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:
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.
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.
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.