Hi All,

I've been flying an analog HGLRC Rekon35 Nano Long Range quad for around 6 months now and I'm struggling to use more than around 60-70% of the 2S Li-Ion battery capacity before it's unable to fly any longer. I've spent the last few months trying to find the cause of the issue and searching online to see if anyone else has a similar issue, but I've not had any luck so far. The spec of the quad is listed here: https://rekonfpv.com/products/rekon35-na...og-version and the quad hasn't been modded (yet   )

I'm aware that Li-Ion cells don't have the same discharge capabilities as LiPo cells, so all of my test flights have been very smooth and steady on the throttle to try and rule out the possibility of over-stressing the batteries. Typically, the cells feel either slightly warm or the temperature just isn't noticeable.

I started off flying with what I thought were Sony VTC6 cells, but quickly realised they were fakes. The batteries were charged to 4.2v and I was able to use around 1400mAh of a claimed 3000mAh capacity before the quad was no longer able to sustain a hover at an average cell voltage of around 3.25v. I then bought some Molicel P28a cells from a reputable source and while these were better, I was still only able to use up to 1700mAh of their 2800mAh capacity and again the quad was unable to sustain a hover around 3.25v per cell. I wondered if maybe Betaflight's power consumption figure wasn't accurate, so I checked my battery charger's figure after fully charging the cells and this was repeatedly reporting figures within around 50-100mAh.

I have also been keeping an eye on the quad's current usage during these test flights and typically it's reporting around 4.5-5 amps in a hover with fully charged cells, which then steadily increases to around 6-6.5 amps shortly before it's unable to hover any longer.

So now I'm wondering if I either have some fault with a component or whether it's just that combination of motors and AIO components isn't able to make use of the Li-Ion cell capacity once the overall voltage hits around 6.5v. I have noticed the AIO board has 10A ESCs and it's rated for 2S-6S input voltage. Could it be that this board or motor spec isn't able to handle voltages lower than the typical 2S LiPo would be fully discharged at? Surely if that's the case, I'd be better off running this quad on something like a 1500-1800mAh 2S LiPo which would be a bit lighter and have a bit of extra punch if needed? I've also considered swapping to more efficient motors, possibly suited to 1S-2S voltages. However, I don't want to be changing multiple components unnecessarily, just to use up most of the rated capacity of the batteries.

Apologies for the wall of text, just trying to gather as much info as I can remember   Any thoughts or advice would be much appreciated!

Liion have a lot of voltage sag (like running a very low C lipo) and the higher your discharge rate, the greater the voltage sag. If you want to make use of the full liion capacity, ideally you want a FC that is rated for 1-2S and not one that is 2S+. Below a certain voltage, the FC can no longer provide power to the ESC controllers, also depending on the VTX setup you may lose video too. There are some videos showing ways to modify ESC controllers to work down to 5v, however it is only for certain boards.

When you say it drops at 3.25v, are you referring to resting voltage (measuring the battery after disconnected) or under load (reading from you OSD while hovering)? If under load, you can see from the 7A discharge curve that means you already used up roughly 2300+mah of the pack or 80+%. The balance capacity is delivered at even lower voltage, meaning you will draw even higher amps, which moves to an even lower discharge curve so what power you can squeeze out of the end may not add a lot more flying time, and the drone would not be very responsive either.



http://lygte-info.dk/review/batteries201...arator.php

Thanks very much for the detailed response and the link to the chart, it makes sense to me now.

The 3.25v average per cell is the reading from the OSD while it's under load. Looking at that chart it seems like the battery would possibly sag and become unflyable quickly, even if my FC and ESC were able to work at lower voltages. It looks like it's possibly not worth trying to get more flight time out of it, just accept it and enjoy the flights!

A 1-2S AIO board is relatively reasonably priced if you want to play around, you can find one for $30-40. It will at least give you the security that you don't have to worry about the ESC cutting out on low voltage, like if your quad is trying to come back from far away.

I think 2S liion is a little too heavy for 3", the battery weight means you have to draw more amps and you basically get less power out of the cell. For 3", I think 1S is the way to go if you want longer flight times. If it is light enough 20-25 minutes should be no problem. For 2S, I have tried some 4" toothpicks, and you can get 30 minutes of flying but it is very tiring flying for so long, maybe more suitable for inav or semiautonomous flights.

i like most work dave c did for the hobby, but that liion mount is a complete fault for fpv use. it might be the connection which doesnt let the enough current pass if your batteries doesnt even get warm.
you might be able to bend the pins and improve it, but that battery case just doesnt match a quads needs

i suggest to solder the two cells to one 2s pack with balance lead and xt30 and balance lead, an xt30 on the quad too - give it a try than ;-)

but try out some lipos too :-)

I just realized your Rekon35 uses the 1303.5 motor. I recently built these motors into a light 3.5" build (though slightly lower kv) and it is a blast to fly on 3S. If you want to lighten your quad, maybe you can pickup a Crux35 frame and move the components over.

