Ideal for indoor applications
The AEM10941 harvesting IC is very suitable for indoor applications because it has an ultra low power startup. The boost converter starts at a very low 380 mV input voltage and 3 uW input power. The IC gets most power out of the solar cells by doing MPPT maximum power point tracking every 5 seconds. In addition the mono crystalline solar cells have a very wide spectral range resulting in 22% efficiency. This makes TSEM very suitable for indoor applications.
How much energy does is harvest indoors?
Indoor light is about 1/100th of outdoor light or 10W/m2. At that power solar cell open voltage is 0.458V and I estimate Vmp at 0.36V per cell. Since we have two in series the booster will work at 0.72V input voltage. At that voltage boost efficiency is about 75% . LiPo charge efficiency is about 95%. Battery current is 5.72 times smaller than the solar current because the voltage increases from 0.72 V to 4.12 Volt. Finally I assume that indoor light is available for 10 hours a day. 44mA * 1/100th * 75% * 95% * 0.72V/4.12V * 10 hrs = 0.55 mAh is harvested every day. So an application must have an average current less than 0.55mAh/24h = 23uA, that’s enough for a simple Bluetooth Low Energy beacon or a very simple LoRa application. However if the device is in full sun for only one hour a day it harvests ~10x more: 7 mAh.
If you need to harvest more then you may connect an external solar panel to the castellated via’s. Below is a comparison between the on-board solar cells and external solar cells with respect to cost, dimensions, and the energy harvested per day.
|Solar panel||Harvested energy per day, indoors (mAh)||Application average current (uA)|
|2x $1.02 On-board 0.5V/44mA 22x7mm Ixys KXOB22-12X1L||0.55||23|
|1x $1.06 External 1V/100mA 30x25mm||1.1||45|
|1x $1.31 External 2V/100mA 79x28mm||2.9||120|
|1x $1.18 External 4V/100mA 70x70mm||6.7||279|