BR289

My Feedback: (106)

Greetings,
A bunch of different Futaba servos say Ni-Cad only on the packaging. My question was sent to Futaba but not answered to my satisfaction......I read their FAQ's but it doesnt really explain much.
I have had quite a few people tell me they use these "Ni-Cad only" servos with NiMh batteries with no problems, but I want to be sure.
Any thoughts would be greatly appreciated.............Thanks

wisdom-seeker

Basically, Futaba is telling you that in order to get peak performance from your servos, you should only use NiCd batteries. What they don't say is that the servos will make very high short-term demands on the battery, and a NiMH with its higher internal resistance will momentarily sag in voltage under the demand. You will not be able to see this with a voltmeter, but the difference will be visible on an oscilloscope. With a voltage sag, the servo may not be able to live up to its specified travel time performance.

mglavin

My Feedback: (31)

Me thinks Futaba is specifically aiming the comment at those that might likely use Alkaline batteries, such as cars and boats with surface equipment which is atypically the norm....

NiMH will work fine with todays battery chemistries. It is true as noted above that NiMH is of higher internal resistance as compared to trusty old NiCd's. That said we've been using NiMH for several years in our GS models as have hundreds of others without issue. Simply make sure your utilizing quality cells with the capacity required for your servo load.

TLH101

My Feedback: (90)

They may also be aimig those recomendations at Lithium Ion packs, as they cannot handle high current drain.

mglavin

My Feedback: (31)

While Lithium is in the same boat as NiMH with regard to higher internal resistance numbers, todays technology is working fine with these applications. Cumulatively NiMH and Lithium cells in this application have similar IR numbers. Again its well recognized that Lithium and or NiMH can and will power a GS model with high current demands. They both work fine with a proven track record providing the cells are properly sized to the task.

I have used five cell 2700mAh NiMH batteries and 4000mAh or better Lithiuom-Ions in many 40% models with a full compliment of digital servos with excellent results, these combinations are commonplace. Shortcomings for power are typically the distribution system not the batteries.

XJet

Yep, the previous two posters are right on the money.

Futaba are just protecting their backside because they're aware that not all mAHs are made equal.

Digital and/or ui-torque servos will work just fine on NiMH battery packs *providing* you use the right cells.

I suspect Futaba are trying to avoid the situation where someone replaces their standard AA-sized 600mAH NiCd receiver pack with an AAA-sized 700mAH NiMH one. In this case, even though the NiMH has a greater capacity, the smaller physical size of the cells is almost certainly going to guarantee that the pack will have a higher internal resistance which may cause problems.

The key, when replacing NiCds with NiMH (especially for hi-torque/digital servos) is to use a pack of equivalent physical size rather than one of equivalent capacity.

In my 3D ships I run AA or 2/3A sized NiMH packs which generally have a capacity of 1100mAH to 1650mAH. This is nearly three times that of the same sized NiCd pack but the internal resistance is only marginally higher. These birds have Hitec HS5925 servos (over 100oz/in of torque and 0.08 second transit) so they draw some *real* current when you're throwing a plane around the sky. Despite this, I've had *no* problems at all with the use of NiMH.

I would never however, dare to use the same servos with one of those AAA-sized 700mAH NiMH packs that are now becoming popular. The smaller size of the cells means that thinner plates are being used and that means higher internal resistances. Under the load of four powerful servos there's a very high chance that the voltage would drop too low for reliable operation at times.

Another good idea when using NiMH cells is to opt for a 5-cell pack. The extra 1.2 volts will give you far more headroom and will more than compensate for the slightly higher internal resistance. You'll also get more torque and higher speed from your servos (although not all Futaba servos are rated for 6V/5-cell operation so check first).

BR289

My Feedback: (106)

Thanks for the replies.......The main reason for using NiMh batteries is the higher capacity Mah-wise. I built a new "Funtana.90" and used mostly Digital servos because of the torque, and I am surely not going to use a small "600 Mah" battery in it! The largest "Ni-Cad" Futaba sells that I am aware of is 1500 Mah , and for the price, I can buy Hydri-Max 2000 Mah batteries for alot less.
Thanks again guys.......

Lynx

Just as an added note. For cell's of the same capacity, Nicads are generally rated for 10C discharge, Nimh's for 5C, and Li-poly's for 2C before the voltage will start to drop. Hi performance Lipo packs have slightly modified internal structures to allow slightly higher discharge and they compensate by simply running more than one cell in parrallel increasing the current handling ability. The same thing could be done with NiCad's and NiMh's but I've never seen anyone do it. I'd actually be curious in the performance difference in a high end electric car when using a pack rated at 2000mah and using two packs in parrallel rated at 1000mah. Considering how hot NiCAD packs get (wasted energy) it might increase battery life for flat out speeds.