Hi people,

I'm working on a University project for a competion involving autonomous flight (but that's another story) and we've got 8 digital servos (Hitec 5925) on an aircraft. What batteries would be best? Due to the design of this turbofan machine, the servos will all be constantly stressed quite close to their max. (testing will reveal if we need stronger servos). The batteries will also be driving 4 spark plugs. Would NiCad batteries be enough? And if so, what's the minimum amperage I'd be looking for? The voltage to the servos will be regulated at 5 volts since it first enters a PWM board which controls the servos.

Thanks,

Marcus

Ok, hmmm..........

Does anyone know of any links which might be helpful?

Marcus

You might want to look into the 'power distribution system' that is available from Powerflite, I believe. If I remember right from a Jim Oddino article in RCM, the unit is driven off of a single 12V or so pack (lithium?) and has a distribution system to deliver the appropriate voltage to various things like reveivers and ignition systems.

You could also post your inquiry in the Battery Manufacturer Support froum - Powerflite is represented there...... if you haven't done so already.

Just a thought or two....

BJH

thanks

If weight is not super critical, I'd stick with multiple nicad packs. You could power your PWM board (what's that ? ) with a separate battery from the receiver battery or power the servoes separately from the PWM and spark system. And/or you can have battery packs in parrallel for redudancy and more capacity. Max current is probably not a problem with nicds. Even little 600 mah packs can put out beaucoup amps, just not as long as bigger packs. What is important is CAPACITY, or milliamp hour rating (mah), to give you the endurance you need for your project.

Eight digital servoes could pull a lot of current; I don't remember the specifics, but seems like I read that stalled, they could pull a few amps a piece, so lets estimate 30 amps peak and 10 amps average load from 8 servoes. Will this overload the regulated output of the PWM board? May be another good reason to power the servoes separate from the PWM; just get the servo signals from the PWM, not the power. Obviously you'll need heavy duty wire; 20 ga or better. 10 A avg load means you'll need 1000 mah capacity for each 6 min of run time. Throw in a good safety margin and we're talking a bunch of packs. Maybe my guesstimates are way off; put a meter in line with your equipment and measure the loads.

One last thought, It would probably be a VERY good idea to power your spark system with a totally separate battery pack to avoid interference with the rest of your system.

Check out FMA's clearance for some great prices on nicad packs:
https://www.fmadirect.com/site/fma.h...roducts&cat=15

Joel

Our PWM board is a circuit board programmed to be the controlling computer of our machine. I will definately check up on the current ratings of it.

Battery packs in parallel sounds good.

What's really surprising is that these servos can pull up to 30 Amps. How much does a regular servo draw?

A separate battery pack for the spark plugs sounds like a good idea, but could you clarrify what interference you're talking about? (so I can explain to the rest of the team

Marcus

Powerflite batteries are the new leading battery technology out there...

They make Lithium Ion and Li-poly cells that are much more advanced then say a Nicad or NiMH...

There packs are much better developed and have much more power and are lighter then nicads or nimh.. The only draw back is the price..

visit there website and start reading! http://www.bktsi.com/

Shortman, being "leading", "advanced", and "lighter" can lightly lead this plane into an advanced position in the ground if you're not careful. I believe all those Li systems have a power regulator which has current limits that may not meet the specs of this project.

Markus, as stated, I was "guesstimating" worse case scenarioes on current draw. All I could find on Hitec 5925s right now was 190 ma draw at no load. But I'm sure this goes way up with a heavy load, and probably goes to a few amps if stalled. Regular servoes don't go as high, but that's the point of going digital: holding power means high current. Multiply this by 8 and add the load of the other equipment and 30 amps peak doesn't seem that high for worst case. Please get a meter in line and get some real life measurements to better estimate battery needs.

If weight becomes a problem, I'd look at nimh batteries next ( www.radicalrc.com ) . They have a higher power density with slightly more internal resistance ( less max current capacity ), but big nimhs should still be able to easily do 30 Amps. I'd stay away from lithiums because of the added regulator and current limits; some of these systems just shut down the battery if the current limit is exceeded ( crash! ).

What interference ?!?!?! Man, you've got 4 sparkplugs! Depending on the ignition system, you've probably got 20 to 50 thousand volts jumping across an air gap. Can you say "make my receiver do the jitterbug"? Does the PWM control the sparkplugs at the turbo? If so, is the signal opto isolated? Are the spark plugs going all the time or just at startup. If you can do it, don't have any wire going between your PWM and radio and the spark system.

Take the time to give Bryan or Kevin a call at Skyborn Electronics (PowerfLite). PowerfLite in most cases offers an educational discount for projects such as your's. I would recommend one of the PowerfLite Multi Voltage Modules (MVM) Systems. If that cannot handle your needs, then we can always custom build something that WILL. Drop them an email ... [email protected] ... or call them at 972-267-5099. Both Kevin and Bryan are former Raytheon gurus. Kevin LOVES to do unique products for special applications.

Rick Head
PowerfLite Consultant

To answer your origninal question I would recommend that you size the pack at 300maH per digital and 200maH per analog servo.