Building a 14Sx15P 18650 Battery Pack

Investigation of 18650 Battery cell Performance.

I have ordered a Batch5 paramotor and have been looking at constructing a 18650 battery pack.
Using 18650s is potentially a lot cheaper than buying the Bonka packs but suffers from not having the same peak current draw capability. A 14Sx15P configuration gives similar energy density by weight but is a lot more compact by volume, than a 4 x Bonka pack.

14Sx15P = $$650 2.1KWh 10.5Kg 5.8Litres => 200Wh/KG 360Wh/Litre 3.2Wh/$
4x Bonka7S = $$1000 2.3KWh 10.16Kg 12.8Litres => 226Wh/KG 180Wh/Litre 2.3Wh/$
At a push, a 14Sx15P battery pack should be able to supply 300A at around 45-49V.

Although there are many videos for constructing such packs on the web, this pack is unusual in that we are drawing 300A. That’s 20A per cell in the 15P configuration. That’s a lot of current to manage. Cell discharge graphs show a large voltage drop with this sort of load, Which affects the power delivery of the cell and generates a lot of heat. Trying to compare the data sheets of various cell manufacturers is difficult as they rarely give the true figures of what happens when you push them to the limit.

I looked at other cell formats (Molicell 21700 is a good contender, as is Samsung 40T) but these gave no power density advantages and were more expensive overall than top of the range 18650s.
So I bought examples of three of the best high discharge current 18650 cells I could find and ran tests on them under various load conditions. These batteries are:
Sony/Murata VTC6
Samsung 30Q
LG HG2

All three are 3000mAh batteries with the data sheets suggesting that they will cope with a 20A discharge. (yes, they are genuine, not fakes!)
The ‘industry standard’ green Panasonic 18650 3400mAh was tested, but the results not included here as it cannot supply the required current.
And the results are in. Here is a set of graphs for the three battery type contenders under 10 and 20 amp loads.


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The data was gathered using a Rigol 3021 Electronic load, and a Fluke thermal imager for temperature measurements.
Care was taken to keep the test leads short and the contact resistance to the battery under test low.
The load current was set to 10A and 20A. This was a constant current load.
Cut off was set at 2.2V (not 2.5V) to illustrate the rapid voltage drop at the end of the cycle.

You can see the near instantaneous drop off from 4.2V (fully charged voltage) to the running voltage, and then quite a smooth slope downwards to depletion.

The amount of voltage recovery after the load is removed is an indication of unspent energy.
Allowing the cell to recover and then discharging at 1A found an extra 0.25Wh. Therefore about 2.5% of the available energy was remaining after the 20A test with a 2.2V cutoff.

The three cells are very similar in their power delivery, with little to choose between them.
The total current supplied (mAh) is not a very meaningful figure here, but battery people insist on quoting it, so there it is!
The real energy delivery figure is the Watt/Hours (Wh). The digital load measurement system integrates the power delivered over time to give the real energy that the cell has delivered into that load – into the motor.

The temperature of the cell after discharge is very concerning. Delivering 20 Amps can easily raise the cell to 90-110 degC (22 degC ambient).

So it is essential to provide good cooling when running at this current.
image
Conclusions:
The Sony/Murata VTC6 just has the edge here. It has the least voltage drop on load, the highest energy and the lowest temperature build up.
This indicates a better quality of construction within the cell.
It is essential to provide cooling of these cells if 300A is to be delivered for any length of time.
Wrapping the cells up in insulation and putting in a closed box is not a good idea!
The contact resistance and the interconnect resistance must be kept very low to deliver the current without heat build up and power loss.

A Sony VTC6 16850 cell can deliver 10Wh and it weighs 48g. This gives it a weight rating of 208Wh/Kg.
I can buy this cell at a price of £2.44 per 200 ($3.13). This gives it a price rating of 4.1Wh/£Pound = 3.2Wh/$Dollar
A cell occupies a 20x20x70mm space – 28mL (with air gaps) This gives it a volume rating of 338Wh/Litre
These figures are close (slightly better) than those quoted in the OpenPPG Battery Comparison spreadsheet.

