Campgems

I have a Ruperts Dad with six Futaba S3151 digitals. Zero flight time yet. I decided to go with a 6V battery due to the size of the ailerons, elevator, and rudder. I first built a 6v pack using 1200mah batteries. On bench testing and doing some engine and wingless runway runs, everything seemed ok. Then I noticed that if the battery had sat for a couple days after a full cycle and charge, and I used all servos, IE running the sticks in circles, I would see the VoltWatch dip down and flash the red light now and then. My first thought was I had a bad cell, so I bought a Futaba 6V 1000ma pack. After three cycles and a full charge, I get the same thing. My guess is that the current draw is pushing the batteries to the limit. I'm not sure I want to fly this way.

My question is, can one use two batteries, using a common negitive and have each battery supply half of the servos via two different positive busses and just supplying the signal from the receiver. Almost all electronics I've worked on uses 0V as signal reference so it seems that it may work. I guess the easist way would to build a bench setup and try it, but I though I would ask first. This is a rough drawing of what I want to try.

Don

redfox435cat

first off the futaba digtals don't like 6volts. also a 6 volt battery will discg\harge faster than a 4 cell with the same capacity. this is becuase it's drawing more current, your batterys are rated by cuttent/time mAh.

you can use mulitple batteries. I'd be allot easyier to just to take a y-harness plug it in to the reciever and plug a battery into each connector on the y-harness. this also double your capacity and the current avaiable.

The solution I've started using on my planes with more than 6 servos is using 7.4V lipos with a voltage regulator. I know allot of people are weary of them becuase of the fire hazard. The thing one needs to relize are the people burning them up are pushing them up to and past their limit in brussless electric motors. Using them to run radio gear is harmless to them.
[link=http://www.duralitebatteries.com/accessories4.html]regulator[/link]

dirtybird

My Feedback: (5)

First of all in the tests I have made servos draw the same current on 6 volts as they do on 4.8V when producung the same torque. If you would like to see a copy of the results PM me.
Digital servos draw higher peak currents. When several are moving at the same time, the high current peaks can cause the voltage to drop to the point the receiver can drop out of operation. Changing to 6V can help.
The best way to fix it is to separate the receiver voltage from the servo voltage.

Campgems

I did find a post about 6v and 4.8 on Futaba receivers. They said that the receivers have a built in voltage regulator and the receiver runs on 3.3V internally. The regulator requires about 0.7v above that to operate, so around 4V is the minimum that the receiver will operate on. That is about 0.7V below the redlight on teh Volt Watch when it is set on 4.8V. Set on 6V, the red comes on at 5.87V, way within the operating range, in fact still above the max range for 4.8V. It kind of sounds like I'm chasing a ghost here. I might just hook up the old trusty O scope and see what is really happening to the voltage. I could see the peaks then. As long as it isn't dropping below 5V, I'm not going to worry. Of course I need to check this with a half drained battery I guess just to be on the safe side.

I do like the idea of a seperate receiver battery though. Something like a little 1/2 - half AA sized.
Don

PS, I'm a bit gun shy just now. I lost a plane a couple weeks back and have no real good explanation as to what happened. It just kind of rolled over and went in like a lawn dart, right into the creek bed behind our field, never to be seen again. I'm doing a lot more checking before I put this one in the air, just to make sure I haven't missed a problem.

dirtybird

My Feedback: (5)

Attached is a voltage plot of the voltage in my Weeks during one flight. The Weeks has four digital servos. Note the the 6 volt battery drops to 4.8 v at one point. I had been using a 4.8 v battery and the PCM receiver would drop into fail safe at times.

Campgems

Very interesting. It conferms what I though I was seeing on the VoltWatch. That is an interesting little piece of equipment. I really like that fact that it responds so quickly. Most voltage modules, IE Digital Volt Meters, only sample every 100 ms or so, and you would miss a lot of the peaks your graph shows. The VoltWatch seems to be way faster, and while you can't get the full picture your graph shows, it does give you a clue as to what is going on.

Thanks again for your help.

Don

alan0899

G'day Mate,
You have not flown the plane yet, & your voltwatch does not measure the voltage with any sort of load, so you really should use a voltmeter that has a known load as well as the servos, to give an indication of how your batteries will hold up in the air.

Rodney

In refute of one of the above comments, the Voltwatch is a very good way of measuring the voltage and uses the actual loads you are putting on the system. Remember, the Voltwatch is measuring the voltage at the point it is plugged in, usually a spare port on the receiver (which is prefered over having it on Y connector). When it drops into the red when you cycle the servos, it is giving you a valid warning. Either the wireing between the battery and receiver (this includes the switch impedance as well) is either to small a gage wire--high impedance switch contacts--defective wireing (may have some damaged strands)--or insufficient battery capacity. If the later and you try to add another battery, it is best to NOT use a Y and the same switch. Put in a seperate switch and charging jack and plug the second battery (through the second switch) into a spare channel on the reciever. If it is one of the first, install heavy duty switches with large gage wire between the battery and receiver.

rmh

A little voltmeter such as EXPERT offers does a bang up job of alerting you to potential problems
insert anywhere in the wiring.
the voltage drop is everywhere.
Just stir the sticks madly --- before take off , with engine running. If you bounce into a red element - you have a low bartter ot something stalling or loading heavily.
Why would a Y connector read differently-as long as it has a proper connection?
My voltmeters do not read same voltages as does my Whattmeter -or a VTVM or my old Hobbico expanded scale or my older huge test meters -- but it does the job needed -

alan0899

G'day Mate,
Agreed, but the voltwatch, by itself, does not have a load, but they are a great tool, I didn't say that they were no good, BUT, I use a voltmeter with a load of 500ma, & I test all the servos at the same time, this gives me an idea of flight load, on the battery pack, loaded voltmeter + servo load.
I had an 1100Mah pack fail in the air, but it showed 80%, when just the loaded voltmeter was used to test it.
Once bitten twice shy

Campgems

But the VoltWatch is loaded by all that is plugged into the receiver. Granted a volt meter, and a load will give you an idea of you battery and system status, but voltmeters are slugish at best to respond to quick fluxiations in voltage. The digitals are the worst of the bunch. The VoltWatch has almost instant readings, and with the six servo load and lots of movement, you can see most of the lights lit at once, in varying degrees of brightness. This is a good quick visual of the graph that DirtyBird posted. It doesn't give you the details, but if the red light is flickering, you are dropping down under 4.60V on a 4.8V setting or 5.87V on the 6V setting. What you don't know is just how far below, and this is cause for alarm and investigation. You just cant see those rapid changes in load, resulting in rapid voltage drops on any volt meter. You could on an O scope, or on a high end digital anylizer, but there is nothing in the relm of something we could take to the field other than the VoltWatch type battery monitors that will give you that speed of read out. The best $10 to $20 you can spend in my mind.

Don