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1S or 2S to power 4 x KST108’s


Rory Hammerson
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Rory Hammerson

Bit confused - any help appreciated. I’m flying a Vibe 2 with 4 KST 108’s.  I’ve got a 1S Radient 600mah slotted in next to the rx. I found that I get slightly longer with a 600mah than I did with a 500mah but not much. I’m only getting about 25mins to 30mins of power before the signal fails. It happened in mid air today and clearly that ain’t good. I was lucky. Long grass good fortune - not a scratch.  Clearly  I need a really slim DSMx to give me info on battery voltage (suggestions please) but I don’t understand what’s happening as the battery stopped powering the system at 78% or 4.010v. So not low nor critical ?  Just didn’t have enough oomf to work the servos or communicate to my DX6. What am I doing wrong ?  The battery is newish (6-9 months old max) and I’m not a heavy duty flyer. Do I need lower mah capacity but a higher voltage - something like a 300-400mah 2S or am I looking at it the wrong way. Hope someone can help. Thanks. Rory 

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Half an hour seems a very short duration for a four servo set up. Is there a stiff linkage in there somewhere that is upping the current drain? Are you really getting the capacity out of the battery that the label suggests you are?

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Wookman is right on the stiff linkage.  I can easily fly a whole contest of 8 x 10 minute rounds on a 1s 600 mAh battery with 4 KST X 08s.

Can you charge the battery to full at say 400mA, then discharge it to 3v per cell at say 100mA to check capacity.

I have noticed that some chargers offer a 'fast charge' facility that lets you set the current (best to use 1C or less) - the 'fast' aspect comes from the fact that they cut off at 4.2v per cell.  (In other words it doesn't fully charge a Lipo battery.)  A normal charge goes past that stage, keeping the battery at 4.2v per cell, by reducing the current to (normally) 10% of the starting value before cut off.  It seems to me that the smaller batteries spend more of their charging time in the 'reducing current' period that the larger capacity ones.

It would be useful to put a voltmeter across the fully charged battery and check the voltage with and without the servos operating.  

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If the cell is only reducing to 4V then the capacity is not really in question, IMHO. What voltage range is the receiver capable of operating at?

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An external variable power supply is very useful for bench testing your kit.  You can identify lowest voltage before RX browns out and when servos stop working.

I know from my own testing that kst08 will go well below safe limit so as above suspect RX is cause.

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Rory Hammerson

Thanks all.
I think my linkages are fine. When I changed the rx I tested the throws and they all seemed perfect (though there is always a slight buzz caused by minor trims.  
I was using a lemon dsmx 6channel 
I had issues with it with a 500mah so I changed the battery for a 600mah. TBH I suspect that I haven’t been very clever with managing the batteries - none are older than 9 months but not looked after well  - just charging them up to ‘full’ and not discharging them and storing them at 3.7v - so I wonder if the battery quality has been impaired by my lack of knowledge. I’ve bought a charger/discharger now and am acting a bit smarter. I switched the rx to an lemon 7ch diversity/vario (taken out of its case) which is giving me masses of inflight info as well as rx voltage. Good to know that some rx’s won’t operate <4v. I never want that stomach dropping feeling of realising I’ve lost signal again. I could probably get smaller higher quality rx with another operating system but I’m limited to DSMx with my DX6 and the vibe ain’t spacious. Thanks again all. 

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On 28/06/2021 at 22:04, Rory Hammerson said:

Thanks all.
I think my linkages are fine. When I changed the rx I tested the throws and they all seemed perfect (though there is always a slight buzz caused by minor trims.  
I was using a lemon dsmx 6channel 
I had issues with it with a 500mah so I changed the battery for a 600mah. TBH I suspect that I haven’t been very clever with managing the batteries - none are older than 9 months but not looked after well  - just charging them up to ‘full’ and not discharging them and storing them at 3.7v - so I wonder if the battery quality has been impaired by my lack of knowledge. I’ve bought a charger/discharger now and am acting a bit smarter. I switched the rx to an lemon 7ch diversity/vario (taken out of its case) which is giving me masses of inflight info as well as rx voltage. Good to know that some rx’s won’t operate <4v. I never want that stomach dropping feeling of realising I’ve lost signal again. I could probably get smaller higher quality rx with another operating system but I’m limited to DSMx with my DX6 and the vibe ain’t spacious. Thanks again all. 

So, does the rx you are using work below 4V, or not?

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isoaritfirst

Also, buzzing servos are taking current, and usually a sign of there being some degree of stiffnes in the system.

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Rory Hammerson

Thanks again all. After some research I’ve found that lemon does takes a wide voltage range but in reality you need more than 4v so my 1S is under gunning the receiver hence the problem. I have replaced the lemon 7channel vario/diversity with a spektrum 6610T which is a shade bigger but somehow I wiggled it in. better (larger) antennas for a start, and it’s voltage range goes down to 3.5v so that’s solved. I reckon my issue was the lack of voltage rather than the lack of capacity. Feel much more confident now that I won’t see my bird nose dive into the Edinburgh landscape.  I can watch the rx power and won’t head home as soon as I was doing. Really appreciate all your help on this. Thank you. PS. I reset the rudder servo to neutral and it’s stopped buzzing. 👌🏼

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