subtleguru

My futaba analogue servos have a "For Nicad use only" warning on the box.

Given that I typically get by on single 700mah Nicad Rx pack for a days flying (usually a couple of planes) can I replace the 700mah Nicad packs with much higher capacity Nimh packs with any risk.

Edgar Perez

My Feedback: (13)

yes

fancman

My Feedback: (16)

Voltage from nicad's and nimh batteries taste the same. Your radio won't notice the difference.

broke_n_bummin

It's not about the voltage ... it's about the current (amperage). Newer analog and all digitals have a current limiting capacitor that regulates the max current going to the servo. Older servos didn't need them because the NiCD batteries were only like 600 to 800 MaH. NiMH batteries have a much higher capacity and will allow more current in to the servo motor. Normal flipping of the control surface on the ground won't hurt it, but when the control surface is under a load, or of the servo binds up, there's nothing to regulate the current going through the windings in the servo motor. The more current the motor uses, the more resistance it builds and the hotter the servo motor and batteries get. When the motor gets hot, the windings swell. Eventually one of 3 things will happen :
The windings will swell and contract enough times to make the varnish coating peel and the wire will touch another wire.
The windings will swell and break causing an open short in the motor.
The windings will swell, break and touch a ground or the field causing a closed short.
BUT ... that's only if the servo either has a load on it or it is in a bind....
Other than that, you should be good to go.

subtleguru

I suspected it must be something like this. Now two questions

1. Would the same apply to a large capacity NiCD pack ?
2. Has this ever happened in the wild or is it merely a theoretical possibility (my servos are Futuba 3310's) running on 6V at close to the max torque values.

broke_n_bummin

Your servos have 2 sets of speed / torque values. One set is for 4.8 volts and the other is for 6 volts. The paperwork that accompanied the servos should tell you the max on the battery capacity. If not, just to play it safe, I would keep it under 1000 MaH.
Oh, yes, it's happened. If it's longevity you're looking for, I wouldt suggest using anything a 6v 1000 MaH NiCD. Set your control survaces up so that the servo range is never maxed out (max control surface deflection), and that the servo isn't in a bind (pushrod not rubbing on anything, control surfaces move freely etc).

garys

The reason many of the servos say "NiCad only" is due to the voltage, not Mah. You'll be fine with NiMh's. Many of the servos are only for use with 4.8V battery packs. If one were to use alkaline or LiPo battery packs, they could easily go beyond the max voltage of the servos. Even with servos rated for 6 volt packs, it's not recommended that they exceed 6.5 volts to the servos (freshly charged 6V packs can exceed 6.5volts). This is the reason voltage regulators are used when using LiPo battery packs for receivers an servos.

Realistically, as long as the voltage isn't being exceeded, and you aren't binding the servos to the point of them stalling and causing a big drain, you can use a battery with as many MaH as you wish.

BarracudaHockey

My Feedback: (11)

And now for the rest of the story, and if you'll pardon me for saying so, the correct answer.

Older generation NiMH cells had high internal resistance and with several servos, most notably digitals, the servos would draw enough current to drop the system voltage to below what would cause the receiver to reset.

Newer generation NiMH cells of sufficent capacity don't suffer that problem, the warning is a hold over.

If a servo is rated "4.8v" its rated for a fully charged 4 cell pack. If a servo has a 6v rating its rated for a fully charged 5 cell pack. The warning has nothing to do with current LiPo powered regulated power systems, again provided the system can deliver the amps that the size, qty, and type of servos you're using need.

subtleguru

Thank you BarracudaHockey,

Ok so here is what I understand ...

I have 5 Futaba 3010 servos and the Futaba online specs are worded as follows

"This servo can produce high-current draw from your batteries.
If using NiMH or LiPo batteries, make sure they are capable
of delivering approximately 2A for each servo."

Now most (all ?) AA Nimh packs that I found have a max discharge of much less than 10A. So I would require a SC pack similar to this

Capacity: 3,700mAh
Measured at 740mA (0.2C)
Weight:352g
Max Discharge Current:35,000mA (35 Amps!)
Max Charge Current:3,500mA

I also noticed that the dinky switch harness I have will only support 3A so I will change that out as well.

I could not find any specs on discharge rates for the NiCad packs but I assume they are sufficient and hence the warning.

I suspect the 2A per servo refers to servos under high load or binding so it is unlikely that all 5 would need 2A at the same time but it looks like I can swop out lead for additional battery weight so this pack works for me.

BarracudaHockey

My Feedback: (11)

We routinely power 5x 9252 high torque digital servos, plus a gyro, another digtial servo and a governor with lipo and LiIon packs on the chopper side.

I have all hitec digital metal gear servos in my Yak and run it with a 2900 mah pack from http://www.hangtimes.com and don't have any issues and thats with a Spectrum AR7000 receiver and I've never had a lockout or reset. That 2a thing is if the servos are stalled.