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High Voltage equivalents for Hitec HS85MG and HS225 servos for 2s LiFe RX battery.


Jonathan M
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I was going to fit HS85MG and HS225 servos in my next build (48" Chilli Breeze IC, original used 1990s standards) but looking to save (further) weight by using a 2s 1600mAh LiFe instead of a 2000mAh NiMh.  However I don't know whether these can take 7.2v fully-charged?  If this isn't safe I could always fit a BEC to step down to 5.5v, but would rather keep the supply as direct and with a few components as possible.

 

So, any recommendations for good quality (not crazily expensive) alternative high-voltage servos I could consider to the 85MG and 225?

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1 hour ago, Graeme White said:

I always thought if a servo said 6v compatible it meant 5 cell nimh, so about 7.5v fully charged, which then meant a fully charged 2s life was OK? Happy to be proved wrong below I blow some of my servos up from over voltage.

 

I'm sure a lot would pass that test, but some of the Futaba digital thin wing servos (3150 etc) are not even rated for 6V so they would definitely not be LiFe compatible. You will probably be ok using them with LiFe 95% of the time, but if one fails you may find it harder to get a replacement under warranty.

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It will be fine, use 2 cell life on those servo's myself years ago, 2 cell LiFe at the end of a charge might be 7.2 V but will never be in real life, decay so quick from peak 6.6V is what you 'll really get 5 minutes later, voltages quoted are always nominal + or - 10% anyway unless tighter (military grade) is specified. 

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5 hours ago, Don Fry said:

Don’t know about those servos, not a brand I use. But for that airframe, I would use a 1000 or even an 800 mAh battery. It would be fine for an hour or so airtime, and as you say, light flies nicer.

 

I did consider that, but the 800mAh NiMhs are based on AAA sized cells and I'd be concerned that they mightn't deal with the load.

 

5 hours ago, MattyB said:

No, both of those at 6V/max according to the spec sheets. I would look at the Savox equivalents, they are much more likely to be LiFe ready.

 

Savox are mightily expensive, and although I could find a Mini equivalent I couldn't see any for the Micro size.

 

5 hours ago, David P Williams said:

I’ve used lots of different servos rated 6V with 2s LiFe and they’ve all been fine, no BECs or diodes needed

 

 

2 hours ago, Philip Lewis 3 said:

It will be fine, use 2 cell life on those servo's myself years ago, 2 cell LiFe at the end of a charge might be 7.2 V but will never be in real life, decay so quick from peak 6.6V is what you 'll really get 5 minutes later, voltages quoted are always nominal + or - 10% anyway unless tighter (military grade) is specified. 

 

Good to hear of that experience.

 

However I've just discovered an unused 5.5v BEC I'd previously bought (from 4-Max).  What would be the downside of playing safe and just fitting this?  Do good quality BEC's ever fail?  After all they're incorporated in the vast majority of ESCs for electric power-trains!

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1 hour ago, Jonathan M said:

I'm going to use the 4-Max BEC I've already got.


That should be fine, just make sure it can supply at least 3A peak and you should be good. I also run all my BECs and servos on a servo tester for at least 90 mins when I get them, as if they are going to fail they tend to do so early on (it does happen) - much better to fail on the bench than in the air!

 

Edited by MattyB
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55 minutes ago, Graeme White said:

Just make sure the voltage drop is high enough. If you want 5.5v you may have to supply more than what a 2s life can give. Sure the bec will have specs stated on it.

 

The input spec for the 4-Max 5A ubec is 6-33V DC (2-8 LiPo), output is 5.5v.

 

A 2s LiFe is fully-charged at 7.2v and fully-depleted at 5.0v, which means (assuming for simplicity a more or less straight line discharge gradient) that the voltage will have dropped to 6v after having used about 55% of the capacity, call it 50% to be on the safe side (and ignoring any peak demands).  For my 1600mAh LiFe, this then equals 800mAh usable.

 

2 minutes ago, MattyB said:

That should be fine, just make sure it can supply at least 3A peak and you should be good. I also run all my BECs and servos on a servo tester for at least 90 mins when I get them, as if they are going to fail they tend to do so early on - much better to fail on the bench than in the air!

 

Good advice!  Do you run the tests on each servo independently or as many as you can fit on your tester (mine does 3 max)?

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1 hour ago, Jonathan M said:

Good advice!  Do you run the tests on each servo independently or as many as you can fit on your tester (mine does 3 max)?

 

It depends. If it's just servos you're burning in then obviously you can run them individually or in sets as required, it doesn't really matter. If I'm testing a BEC I've historically connected all the servos I could reasonably expect to operate simultaneously (i.e. 6 for a 4 servo wing F3X model) using Y-leads as necessary, then run the test.

 

However, the last time I did this it was on a secondhand model and wanted to test loads on the servos in situ, so I utilised my Taranis instead. I setup (or downloaded, I can't honestly remember!) a servo testing model setup that exercised channels 1-16 with variety of options/speeds. I then disconnected the linkages to prevent anything getting overdriven and left it all to run for a couple of hours with telemetry logging enabled so I could see the effect on the RX battery voltage over time. It worked fine, and as I know you have a Taranis it may be worth you investigating this method; once setup the first time I find it easier than trying to find my servo tester and enough y-leads to do the job in the morass that is my office! ?