With the Crux35 frame, my setup weighs 76g. It just so happens it has 1-3S AIO inside so I could put on a liion pack and give it a test. Too bad the FC does not have a current sensor, but I would guess more than 25 min flight times since it is so light. But honestly if you don't need such long flight times, I think a larger low C rating lipo is a better option.

As for the battery tray, I also think it adds unecessary weight although the idea of simply plug in the cell is nice. I have an XT30 on my liions just easier to swap. However I have seen one youtuber simply solder nickel plates to the drones power leads, and use tiny magnets to attach the plate to the liion. Supposedly he has never had it fall off. The packs are held in place with a 3D printed clip, but the wires are just held by the magnet.

Thanks for all of the suggestions, much appreciated! It certainly seems like the FC not being able to handle 1S voltage is holding it back, which is a shame since that was how it was designed and shipped...

However, I have an 850mAh 2S LiPo I could try if I solder on an XT30 connector, so I might give that a go and see how it handles. If it flies well I could maybe look at getting a larger 2S LiPo that would still be lighter than the Li-Ion cells.

If that doesn't work out very well, I might look at getting a 1-2S or 1-3S AIO board and lose some weight from the quad to take advantage of the extra capacity of the Li-Ion cells.

Overall it seems like a nice quad to fly, so hopefully with a few little mods it will be a decent long term quad to build up some flying experience with.

you might not believe in the missconstruction of tht liion battery mount

additional to explain, if the power connection doesnt pass enough current, it goes worse with lower voltage. to get same power, same watts, you need to pull more amps on a lower voltage to achieve the same.

you should primery try your liion batteries with a decent and -regular- battery plug. its realy a mistake which was done when rc started, its truly a shame something like that comes from the fpv community which is way more sensitive to all that kind of details... than normal rc is. we take the right rated esc and the right plug, we dont messup with unnessecairy weight for an overrated plug.... but we are also known where thes plugs start to suffer... we are we ahead of rcs first days about optimize our power supply.

a 1-3s board wont improve anything on the power connection, if the current is the issue, im pretty sure, it wont change anything.

(18-Mar-2023, 07:50 PM)hugnosed_bat Wrote: you might not believe in the missconstruction of tht liion battery mount

additional to explain, if the power connection doesnt pass enough current, it goes worse with lower voltage. to get same power, same watts, you need to pull more amps on a lower voltage to achieve the same.

you should primery try your liion batteries with a decent and -regular- battery plug. its realy a mistake which was done when rc started, its truly a shame something like that comes from the fpv community which is way more sensitive to all that kind of details... than normal rc is. we take the right rated esc and the right plug, we dont messup with unnessecairy weight for an overrated plug.... but we are also known where thes plugs start to suffer... we are we ahead of rcs first days about optimize our power supply.

a 1-3s board wont improve anything on the power connection, if the current is the issue, im pretty sure, it wont change anything.

Thanks, yes I can see what you mean. Those terminals aren't going to be great for efficient transfer of power. I'm really not confident to solder my own Li-Ion battery pack at the moment, which is why I thought I would add an XT30 connector to my quad and try my smaller LiPo pack.

maybe you know someone around you, can craft it for you?

A 1S capable FC will definitely make a difference for flying li-ion, but unless you really need the long flight times I think you will enjoy your drone a lot more with a lipo. I tried out 2x18650 on my 3.5" today despite the 25mph winds, and it was no fun to fly at all. It was controllable, but very clumsy. I imagine the Rekon35 is quite a bit heavier, so it would be even more clumsy and less enjoyable to fly with liion, but if you are going long range and need the flight time...

Just for reference, my flying voltage reached 3.25v after 20 minutes. I landed at the 30 minute mark because my fingers were just too tired. The drone was still controllable but the pack could not deliver any full throttle punches at that point. Flying voltage was 2.75v, and resting 3.20v, so according to the discharge curve on these 3400 cells, there is still around 500mah capacity left and might give a few more minutes before it becomes unflyable.
I did not have a current sensor, but based on flight times I guess it draws around 5-6A cruising around the yard.