Guess the fault

[Cuban8]

Last time I went flying was a couple of Sundays ago and had half a dozen flights with my H9 P47. On the last flight after landing, I was about to stop the engine from the tranny and I happened to notice that the LED battery indicator in the cockpit (strip type with eight lights - green to red) was flickering past the greens and into the yellows. Usually rock solid greens. When I throttled up I was startled to see the red lights coming on. When Idling, just the second green light would flicker. Engine off, and a steady single green. Not right, so needed investigation.

Took a look at it on monday on the bench and at switch on (no engine running of course) the indicator was a steady green. I happened to knock the fuz with the screwdriver I was holding and.......the battery indicator flickered. Tapped the fuz with my hand and found that if I knocked it at the front, the lights remained solid, but as I tapped further towards the tail I could get the yellows to come on and if I really tapped it (careful) I could just get a red to flicker. That's all I'll say for now.

Fixed it this afternoon...........what do you reckon it was?

[kc]

Poor contact somewhere - likely to be the switch.

[Bob Cotsford]

Rear mounted servos? If so an intermittent short in the extension leads. Switch on the blink?

[J D 8]

Duff indicator? got one in an old Katana and switched on with nothing moving will be first green. Move the sticks and it goes down to red and everything in between, sticks centred back to green.

[Former Member]

[This posting has been removed]

[Nigel R]

Black wire

[Brian Cooper]

I had this happen on one of mine a few years ago. . It turned to be a broken connection between two cells on the Nicad battery. . At rest, the connection was okay but when the engine was running, the vibration made the connection flick between on and off.

The strange behaviour of the battery indicator lights saved a model.

B.C.

[CARPERFECT]

First charge the battery to full, then check with a battery checker.Then test the switch, by simply plugging the battery direct into the rx. Then do the vibration test again, if still there try a different battery but connect via the switch. Only ever change one thing at once, Put the battery checker( the one with lights) into a different rx socket.

[Cuban8]

Couple of you are getting warm. Clue....not a bad connection.

[Former Member]

[This posting has been removed]

[Dave Cunnington]

Worth putting on this thread...may have some bearing ?

Yesterday I was readying stuff to go the field and put an rx battery into a plane to check it out.

Removed battery after a few minutes and it was unusually quite warm, so looked closely then

quickly separated the red and black wires, which were shorting together

The plastic shrink wrap is quite hard and had cut part way through the insulation

Good lesson, something else to check - or if you ever lift them by the wires ….don't !

Edited By Dave Cunnington on 04/10/2019 08:34:56

[Devcon1]

Anything to do with a stabilisation Rx with the movement provoking a servo response with that servo possibly having a fault.

[Trevor]

My money would be on the tailwheel steering servo pulling the battery voltage down when the rear of the fuselage is knocked. Could be a faulty servo or high resistance anywhere from battery to receiver.

[gangster]

Black lead or switch harness. Could also be a pin on a plug or socked retracting a bit. I still prefer the old Flair battery checkers that lock d to a bright flashing led it there had been a temporary issue such as high resistance or heavy servo. Wish you could still get them

[Bill Reed]

Climate change? Sorry Cuban8 just watched the news and its getting blamed for everything!

[Cuban8]

OK, all good and plausible explanations and ones that on first looking over the model, I did check as a 'keep it simple' approach before delving deeper.

Revving up the engine and tapping the model near the rear of the fuz was actually causing the elevator and rudder surfaces to load the servos very slightly thus causing the servos to correct and draw current from the battery. After a minute or two on the bench, just manually deflecting the elevator slightly and getting the servos to buzz would have the LED indicator in the red. The battery, off load measured a tad over 6V (with my AVO) but after stirring the sticks to elevator and rudder would drop to 5.2V. (five cell Nimh pack)

I'd seen this before and was confident from the evidence that a weak cell in the RX pack was the root cause of the problem. So it turned out to be. After removing the heat shrink cover, cell No 3 measured more or less the same as its partners when off load but put any sort of drain on the pack and no 3 would rapidly fall to 0.7V. hence why the expanded scale LED meter was flickering down to red. The meter, when set to five cells, will be monitoring the range between 5V (flat) to >6V (OK).

Ordered a new battery that afternoon and it arrived yesterday and no further problems. Voltage with everything going full pelt is rock solid.

This raises a point that I've mentioned before. If you get a similar weak cell in a four cell pack, the 'head room' available before the pack collapses towards the receiver's shut down voltage (applies to any make of RX) is much reduced. This will be made worse with current hungry digital servos and towards the end of a lengthy flying session when the other good cells will be declining quite normally anyway. Servo efficiency and power will also suffer and this might be evident before the RX 'browns out' by control sluggishness. You might notice it on an aerobatic model that pulls out late from a loop, but a vintage model pottering about will be unlikely to warn you until too late and it chugs off into the distance.

Without the little strip LED indicator (all my models have them, a couple of quid from HK or Ebay) I'd not have had a clue as to there being a problem, until I'd picked it up when doing a discharge check. Having the extra cell in the five cell pack gives redundancy of a sort; with just a four cell pack and being otherwise unaware of the failing cell could have lost a model. The pack was four years old - I always date RX packs when putting them into use.

