Power Wall

My recycled 18650 DIY "powerwall" project.

For background see:

My current setup (as at 28 May 2018):

  • 1000/750 Watts PV [one 250 Watt panel is on a relay which auto redirects to a micro grid-tie inverter when the system is above 27 volts)
  • MPP Solar PCM60X solar charge controller (60A, 12V/24V/48V)
  • Invertek 3000W pure sinewave inverter 24V
  • 24V battery packs made from recycled 18650s:
    • 2 kWh Powerwall, 7S48P with 30 Amp 700 Watt BMS
    • 1 kWh PowerDell, 7S24P (built for the go-kart)
    • 1 kWh Powershelf, 7S24P (added late July 2016 and repopulated each time I build a new Small Blocks pack)
    • 2 kWh Small Blocks, 7S24P x 2 (added early Aug 2016) with * 30 Amp 700 Watt BMS
    • 1 kWh Small Blocks, 7S24P x 1 (added early 2017)
    • 1 kWh Powershelf, 7S48P low capacity cells (added mid-2017)
  • Powering:
    • the web server that this site runs on, and a router
    • charging two 0.6 kWh e-bikes some nights
    • running all house lights, and the fridge-freezer in summer. Just the house lights in winter.

The goal:

  • 3000 Watts PV on the garage roof
  • MPP Solar PCM60X solar charge controller
  • ElectroDacus Solar BMS SBMS60 or SBMS100 maybe
  • 3000W pure sine wave inverter
  • 48 volts 10 kWh pack
  • powering the web server this site runs on and a router
  • charging two 0.6 kWh e-bike every night
  • running all house lights
  • and the fridge-freezer and other small loads

YouTube videos of the process: https://youtu.be/UTiqkOaNElY

Old bits:

Cheers, Paul
Email me.

6 August 2018

A new 3D printer


29 May 2018

Battery holder design improvements

I've improved the design of my 3D printed battery holders. It will now accept 7mm wide nickel strapping OR twisted copper mains wire.

The standard model will take 12 pairs of 18650s in two rows of six. And the longer versions will do 18 pairs, 24 pairs and 28 pairs.

I've also left off the skirt for the new designs (because my new printer has a heated bed).

All the 3D printer (STL format) files are available free on Thingiverse: https://www.thingiverse.com/thing:1729730

4 April 2017   18650 Merch

I've created a Teespring Store for the 18650 "merch" Nerdville and I designed:


1 Feb 2017   Monitoring groups across the pack

Attach:voltmeter-clipon.jpg Δ

I'm going to use 7 of these low voltage meters to indicate the voltage of each of the 7 groups in my 2kWh "small blocks" pack.


I'll test them to see how accurate they are.

Here's the quoted specs:

  • Operating voltage: 2.4V-30V
  • Operating Current: less than 30 mA
  • Low operating voltage: + 2.40V
  • Dimensions: 30 * 11.7 * 9.2mm
  • Accuracy: 0.3%

16 Jan 2017   Powerwall powered cell charging

Attach:DC-DC-converter-35-5v20a.jpg Δ

Here's the DC-DC converter I used: 10-35V to 5V 20A DC-DC converter

And here's a better one: 8-40V to 5V 40A DC-DC converter

15 Nov 2016 DIN rail mount for BMS PCB

Attach:DIN_rail_mount_for_BMS_PCB.jpg Δ Download the STL file from Thingiverse: http://www.thingiverse.com/thing:1892392

This is one side of a DIN rail mount for a 105x60mm BMS board. To make the other side mirror the SLT in your favorite slicer, then print.

4 Nov 2016 cascading small blocks

Small blocks extra connector clip-on

3d printer files

The flat "T" section is a custom raft to help it print nicely in PLA on a non-heated printer bed.

24 Oct 2016 DIN rail mount for "standard" 18650 clips

DIN rail mount bracket for 1x2 18650 clips:

DIN rail mount bracket for 4x5 18650 clips:

15 September 2016 The Schpankme Sort

Here's a google spreadsheet for sorting a 7S12P pack: https://docs.google.com/spreadsheets/d/1DzHPjOyKAWf_BDvVjKvI2jfDCsEAXT9AFQznFGO5Ml4

I'm looking for volunteer developers who could help create a generalised app to generate sorted packs from a input list of cell capacities.

If you're interested please email me.

My primary discharger is now an OPUS BT-3100: http://lygte-info.dk/review/Review%20Charger%20Opus%20BT-C3100%20V2.1%20UK.html


1 September 2016 DIN rail mount for small blocks

Thingiverse: http://www.thingiverse.com/thing:1747791

Here's the original design on Tinkercad (without the skirt): https://www.tinkercad.com/things/3MQdVcH7bOt-din-rail-mount-for-18650-small-block-v2

Attach:18650-battery-holder-2x6-pairs.jpg Δ

21 August 2016 18650 battery holder 2x6 pairs

Here's the STL file for the small blocks battery holder I used: http://www.thingiverse.com/thing:1729730

And here's the Tinkercad source: https://www.tinkercad.com/things/lft19zWDf7f-18650-battery-holder-2x6-pairs-v6b-no-skirt

Feel free to copy and modify.

Attach:server-rack-cabinet-162x300.jpg Δ

10 August 2016 Bought an old server rack cabinet

I picked up an old server rack cabinet for NZ$32 today. My plan is to clean it up and use it to house all the batteries.

