What would be a solar system without a battery ?

A power bank is especially helpful if you hope to use solar energy when there’s no sun: illuminating a campsite at nighttime, charging your phone when raining, or keeping your laptop running on a cloudy afternoon are all good examples. 

However the price of a power pack can be as high as the rest of the system... Well unless we try to build one for free!

There are a lot of tutorials on internet to build 18650 lion cells packs. I decided to build one for my panel, reusing old cells scavaged from laptops packs.

Very often laptops packs die only because a few cells are dead. The Battery Management System shuts down the whole battery and you have to change it... But some of the cells are still into good condition.

This was the principle of "Reuse, Recycle Revamp" Challenge of Hackadays'Prize. So why not trying to keep this challenge's spirit and reuse laptop cells for our battery pack !

It will also have the advantage to stick to the Planet friendly challenge's objectives : "For this challenge, we want you to focus on minimizing the costs around building and installing clean energy solutions. This extends to the way energy is stored as well, to maximize the efficiency of the entire infrastructure."

Energy storage wasn't yet covered into my project, it will be done now!

Going back to our 18650 cells, we will have the delicate (and long) task to evaluate the health of each cell individualy. This is done by charging/decharging each cell and taking note of the capacity in mAh.

I succeeded in scavaging 3 laptops and finally got 12 "good" cells with almost the same capacity. 

Then we have to design a pack...obviously coping with the number of cells we have got and the capacity of your panel.

My panel is given for a MPPT voltage of 16.1V, so I decided to build a 4s pack (4 cells in serie).

Why 4s ? as I can barely reach 4*4.2V = 16.8V with my panel (4.2V being the max voltage for a 18650 cell)... well simply because I do not plan to balance my pack every time I charge it... So a safety margin can be applied and charging the pack at 16V will be sufficient to keep it into good health even if a light unbalance is present!

Having 12 cells my pack will thus be a 4s3p

The pack can be composed using the pack_builder tool proposed on repackR web site.

simply enter the capacity of each cell, and the site will tell you the best configuration and dispaly a nice summary

That's it , 

• print in PET the battery supports provided on my thingiverse account, 
• install the bats, 
• use 2mm copper wire as bus bars and solder the pack. I use thin wire as "fuse" to connect the cells to the bus bars. This is a safety protection to "fuse" these wires if too much current is drained from the cell.
  • Be careful to avoid any short when soldering.
  • Precharge all the cell at the exact same voltage before soldering
• Then add a balancer plug, 
• Then use an old lipo pack connector for plus and minus.
• Then use another plastic bottle as heatsrink tube!

You are now ready to use the balance plug to balance your pack and charge it.

A few words of caution 

This pack is in the range of 5000 mAh at 16V so can be "powerful enough" to be dangerous...

• the pack is not equiped with BMS. 
• Avoid any overcharge (it would take fire...) 
• Avoid any underdischarge (less than 12V) the pack would be totally dead...

I will use this pack on my solar system. I will balance the pack only from time to time. 

I do not intend to drain high current from this pack (1A to 6A max). 

As it sustains 5Ah you can charge it at 1C that is between 5 to 6A. This is perfect for the panel (100W at 12V --> 8A absolute max.. rather 4A in real life).

Once again be careful with these cells they can be very dangerous... this videos on youtube demonstrates the effect of overheathing a lion cell up to catching fire...