Solar linear lithium ion capacitor charger board

What is it?

When combined with a 5.0V solar panel this board efficiently charges a 4.0V Lithium Ion Capacitor, it even charges from indoor light, 400 lux. Due to the low voltage drop of the LDO (0.04V) and the reverse current blocking diode (0.2V) this is more than 90% efficient and the solar panel works near to maximum power voltage. This board uses a 4.0V low quiescent current LDO and a low reverse current Schottky diode to charge the capacitor to maximum 4.0V. The board has 2.5V under voltage protection and 3A over current protection for the Lithium Ion Capacitor. It was tested at both indoor light level 400 lux as well as outdoors on a cloudy day 2500-5000 lux and compared to a highly efficient switching energy harvesting chip. Outdoors it slightly outperformed the switching chip making this a very efficient and cost-effective alternative to expensive switching ICs for applications that sometimes see outdoor light.

Specifications

1. input voltage max 30V (5V solar panel recommended)
2. LIC output voltage 4.0V max
3. LIC charge current 100mA specified, measured 150mA
4. LIC under voltage protection 2.5V
5. LIC over current protection 3A
6. quiescent current 3.5uA typ
7. use with 4.0V or higher voltage Lithium Ion Capacitor
8. Use with 5V solar panel

What you get

1. A soldered and tested board
2. 0.1" pin headers

Pinout

1. IN - connect to solar panel positive terminal, max 30V
2. GND - solar panel and LIC ground
3. LIC+ - Lithium Ion Capacitor positive terminal, max 4.0V
4. LIC- - Lithium Ion Capacitor negative terminal
5. LOAD - output, connect you load here

Tested with

1. this 5V 120mA solar panel but it works with other solar panels too
2. this 1100F 4.0V lithium ion capacitor
3. this 1300F 4.2V lithium ion capacitor

Why did you make it?

I made it because I found that linear charging circuits are very efficient when they have little voltage drop and when the solar panel voltage is well matched to the storage unit voltage keeping it close to maximum power voltage. I was inspired that for outdoor applications it is absolutely not needed to buy an expensive switching energy harvesting chip with maximum power point control. I have designed this board because because of ever increasing developments in LICs capacities exceeding 1000 Farads, and voltages exceeding 4.0V. I believe LICs combine many advantages of Li-ion batteries and supercapacitors making it a perfect choice for batteryless IoT applications.

How long can an application run on a fully charged Lithium Ion Capacitor?

Rule of thumb: When a 1F capacitor is loaded with 1A the voltage will drop 1V in 1 second. The board charges the capacitor up to 4.0V it can be used down to 2.5V. If your application draws 30mA from the output, then it will run for: 1100F(4.0V-2.5V)/0.03A= 50,000s = 15 hrs

Can I put two Lithium Ion Capacitors in parallel?

Yes, When two of the same capacitors are put in parallel the capacitance doubles. LICs have a very low internal leakage. I measured that a 250F LICs only have few uA leakage and that is about 5 times lower than supercapacitors. Make sure to equalize the voltage before connecting in parallel.

How to solder the pin headers?

Use an old bread board, stick the pin headers in and place the PCB on top, then start soldering.

How does LIC compare to other storage devices?

Compared to Li-ion batteries LICs are non-toxic, don't need protection circuits, they don't have shipping restrictions and can be disposed with normal electronics. Compared to supercapacitors LICs have much higher energy density and a much lower leakage. A 1100F has the same capacity as a 400 mAh Lithium Ion battery.