You probably see is electronics shops these chinese powerbanks with integrated solar panel? Idea looks very good – let’s collect energy when intense sun is available, and use it when it is needed!
But, mostly avaliable solar powerbanks is just waste of money. Let’s make own solar powerbank, which will really work!

At first – you may ask – what’s wrong with that compact powerbanks with integrated solar panel? There is two problems: first – very limited power of solar panel inside them. It may take weeks to fully charge integraged lithium battery. And second – it is heat: solar panels usually black or deep blue, and they will get hot when placed under direct intense sunlight – and they will warm-up internals of powerbank, including battery. Intense heat is not too good for all electronics, and for lithium batteries too. They can rapidly loose capacity, bloat, became unstable, and even explode when overheated.

Bloated li-ion cell from Samsung tablet.

So, in good system we will need to have separated solar panels and battery block to solve these two problems.
To make our powerbank, we will need, as minimum, these things:
– Solar cells. Better to use ones with open loop voltage at 5-6 Volts. About number of cells – more is better, we need at least 500 mA of current.

– Li-ion charger board. Important to have very simple charger board, but with protection for lithium cells. Complicated charging circuits, like ones in powerbanks with LCD displays, designed to work with virtually-unlimited power source – and they have relatively high current consumption. We have limited power source, so we want something with minimal features (protection + status indication (charging in progress/charging complete)) to have much as possible energy comes to battery instead of wasting it in charger board.

– DC-DC boost converter (li-ion discharged board) – to convert 3.7V from our battery pack to 5V for USB devices. Again, we need something simple and energy-efficient, something with low current consumption when idle and good efficiency.

Charging board (at right) and step-up to 5V converter (at left).

– Battery itself. Hard to overestimate importance of this component. You can buy cells from Ebay/Aliexpress, or use cells from old tablets, cellphones, laptops, etc. Old batteries, that will not run you laptop even for couple of seconds, actually can work here. When battery aging, internal resistance of it becomes higher – this means, battery can’t power same load as when it was new: there will be too big looses on internal resistance and voltage will quickly drop. But if you will discharge it with small current – you probably will find that actual capacity is about 60% from new. And if you have lots of batteries from old electronics – you can connect it all in parallel – and will have relatively low current on each cell when you charging you device from this array.

– Optional – some charge indicator – to estimate how much energy we have in battery now.

Testing assembly of one cell, charginb board, step-up converter and chage gauge board.

 

Let’s make it!

First – we will need solar panels itself.
You can buy cells at Aliexpress/Ebay. We will need cells with open loop voltage about 6 Volts. As usually, one cell have very limited power (and current) – and to make battery charging in reasonable time – we will need several cells.
If you use cells with 5-6 Volts open voltage, just connect them in parallel.

Solar cell from Aliexpress.

If this array will stand inside you house, just make some mechanical support at back side of cells. If you’ll use some glue tape – only place it at back side of cells, do not place anything at front side – this will decrease power output.
If you plan to place solar cells outside, you must to carefully cover all electrical connections with epoxy glue or silicon sealant – otherwise any contact with water (such as rains, dew) will cause fast corrosion.

To make it easy to use – connect out of your solar panels array to USB socket. Since we will not connect li-ion cells directly, you can not use blocking diode on solar panels.

 

Next, assemble your battery pack. Depending of what cells are available to you, it can vary. If you do have some used cells, firstly do this:
– Fully charge all your cells. Better to use specified chargers like imax to see, what capacity batteries will take from charger – to estimate how much capacity remains in cells,
– Leave cells at least for one weer each, don’t connect something to them, just leave them charged,
– After at least one weer for each cell, check voltage on cells – if there is 4.10V or more – use this cell as part of you battery, if less – it have too much self-discharge, don’t use it.

Here is reference circuit diagram of all parts connected together:

 

Just connect all suitable cells in parallel (observe polarity!). Connect charger and step-up converter to battery, and connect your solar cells array to charger board with USB cable. If you have some board with charge indication – connect it too, and it will be good to have some enclosure for this all. So, there is it, power bank ready!

A few common moments:
– I’am recommend to use only protected cells. Yes, we have charging and discharging boards that have protection features – but if there will be short-circuit inside lithium cells connections – it will not help, we need to have some protection against it. Lithium cells are potentially dangerous, so generally it is a good idea to have protection board close as possible to cell itself.

– When you charge tablet – turn it off. Mobile devices “thinking” that they connected to infinite power supply when charging – so they often turn off power-save features when plugged in. In our case source is limited, so it will be good idea to turn devices off.

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