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Why sudden dramatic voltage drop in 700mAh 4.8v NiMh pack?

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Frank Skilbeck31/05/2020 08:03:30
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That's quite some heavy use in a glider, I use LiFe cells in a variety of gliders and don't think I use half that consumption. Based on your use the system is drawing an average of 0.6A and as your Rx will typically draw less than 80 ma the servos are averaging over 0.5A, might be worth checking you don't have a faulty servo drawing more current than it should.

Remember for years the standard battery for all models was a 600 mah Nicd and we used to happily fly for hours on one of those.

Jonathan M31/05/2020 08:21:12
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How would I check the current draw of the servos?

They are quite hefty units for the light weight and small size of the Ahi, but it's got relatively big control surfaces and I'm running big travels with lots of expo.

This is how they tend to be flown (I'm scrappier on the sticks and have been flying on trickier slopes in gustier conditions): surfimp

Dream Flight DFFA009 12.4g Specifications
Body dimensions 23.4 mm wide x 9.4 mm thick x 26.2 mm high  
Max dimensions 32.1 mm wide x 9.4 mm thick x 31.5 mm high  
Stall torque (4.8V) 2.40 kg.cm  
Weight 11.5 g  
Lead length 15 cm  
Operating voltage 4.8V to 6.0V
Bearings 2 Ball Bearings
Gear material Metal
Case material Plastic
Motor type DC motor
Digital Yes
Programmable No
Supplied accessories 2 Plastic output arms, 2 Arm retaining screws, 4 Mounting screws.

 

Edited By Jonathan M on 31/05/2020 08:22:03

Steve J31/05/2020 08:44:30
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Posted by Jonathan M on 31/05/2020 08:21:12:

How would I check the current draw of the servos?

With something like this.

**LINK**

I bought one out of curiosity, but to be honest, I can't remember the last time I used it.

Steve J31/05/2020 08:45:54
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Posted by Frank Skilbeck on 31/05/2020 08:03:30:

Remember for years the standard battery for all models was a 600 mah Nicd and we used to happily fly for hours on one of those.

I always take such claims with a pinch of salt.

gangster31/05/2020 08:57:30
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Posted by Steve J on 31/05/2020 08:45:54:
Posted by Frank Skilbeck on 31/05/2020 08:03:30:

Remember for years the standard battery for all models was a 600 mah Nicd and we used to happily fly for hours on one of those.

I always take such claims with a pinch of salt.

no that is quite true With standard 40 and 60 size models Four servos my mate and Would often buy a gallon of fuel between us on a Friday and by Saturday teatime little was left and we had literally flown all day on a standard 500 nicad What we need to remember is that you cannot get a quart into a pint pot and the capacity figures quoted are dubious, not lies, but clever mathematics that don’t reflect what we do down the field The second thing is there is no such thing as a smart charger they can be pretty dumb in fact and detect false peaks There is no substitute for a long slow charge that allows all cells to come up to peak capacity

Peter Christy31/05/2020 09:15:57
1820 forum posts
Posted by Steve J on 31/05/2020 08:45:54:
Posted by Frank Skilbeck on 31/05/2020 08:03:30:

Remember for years the standard battery for all models was a 600 mah Nicd and we used to happily fly for hours on one of those.

I always take such claims with a pinch of salt.

Not really. My Bonner Digimite (the first mass produced digital proportional - c.1966) came with a 600mAH NiCad pack, and would last a days flying.

Bear in mind that back then, we didn't have what are now referred to as "digital" servos. Current consumption was substantially less than with modern, high torque, high speed servos.

The Digimite servos may have had several pounds of thrust, but they were very slow compared to modern servos, not that we noticed at the time! They were used for quite some time by all the major American RC manufacturers, except Orbit. (Kraft were fairly late to the party!). Despite their slow speed, they were used at all major aerobatic events, National, Continental and World Championships and no-one complained about their performance!

Even in the 70s and 80s, most of the popular radios came with 500 or 600 mAH packs which were quite adequate for most flyers, including contest types!

It is only the relatively recently developed servos, that are not only very powerful, but also very fast, that have forced us to increase battery capacity dramatically. And while such servos may be a requirement for jets or 3D helicopters, I do wonder how many people would notice the difference if someone magically replaced their modern servos with ones from 40 odd years ago overnight!

Of course, the other down-side of such powerful servos is that the peak current draw of four or five of them can be a few amps, pushing the standard 3-pin connectors used for the battery packs, as well as the servos, well in excess of their design limits!

--

Pete

Engine Doctor31/05/2020 11:12:48
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2511 forum posts
39 photos

Just looked at your flying style . Yes you really put the servos to work! That would really take the juice out of a 700 AAA pack . Why not keep it simple and take a spare pack ? Assuming they are easily changeable? All the speculation about different chargers etc won't change the fact that running digital or high draw servos will require more power . An hours continuous flying in your style is a good workout for the battery and modern gear doesn't give warnings that a battery is getting low unlike the older 35 met gear that often recovered enough to get the model down For me it's an easy equation 2 X 700 batteries = X : crashed model with flat battery = XXXXX?

Jonathan M31/05/2020 11:51:02
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You've hit the nail on the head ED. There's clearly a world of difference between nudging a large soarer around bigger skies with 2000mAh for nose-weight and constantly whipping something like the Ahi with a third of the battery capacity close in to the slope.... although I'd add that the video link isn't of me flying (for a few years yet), that's Steve Lange on the US Pacific coast !!

