[bbbair]

Hi Guys;

This is a repost from my AT6 build - I think that this problem bears a look by more people than just the War Bird followers.

I have been having some problems with my servos in the Harvard – I have solved my issues, but in the process I’m feeling rather stupid – but the hobby is all about learning so I guess that I’ll get over it.

The Harvard has seven servos and each servo has it's own channel to control it, Ailerons and Elevators worked hesitantly and not in a uniform manner - certainly not the way that would allow safe operation of a RC Aircraft. While all the rest of the servos worked flawlessly.

Yes - the problem servos are Hi - torque to handle the pressures of the control surfaces and they require MORE Power to operate than the 'Standard' servos.

NORMALLY I use two receiver packs in my giant planes – but I have noticed that the average charge after a day of flying was only 400 ma on a 2300 ma battery pack. So I reasoned that a second battery and the ‘Battery Share’ device was not needed (it was also going to save me a bit of money...). So I left the second battery out this time (besides there isn’t much room in the nose of the Harvard) – What a Mistake!

After a lot of head scratching, charging batteries and changing of servos etc, I dropped in a second receiver battery in parallel using a Battery Sharing Device and the problem disappeared! It would seem the issue was current demand of the servos vs current capacity of the battery.

Now I have to install another battery as well as the switch and then re-balance… not a really big deal but – frustrating none the less.

Not AS frustrating as loosing a plane but still frustrating.


Happy Landings

[Max]

How many C was the battery in the plane when you tried it as a single pack?

This sounds like low current availability, and if the C rating of the battery is low, it doesn't give the required amps.
Pairing up the batteries in parallel, the C rating is doubled

[bbbair]

Max;
The battery pack is a series of four AA Cells rated at 2300 ma - as to how many 'C' it is... I have no idea.

I have been an Electronics Technician since 1979 and I have never seen that term used in the profession.

[Terry Gauvin]

C rating is simply the max current your batteries can supply, a 2300 ma pack at 1C could supply 2.3 amps max, 2C 4.6 amps etc...

[davidmc36]

Originally Posted by bbbair

.....four AA Cells...

The defacto standard these days has become 5 cell NiMh or LiFe 2S

If your 4 cell is NiMh it is easily conceivable with high demand servos that the C rating is not adequate and voltage is sagging to a point the servos do not operate properly.

Even though overall you did not draw a lot of charge out of the battery at the end of the day the instantaneous draw may be too much for it.

[davidmc36]

Originally Posted by Max

....C rating is doubled

C rating remains the same.

Capacity doubles.

Ergo: 2C x 2300mah = 4.6 amps

2C x 4600mah = 9.2 amps

[davidmc36]

Originally Posted by bbbair

.....I have never seen that term used in the profession.

http://www.batteriesinaflash.com/blo...d-demystified/

C Rate is derived from Coulomb's Law developed by French physicist Charles Augustin de Coulomb. The c-rate is the governing measurement of what current a battery is charged or discharged at.

[bbbair]

Thank you David!

I always learn on the RCC.

[Max]

Originally Posted by bbbair

Max;
The battery pack is a series of four AA Cells rated at 2300 ma - as to how many 'C' it is... I have no idea.

I have been an Electronics Technician since 1979 and I have never seen that term used in the profession.

Sorry, I didn't realise you were using AA size batteries. I though you were using a Lipo and some sort of BEC. Those type batteries are usually not rated as we we do with Lipos and LiFe's.

Still, it sounds to me like the batteries aren't able to produce the capacities required to drive the servos.

[Max]

Originally Posted by davidmc36

C rating remains the same.

Capacity doubles.

Ergo: 2C x 2300mah = 4.6 amps

2C x 4600mah = 9.2 amps

Hey David. Though you are correct, that is not what I am talking about.

I am talking about the capability of each battery to deliver the required amp draw. So I am speaking from a position of constant power demand's affect on one vs 2 batteries, and you are talking about the maximum capacity of a combined set of batteries in parallel. We are really talking about the same thing but from different positions

In the OP's problem, if a single battery is not able to deliver the power, I'm suggesting its because he may have exceeded the batteries C rating. Putting 2 in parallel divides the power drawn from each battery in half.

Using your exact examples, lets say the device draws 4.6A when in use.

A single 2C 2300mah battery has to give up its full 2C to deliver that 4.6A (100% if its C rating)

But if you have 2 2C 2300mah batteries in parallel, and the device still draws 4.6A, then each battery only has to deliver 1C (50% of its C rating), thus putting half the load on either battery.