Supercapacitors for Space, Qualification of COTS

Supercapacitors for space Introduction

Tested Samples

For the measurement set up an assessment, we have chosen 4 different top manufacturers making high capacity supercapacitors of two different capacitance levels available on the market. The first capacitance level is 3000F + and the second one 1200 F. This should prove the measurement technique enough in order to rely on during the consequent reliability/life tests.

The list of selected manufacturers:
A) Skeleton
B) Maxwell
C) Nesscap
D) LS Mtron
E) IOXUS

Group – 1200 F

• 1200 F SKELETON SCA1200 1200F 2,85V 1,35Wh
• 1200 F LSUC 2,7V 1200F 1,22WhLs , Mtron Ltd
• 1200 F Maxwell Tech BCAP1200 P270 1200F 2,7V 1,2Wh

Group 3000+ F

• 3200 F SKELETON SCA3200 3200F 2,85V 3,61wh
• 3000 F IOXUS i CAP iRB3000K270CT 3000F 2,7V 3,04Wh
• 3400 F MAXWELL Tech K2SERIES BCAP3400 P285 3400F 2,85V 3,84Wh
• 3400 F NESSCAP NE03V03400SW001 3400F 3,0V 4,2Wh
• 3400 F Ls Mtron Ltd LSUC 2,85V 3400F 3,84W

Test Description

Initial capacity measurement methods have been applied in two specifications according to IEC 62391 and the measurement set up should be able to do so. Also, the DC ESR measurements have been included in the measurement cycle. Capacitance measurement theory.

Simple formula how to determine capacitance from discharge region:

Fig.3. Time vs voltage during the capacitance measurement.

Experimental Setup

The experimental setup was the key part of the project. The setup should handle high passing currents with very low losses as well as high accuracy voltage readings with reasonable sample rates. In our setup, we have used two power supplies able to deliver up to 600A and DC load able to dissipate up to 500A. Since this is a modular thing that can be upgraded either to do higher current job or work at higher voltages e.g. at capacitor banks. The first thing we had to do was to remove all ballast devices from power supplies on the outer side because they negatively affect both capacitance and ESR measurement results.

This is a simple action but then we have realized, that it’s impossible to stabilize the output voltage. Therefore temporary load has to be applied during the steady voltage conditions and must be released prior to main load activation. The other issue we have faced was the accuracy of voltage readings across the capacitor for ESR measurements, to be able to read and repeat values in
hundreds of picoohms.

You should be aware when measuring the voltage across the capacitor around rated voltage you have to use the range according to this level, but on the other hand, you are losing accuracy even if the precision meter is used. Higher precision also means a slower sample rate. To overcome this point we are applying shift voltage to the meter close to the rated voltage (lower by 0.1-0.2V), which allow us to use single e.g. 0.2V measurement range. It’s essential that the four-wire measurement technique has been utilized.

The last but not least part is PC software written in Keysight VEE environment. The software controls the power supplies, load, and all other equipment. At the end of the measurement calculates Capacity, ESR values via mathematical treatments as well as always finds the exact time of any performed event to prevent time delays caused by control lines or PC to have an impact on result accuracy.

Fig.5. Simplified wiring diagram

Fig.6. Experimental setup view – see featured image

Results

All measurements have been taken on the equipment described above and used as input data for further testing (life testing, cycle testing…). Due to the tests are ongoing no other results can be presented within this paper.

Group 1200F

Capacitance plots

The resulting capacity of group 1200F shows higher initial capacitance than one should expect. The highest values have the Maxwell cells up to 20% above specification.

DC ESR plots

The ESR values for the Sceleton have three times lower ones to Maxwells and two times lower ones to LS supercapacitors. This will results in lower thermal losses during the cycle test and lower degradation caused by lower temperature.

Group 3000+F

Once again the initial capacitance is higher than specified in datasheets.

Method 1 B 3200 + cells

The ESR for the Skeleton cells is below 100 microohms, that is extremely good value.

Source: EGGO Space s.r.o ; ESA SPCD 2018 Symposium

Published by EPCI under approval of ESA SPCD 2018 organizing committee.

REFERENCES

Supercapacitor information resources:
1. www.skeletontech.com
2. www.nescapp.com
3. www.maxwell.com
4. www.ioxus.com
5. www.lsmtron.com