[Sparkfarmer]

I have a 55cc MSX-R that I had bought basically new from a fellow RCC'er. The aircraft is running 5 Turnigy 9150MG servos, powered by a LiFe 6.6V/2100/10C through a AR8000.

I'm wondering if I should move to a higher capacity battery. Anytime I perform a snap roll, or a maneuver that requires quite a bit of surface deflection from multiple control surfaces, my DX8 telemetry Rx voltage alarm goes off momentarily. Alarm is set to 4.3V. I've flown it all last supper without any issues, and the batteries always charge fine and maintain a good balance.....but that's quite a draw!


I suppose it wouldn't hurt to move up in capacity, and I probably will. Worse case I get longer cycles out of it.

Just wondering what the pros think!

[kwm]

I'd get rid of the Turnigy servos and get some good quality ones in, you have a big expensive airplane and you put cheap servos in it.

[Sparkfarmer]

Changing servos would reduce current draw?

[dirtdr]

That is a lot of draw.

I do not use life batteries, but if you are pulling down to 4.3volts you must have some serious current there. Before you just replace the battery with a bigger one, i would suggest adding a second battery and switch (temporarily) and doing a flight and checking voltage during these snap maneuvers. If it in fact drawing that much you could be overloading the switch as well.

About the servos, it is very likely that these hobbyking servos, although sped'd well, could very likely draw double the current than a brand name servo. This has been my experience with the smaller ones anyway.

[grizak]

It sounds as though the LiFe battery may not be able to produce the current necessary for the your setup. Changing the battery may help - there are many flavours of LiFe batteries on the market, and not all are created equal. Some just do not generate enough current for larger planes. We have evaluated LiFe packs that can only produce about 3Amps before a dramatic drop in voltage.

You may want to try a battery based on A123 cells - these will definitely generate enough current. We have these in stock here: http://www.thunderboltrc.com/index.p...ahv0mbqg4cddq5

If you replace your battery and find that you have a similar issue - you may want to switch to a 2 battery pack setup - 2 battery packs will supply twice the current to your setup.

Also, check the geometry on your control surfaces - make sure you don't have any binding, etc.

Grizak

[Sparkfarmer]

Originally Posted by grizak

It sounds as though the LiFe battery may not be able to produce the current necessary for the your setup. Changing the battery may help - there are many flavours of LiFe batteries on the market, and not all are created equal. Some just do not generate enough current for larger planes. We have evaluated LiFe packs that can only produce about 3Amps before a dramatic drop in voltage.

You may want to try a battery based on A123 cells - these will definitely generate enough current. We have these in stock here: http://www.thunderboltrc.com/index.p...ahv0mbqg4cddq5

If you replace your battery and find that you have a similar issue - you may want to switch to a 2 battery pack setup - 2 battery packs will supply twice the current to your setup.

Also, check the geometry on your control surfaces - make sure you don't have any binding, etc.

Grizak

Thanks for the info guys!

Based on what you guys are telling me I think I may try doing a few tests on my servos with an ammeter. I have a few different servos here to compare with.
I have a second LiFe pack that I thought of putting in parallel but I don't have enaugh room.

I've heard very good things about Thunderbolt RC on here. I'll place my order as soon as I figure out what I need.

[Cougar429]

My last two larger projects are powered with the Thunderbolt 2S LiFE packs with no problems. Since my servos are not HV I run each pack through a Castle BEC PRO rated for 20 continuous amps and set 6V output.

If your voltage falls under load that is an indication the capacity of the pack is depleted for the conditions and/or the battery simply cannot supply your requirements even if fully charged. This is true for any battery chemistry. It will also occur if the batteries are damaged or they are near the end of their service life.

The key is to have an adequate source that will NOT suffer a severe voltage drop during normal use under any conditions that will risk loss of servo or receiver function.

One overlooked item is how you feed the source to the load(s). The battery may be sufficient for the system loads, but if any of the connections or wiring are substandard then function will suffer. In that case the warning may not be of the battery state, but of the voltage present downstream of the pack. Most standard RC flight system switches are only rated for 4A and smaller diameter servo wire will not feed much more, especially if the leads are longer.

Another thing to watch for is feeding high current through the common buss inside the receiver. If you expect your current draws for all the servos to exceed 4A you should look to bypassing the receiver to supply power to the servos. You can still tie the main battery to the receiver, just feed power to the servos directly.

Another option is to use a much smaller battery to feed just the receiver itself. Their current draw is minimal. One other advantage of splitting them this way is to help isolate noise from long servo leads reaching the receiver.

Here is an example of splitting the system. You don't have to be sophisticated and can wire the common battery up with two switches. In this example, due to the fact Rare Bear is notoriously tail heavy one A123 pack is stuffed as far forward under the tank as possible and a separate receiver pack is mounted above the motor. The VOLTWATCH monitors the receiver battery state only:

[Sparkfarmer]

That's not a bad idea Cougar. Very nice clean install on there too!

