gnirwin

My Feedback: (25)

I bought a pair of Futaba S3305 metal geared servos to use in my new Katana. The instructions that come with the servos is as follows:
"This is the Futaba S3305 High Torque, Metal Gear Servo.per Futaba this servo is approved for use with NiCd batteries only. This servo can produce high-current draw from your batteries. If using NiMH or Li-Po batteries, make sure they are capable of delivering sufficient Amps."
What gives. I was planning on using a Nimh 1100 or possibly one of my older Lipos, 1200 2p with regulator. I was not planning on buying a new battery just to use these servos.

dirtybird

My Feedback: (5)

The low internal resistance of NI-CDs make the servos perform better. You can use the other batteries without problems. You will just not get as much torque out of the servos.

Silent-AV8R

It is exactly what it says. Many NiMH cells have a relatively high internal impedance. This limits their ability to deliver satisfactory voltage under load which reduces the performance of the servo.

For instance, a 2700 A cell has an internal impedance of 20 milliohms while a 3300 Sub-C is 5 milliohms. The difference is that under a 1 amp load the A cell will see a voltage drop of nearly a full volts (0.85 volts) while the sub-C will drop only 0.24 volts. This is a huge difference. It is critical that you make sure when using cells other than Nicad that they can perform as desired.

JoeAirPort

My Feedback: (41)

The previous poster covers it pretty well. Futaba made a blanket statement to use NiCd to keep it simple. The NiMh are getting better now, if you keep your total battery impedence to 10 milli ohms or less you will be fine. Using a NiMh 1100 that has a 25 milli ohm impedence is asking for trouble IMO (unless you keep your flights to 3-4 max). Parallel two of that same battery and you're fine (12.5 milli ohms).

dirtybird

My Feedback: (5)

At one amp current draw most servos produce nearly 100 oz-in. Thats more than 6 pounds. I doubt you are going to need that much torque very often

captinjohn

Will standard size servo wires handle 1 amp in a good manor. Seems like a lot of juice for small wire? Thanks Capt,n

blikseme300

My Feedback: (118)

Depends on the length of the wire. Think hose pipe and water. A thin one that is long does not deliver nearly as much water/minute as the same diameter, but shorter hose would. The connectors that we use on servos are typically rated at between 2 & 3 Amps max. I always recommend that any extension be at least 22awg.

Safe Flying!

XJet

How do you calculate those voltage drops?

According to my Ohms Law cacluations:
1A x 0.025Ohms = 0-.025V of drop within the cell for the A and
1A x 0.005Ohms = 0.005V of drop within the Sub-C

V= I * R, it's the law :-)

Silent-AV8R

I didn't calculate them. I measured them. These are real life numbers as measured on packs that I have here. And I am not certain that you can use the internal impedance of the cell, which some call internal resistance, in Ohm's law.

At any rate, as I said, my numbers are actual measurements and not theoretically calculated values. Also, these measurements are for a 5 cell packs.

Regardless, the point remains that the lower the internal impedance the lower the voltage drop under load.

XJet

It's been my own experience that neither the mAH capacity, nore the physical size of a cell re particularly good indicators of their internal resistance or performance under load.

I hve 1650mAH AA-sized packs that I would only use for "sport" flying with standard servos, because they do have a high internal resistance.

However, I also use a lot of 1050mAH 2/3A sized cells which I use with up to 5 hi-speed digitals (HS5925) on 3D ships and they work superbly. These cells (despite their lower capacity) will deliver 20A continuously with minimal loss of voltage and outperform most 1000mAH NiCd packs I've tried.

A good (but not infallible) rule of thumb is that: as the capacity of a cell goes up for a given physical size, so will the internal resistance. A 2500mAH AA cell will almost certainly have a much higher internal resistance than a 1000mAH one. And, as a result, when subjected to heavy loads, the lower-capacity cell will often actually deliver a greater amount of total power to the load -- making it better in all ways than the hi-capacity one.

If you've got a 2700mAH A cell that loses almost a full volt at 1A then it's a very poor cell and I would certainly put it nowhere near a model! Any cells used in a flight pack ought to be able to deliver twice that with little more than 250mV drop when charged.

3D Joy

I also did some measurements when I was setting up my 33% Extra and did end up with comparable results. The cells were 2300 Mah AA at 6V. I saw more than 1 volt after an extension at 2 amps. These cells were of 25 miliohms resistance. Performed the same test with heavier less capacity and more expensive pack to see that the voltage drop was MUCH lower (internal resistance of 4.5 miliohms only). Actually, those cells deliver more Mah under load than the 2300. The 2300 cells don't deliver more than 1900 at 2 amps and the 1950 4/5FAUP are litterally loafing at 2 amps. I can get close to 2000 Mah even at 2 amps. There are some guys that use these cells in cars and pull close to 40 amps continuous (that's more than 20C).

And yes those are Nimh. New cells are coming out just about every week and some are IMO very useless for us but some are very good. I can't see the point of using Li-po's IMO as those cheaper Nimh do the job nicely and are safe to boot.

PIRANA

Hi: i have a futaba 3pj transmitter, i have fitted a 2200Mah Ni-Mh battery pack into it, can someone tell me if it is safe to use a Std Futaba TX charger, and will this charge the Ni-Mh cells, or can i get a phisicaly simular type of charger which will charge my Ni-Mh battery pack, while still in the Transmitter.

Thank you.

XJet

Yes, you can use the standard wall-wart charger. Since its charging current (60mA) is well under 1/10C for those cells (220mA) you can't harm them.

Be aware however, that they would take about two days continuous to charge from near-flat.

I have two transmitters, one with 1650mAH NiMH and another with 2200NiMH packs. I have been charging these using the regular wall-wart charger for nearly two years and had no problems at all.

suitcase

My Feedback: (1)

I bought those servos (3305)as a gift for Christmas. I bought a 1200 mah 4.8v battery from Tower at the same time. Will it be powerful enough? I'm also building a Mayhem 3D and am using the Hitec Hs-6635 from my crashed VF extra. They were'nt hurt in the crash. I have a new Hitec Supreme 8ch RX and a 5645 for the rudder. Was going to use the 1700 mah nimh from Servocity that was in the extra. But,,I ordered a 2200mah nimh 6v to replace it. After reading this,, I don't know what to use.

dirtybird

My Feedback: (5)

Any of those batteries will be just fine.