My advice for what it's worth is.......individual cells do fail, so have a method of visually monitoring the pack voltage and make it part of pre and post flight checks. Use a five cell Nimh pack. Perform regular discharge capacity checks to flag up failing cells. Most four button Lipo chargers will have all you need to maintain and check RX packs. Might just save you a model.

 

 

 

Edited By Cuban8 on 04/10/2019 10:51:00

[Nigel R]

C8 thanks for sharing the issue and solution.

What brand and how old are the cells?

[Engine Doctor]

Hi Cuban 8 . Since loosing a good model some years ago due to 1 cell going down in a nearly new pack ,I now ckeck packs regularly with a load applied . This shows up any weak cells after a few seconds . I also use two packs with a diode on each pack on bigger models to insure against such a problem occurring again.

Lady luck was on your side that day , well spotted.

[J D 8]

Hi C8, As noted in earlier post I have my only strip indicator in an old Katana and it has been like that for years so not sure it tells me anything.

I test my rx batts with an SM SERVICES 4/5 cell battery checker before each session. [ Don't think they are available now ] It is the type that you push a button for 30 secs and put a load on the batt. A good batt will only drop point one or two of a volt

[Bob Cotsford]

I used to rely on those strip indicators but it didn't help when my Katana lost a cell in flight, the 4 cell pack had looked fine stirring the sticks before take-off but died during a roll. There had been no sluggish response or anything to give a warning, it just died. Now I have telemetry but I'm not sure even that would have helped with such a dramatic power drop. It put me right off NiMh packs I can tell you!

I still have a few bar indicators new in the packs somewhere, iirc they were for LiPo/LiFe though.

[Chris Walby]

Thanks for the info, I am newish to the hobby so use lipos and UBEC's for RX power. IMHO with digital servos I am not sure these RX packs are up to the job.

I was discussing dual supplies with a UK supplier of batteries, UBEC's ESC's and motors and for the model I had the recommendations was a 3S2200 lipo 20A UBEC as it has the capacity to maintain the RX even if a servo failed (burn the servo out).

Another dual supply UBEC I have discharges both batteries at the same rate, be interesting to see how the battery with the faulty cell would preform. I think you would only find it had a fault when recharging it as it have a different recharge Ah value.

Does anyone do a load test on RX packs as a maintenance check

[Nigel R]

"Does anyone do a load test on RX packs as a maintenance check"

I periodically do a charger based discharge/charge cycle to check things, using a load of about 1C (i.e. 2.5A for average AA size pack). That should result in approx. 50 minutes of discharge, with the cutoff at 1V/cell. Capacity should show around 80% of nominal. Ideally, trickle to charge.

"IMHO with digital servos I am not sure these RX packs are up to the job."

My take

RX pack choice depends on your servo load (somewhat obviously) - a "standard" nimh RX pack with a good switch, standard wiring (but quality, i.e. silicon insulation, 22AWG up to the RX) etc., are all good for a few amps (5A or more IIRC), which is enough to safely run four standard size digitals. Anything beyond that and yes, you're into lipo + reg territory, or sub Cs, or something along those lines, but also beefy wiring and the entry point for power distribution blocks and backup packs and whatnot.

I fly (max) 10cc sized sport stuff as a rule, and universally use analogue servos and standard nimh packs (or, switch mode BEC on an electric) - nothing up to that size class really needs the juicy servos. I'm not fast enough to detect a difference with fancy fast / digital servos. If I went the far side of 10lb then the picture would change (as it would if I had a 40 class or above 3d airframe).

[Braddock, VC]

With the duest of respect I think the problem is/was you're stuck in the last century; change over to 2 cell LiFe batteries and a balance charger all faults will show when you charge.

I think I dumped all my nimh and nicads over 10 years ago and never looked back.

Rx and ignition batteries are all LiFe 2 cells direct into the rx, never any problems with them except with old JR servos which let the smoke out immediately when over 5 volts is applied.

All bar 1 of my Tx have LiFe also. The odd one, a futaba 6 ex 2.4, has a 3 cell lipo in it as it's built to handle 8 dry cells.

I've never had any battery problems since my change over, SHMG.

Good LiFe batteries are both lighter and cheaper than the best of canned cells (like for like), which,for a 5 cell Ni** battery, have 4 welded terminals and two soldered ones that may fail plus they can leak and that causes all sorts of problems.

You don't use castor as well do you?

[Nigel R]

"never any problems with them except with old JR servos"

There's a rather large problem for me there, as I have a big pile of JR servos. I guess I could spend a pile and replace them all but I have no real need to.

I'm not adverse to LiFe batteries, my TX is LiFe.

At some point I'll move over to LiFe RX packs, I'm sure.

Simply that "not up to the job" just isn't true of nimh cells.

[Denis Watkins]

Similar to you Nigel, I think I fly regularly about 8 models, mainly 4 stroke

But a .60 is my max, and all models fly on Nimhs.

My routine is to check at the field for full charge, then recheck after 3 flights.

Never had a supply problem on those