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Solar Tower @40.3,21.8
This project took some time, since I'm mostly crazy-meticulous about anything I do and this has also bubbled over into my hobbies. I started gathering laptop batteries around a year ago and have gone through just around 120 of them so far. It's somewhat difficult to find used batteries in Greece, let alone for free. I have spent around €90 for the batteries so far.

Here's a link to my on-going harvesting spreadsheet. I basically started it so as not to lose track of the cells, in case something would go wrong, I would have better chances of knowing the condition of the cell in the beginning.

I started this project mainly "just because", but also because I built an e-bike around the same time, and because I had access to a few used solar panels from my job's secondary activities (I am a Surveyor Engineer).

Basic goal was to build a 7s battery. Started sourcing laptop batteries with the first goal being to put together 7 20p bricks, as I've been seeing on all these videos around. I would be OK with nominal 1400W that the battery would be able to provide. However, since I am almost at 200 cells that have passed my thresholds, I will be going for a 7s40p battery.

While on the "my thresholds" issue, I should note that I am testing the cells at 1A discharge, they usually stay for 15 days (minimum) to test self-discharge, and I only keep for the main project the cells that are over 70% of their original capacity.

All other cells go into the "<70% norminal" spreadsheet tab, they get a second discharge test at 500mA and that's actually the cells that I am using into the "Solar Tower" project, that is, this one. I am using Jehu's boards (hence the "tower") and been swapping cells from time to time, in order to be using the ones with the greatest capacity (that of course are below 70% of norminal).

So, here's my setup so far:
  • 3x 90Wp thin film solar panels in parallel that only give out 180W in total, because they are old
  • a Victron 100|15 controller
  • 10x Jehu's boards populated with my "<70% norminal" cells
  • an AliExpress "Smart BMS" with bluetooth
  • an MPT7210a used as a DC-DC converter to 41V for my e-bike
  • a buck converter down to 12V to run a few lines for LED lighting (internal and external), the iMax charger and another Maha one.
  • a buck converter down to 5V for USB power
  • a panel 12-24V USB-c + USB QC socket with display
The BMS is set to bleed-balance (60mA) the cells starting at 3.95V and I have set the maximum through the charge controller at 4.1V. Have been quite happy with this BMS, it's keeping the cell groups in sync.

I am currently using the above setup to charge my e-bike (which has a minuscule 140Wh battery), as well as my "Giveaway Powerbanks" I am building out of the worst cells, but it will soon grow to include the following as soon as the stuff I ordered a couple of months ago finally arrive:

[*]another DC/DC converter to power a laptop I have laying around in the shed, if it doesn't work on 12V DC
[*]a cheap true sinewave inverter to be able to run a couple of other stuff (TV, 220VDC lights and stuff)
[*]something else I am forgetting right now for some reason

I will be building my next "Solar Bricks @40.3,21.8" project as soon as I get to 280 cells.

Please let me know if anything is far off... Also happy to reply to any questions.
Korishan, omurphyt, hbpowerwall And 1 others like this post
Very nice post and good job
OK, I wasn’t expecting this... the iMax B6 Charger charging at 5A gets up to 37C when running at 12V input, but jumps to 41C when running at 16V! I was expecting it to be the other way around, due to lower current on higher voltage, but seems like the DC-DC inside the iMax is not extremely efficient...

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