I would love to know what the Bonka pack can do under load.
Has anyone measured the temperature of a pack When run at 150Amps in still air? It must get very hot.
How much energy (Wh) does a Bonka pack actually deliver, running at 150 Amps to depletion? Anyone measured that??
If there is anyone in the UK who has a Bonka pack?, I would be happy to test it.

I hope you find this investigation useful - and that I havn’t made too many glaring errors!

I’ve just ordered 210 x Sony VTC6s and all the bits to build up a 14Sx15P pack.
If you are interested(!) ,I’ll keep you posted how I get on. Advise would be appreciated.

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Hi Richard,

Great tests and info, thanks for sharing.
I’m in the UK (CV31) and hope to be ordering some 22Ah 6S Bonkas in the next few months. I could order more if you want one. Be interesting to know where you are.

Cheers,
Richard

I built a 14s20p pack with vruzend connectors. I used lghg2 cells. It performs really well. I’m going to downsize the pack to 13s 18p as I find it too heavy. I also have 160kv motors and you have 150kv so I can run 13s no problem.

You will want a bms on your pack, so that you can have some safety built in, monitor voltage and temp of pack with probes inside the pack. You will be surprised by the weight of the pack once you put the cells all together with your wires, bus bars, case for the pack. It adds up and will be heavier then the lipo bonkas. Your really pushing those cells with 15p pack. You will be over 300 amps at max throttle. It’s around 340 that I’ve noticed so you might want to consider slightly larger pack or you could just put a spacer on your throttle so you never go over a certain amount of power. But I would highly recommend a battery management system (bms). I’m going to be adding an active balancer as well so the pack stays balanced properly.

hi, after long-term test and about 500 flights (me and friends together also fly my packs) I can tell you to the sony vtc 6 that the maximum on continuous load is about 10A. then the cell lives long. in other words. Constantly above 55 degrees Celsius damages the cells. I have a test set-up 1 year to load and unload in the office. meanwhile I fly the samsung 21700/40 T which has a really better performance than the sony vtc6. Here, with good cooling, almost 20 A continuous load is possible. but your setup with 14 S / 15 P is perfect for your project! the max power you need only briefly to start.

Hi Richard, I’m in Oxford so not too far away. The simplest thing for me to do is buy 4 x 7S Bonkas and just fly. I may well still do that if I have problems with this pack construction, in which case sharing postage costs would be a good idea.
Why would you not go for the 7S bonkas? Are you ordering from OpenPPG? What charger are you looking at? I presume you’ve ordered a batch 5 paramotor?
So many questions! We will have to compare notes soon.
Regards, Richard.

Hi Bratwurst. I’m interested building a battery using the 21700/40T. What size pack did you make? What motor and ESC config are you using?

hi, you can find a lot of information in my videos in the descriptions. in facebook in my page. And there are many more in my facebook page. There are also some newspaper reports that you will find linked in facebook. regards https://www.facebook.com/customparamotors/

Hi Foiledagain,
If you’re doing a 21700 pack, you might want to concider the Molicell P42A cell over the Samsung 40T. It’s a bit more expensive, but theoreticaly has more (5% more?) energy density.

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yes richard, the molicell is about 3 -4 degrees celsius cooler after the first tests and can deliver more by about 80 mah at the same final discharge voltage. I can not say anything about the cycle life. at the price it is in the wholesale still very different. samsung around 4.4 euro. vs molicel 5.7 euro incl. tax. ( in Europa) i’m still waiting for spring to buy the molicel. until then, the 200 test cycles are done.

@RichardG Any updates on your VTC6 battery build? I’m really interested.

Hi @qbplus9 .

Things are progressing slowly, but I have fresh impetus now that my Build 5 paramotor is on its way! I have accumulated all the parts and tools needed to build my battery pack and I am taking lots of photos with the aim of writing up its construction.
DSC00830
My latest problem is that I have been sold nickel plated steel strips being passed off as pure nickel. I was aware of this risk and so had tested a sample of the strip some weeks ago when I first got it and it appeared to pass. Basically I filed back the nickel coating and put the strip in some salty water. After 24 hours it hadn’t rusted so I figured it was OK.