As far as your comment regarding voltage falling under load, this is where I'm not sure. Because it isn't completely true that battery voltage falling under load indicates a bad battery. In alot of cases it's perfectly normal. Like when your lights dim while starting your car, your battery voltage takes a dive momentarily. Even in full scale applications.....when I start a PW120 on one of our Dash-8's bus voltage drops from 28 to 22 volts during starter engagement. The advantage I have on a Dash 8 though is I can monitor internal battery temp, where as in my MSX-R I cannot. Hence my query on what is acceptable for voltage drop in my setup.

Seems everybody is in agreement that it is too much of a drop, so I will definately change things up. I'm working on a test bench setup to see how much current one of my servos draws under a given load. Probably not necassary but...interesting!

[Cougar429]

Perhaps I could have been more specific in what I stated in the second paragraph. When current demand comes close to or reaches the limit the source can supply, (in this case the battery) then output voltage will fall. This is normal and indicative of that condition.

It can also indicate a weakness in other components. If the wire cannot carry the required current or any of the connections, such as wire terminations, are corroded or too light then what is seen downstream will suffer.

What becomes critical is if current demands under normal flight conditions reaches that limit and the voltage falls to the point where system degradation or failure occurs. This points to two possibilities. Either the current demand exceeded the source from the get go or the source has degraded. In this case there is no background history to tell if the battery ever had enough juice to supply the flight system demands or if the conductors and associated switch and connections were capable of carrying the required current.

My own suspicion is the problem exists upstream of the receiver as that is what is setting off the low voltage alarm. Either the battery, switch or wiring are the culprit and excessive current demands from the servos simply exacerbates it.

p.s. Sparkfarmer, I was a heli pilot and AME, so know the systems you describe well enough. The reason your battery voltage drops is because you are pushing the upper limits of what the battery can provide. The fact you monitor battery temp is a good indication that this is the case as internal temps will climb during this abnormal condition. The battery can provide a certain amount of amps with no adverse effects, but will when pushed hard. You can get away with it temporarily, but under normal conditions watch the temps and cycle times.

You will also note that the battery is likely the only thing to show these effects, (if you ignore the limitations on the starter/generator). The wiring and associated systems should NOT be stressed even during starting.

As an aside, the reason aircraft went to 28V systems is due to what can be described as "POWER" or WATTS. This is the combination of both current and voltage to tell what your total system power can be. Raising the system voltage meant the amps could be reduced and still provide the same amount of power. Since current demand denoted conductor size then that could also be reduced. It was all in the pursuit of weight reduction. When you think of aircraft carrying miles of wiring this can be significant.

[Sparkfarmer]

Originally Posted by Cougar429

Perhaps I could have been more specific in what I stated in the second paragraph. When current demand comes close to or reaches the limit the source can supply, (in this case the battery) then output voltage will fall. This is normal and indicative of that condition.

It can also indicate a weakness in other components. If the wire cannot carry the required current or any of the connections, such as wire terminations, are corroded or too light then what is seen downstream will suffer.

What becomes critical is if current demands under normal flight conditions reaches that limit and the voltage falls to the point where system degradation or failure occurs. This points to two possibilities. Either the current demand exceeded the source from the get go or the source has degraded. In this case there is no background history to tell if the battery ever had enough juice to supply the flight system demands or if the conductors and associated switch and connections were capable of carrying the required current.

My own suspicion is the problem exists upstream of the receiver as that is what is setting off the low voltage alarm. Either the battery, switch or wiring are the culprit and excessive current demands from the servos simply exacerbates it.

p.s. Sparkfarmer, I was a heli pilot and AME, so know the systems you describe well enough. The reason your battery voltage drops is because you are pushing the upper limits of what the battery can provide. The fact you monitor battery temp is a good indication that this is the case as internal temps will climb during this abnormal condition. The battery can provide a certain amount of amps with no adverse effects, but will when pushed hard. You can get away with it temporarily, but under normal conditions watch the temps and cycle times.

You will also note that the battery is likely the only thing to show these effects, (if you ignore the limitations on the starter/generator). The wiring and associated systems should NOT be stressed even during starting.

As an aside, the reason aircraft went to 28V systems is due to what can be described as "POWER" or WATTS. This is the combination of both current and voltage to tell what your total system power can be. Raising the system voltage meant the amps could be reduced and still provide the same amount of power. Since current demand denoted conductor size then that could also be reduced. It was all in the pursuit of weight reduction. When you think of aircraft carrying miles of wiring this can be significant.

Well put.... You should be a technical writer! keeping things lite to pack more people is the name of the game! It's also the reason almost all larger airliners are 115 AC. Were you an AME M or E? Actually from what I've read from your past posts you were military right? I have a few co-workers that are ex-military, and your rating system is totally different. I'm an E ... but I digress.....

I've done some research on flying giants and have seen alot of setups similar to mine, with the exception of my switch. Here are a few pics of my setup: