Solar harvesting into Lithium Ion Capacitor

Recently I bought a 250F and a 750F 3.8V Lithium Ion Capacitor from Aliexpress, it is manufactured by Cda. Are they good?

250F 3.8V

750F 3.8

I compared it to other LICs from well know brands like Vinatech and Eaton. I compared datasheets, measured capacitance and measured leakage. To measure the capacitance I used my own electronic load set at a constant current of 500mA and and measured the time it takes to drop from 3.8V to 2.5V. Capacitance is calculated Q(F)= )=t(s)*0.5A / (3.8-2.5)

To measure the leakage I charged them up and every day I measured the voltage decay with a bench  multimeter and calculated leakage current using following formula leakagecurrent(A)=capacity(Farad)*(yesterdaysvoltage[V]-todaysvoltage[V])/t[s]

	Vinatech VEL13353R8257G	Cda LIC1620Q3R8257	Eaton  HS1625-3R8227	Cda LIC1840Q3R8757
specified capacitance (F)	250	250	220	750
dimensions (diameter x length mm)	12.5x35	16x20	16x25	18x40
operating temperature (Celsius)	-25 +70	-40 +85	-15 +70	-40 +85
life cycles	20k (10%)	30k (30%)	500k (30%)	30k (30%)
Current continous A	0.75	0.75	1.1	3
Current peak A 1s	5	10	15.3	30
ESR AC 1kHz mOhm	50	50	100	25
specified leakage current 72h/uA	10	5	12	23
price at qty1000	$1.90 at Vinatech	$1.90 at a Aliexpress seller	$8.60 at mouser	not requested
measured capacitance CC 0.5A	301F (+20%)	220F (-14%)	228F  (+3%)	761F (+1%)
measured leakage 72h/uA	3	1	2	4

The CDA 250F LIC has 14% lower than specified capacitance, not perfect but not bad either. The leakage of all LICs after 72hrs is very low, less than 5uA, so it becomes insignificant for most energy harvesting application. 

Conclusion: the 250F and 750F 3.8V LICs from Cda are not bad at all. The specs are good, the measured capacitance is not bad and leakage is good. For the Cda 250F the price is $1.90 in qty1000 same as the Vinatech 250F.

I have compared the leakage current of a few Lithium Ion Capacitors (LIC) to a few supercapacitors (EDLC). I charged them up and then almost every day I measured the voltage decay with a handheld  multimeter and calculated leakage current using following formula
leakagecurrent(A)=capacity(Farad)*(yesterdaysvoltage[V]-todaysvoltage[V])/t[s]

The chart shows for both LICs and EDLCs that after 72 hours the leakage current is less than 5uA, which is good news for low power IoT devices. Interestingly the high capacity (>200F) LICs have a lower leakage than the 10F supercapacitors. And if you plot the leakage per Farad capacity, then it becomes even more clear that LICs have about 5 times lower leakage than supercapacitors. 

I didn't like so much that the AEMLIC V1 has a 2.2V/80mA output while many wireless development boards need 3.3V and a higher current. Therefor I have designed the new AEMLIC V2. It has an TPS63900 buck boost converter that delivers 3.3V/400mA. It is again based on the trusted AEM10941 (there was a problem with the newer AEM10300).

When AEM10941 storage voltage drops below 2.5V STATUS1 pin goes high and the host processor is immediately informed of an impending shutdown so it can gracefully end EEPROM FLASH/EEPROM write operations before it's power is shut off. The buck booster enable pin is controlled from the AEM10941 VHV output that goes low 600ms AFTER the host processor is warned. 

The bare PCB looks like this

The assembled PCB looks like this

And after soldering onto headers and adding a 250F Lithium Ion Capacitor and a solar panel it looks like this. 

I have performed full hardware design verification and found no problem. 

Then I felt confident enough to order the first batch of 50 from Elecrow. They are currently being produced and I expect that they are available for sale end of March 2022. Elecrow already share an image of the bare PCBs

Normally the TPS63900 buckboost converter is $0.80USD in qty1000 but the stocks were drying up at the normal distributors and so in January 2022 I have bought 220pcs from ICSOSO for $2.10 each. 1 Month later all stocks were completely dried up and Elecrow could source it for $39 each! So I was happy I bought some.

I am so excited about this new board because it uses an awesome solar energy harvesting chip, it stores the energy in a truly green storage device and it is can supply high enough current to power most IoT projects, not only BLE or LoRa but also NB-IoT, LTE-M, and wifi.  This week I expect shipping info, which means the board will arrive next week around wednesday. I am so excited!

Today I have receiv

The Chinese PCB assembler Elecrow has recently shared with me the pictures of the panel and of the first assembled board, for my approval. 

I see some solder mask issues between the pads of the IC but Elecrow assured with me there is no problem and that there will be no short circuits.