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

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In the past I have compared a linear (MCP73831) with a couple switched mode (LT3652, BQ24650, CN3722) Li-Ion battery chargers, powered from solar panels, and found linear chargers actually don't perform so bad. Andreas Spiess concluded the same in this video. When the panel voltage (~5-6V) is matched to the battery (~3.7V) the losses should not be so bad. Switched mode chargers price is about 10x, so if more power is needed it is cheaper to buy a larger solar panel than a switch mode charger.

I really hoped the input under voltage lockout voltage of these chargers (~3.5V) could be matched to the maximum power point voltage of the solar panel, just like in the expensive switch mode chargers that have maximum power point approximation. I ordered below boards from aliexpress and put them to the test. All chargers are cheap and intended for USB or wall adapter.

V in (V)I charge-max (A)V uvlo (V)board price eachchip price @ qty100

3.75 - 60.5 3.41.25 euro0.38 euro

3.75 - 613.46.92 euro0.60 euro
4056E / TC4056A

4 - 81?0.25 euro0.04 euro
4.4 - 613.70.91 euronot available

I had some old 4054E and some newer TC4056A, I don't know there difference. There must be several manufacturers of the same chip and TP-ASIC is the original manufacturer

The input voltage range of the MCP7383x is limited to max 6V and so I decided to buy 5V solar panels. A 5V panel would also fit nicely with the 3.5V under voltage lockout voltage level, because that is near to the maximum power voltage of the solar panel. To make a fair comparison I have limited charge current to about 500mA on all boards.

Following solar panels I have bought from aliexpress and tested at 600W/m^2 solar irradiance.

Solar panelMeasured open voltage (Voc)Measured short circuit current (Isc)Estimated powerPrice
142x88mm 5V/5W
5.75 V325 mA3W3.48 euro
110x69 5V/1.25W
5.75 V187 mA1.7W1.17 euro

I measured solar panel short circuit current and nominal voltage and I have estimated maximum power at 1000W/m^2. Calculation is: Estimated power = 1.1*Isc * 5V * 1000/600 (watt). This shows that the specification is not correct at all. 

Then I have measured charger input voltage and current, and output voltage and current as shown in the measurement . I have tested with above two solar panels, and two Li-ion batteries charged to different levels.

First I tested indoors. At 500 lux non of the chargers was able to charge at 1mA or more. 

Then I tested outdoors in full sun, it was about 600W/m^2. 

Below is the table for the larger solar panel and the lower battery voltage (~3.67V)

Charge current is a lot less than the programmed 500mA. Input voltage is still pretty high. And overall efficiency is poor (70%) due to the large voltage difference between input and output. There is no clear winner. 

Below is the table for the smaller solar panel and the lower battery voltage (~3.67V)

Charge current is not so much worse compared to the larger panel. 4056E is the winner. MCP73833 somehow performed really bad. 

Below is the table for the larger solar panel and the higher battery voltage (~3.96

The charge current is much lower now, also input voltage is so much higher now. The MCP73831 is the loser.

Finally I tested the smaller solar panel with the higher battery voltage (~3.96V).

The results with the smaller solar panel is not different from the larger panel. Apparently it is mainly dependent on battery voltage. The MCP73831 is the loser.


  • you can not trust solar panels specifications on aliexpress
  • at indoor light levels (500lux) the chargers do not charge the battery (>1mA)
  • outdoors the solar charger are capable of charging the battery from solar power, but current was lower than the solar panel max current or the programmed current
  • I hoped charger input under voltage lockout voltage would work as sort of maximum power point control, but it doesn't. Efficiency...
Read more »

  • Outdoor applications may use a linear solar charger

    Jasper Sikken12/21/2022 at 10:11 0 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.. 

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Dan Maloney wrote 09/26/2019 at 15:21 point

Surprising and disappointing results. Thanks for taking the time to experiment and post.

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