XJet

Be *very* careful.

The higher capacity AA cells generally also have a higher inernal resistance, which means that when you start drawing heavy currents (ie: digital servos) they not only show a marked voltage drop but they seldom deliver their rated capacity.

Some tests I did indicate that under conditions of high-load, a 1600mAH AA pack can often provide *better* voltage regulation and *greater* realized capacity than a 2500mAH AA pack -- due to the former's lower internal resistance.

I try to steer people away from AA packs for digital servo use -- most AA cells have a significantly higher internal resistance than A or 2/3A cells of equivalent capacity.

The mAH rating printed on the side of the cell is only *one* factor in selecting a good pack.

leszek_k

dirtybird - if you want the same torque, using a higher voltage, servo draws less current. Torque over time translates to power (kind off). So more voltage, less current. As power equals voltage * current.

But - higher voltage gives you more potential power and sevo does not care that you just want X torque. It always gives all. That means it draws more current when it moves. Or rather when it moves your elevator, at, say 80mph, when you yank a stick.

You also have to remember that drawing more current, like digital servos do, means you also need heavier wires.
You put 5 cells and by using standard wiring - you reduce it effectively to 4 cells. Have it in mind.

Also, and this is from my own experience, if you leave NiMH battery unused for some time, check it VERY carefuly. In my case, 2300mAh batttery, after 3-4 months was down to 200mAh. I discovered it after......... Tai Ji 60 is no more, YS 110 in pieces.

rainedave

My Feedback: (1)

Now you guys are starting to scare me. So what are the recommended cell sizes for digital servos? 2/3A, 4/5A, SC, or something else? I'd really like to stay under 6oz for a 5-cell pack if I can. Thanks.

nonstoprc

My Feedback: (90)

Here is the setup on my Hangar 9 showtime 90: 1 5945, 2 5645, 2 645, and 1 standard = 3 digitals and 3 analog servos. The receiver battery is Sanyo Nicad 1500 mah (about 5 oz). 4-5 flights per charge.

You can check out the 1400mah one at [link]http://radicalrc.secure-mall.com/shop/index.php?rec=24&shop=1&cart=502602&cat=19&keywords=&match_criteria=&searchCat=[/link]. Pretty close to the 1500mah I bought from them a while back.

Because of high current draw from the digital servos, I stick with Sanyo Nicads.

At long as you periodically check the capacity of the pack, you should be fine.

rainedave

My Feedback: (1)

Thanks, nonstoprc. Is this the one you are referring to?

Radical RC 1400mah Sanyo NiCad 5-AE cell 6V Flat RX Pack

Weight= 166.4g/5.868 oz. Extra low weight/high capacity pack. Very popular pack. Same length cell as AA pack, just larger diameter. Good choice for 5 to 8 servo ship. AE-Sanyo 1400 Cells. Tired of recharging that tired factory pack? This one will last double the time!

SKU Number:RRC05N1400F

Price: $17.00

nonstoprc

My Feedback: (90)

Yes. Its internal resistence is pretty low (10 mΩ) - [link]http://www.batterystore.com/Sanyo/SanyoPDF/KR1400AE.pdf[/link].

XJet

I buy my packs from Radical (excellent service and quality) but I go for the one that's still listed as a 1050mAH 2/3A cell. They actually ship them with 1400mAH cells now "Intellect" brand and I know that Dave wouldn't use anything but the best.

I just weighed one of these 5-cell packs and they are 4.25oz (120g) including lead and plug.

rmh

those are killer good cells -

rainedave

My Feedback: (1)

Thanks again everyone. I just ordered a Radical RC 1400mah Sanyo NiCad 5-AE cell 6V Flat RX Pack for my UFO.

XJet

Why a NiCad?

That pack is 20% heavier, quite a bit bigger, and has a *higher* internal resistance than the 2/3A NiMH pack Radical also sells.

nonstoprc

My Feedback: (90)

Just for my info, what is the internal resistence on 2/3A Nimh? No vendor's name is listed.

For Sanyo Sanyo HR-AAU 1650 NIMH, its internal resistence is way high (25mΩ)
[link]http://sanyo.wslogic.com/pdf/pdfs/HR-AAU.pdf[/link]

dirtybird

My Feedback: (5)

Theory is great but with the servos I have tested the current drain of the servo in almost the same for a particular servo on 4.8 or 6.0V at the same generated torque. Apparently the extra voltage is absorbed in the amplifier.

rmh

there is an " accumulative resistance" for any servo-
hows that for a coined engineering term--?
anyway -what I think most neophytes miss is the fact that when more power is called for - it all comes from the same point -- the battery
The efficiency of ANY MODERN servo we use is pretty much the same- so there are no servos which put out more power and consume less IF the work they do is the same.
The speed and torque game is a joke -
under load- the speed is reduced by any increase in load-so a fast rated servo may work slower (and harder ) than a slower rated -higher torque rated servo --which does the job faster .
The best servos on the market -in my book- are the latest designs - I have extremely inexpensive JRSport servos and some identical sized 10-15 year old expensive JR servos. The new ones are quicker and smoother and cost a fraction of the old technology. Ditto for batteries
they are yet another confusing issue
The MAH rating is only 1/2 of the issue
on the old design servos with high working resistances (low torque), battery packs could be smaller -(less amps and higher impedance) simply because the servos could not suck much power
(for you farm boys-a small calf can't drink as much as a large calf)
The modern servos - standard servos now are 70-90 inch ounces of torque and some good ones are digital -which consume MORE power as they look for and grab power at a faster RATE
Bottom line ---------------------------------------------------------
when you go shopping for batteries - look at impedance and if yo u don't know what that means - go to Cheap Battery Packs .com or No BS batteries and read up - then - select your battery type
If you select packs with high internal resistance - the fancy servos you bought are of little use
One of my favorite gags is about the guy (s) who bought into the LiIon packs of smaller size --and the attendant regsulators
They swore up n down that a 2400 ma , regulated LiIon setup lasted longer than a 2400ma NiCad setup - and more constant power to the batteries - longer is better (the John Holmes factor)
Of course they did- the current was limited .
IF --they used batteries with less impedance , they could have quickly found that the servos operated faster under load .

rainedave

My Feedback: (1)

That is some good info. The "advertised features" of consumer products are often times not the most important features to look for. Like when engine manufacturers claim 2hp at 18,000 rpm (on a 9x6).

According to the link above, the cells I bought have an internal resistance of 10 mΩ which is half the resistance of the Panasonic NiMHs in my other 6v pack. I'm sure I'll be happy with the Radical r/c Sanyo pack I bought.

XJet

I believe that the resistance of those 2/3A cells is around 9 milli-ohm, every bit a match for a good nicad.