any specific throttle
setting compared to a 20C pack, or lower discharge rated pack..

3. The 40C battery will have a higher supplied voltage under load resulting
in a measurable amount higher RPM.
	a. The amount of the RPM gain is dependent on ESC, Motor, and Prop
choice.
	b. This is due to lower internal system resistance directly
proportional to the lower internal battery resistance.

If the points above are true, then the considerations in the set up of a
throttle curve might be:

A. As the battery is discharged a specific throttle setting position will
result in a loss of RPM over the time of flight.
	* This is due to the depleting voltage under load vs discharge time.

B. The throttle response will seem to be more consistent from beginning to
the end of flight with a 40C pack.
	* This is due to the 40C battery over the 20C battery having a
shallower discharge curve for voltage under load and lower system internal
resistance.

C. --->>>Do not set the throttle curve based on a fresh battery or a drained
battery (at end of a sequence).

D. Set up the throttle curve based on the middle of the sequence.
	* This should result in a better balance from beginning to end of
flight.
	* The Tx throttle curve will change (shift lower) as the battery
voltage under load drops over time of flight.
	* A Tx throttle point established for cruise speed will result in a
gradual decrease in RPM over the time of flight.

***Myth Busted***
Myth - All things equal in the electric propulsion system and only changing
from a 20C battery to a 40C battery to fly a higher class will make the
plane too hot for a lower class.

--->>>Reality - Proper throttle curve settings and in flight throttle
management should be sufficient or changing the upper end point for the
throttle to limit max RPM.

*****Conclusion - As everything else in pattern it really all boils down to
flying style, preference, budget, and weight*****

Looks like the best battery choice for me will be to see what the weight of
the 2M Monolog is without the battery and then buy batteries to try and keep
the weight under 5Kg.

I'm not sure if this helps anybody else, but this thread has corrected my
mindset and gives me a bit more confidence going to electric.

Very curious to read others comments regarding this post. Especially if my
understanding is much better than before...

Larry Diamond


-----Original Message-----
From: Larry Diamond [mailto:webmaster at diamondrc.com] 
Sent: Friday, December 07, 2012 11:49 PM
To: 'General pattern discussion'
Subject: RE: [NSRCA-discussion] Going Electric - Battery Questions

Thanks Duane....

The little light went on in my head when Keith's response... My mind was
locked onto voltage control by the ESC, which is evidently not how it
works....

I now understand how the battery voltage directly affects the RPM. And what
your stating makes sense to me. Great explanation by the way...

This helps going forward....

Thank you very much!!!!

Larry Diamond

-----Original Message-----
From: nsrca-discussion-bounces at lists.nsrca.org
[mailto:nsrca-discussion-bounces at lists.nsrca.org] On Behalf Of Duane Beck
Sent: Friday, December 07, 2012 11:30 PM
To: General pattern discussion
Subject: Re: [NSRCA-discussion] Going Electric - Battery Questions

> The ESC controls the voltage to the Motor, not the battery.

This is not strictly true.  The ESC controls the duty cycle during which the
battery's voltage is applied to the motor.  This has the effect of
controlling the average voltage, but as some fraction of the battery
voltage, not referenced to some absolute voltage.  At 100% duty cycle,
obviously the full battery voltage is applied.  At 50% duty cycle (1/2
throttle, more-or-less depending on the ESC and your throttle curve), about
half the battery's voltage will be applied to the motor.  If the battery's
voltage sags less under this load (because it has a higher C or lower
internal resistance), then the motor will try to spin faster (based on its
KV).  Spinner faster with the same prop (load) means more torque is required
and, with electric motors, current is proportional to torque.  So, for the
same throttle position, you end up with more voltage, more current, and more
power (proportional to the square of the voltage increase).  You can get the
same power output as the lower C  battery by reducing the throttle, but
you're effectively reducing your usable throttle range - and thereby making
it more sensitive.

Duane
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