# Powering a power bank with solar panels

Excuse my ignorance in electrical engineering, I am not familiar with the field.

I am trying to power a power bank using solar power. I have settled on the following components:

To my understanding, I am able to hook the solar panels in series and then power the power bank using the USB outputs on the battery regulator. Although, I am unsure if I am missing any components or naive in my approach.

Here is a rough diagram of my approach:

I will hookup my panels in the following way:

The outputs will be fed to the battery regulator and then from the battery regulator USB OUT to the power bank.

If my concept is correct, it should take 36hrs(?) of direct sunlight to power the battery bank.

battery = 20Ah
panels = 4*(0.138)A

totalTime = battery/panels
totalTime = 36.231884058hrs


Will this work as expected?

• Don't connect solar panels to usb output ever
– asim
Feb 26 '21 at 19:35
• @asim Could you elaborate? I'm not directly connecting the solar panels to USB out, they will go to the battery regulator and then using the USB out on the battery regulator to power the battery bank. Feb 26 '21 at 19:37
• Oh, i might have misread then, yes it should be fine but there could be losses in efficiency, you can achieve the same result by just using a buck regulator without mppt if you are not going to charge any 12v batteries
– asim
Feb 26 '21 at 19:40
• You state "I am able to hook the solar panels in parallel", but the diagram you have shows them connected in series. Feb 26 '21 at 20:12
• The MPPT controller has a USB output, but the description doesn't say what current it can output, if it is limited to 500mA it will be a waste of money. Feb 26 '21 at 20:29

Usb ports are usually designed as an extra feature, mppt may not provide maximum amps (maybe 2Amps only) through usb port which will result in power wastage of solar panels

You can instead use all 4 panels in parallel and use buck converter to regulate voltage to 5v which will yield maximum efficiency and can utilize maximum power from panels

Connections illustrated in diagrams are in series not parallel

Edit: Just noticed your solar panels are only 10watt max, for them, the mppt is overkill and wastage of money, try xl4015 buck converter instead, it is more suitable, solid choice for your project and can easily handle 4~5 amps (your panels can only produce 0.552 amps at max when parallel), make sure you don't connect more than 36v on input side of xl4015 (and that your panels are in parallel not series like illustrated)

This despite being a better option is not a good choice because of low wattage solar cells, try solar cells between 6v to 10v voltage rating (voltage higher than 6v but not too high) and higher amp rating, buy more cells from the money you will save on mppt, I recommend atleast 20watts of combined solar panels power

Answer to your comment: putting panels in parallel will multiply amps but the voltage will remain the same which is better for you, putting them in series will multiply their voltage but amps will remain the same which is bad for both you and your use case because 72v are not that dangerous but not safe either, they will shock you, anything over 40v should be handled with caution

Rest assured, putting them in series or parallel, both configurations multiply total power and total power will remain the same (10watt), you need higher amps so you will connect them in parallel, if you needed high voltages you would use series configuration. 72v may have already exceeded max input of your mppt so they are not safe for mppt either, don't put them in series.

I have a transformer that outputs 2v 1000+ amps and it can melt steel but a 9v toy battery cannot, even 10 9v toy batteries in series will not melt steel, less voltage does not mean less power, power (watts) is total voltage multiplied by amps

• If my panels are in series, won't they be producing 72v? Therefore higher than the 36v input on the buck converter? Feb 27 '21 at 10:40
• See my updated answer @jacques
– asim
Feb 27 '21 at 11:12
• Don't forget to set output voltage of xl4015 to 5v before hooking them up with the powerbank, i don't wanna be cursed at for burnt powerbank, you will have to cut a data cable to connect output of xl4015 to powerbank @jacques
– asim
Feb 27 '21 at 13:52
• hahah don't worry you wont. Thank you very much for your help, much appreciated. Feb 27 '21 at 14:07

The current needs to be determined at the MPPT tracker since it is converting power, not current. For some rough cut numbers 10W max and a 5V usb output for the battery, the minimum charging time would be

panels => 4*2.5A = 10W

current output at MPPT with 10W/5V => 2A (max)

at 20Ah/2A would be 10hrs. Which would be a maximum.

However this is assuming no inefficiencies and that the panels are in full power. The MPPT tracker will have an efficency in the 90% to 70% range. There will be losses from the cable (make sure you use a short one with a large AWG). And the battery will also have it's own inefficiencies in the converter/charging circuit on the order of 90% to 70%.

so worst case it would be about 0.7*0.7 or 50% on efficiency which would double the charging time to 20hr.

In many cases solar panels are tested and rated at 1000W/m^2 if your running these at a higher elevation than you might be getting 600W/m^2 so that would also decrease the power by 40% and the best case times by 66% to 16hrs (best case) to 33hrs (worst case)

These are just rough cut numbers, but there is a good chance you'll get charging times between those two numbers at full sunlight. Clouds, the angle of the panel (needs to be incident to the sun to get full power) and time of day will all increase the charging time beyond 33hrs worst case.