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All Forums >> Radios, Batteries, Clubhouse and more >> Batteries & Chargers >> What's the difference
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What's the difference - 12/14/2001 8:54:30 PM   
TopShelf



Posts: 648
Joined: 12/6/2001
From: West Linn, OR, USA
Status: offline 		If I have a 4 cell receiver pack that is rated at say 1200Mah does the cell size (Sub C vs. AA) make any difference?

Kent

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What's the difference - 12/14/2001 10:26:57 PM   
mglavin



Posts: 5203
Joined: 11/29/2001
From: Elverta, CA, USA
Status: offline 		Weight and the available loaded current for discharge is the main consideration for our use.

Meaning that the sub-C's will provide a higher level of current/amperage at a given discharge voltage.

For our needs the sub-C's are heavy and will not realize there full potential...

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What's the difference - 12/14/2001 10:49:00 PM   
TopShelf



Posts: 648
Joined: 12/6/2001
From: West Linn, OR, USA
Status: offline 		Thanks for the reply. I figured weight was one issue. I'm an electronics ignoramous so don't really understand the "...current/amperage at a given discharge voltage..." part. If you would care to explain, I would greatly appreciate the education.

So the additional current/amperage has no impact on the servos? Speed? Strength? or ?

The plane I am considering will have 8 servos (1 std for each elevator half, 1 std for each aileron, 1 std for the rudder, 1 std for throttle, 1 mini for nose gear stearing, 1 micro for air valve actuation)

Kent

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What's the difference - 12/14/2001 11:18:07 PM   
mglavin



Posts: 5203
Joined: 11/29/2001
From: Elverta, CA, USA
Status: offline 		Kent

I'll try and describe a user freindly ascertation..

Many different cell types make this description rather generic.

The sub-C is capable of delivering 1.0 volts at 20amps for approximately 4 minutes.

The AA is capable of delivering much less amperage over the couse of the discharge curve.

These numbers are just approximate guess's, not to be confused with reality or actuall cell performance. Maybe Red S. will see this and offer some known specifications...

Total cell count does make a marked improvement in servo performance. Total cell count and capacity offer the user the ability to increase servo speed and torque at the sacrifice of higher discharge rates and decreased servo motor life. Meaning 5-cell will deliver approximately 6 volts as compared to a 4-cell at 4.8 volts, 6 volts equates to more speed and torque.

Larger capacity batteries have the abiltiy to offer more run/use time and typically offer more reserve for higher current load situations. Again cell type is a decided factor when discussing available loaded current/amperage discharge voltage.

The cells we generally see used in our radio systems are designed for lighter use than the cells one would use in an electric powered airplane or car.

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What's the difference - 12/14/2001 11:36:04 PM   
TopShelf



Posts: 648
Joined: 12/6/2001
From: West Linn, OR, USA
Status: offline 		Michael,

Thanks, hopefully I wont frustrate you with my thickness.

Ok, so I understand that more voltage means more power so if one is running 6 volts, the servos will move quicker and have more holding power thant 4.8 volts but they will wear out sooner along with reduced battery capacity.

What I am still struggling with is the impact of amperage/current is on the system. I guess I don't really understand what amperage is/means. Webster defines it as "...the strength of a current of electricity..." but strength seems to me like voltage. I know at work when we need to add more equipment (computers, printers, lights etc.), the electrician makes sure the circuit has sufficient amperage. So it would seem the more servos I have, the more amperage I would need?

Ignorantly yours,

Kent

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What's the difference - 12/14/2001 11:47:28 PM   
TopShelf



Posts: 648
Joined: 12/6/2001
From: West Linn, OR, USA
Status: offline 		Michael,

Ok, after I posted the previous reply, I did some searching and found the following.

"Ohm's Law states: In a DC circuit, the current is directly proportional to the voltage and inversely proportional to the resistance. In other words, the water flowing in a pipe (amperage) will be increased if the water pressure (voltage) is increased. And, if the restriction (resistance) in the pipe is less, the water flow ( amperage) will be more"

So, does this mean that in our cicruits, amperage is irrelevent since it is directly proportional to voltage? And, if this is true then does that lead us back to were we started i.e. Sub C cells are just heavier and don't offer any real advantage.

Kent

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He Who is a Slave to the Compass has the Freedom of the Sky.
www.TopShelfRC.com

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What's the difference - 12/15/2001 12:42:26 AM   
mglavin



Posts: 5203
Joined: 11/29/2001
From: Elverta, CA, USA
Status: offline 		Kent

Well, kinda maybe...

[QUOTE]Originally posted by TopShelf

In other words, the water flowing in a pipe (amperage) will be increased if the water pressure (voltage) is increased. And, if the restriction (resistance) in the pipe is less, the water flow ( amperage) will be more"
[/QUOTE]

Decreased resistance will be realized in lower amperage if the load stays the same, irregardeless of the voltage. If I raise the voltage, an incresae of the amperage draw will be realized, due the fact that I am absorbing more power to obtain additional torque and speed.

[QUOTE]Originally posted by TopShelf

So, does this mean that in our cicruits, amperage is irrelevent since it is directly proportional to voltage? And, if this is true then does that lead us back to were we started i.e. Sub C cells are just heavier and don't offer any real advantage.
[/QUOTE]

The actual demand as seen by the flight pack vairies considerably based on voltage, servo count, servo type, system resistance and flight surface loads.

Digital servo's are power hungry, they update there position at much greater rate than there analog cousins. Meaning they are continually attemptting to provide the desired location as required by the RX, this is known as "jittering" or something like that... While the servo is looking to maintain this position analog or digital, the amperage required to maintain this position is inversely proportionate to the flight surface load. Faster update's of the required position is realized in an increased power demand.

Therefore if I am always attempting to maintain the desired position, the end result will be the ability of the servo to resist control surface deflection, proportionate to the update rate, which equates to more power consumed.

The big queation is what is the actual "average" load on the battery pack under normal flight conditions in amperage.

If we knew the load we could choose a battery pack accordingly, providing we had the specifications of various batteries/cells...

It is commonly recognized that the AA cells will provide the required current/power/amperage capacity to run many servos... Just how many, under what conditions is the variable...

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What's the difference - 12/15/2001 2:11:15 AM   
TopShelf



Posts: 648
Joined: 12/6/2001
From: West Linn, OR, USA
Status: offline 		Ok, I think I get the just of it.

Thanks for indulging me

Kent

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He Who is a Slave to the Compass has the Freedom of the Sky.
www.TopShelfRC.com

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Batteries.............. - 12/15/2001 12:11:56 PM   
Jackjet



Posts: 1023
Joined: 12/9/2001
From: Hesperia, CA, USA
Status: offline 		Kent,
lets say you have a airplane that has 13 servos in it(I do )-and I am running a 5 cell sub C 1700mah sanyo nicad-I assure you that you WILL get much more flight time out of that than you would flying the same amount of servos with a 4 cell AA pack-but you can always fast charge between flights.........


Jackjet

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Jack Jet Williams
Hesperia,Ca

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       Post #: 9

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