Foldable Solar Charger, Cloud-Restart Test - i-surplu 21W IL (IW-FS21W03il)

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Foldable Solar Charger, Cloud-Restart Test - i-surplu 21W IL (IW-FS21W03il)

I recently purchased too many of these fold-able type solar chargers.  Some of them have many reviews, and some have very few, but almost no reviews discuss how the panel deals with shady interruptions like clouds.

NOTE: This is only a test of one aspect of this charger, not a full review.

Charge Restart Feature


These solar chargers are marketed toward backpackers, campers, and/or emergency situations.  In my opinion it makes sense to charge a battery bank with one of these, and then charge your mobile phone with the battery bank later on (after dark?).  These chargers can also work directly with your mobile phones, but that is a tricky endeavor. 

Many mobile phones (and some battery banks) will not adapt to the changing power provided by a solar charger.  For example, current generation apple devices will start charging at full power, but if a cloud reduces the output temporarily, the apple device will not resume charging at full power. 

In response to this, some solar chargers have begun adding charge-restart features to their circuitry.  The theory is nice, the panel detects an interruption in solar energy, and briefly cuts power to the usb ports.  This should trigger an apple device to automatically resume charging at full power.


Depending on how the charge-restart feature is implemented, there could be some serious drawbacks.  Ideally the solar charger would reset the usb ports only once, after the cloud has passed.  That means the charger has to notice a sudden increase in sunlight, and reset the device.

Some solar chargers are taking the easy way out, and just cutting the ports off when the output voltage drops below a certain threshold, say 4.4v.  This method means that when a cloud obstructs the sun, power is cycled immediately.  But as soon as your device begins charging, the voltage will drop again, and power will be cycled again.  This will repeat until the obstacle passes.  This method could be very bad for your device.  This method also makes it impossible to trickle-charge your device.

In this test I will try to determine if these solar chargers have a 'charge restart' feature, and if so how well it works.


The i-surplu 21W IL (IW-FS21W03il, purchased on amazon, Oct 30, 2016)

Some Pictures






The Test

In my test, I tried to obscure the sun as much as possible without cutting power to the device being charged.  For this charger, I closed the third panel over the second panel, leaving only the first panel exposed to sunlight.  Then I used a semi-transparent piece of plastic over the last panel to further reduce the output.  I monitored the charge rate with a simple USB charging meter.

After the charge rate slowed to almost nothing, I removed the plastic and opened the panels back up.  Here are the results:

This charger has voltage based charge restarting.

When the voltage reported by my usb meter reached approx 4.35v, the usb ports would reset.   This is much lower than the first 21w iSurplu charger I tested.  This is almost too low to be very useful for maintaining full charge rate on the iphone I tested.  I was easily able to reduce the charge rate on the iPhone without triggering the usb port reset. 

In my opinion, with the reset voltage set so low, I would call this more of a low voltage protection.  Some usb devices wont function (lock up, etc) with such a low input voltage, and this could serve to keep those devices functioning properly.

One area where this might work VERY well is leaving the charger out during a sunrise.  The very slow rise in voltage can easily lock up some devices, and this low voltage protection might work perfectly for avoiding that.  (might be able to test and report back)

BONUS Amp AND Light Meter

This solar charger has a front facing power and light meter. 

The power meter shows about how much power your device is drawing, with all 4 red leds meaning it is drawing at least 1A

The light meter shows about how much sunlight your panel is receiving, with all 4 blue leds meaning the sun is at its brightest.

I love the idea of these front facing meters.  You can easily read them in direct sunlight, even though my camera cant seem to capture the LED light for some reason (PWM?)

The Specifics

Resting Voltage
3 panels exposed
Charging Tesa PB
3 panels exposed

Charging Tesa PB

1 panel exposed

Charging Tesa PB
1 Panel with Plastic Cloud

5.17v 4.75v/1.55a 4.4v/0.66a restart!



In my opinion, the i-surplu 21W IL does technically have voltage based charge restarting, but the trigger voltage is set so low that it might as well just be considered a low voltage protection.  You probably should not rely on it to keep an iPhone charging at full power.  This solar charger would work very well for phones and power banks that keep charging at full power after temporary reductions in power. The low voltage protection MAY work well for kick-starting your devices when the sun is rising.

The front facing power and light meters are awesome, I really like this feature.

I always recommend you fully test your setup in varying conditions before you rely on it.


EDIT: cleanup

Edited by: netprince on 02/15/2019 - 12:42