New battery option: better power density!

a constant test is not possible with a resistor because the voltage drops and therefore the current decreases. so i wrote in 12 minutes for the whole discharge. yes of course an experimental setup on the floor in the workshop is the best thing when it comes to safety. It only becomes dangerous if people are now of the opinion “ok. 150 A is possible” now I’m flying with such a pack I want to use a light setup by weight and only gain a little height for thermal flight. you have to consider that a new pack has a good performance. if it gets hotter than 55 degrees celsius several times, the chemical composition changes somewhat. that means the inner resistance gets worse and the capacity decreases. that means after 20 cycles, the pack becomes noticeably hotter than the first try. this means that the conversion is even more noticeable. It is a doom-loop. this is the reason why lipo can often no longer perform the service after a short time. this is dangerous for people who do not fully understand the background when flying with it. that’s why I always repeat not to use a system based on manufacturer values ​​but after extensive tests under real conditions. and the most important thing is to have a 30% reserve at the temperature. only then will I describe a battery as suitable for man-carrying. to fly a pack that already reaches 60 degrees celsius with 5 C load is tired of life. the same applies if the cables and arresters are not designed for this. imagine using 2 packs in parallel. one of the plugs fails. the pilot does not notice it and remains at 200A. if he is lucky the other plug will melt. if he is unlucky, the lipo ignites beforehand through the arrester, which begins to glow. i often think many believe flying with eppg is a game where you can get out at any time. this is only possible if you fly very low only on the sports field ok. if you fly properly at great heights or longer distances you can’t just say: ok my battery is starting to burn, i’ll land you quickly … some will understand. some not. warnings have been written enough. opportunities as well as how to fly safely. Many here build the li-ion batteries themselves are proof of this.

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So to have a good cutaway for the battery could be the best …

Here is the voltage graph of the discharge into 150Ohms. Because this is a constant resistance load, this graph is relatively flat as the current reduces as the voltage reduces.

The demand from a motor ESC will be more of constant power demand rather than constant resistance. So in the real world, the tail end of the discharge will have greater current demand on the cell, not less. The voltage graph would be much steeper.

I think I need to run the 150Amp test again with the battery opened out to let me see how hot the tabs on the cells get.The Bonkas would have a similar problem. Do they? Has anyone done these sorts of tests on the Bonkas??

I think that with the props running, there will be plenty of air cooling to keep the cabling and batteries to 60 degrees C.

OK, now you got me thinking about emergency eject mechanisms for a burning power pack!!

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Cool thank you - did the temperatur rise to 60 c at the end or did it hold the 60 c for 5 minutes.or so?

An openppg eject mechanism for bonkas was already build by voltair:

Hi, As I recall, the temperature built up gradually and only reached 60 degrees towards the end.
I think any battery holder should allow a free flow of air so these battery bags dont look quite such a good idea.


Battery Pack Comparison Conclusions.


I have just about finished my comparative testing of Bonkas, Foxtech and 18650 cell battery packs.
I have put all batteries on full load, compared energy delivered and looked at what gets hot.
Here are my conclusions so far:

The ultimate, money-no-object highest power density per weight is the Foxtech Diamond. A similar pack configuration to the Bonkas. I have posted my capacity investigation on these in this thread above.

These are what Foxtech call semi-solid lithium. They have impressive energy density specifications, which my testing has confirmed.

  • 4 x Foxtech Diamond gives 7% more energy for 15% less weight than 4 x Bonkas.
  • But costs 28% more.
  • The Foxtechs are very difficult to get hold of ( but I now have four!)
  • They need re-wiring with thicker cable to work with OpenPPG.
  • The Foxtech’s longevity is yet to be tested.
  • They are currently only available in a 6S configuration.

18650 pack:
If low cost is everything, then constructing a 14S16P battery pack from 18650 cells is the cheapest option. A 14S16P battery pack constructed from 224 x Sony VT6 cells will cost about 60% of the equivalent four Bonka packs. Even with all the additional holders, nickel strip and wiring, a 14S16P battery pack comes in at about the same power and weight as 4 x Bonkas.

  • 11.2KG for the 14S16P vs 11.84KG for 4 x Bonkas.
  • 2.12KW/H for the 14S16P vs 2.13KW/H for 4 x Bonkas.

The problem with a pack made from 18650 cells is that you have to do a lot of construction yourself.

Although this may be an enjoyable challenge for many, it requires tools that you may not have to hand and so need an investment in a Spot welder, High power soldering iron, and Hot air gun. Considerable care is required to do it properly.

Most online construction tutorials/methods are not intended to cater for the current draw that we need and so may not be appropriate. Sourcing all the components takes time.
It takes many man-days of work to finish a pack and ensure it is reliable.
It is very difficult to test/replace faulty cells.
16 x 18650 cells at 20 amp per cell (320Amp) puts them close to the edge of specifications for those cells so do they need to be kept cool and not enclosed.

I will do a post soon on how I constructed my 18650 pack (above center) and the problems I had to overcome.

Bonka cells are rated for much more current 25C = 550A continuous from a single pack.
But there is no way a Bonka pack could sustain that. The 18AWG cabling isn’t up to it, and they get very hot at 150 amp continuous load. They rise in temperature by about 55degrees C (above ambient) after a continuous load of 150 amps (per pack) down to depletion.

