Fail Safe caused FULL THROTTLE at low voltage!

[DarkEnergy]

HPI Savage SS built myself

Relevant parts:
JR Racing RS300 receiver
JR Racing Z590M
IMEX Fail Safe No.5000

I just replaced my 4AA cell pack with a 1200 NiMh and I haven't sufficiently tested this problem it with the old battery pack.

Long story short, my truck went wot and I had to chase it down and wrestle it to the ground just short of darting out into traffic. This was almost 100 feet from where I was playing. First it oscillated full wot and 1/2 wot for about 20 seconds, then it just layed it on full until I was able to stop the engine. Holy CRAP it was scary! I had partial control of steering in brief spurts as I chased after it.

Turns out it was the FAIL SAFE that caused the full throttle. With the engine off, I am able to reproduce the issue as the battery dies. The fail safe works correctly when transmitter is turned off (snaps to exact position I set).

When I remove the fail safe, the throttle stays solid with much less power left on my battery. I can play with the servo for another few minutes on the same charge and, while it becomes somewhat erratic right at the end, it never yanks over to full or oscillates.

The battery died before even a single tank. I'm wondering what is causing such high drain. I have a HiTec servo for my steering that is very beefy (sorry I can't remember which model# it is but its pretty much the highest end possible for steering - its "that one"). I'm also thinking it might be the fail-safe that causing drain because there seems to be more power available to the servos when I do a test with and without.

A different fail-safe, the futuba micro fail safe, has this note in the description:

The discharge curve of alkalines and NiMH cells is different from that of the NiCd cells for which this unit was designed. Non-NiCd cells will not provide the warning at the appropriate time. Does not work with transmitter in PCM mode.

One last note: I remember when first setting the fail-safe, it was actually set on the opposite end of the servo. Since servos can reversed depending on the orientation of the installation, does the fail-safe account for this? It almost seems like the servo uses the set position for reception problems but the "default" position for voltage problems. The way it was oscillating almost makes it seem like the two fail points were fighting as the voltage spiked up and down.

Sorry so long... Comment and recommend me a proper fail-safe. This thing almost caused me a heap of problems.

 

[beason]

mine works the same weather its the battery dieing or loss of signal. maby you got a faulty one?

 

[robmob]

You must be pretty quick to chase it down while going WOT.

World class sprinter caliber.

 

[savagewild]

i would take it back, explain what happened & get a replacement...........my bet is it is faulty...........

 

[DarkEnergy]

You must be pretty quick to chase it down while going WOT.

World class sprinter caliber.

Hard to explain - at some points I had enough control to get it turned around so I was never that far from it. It hit an obstacle and I caught up to it as it was tumbling. No, of course I cannot really catch it it moves! Damn! I wish I had video, I'm sure I would look ridiculous.

 

[dougc]

I use to have a fail safe... I would set it up right and it would never work right...Also you mention that you bought a new battery pack... did you cycle it at least once... and how many cells does it have?

 

[HeartBreak]

The fact that you were able to catch it before it got to traffic :thumbup:. I've been thinking about getting a failsafe for some time now, and have been lookin at the Ofna model.

 

[SMaxxin]

Sounds like you got a crappy battery and fail safe. I have used the Ofna micro and now have two Dynamite fail safes, both brands have served me well.

 

[Plaidfish]

your battery draining that fast is definately an issue....do you have a digital Hi-tec servo? and did you set the endpoints? if not that can do one of 2 things, break your servo geartrain or drain the battery at a super accelerated rate (one tank sounds like a real bad drain still)......next I have never heard of ANYONE recommending the Imex failsafe.....I've heard of problems w/ every brand except the OFNA and Venom in-line Fail safes.....I personally had bad luck w/ the OFNA one and scrapped it altogether....if you can I'd suggest getting one that's integrated w/ your radio setup...much more reliable IMO.....

 

[DarkEnergy]

I attached a volt meter and looked at voltage drops.

The biggest voltage drop I was able to produce came from running the steering servo while holding the wheels in place. Going to either end of the steering servo produced much less drop than holding the wheel in place, so I think the end points are ok. No, I didn't set endpoints. I guess I'll look up online how to do it because I didn't get a manual with my servo.

There was no different in voltage with or without the fail-safe so I guess thats not the problem either.

My AA bat pack held for several tanks or more so I guess my NiMh is crap. its five cell btw. Is 1200 enough?

Also, quick random question for anyone reading: Two examples: 1200 mah 6v and 1200 mah 9v. Do these have the same amount of energy storage? Or do you still have to multiply by voltage eg: 1200 x 6 = 7,200 milliwatt hours, 1200 x 9 = 10,800 milliwatt hours... or something.

 

[SMaxxin]

No, I didn't set endpoints. I guess I'll look up online how to do it because I didn't get a manual with my servo.