Slope Aerobatics

Mike Blandford31/05/2020 12:30:13
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25 photos

Also for some time the NiCd batteries were centre tapped (4 wires to the servo), and only 2 cells were used to move a servo, the "other" 2 moved it in the opposite direction. Many servos also used a 5 ohm motor, so with only 2 cells driving it you could not draw more than 0.5Amp (from half the battery) in any one servo.

Mike

Frank Skilbeck31/05/2020 13:20:36
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4729 forum posts
101 photos

Was out flying my Vegas Hotliner this morning which has a telemetry amp sensor, with motor off, and no servos moving it measured 0.1A, waggling all 4 servos and it registered 0.2/0.3A briefly, held a servo to stall it and most I could draw was 0.4A..

A few years back we did have an Atom Autogyro that would brownout when the battery was connected, we thought it was the BEC as it all worked when we used a regular battery, a 2nd ESC did the same, by process of elimination we found it was the rudder servo, a small Savox digital, drawing excessive current. We sent the servo back to the shop with an explanation, they returned it saying they couldn't find a fault! The Atom had many successful flights with the original ESC and a different rudder servo (until dumb thumbs finished off!)

Peter Christy31/05/2020 13:24:45
1820 forum posts

Also bear in mind that that 0.5 amp would only be if the servo was stalled or starting! Once the motor starts to rotate, the back emf reduces the current draw even further!

There is something to be said for slow, low geared servos for a lot of models! As I said earlier, I doubt if 95% of us would notice the difference!

--

Pete

Martin McIntosh31/05/2020 21:14:35
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My gear in the 70`s was supplied with 500 mA/h Deacs, later upgraded to 600`s but we were at the time rather concerned that after four competition flights there may be a fly-off which could be too much for the packs. Other than rigging up a discharge set up and using a stop watch there was no way to determine how much was left.

Much more recently (although in the late days of 35mHz) a particular model with a couple of `digital` servos kept glitching despite many types of Rx being tried. The culprit turned out to be a high capacity NiMh pack which simply could not supply the required power, much the same as using AAA`s which due to the slender shape are unable to supply power without a large voltage drop.

I am now gradually changing to LiFe packs and even a tiny 700 will happily supply digi mini servos or none digi standard size with ease and they seem to last for ever.

Jonathan M31/05/2020 22:32:44
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Had another sesh with the Ahi this afternoon on a more local slope (five mile round-trip walk from home) but only marginal lift so a much gentler, more careful half-hour's flying: only 170mAh used, half that of the active aerobatics in the same timeframe previously.

Jonathan M31/05/2020 22:34:40
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Posted by Martin McIntosh on 31/05/2020 21:14:35:

I am now gradually changing to LiFe packs and even a tiny 700 will happily supply digi mini servos or none digi standard size with ease and they seem to last for ever.

Are you using a voltage-regulator for the servos? Mine are rated for 4.5/6.0v only.

Martin McIntosh01/06/2020 13:56:02
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Well, yes and no. One of my recent builds uses the HV version of the servos, most of the larger stuff has 5.9V Failover switches on twin packs and some are directly from the 6.6V 2s LiFe since a 6V NiMh is actually way above this anyway for most of the discharge.

I believe that LiFe`s are available in round cells similar to AA`s so two should fit almost anything.

Further, I would never now operate 2.4 sets with only 4.8V since a voltage drop to below 3.7V can cause a brown out on some (ask me how I know).

Jonathan M03/06/2020 18:37:37
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Now taken delivery of a 2nd 700mAh NiMH, so a question on 'best practice' with a new battery. Just charge and go fly, or cycle up, down and back up again, then go fly?

Peter Christy03/06/2020 18:58:10
1820 forum posts

NiMhs don't suffer from "memory" effects like NiCads used to, but can still lose capacity if abused.

Do the initial charge as a trickle charge, ie: charge at 70 mA for 12 hours. After that, just charge with a peak detect charger at 1C (700mA). Set the maximum charge time to 1 hour, but it should trip off well before then.

Once every couple of months give it a 12 hour trickle charge to balance the cells (depending on usage). If not used much, you don't need to trickle charge so often.

Charging at 1C should provide a nice voltage drop for he peak detect charger to see. Trickle charging will wipe out any imbalance in the cells, but should not be done too often. It will result in over-charging, causing the cells to emit gas and dry out the electrolyte. Only do it occasionally.

Its amazing how the old problem of "black wire corrosion" has gone away since the advent of peak detect chargers....!

--

Pete

Denis Watkins03/06/2020 18:58:53
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No need to cycle a Nimh Jon, just ensure full charge

Simon Chaddock03/06/2020 19:10:15
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It is all too easy to assume a bigger capacity battery also means a bigger current capability. This true only if the construction of the battery remains the same.

High capacity (2000 mAh) AA size NiMh are a case in point. Their maximum current capacity is actually worse than a 500 mAh NiCd. The extra capacity thus offer no better performance when driving high current servos.

Many high capacity NiMh only display their full capacity at a 1/10 C discharge rate. At a higher discharge rate their voltage falls simply away.

For a good NiMh servo battery you need battery 'volume' rather than simple electrical capacity or use a higher rate discharge chemistry like LIPo or LIFe.

Steve J03/06/2020 20:07:39
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Posted by Jonathan M on 03/06/2020 18:37:37:

Now taken delivery of a 2nd 700mAh NiMH, so a question on 'best practice' with a new battery. Just charge and go fly, or cycle up, down and back up again, then go fly?

If your charger has a forming setting, use that. If it doesn't, I would suggest a 0.1C charge overnight followed by a delta peak cycle to make sure that it is working. I would also advise cycling conventional NiMHs every 3-4 months.

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