So, in practice, the practical continuous load currents are similar for 14S16P as the Bonkas.
They also need to be kept cool

For an easy life, go with the Bonkas – but I do not recommend enclosing the batteries like I have seen in some paramotors. They need airflow to keep cool and last long. Even to the point of putting spacers between and around the packs.

18650 cell batteries are cheaper and more flexible but are tricky and time consuming to construct.

The Foxtech diamond give an edge but are, as yet only available in 6S configuration and are untried in the field.

Whatever you do - get cabling and connector current capacities right and make sure the batteries have good air flow around them.

Hope this helps.


Thanks for this really good summary Richard.

It remains clear that in terms of value for money, Bonkas are way ahead of the competition. It is really lucky that Zach and Paul could get such amazing deal with Bonka, cutting out the middle man.

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Browsing around, I found this kind of crazy website that sells used electric car batteries at killer prices.
This Volvo battery caught my attention in particular:


Did your battery adventures take you any further? The semisolid Foxtech packs look like a real winner if fire is a concern, but every battery manufacturer ever tells you about how safe their product is, despite pictures to the contrary.

Actually all big Lipo battery producer go with high voltage Lipo’s…
They have the same high current properties and have 15-25% longer flight time with the same weight and volume.

These are 4.35 Volt per cell Lipos’s and they are working on the next generation with 4.40 V per cell.

You could charge them just to 4.20 V so they are compatible if you use them together with your old Lipo’s.

With 52.2 Volt fully charged as 12S these would be great for the X4 KV150…

They would but the price!! YIKES

If I got double the capacity for double the price I’m in! A 15% increase is only a couple of minutes (for four packs) and is about $100CDN more. Not worth it for me.

“We recommend limiting the discharge rate to 170a continuous …” We’d need to run dual packs of these to get level flight and acceptable climbout power. That’s over 50lbs of battery. Nice, inexpensive option for a longer flights if you can handle the weight. Too bad they’re out-of-stock.Since they’re 16 cell in series, you would have to add a converter to get the voltage down or open them up and remove 2 cells.
I wonder how many charger options there are for 14S liPo?

Hi Kubark42,

I’m afraid my ‘battery adventures’ came to an abrupt halt 3 months ago. I have broken my leg paragliding.

I ‘only’ broke the one leg. I had the full works - emergency services, helicoptor rescue, even got into the national newspaper. So the finer points of battery technologies are not highest on my agenda right now!
I still havent flown my X4 paramotor - maybe next year. Then I can compare battery packs like I intended.


The Tattu HV batteries are very good. I’m using 2 of the HV 6s 25Ah for my ehelp Previously I was using 2 of the Hobby King 6s Graphene batteries

The difference is night and day. With the Hobby King batteries I was climbing out at 150-200fpm and with the Tattu HV I was climbing out at 350-400fpm. Also flight time was 20min vs 4 min which is 5x compared to a slightly more than 2x the Ah.


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Great work charles. Please keep sharing your progress!

Thanks, the carbon ehelp really isn’t my focus though. I’ve been working on a twin electric rig for about 5 years, the XTM 5500, and stumbled on the openPPG project about 3 years ago overview here: The twin is really nice and I’m still refining since making the overview video I’ve had multiple test flights and it works really well. Its much more technical than an ehelp or openPPG due to thrust line issues and the need for wireless controls. I’m also using Tattu batteries with it but the 14Ah 6s batteries which are a great value, only $150 a piece


Climb rate = 300-400 fpm drops to 100-200fpm as battery voltage sags
Batteries = 12s (2x6s) Tattu 25Ah HV
Current draw = 162 amps

Guessing we’re looking at around 8kW (~11HP) when fully charged dropping to 6kW (~8HP) when at the bottom. If the trend held it sounds like you could hold altitude with about 4kW (~5.4HP). Not bad. I like the look of that motor better than Reidar’s re-wound Turnigy 150cc models - seems to have better available cooling flow.

Spot on 8.1kw at full charge (measured, not from specs), not sure what it drops to though. Need about 5-5.1kw to hold altitude, its a small falcon after all. I avoided the Turnigy motors as they have poor cooling and need to be rewound to really dial in. My first motors were Alien Power Systems which likely come from the same Chinese mfg as the Turnigy but for an extra couple bucks could get custom kv’s so tried 130, 110 etc. The Mad motors have amazing cooling, I ran one in my office with no prop and it blew all the papers off my desk it was like a leaf blower coming out the sides. Ideally I would get a 105kv or 110kv but the closest Mad has is a 100kv. In the end its not really a big deal, I just moved from a 34 to a 36 prop, which is likely more efficient anyway. Plan to make a mount for a 37in folding prop or bigger with the goal of pulling 190-195amps, however, will need to go up one size in ESC likely to a 250-300amp and move from a M20 to an M30 motor.


How was your leg recovery, Richard?
I’m a new “future PPG-er”, and I really appreciate the testing and sharing you did on this thread. I was saddened to see you crashed a few months later.

Steve M.

Wow, your conern is really appreciated. Thank you!
My leg is mending well with no complications. Its just difficult to get stuff done, and so annoying to see others flying while here I am stuck to the ground!!
My battery investigations are just part of a larger project I have in mind. I hope to get back to it all one day.
Stay safe, Richard.

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