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 crystaline 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.

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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

The harvested energy can be stored in this 110mAh 3*20*25mm LiPo battery. This battery fits very nice below the module.

Read more: Tiny Solar Energy Module (TSEM)