Outdoor applications may use a linear solar charger

A project log for Testing cheap linear Li-ion chargers for solar

I have tested a couple cheap USB linear li-ion chargers and connected them to a solar panel

jasper-sikkenJasper Sikken 12/21/2022 at 10:110 Comments

Recently someone consulted me for solar energy harvesting in sensor device that is used in a green house. This is basically the summary. 

Basically all low power solar energy harvesters like BQ25570, AEM10941, SPV1040, SPV1050 have similar performance excpt that AEM10941 has lowest external components. I am very comfortable with AEM10941. All these ICs are very expensive 3-6 USD in qty100. I know that AEM10941 is around $2.20 in qty1000. 

For outdoor applications, that sometimes see direct sunlight linear chargers works very well and are a much cheaper choice. Reason is that outdoor light level is 10 to 100 times more than indoors. Even on cloudy days (100W/m2) linear charger can still charge a storage unit. They don't charge at 50W/2 or less. One 1 hour of sunlight is more energy than can be harvested than in 4 days from an indoor environment. Indoor light is very poor light to harvest energy from because compared to sun light, LED and fluorescent light have very narrow spectral peaks and no energy in the infrared spectrum. 

Linear chargers are pretty efficient when solar panel voltage is well matched with the storage unit. For example for a 5V solar panel and 4.2V battery you lose only 20% power because of voltage drop between battery and solar panel. 20% is comparable to any switching converter. I have compared a few linear li-ion battery chargers and found TP4056 to be best, because price is low and compared to other chips it has a large input voltage range. TP4056 does not need a reverse blocking diode between solar panel and charger because of internal mosfet architecture, which saves 0.3V voltage drop typical. 

Alternatively, Lithium Ion Capacitors can be used to store energy. I know the price of a 250F 3.8V LIC from Vinatech is $1.90 in qty1000, so price is much lower then most people think. LICs have many adbvantages over LIBs. They can be recharged almost endlessly, they have a much wider operating temperature range (-25 to +85 Celsius), they don't need special protection circuit, they don't burst into flames, they don't have shipping restrictions and don't need to be disposed with chemical waste. Only disadvantage is that energy density is 6 times lower, for example a 250F LIC is about AA battery size but has only 90mAh capacity. 

Lithium Ion Capacitors have very low leakage, about 5 times lower than supercapacitors, less than 1uA after 72hrs. Thefore you can put 4-5 LICs in parallel and leakage is still not significant for most energy harvesting applications. Yes, it is OK to LICs in parallel. 

Lithium Ion Capacitors can also be charged from a linear regulator. They need to stay within 2.5V and 3.8V voltage range. So you can use a 4.0V LDO followed by a 0.3V schottky diode to prevent reverse current, then then the LIC is charged up tp ~3.8V. For the application a 3.3V buck boost converter can be used, these are pretty expensive. I have tested TPS63900 (400mA out) and MAX77827 (900mA out) with a 250F LIC. For the  under voltage protection of the LIC you should add a 2.5V undervoltage protection which is 20 cents in qty1000. It disables the buck-booster when the voltag eis too low. LICs have a maximum chrge/discharge current, for example 750mA for a 250F LIC. When you select a solar panel makes sure it does not exceed this limit. The current limit protection on the load side can be a simple 0.75/1.5 hold/trip PPTC/polyfuse..