Yesterday I sat down to start spot welding the nickel strip. I had a quick tidy up before setting to it, and found the test piece - brown with rust!! With the currents required, nickel coated steel is just not good enough.!
DSC00835
There are a few youtube videos on testing for nickel coated steel imposters. I happen to have some “Blu gel” steel blackening solution for antiquing fresh steel and treating gun barrels. It instantly turns steel black – but not nickel! That has allowed me to quickly test the other nickel strip that I have and that’s fine.

So, construction continues…

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Cheap Battery Construction Alternative for OpenPPG.

18650 (21700,etc) cells are the cheapest, most flexible way of putting a battery pack together. 2/3 the price of equivalent Bonkas. But constructing them takes time, skill and investment in equipment.

So, I have devised an alternative construction method suitable for the ‘hobbyist’ and OpenPPG

I have put together a few cells in this way as a test and they perform really well.
This alternative method has the following advantages:

  • Easy to put together.
  • No spot welding required.
  • Very little soldering
  • Individual cells can be removed and replaced.
  • Materials readily available
  • Very high current carrying capacity.
  • Flexible configurations.
  • Cheap,
  • Lightweight,
  • Robust,
  • And I believe it is would be reliable.
  • (No, It is NOT that awful vruzend system!)

Does this sound like a set of features you guys would want?

Forgive me, but I would want to fully try this method on a flyable pack before publishing the details.

As I now already have my battery pack constructed using conventional methods, I need to know how much interest there would be for this, to see where my priorities should lie in developing it further.

So, what about it? Would this be of interest to you guys out there?

How many pilots want to save money on their battery pack, but don’t feel they have the time, skills or equipment to construct a battery pack in the conventional manner??

Should this become part of this open source project or are you all happy with your Bonkas?!

Feed back please!

We just want to know how you did it. :wink:

I will soon be build an 18650 battery pack for a paramotor. I’ll be interested to see what you have come up with as I may decide to use it…

What you’re describing is the holy grail of DIY battery packs. I’m pretty sure more than just the openPPG crowd would be interested.

At the moment, it’s just an idea and a small test pack to prove the concept.

If you have a spot welder and know how to use it, I say carry on with that method.

If you have Bonka packs – they are fine – carry on. (Just keep them ventilated!)

This alternative method needs thoroughly testing as it would be aimed at those that don’t have the skills, knowledge and equipment to build in the conventional way and to test the result thoroughly.

So I am reluctant to release the details without properly testing it and confirming that it is reliable and largely fool proof. I don’t want to go off half-cocked on this. Sorry to be a tease!

My dilemma is that I already have myself a conventionally constructed 14S16P 18650 pack, fully tested and waiting to go flying. (construction details of that to follow)

To investigate this alternative method would not benefit me personally, I would be doing it for the good of the community. I am putting this call out to make sure that if I did put my time into this alternative construction project, that there is a need for it!

Please persuade me!!

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There is obviously need for a good 18650 battery construction method. Hence vruzend. Something as easy that can handle more draw would be of interest for sure.

But why not let this crowd s share the work of testing the idea and working out the bugs. That is what oppen source is all about. Those of us that have the skills and are willing to work out problems and have the time to tinker will be all over it with you. Those that need the final solution will sit back and watch and wait.

Unless your planning to develop a product and sell it…

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I’d be willing to build a small test pack that’s not for flight use, just because you’ve piqued my interest.

I certainly wouldnt want to sell it. it uses commonly available and cheap bits and pieces (apart from the batteries!), so there’s not that much opertunity to make money.

OK, I have decided, I will throw the idea out there as long as there is someone to catch it !
I think this deserves a seperate thread, so I will start one just for this.

I will produce models, photos and descriptions to show how it all connects together. I may also post some .STP models on GITHub so that people can contribute there.

In the mean time, here’s some tantalising CAD visualisations of it fitted on the OpenPPG. This is a 14S16P configuration with a nice healthy gap between cell blocks for a cooling breeze!



I will leave you guessing how the cells make contact with each other - which is key to this thing working.

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Nice! Down one side and up the other. Simple, solid, flexible. Ingeneous. This looks well thought out.

This looks super simple! (Link to the new thread here when you start it)