 

 

OpenTX servo tester setups:

 

 

Edited by MattyB
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I have used 4 max Ubecs in the past and they were fine, until one wasn't. 

A 6 cell LiPo supply and the 20 amp Ubec failed open circuit to let all 24volts through to the RX and servos. 

Unfortunately it was my 5m Thermik XXXL and it took out a 12 channel MPX Pro Rx as well as all 10 servos (around £600/£700 worth).

Good job it was on the ground so no crash damage to the airframe. 

So now I will always use a separate RX supply battery in isolation from a motor drive battery and will never rely on a BEC in an ESC, or a separate UBEC ever again.

However ymmv and everyone to their own decisions.

 

John M

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Thanks Matty, that's great.  The programming looks quite straightforward, and I like the idea of being able to monitor the RX voltage in real time (having calibrated the telemetry to the battery first of course).  I expect this to simply read 5.5v until the LiFe supply voltage drops below 6.0v, after which it'll be very interesting to see what happens.  Will the BEC output voltage then drop below 5.5v or will it cease supplying any volts at all?  All this can be done on the bench as part of the pre-build cycling exercise on the four main servos (wings and tails); the throttle one doesn't need any real torque so will be a diddy little EMAX one to save weight which can be cycled on the tester separately.

 

John, I hear your warning!  My only observation is that your UBEC failure was about letting 24v through the gate, whereas my concern here is whether the UBEC simply works per se to supply 5.5v from a more modest maximum of 7.2v, and then keeps working once the LiFe is below half-capacity and for how long.

 

Edited by Jonathan M
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Yes, like any electronic component, they can fail. Like a lot of power controller type nick nacks, you got two failure modes, FET off and FET on. Off, would be instant power off. On, is full battery passed through. My guess is that fail off is more likely mode than fail on. Either is likely game over if you're airbourne, I wouldn't trust fail 'on' to stay 'failed on' for long.

 

Last thing (and like any free advice you can have your money back if you don't like it)... A 4.8V 2000mAh Eneloop pack weighs 108g. A 1000mAh LiFe pack, plus the regulator; 50g and 15g respectively? 65g vs 108g, ~40g diff, or 1.5oz in old money, just throwing that in there. The original breeze ran 4 standard servos at 45g each, say 180g total. You're looking at 2x85Mg (22g), 2x225MG (28g) and (guessing) an 81 (17g) on throttle, say 120g total, you bought 60g of lightness from the servo weight, on a model that will be near 1800g total (originally). Either improvement is around 2% of total weight. Not worth sweating over, I would suggest, either way, even if you went all old school with AA and standards.

 

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Getting back to the original question, 4-Max do some reasonably priced HV servos.

 

But I have run 2s LiFe batteries on many standard servos with no issues, the voltage on a LiFe pack drops to 6.6v very quickly and then stays there for quite a while, depending on the pack you will see some voltage drop off underload as some packs do have a larger internal resistance than LiPos.

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  • 2 weeks later...

Set up a programme on the Taranis for servo-cycling (with RX battery telemetry logging) and ran some NiMh tests.

 

Slightly differently to the video instructions above, I set the speed to 2.5secs each way with no pauses at the end-points, so 5.0 secs complete cycle.  This is more similar to but very slightly slower than a standard servo-tester cycle.

 

Used the five servos I'm due to fit to the Chilli Breeze (which are now well burnt-in!)

  • 2 x Hitec HS225MG (rudder and elevator)
  • 2 x Hitec HS85MG (ailerons)
  • 1x Emax micro (throttle).

All three batteries were fully discharged to 4.0v then re-charged using my standard routine for consistency, and this was done the night before so they were rested.


The graphs below are self-explanatory.  They were the second set of tests which were almost exactly the same as the first set.

  • The AA 2000mAh Eneloop (graph shown in the raw without annotation) finished the hour with the voltage still nicely above 4.9v.
  • The 2/3AA 800mAh held the voltage above 4.8v for about 50 minutes.
  • The AAA 800mAh Eneloop dropped below 4.8v after only 20-25 minutes.

1818551420_AA2000mAh4.8vcopy.thumb.png.3a4bcc5ae3eaafbbd59070ab74d0c9bf.png

 

1907796475_23AA700800mAh4.8vcopy.thumb.png.d82f4926d059fd49cc5c572cb82e4053.png

 

544342125_AAA800mAh4.8vcopy.thumb.png.5301226364a07ee7dec6f0ae8d7b5535.png

 

So the answer is to fit the 2/3AA 800mAh (no point in fattening the 48" CB with an unnecessary extra 2oz that comes with a 2000mAh).  This is the spare for my Ahi (usually flown for extended periods in bouncy conditions on high-rates with large/frequent surface deflections off four 12g metal-geared 2.4Kg.cm servos), so I'm sure will be fine in the CB for several 8-10 min flights off a full charge.  I can check the current voltage at any time by just glancing at the large telemetry screen on the TX.

 

My normal charger also runs off 12v from the field-box Pb and would take say 30mins to top-up, else I can carry a spare.

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Very thorough ?

 

When I've checked, my 60ish airframes are taking about 80mAh per 10 min flight. The smaller CB would presumably take a bit less. 800mAh nimhs should keep it going for a days flying without issue. 

 

As stated, old days, just used a 500 or 600 for the day.

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