You adjust endpoints on your radio (if available) not the servo. Do you have a digital servo? What radio do you have?

 

[4u2nv]

Failsafes ALWAYS made my stuff glitch. I couldnt find a problem anywhere and tried a couple of them. Still glitched. I havent used a failsafe on ANY of my rc's for a couple years now and never had any probs. (I do kinda live in the sticks though)

 

[dougc]

Many people have the same question... mah is the size of the "gas tank". You pack will be 1200mah whether it is 6 cells or 19 cells ... You should never use more then 6 volts when plugging directly in to the receiver.
What did you charge this battery pack at? Also if you have a discharger I would at least try to cycle it 2 times. This will "wake up" the battery pack

Also, quick random question for anyone reading: Two examples: 1200 mah 6v and 1200 mah 9v. Do these have the same amount of energy storage? Or do you still have to multiply by voltage eg: 1200 x 6 = 7,200 milliwatt hours, 1200 x 9 = 10,800 milliwatt hours... or something.

 

[DarkEnergy]

Many people have the same question... mah is the size of the "gas tank". You pack will be 1200mah whether it is 6 cells or 19 cells ... You should never use more then 6 volts when plugging directly in to the receiver.
What did you charge this battery pack at? Also if you have a discharger I would at least try to cycle it 2 times. This will "wake up" the battery pack

This doesn't make sense to me. To me, the "gas tank" means how much power it has. For example, the electric company charges in killiwatt hours.

A milliamp-hour is the amount of charge that is removed after drawing 1 milliamp of current for 1 hour. Measuring amps over time is great for comparing batteries of the same voltage, but if you want batteries with different voltages, you need to measure power/time not amps/time.

A milliwatt-hour is the amount of power that is removed after drawing 1 mW of power for 1 hour?

So if battery a has 1200 mah at 7.2v and battery b has 1200 mah at 9v, then battery b has more power stored in its "gas tank". Sorry to come in here, ask a question, and then argue with the answer, but it seems like there is something that I'm just totally missing or everyone I have asked this question at the rc shop is just wrong.

Maybe the gas tank analogy works if you consider "octane" as the voltage, dunno .

I'm going to scrap the fail-safe altogether, cycle my battery like has been suggested, give it another shot next weekend. Thanks for all the replies!

 

[Ryan with savage]

I've always thought of it a bit like this. (And I'm probably wrong so someone please put me right)...erm....

Voltage is like a bucket
Mah/watts/amperes is the capacity of the bucket.

If I'm filling a bathtub say and only have 1 bucket of 1 litre I have:-

1 Volt (bucket) X 1 Litre (Mah) to deliver and it takes 10 seconds to pour.

Whereas if I have 6 full buckets i have

6 Volts (6 buckets) x 1 (Litre) to deliver. It will take me 60 seconds to pour.
I can only pour 1 bucket at a time but I have 6 times many to pour.

Does that make any sense?? Or am I completely wrong?

 

[godale03]

Hey DarkEnergy,

I personally use the OFNA failsafe and have had no problems. I also use a throttle return spring in conjunction with the failsafe. My advice is that if you just scrap the failsafe at least put a throttle return spring in place. An 1/8 scale Nitro is very dangerous thing to have running away on its own. Just my 2 cents

Take care and good luck.

Tom

 

[Plaidfish]

The biggest voltage drop I was able to produce came from running the steering servo while holding the wheels in place. Going to either end of the steering servo produced much less drop than holding the wheel in place, so I think the end points are ok. No, I didn't set endpoints. I guess I'll look up online how to do it because I didn't get a manual with my servo.

There was no different in voltage with or without the fail-safe so I guess thats not the problem either..

Okay, the fact you get a larger voltage drop at the ends of the travel should tell you there is more Current draw on the battery right?
that's from the endpoints not being set....your servo has more travel than the steering may need....so when the servo gets to the end of the steering linkage's travel it will try to keep going...when you set the endpoints it's the radio telling the servo where it needs to stop.....this is VERY critical w/ digital servos...they will keep trying at 100% torque to make it to the end of it's travel...this is where you are getting the higher current draw (=voltage drop).....
as for the capacity of the pack and the gas tank analogy...I always thought the engine was where the "power" came from and the tank was the supply for that power......mah is a capacity rating similar to a gas tank having a capacity of 30gal...the higher the number the greater the capacity.....these servos will work on a voltage between 4.8 and 6VDC.....they will get more power from a 6V battery, yes....but they will draw more current at the lower voltage......I don't know why you would be even looking at 9V batteries....the electric cars that run servos from a pack are dropping the voltage through an ESC w/ a Battery elimination circuit (BEC) .....not sure if you were considering 9V or not......but it seems like you were......the End point adjustment (EPA) is very critical do that first and see if you are still getting funny voltage issues....