I have to admit (not that it is hard to admit) that I paid someone I know (Thank you Jörg!) to do the proper planning and drawing.
I just told him how I wanted it, he added some ideas, went through some iterations in the design process of the mounting brackets and then I went for it.
I am not sure what you mean with “low voltage” switch - but the BMS takes care of shutting off the current when voltage goes to the lower limit.
turning on and off the battery manually is done with the button that i wired and tied to the shoulder straps. I will take a few more photos tomorrow.
Here’s a screencast of the BMS’ App, testing the battery WITH the case and the harness on my back.
It took me a few hours and watching that screen cast again and again, until I figured why the battery kept shutting down:
The Voltage Sag when depressing the throttle is so high, that the “low voltage” protection is triggered:
I received some more nickel strips and the 65 additional LG HG2 18650 cells, but I am still waiting for the plastic cell holders/spacers to arrive, to be able to mount and add them to the battery pack.
@GliderPilot any comments on why the voltage sag is so heavy?
Do you think it will change when the additional 5 parallel groups are added, because there is less stress on each parallel group thereafter?
I agree… I used the same foamed rubber as padding between the cells and the case, but this time it was not wrapped around, AND I cut out holes in the same position as the holes in the aluminum case.
So the air should be sucked very nicely through in between the cells and keep them from overheating.
I did not do any time measuring, but I did let the props run for a little under ten minutes yesterday, I guess, and the temperature did not exceed 32 degrees celsius.
I do know that doubling the number of parallel cells will cut the voltage drop in half… so yes, adding any number of rows will help.
How full were your batteries to begin with? If they are 50% charged it’s easy to drop to zero% with full throttle. Full batteries drop to about 50% with four bonkas at full throttle.
There are two reasons for voltage drop. One is the resistance in the batteries. More significantly, as you draw current from them, you deplete the chemical reactants near the plates, and it takes time for new reactants to take their place. When the batteries get warm the recovery is quicker and the voltage drops less. So running it down to 50% during a flight and getting the batteries warm in the process will give you a different result than a cold start at 50% capacity.
For the busbar, I purchased a new set of flat copper material, a bit smaller, with round edges.
I also decided to not solder the wires, but to connect them with nuts and bolts. You can see the bolt that will be used for this purpose on the top of the copper bus bar:
Here’s the negative terminal, connecting to the input of the BMS.
I used some duct tape (probably should have used heat resistant kapton tape, but well… ) to avoid accidentally touching both terminals with the conducting aluminum case.
The thinner wires on top of the array of cells connect with four thermistors to measure the battery’s temperature. I also rewired and shortened those.
Here’s a top view of the battery where you can see the BMS. it has aluminum plates on both sides for heat dissipation. the other side directly connects with the aluminum case with the help of heat conductive material.
You can also see the negative output of the BMS with the positive terminals connected to the bus bar.
Here you can see the three fans that are for cooling the battery… I haven’t tested yet whether all three are really needed, or wether just one or even none at all would work as well.
@tarifachris - are you from Tarifa in South of Spain? I went there once for a week of kitesurfing
Hope this works out for you…
I think it does, the amp draw does not seem to be an issue. after I added 5 parallel groups there’s even less stress on each individual cell when going full throttle - even thoug the LG HG2 cells are rated for the minimum setup of 15P as well.
if not change it to 12s with the KV180 motors or 14s with the KV150 motors!
I am confused, I don’t even know which KV rating my motors have. I ordered and received the batch 3 kit …
but switching isn’t so easy, as I spot welded all the connections in between the cells (except for the bus bar, which uses screws) and since the case has been designed around the 13S width, and the 20P height.
If I remove the fans (as it looks like, they are not needed) and place the BMS elsewhere, I’d have additional space for another 5 parallel groups, but no space for an additional series connection.
Could you explain, in which way the KV rating changes the Amp draw?
from what I know is that the KV rating determines the revolutions per minute if it is multiplied with the Volts.
so 50 Volt input on 150KV means 7500 revolutions.
I can not have less thrust than what I have now:
When the battery is full (and the full voltage is at my disposal) I have enough thrust to lift off and gain altitude. However when the battery is only at 50%, the thrust is only sufficient to keep level flight.
I think the question with the flight time is extremely difficult to answer.
I don’t have many flights with the new battery and the OpenPPG on my clock yet, but let me give you some insight as follows:
Flight time
(Info: 1 Meter = 3,28feet)
Flight A: It was around 5pm on a sunny, cold day, thermals were looking good.
I launched and climbed to around 400m over ground, then found thermals and climbed further up around 700m slowly (it was already late in the day, so slow but steady thermals).
it probably took me 10 minutes to reach the 400m on full throttle.
I was then stupid, flow over a small hill range in our area and went into a Lee situation. The wind really pushed my down hard, I could not climb back up because of the half depleted battery, so I went half throttle trying to keep altitude, I barely made it back half way around the hill, had to walk the rest of the way to the launch area. total flight time: 40 minutes.
If I hadn’t been so stupid to fly behind the hill range into the Lee situation, I probably could have kept flying for another hour or so… (using the rest of the thermals.
Flight B: I launched, climbed to 400 meters over ground or so, stayed there for a couple of minutes, total time probably 15 minutes. No thermals at all, rather the opposite: downward moving air. then noticed on the throttle controller that the battery was almost empty (I trusted the pourcentage shown, which was stupid - the throttle controller is set to a LiPo 12S battery voltage than the actual 13S Lithium Ion that I use, which has much higher voltage when full, and much lower voltage when empty. So I can not rely on the pourcentage, rather than the voltage shown.)
being in the air, I did not remember how to change the display to the actual voltage. So I landed.
I then noticed that I still had some capacity left, launched again and flew another 3 minutes at low altitude. with the battery almost depleted I can not go full throttle to climb again, and the thrust is also lower due to the lower voltage. If I stayed higher up, I could have flown for probably a lot longer
Flight C I launched, climbed to 300m or so and was looking for thermals, did not find any, after 10 minutes of flight time or so I decided to practice spiralling, did like 4 turns which made me loose a lot of altitude of course. with the battery half empty I decided to fly very low over the ground (1-3 meters), which is a lot of fun, but of course also kinda dangerous.
Stupid me did ignore that with half empty battery I have not much thrust anymore, to stay level, I pushed the throttle once too hard (Voltage Sag) which led the BMS to shut off the battery due to low voltage.
I immediately landed hard, with the wind - very very stupid maneuver!
Bottom line: The loose of thrust on an half empty battery is a problem, if your plan is to fly around, up and down.
You will get most out of the battery and have it be as efficient as possible when you only gain as little altitude as necessary to fly safe and comply with aviation rules, and then just fly level.
I have yet to test that, to really be able to tell you the possible maximum flight time.
What really is a good scenario, should I not make the battery even bigger in the future, is to fly midday, climb to a thousand feet or so, let go of the throttle and continue flying by riding the thermals!
By the way: I have IGC tracks from my Vario of all the above flights. if anyone wants to see them, I’d be happy to upload them
Battery Weight
the original 13S15P pack was 10.2 kilograms without the case.
I then added another 65 cells (5 parallel groups) and an aluminum case.
I don’t have the weight of the aluminum case, maybe 1-2 kilograms
but the cells weigh 48 grams per cell, that’s a total of 3,12 kilograms
so total weight now should be around 15 kilograms.
The KV 180 MAD motors where not designed for 13s - sure you have great power but your prop tips
speed is at full throttle at 800 kph and should be very loud. Ideal noise reduction would be in the lower 600 kph tip speed region…