And here's some 18650 amusement: https://www.youtube.com/watch?v=Bn5zhqz4LsE

8 August 2016 Small block pack wired up

Attach:500_YouTube_subscribers.jpg Δ

28 July 2016 Powershelf power-up and monitoring

I'm experimenting with not using a BMS on the 1kWh Powershelf. I'm will be monitoring the cell voltage levels manually using a cheap cell level alarm. If I see the cells drifting apart more than 0.15 V then I'll slap on a BMS.

<iframe width="1024" height="600" src="https://docs.google.com/spreadsheets/d/1dkU_BArACR__KGEuYt_-R2Wxp4mqfpFrZ2fla-R2o_M/pubhtml?gid=1482537636&single=true&widget=true&headers=false"></iframe>

The 1-8S Lipo/Li-ion/Fe Battery Voltage 2IN1 Tester Low Voltage Buzzer Alarm I'm using to monitor the cells costs US$1.13 each on Aliexpress.

1 July 2016 18650 testing flowchart

Attach:18650-testing-flowchart.jpg Δ

Download PDF copy

Here's the source file which you can download (right mouse click and chose "Save link as..."), then open and edit it using Flowchart Maker online chart tool.

Have fun.

24 June 2016 18650 Pairs vs singles

In many recycled laptop batteries the cells are arranged as three pairs of cells, which are connected in series (3S2P).

My contention is that keeping the pairs together saves a lot of processing time without much downside.

Here are the results from my testing to see if the capacity of single cells is better or worse than keeping them in pairs.

<iframe width=1300px height=470px src="https://docs.google.com/spreadsheets/d/1crlV7mE-kN7z0iLgGKBdaOq3TcEk3qKe1CZ9j13yFEA/pubhtml?gid=0&single=true&widget=true&headers=false"></iframe>
Raw Google Sheets data

This test suffers from very small sample size. That's because I don't want to break up my good pairs. A better test would be if one of the powerwall DIYers who does do single cells would number their next batch of cells they process then we'd get a lot more data.

Is there anyone up for that task? Let me know if you are!

1 June 2016 Monitoring and testing

Attach:2kW-pack-22W-load-then-177W-load-201606012012.jpg ΔAttach:Logview2-Legendv2.gif Δ

Graph showing the voltages of the 7 groups in my 2kWh pack.

I ran a 6 hour test with the 2kWh pack powering my Sunraiden 1000W pure-sine wave inverter driving a 22W CLF lamp (about 1.5Amps drain), then a 177W incandecent lamp (about 7Amps drain).

As you can see group 1 (red) and 2 (blue) of my groups started sagging faster than the rest so those are the ones that need upgrading.

The graph was generated by LogView from data coming out of my iCharger in Monitor mode.

23 May 2016 Powershelf design version 0.1

<iframe src="https://drive.google.com/a/kennett.co.nz/file/d/0B5exHJfGwNJ0S3BKNGJ2aXN6WFU/preview" width="640" height="480"></iframe>

17 May 2016 Closer look at my 20-way 18650 charger

<iframe src="https://drive.google.com/a/kennett.co.nz/file/d/0B5exHJfGwNJ0M2JIbUlOQjFrSFE/preview" width="640" height="480"></iframe>

  1. 7 mm * 0.2 mm nickel strap roll 1 kg
  2. ATX Breakout Board
  3. YouTube search for 'ATX bench power supply'
  4. TP4056 18650 charger module
  5. 18650 battery holders

2 May 2016 Battery holder version 8

Here's the latest version of my 18650 cell pairs battery holder: http://www.thingiverse.com/thing:1534159

28 April 2016 Why do Li-ion Batteries die? and how to improve the situation?

Some good stuff in this video: https://www.youtube.com/watch?v=pxP0Cu00sZs&list=PLia3mEYsCVebndk_2F2bacuKkoYOKSWAt

26 April 2016 Parallel and serial battery pack examples

Battery pack grid template to download:

Attach:18650-cell-grid-with-voltages.jpg Δ Δ Download PDF version Δ

22 April 2016 Simple temperature controller for cooling fans


And now I'm wondering about a smoke detector too: http://www.aliexpress.com/item/Freeshipping-Smoke-Sensor-Module-use-for-Detection-smoke-Detector-W-Relay-Output/1956798697.html

15 April 2016 Installing BMS and breaker switches

BMS: http://www.aliexpress.com/item/24V-30A-7S-BMS-PCM-PCB-24V-Lithium-battery-BMS-for-7S-3-7V-lithium-cell/32469245428.html

DC 20A Breaker switch: http://www.aliexpress.com/item/1P-20A-20Ampere-DC-125V-Circuit-breaker-MCB-direct-current/32267763380.html

13 April 2016 Where to get cheap recycled laptop batteries from (in Wellington, New Zealand)

I get my recycled laptop batteries from my local e-waste recyler IT Rercycla in Lower Hutt (there's also a branch in Auckland).

They are currently selling them for NZ$1 per laptop battery and they have plenty.

If you have an e-waste recycle facility operating near you - go and talk to them.

7 April 2016 Lithium battery failure modes

Attach:lithium-battery-failure-modes.jpg Δ

From: https://youtu.be/DyRvLiKNKF0?t=1123

6 April 2016 Blinkenlights

I used 7 tri-colour auto transitioning LED's wired in series across the battery, plus the diffuser and polarizing filters from a old laptop display to produce the cool & silly light effect.


4 April 2016 Front panel

Made the panel by laser cutting 6km MET 'wood'. I chose MET because I suspected I might want/need to tweak the design so I didn't wean make it from something too permanent.

I probably should have chosen cardboard.

The design file was done in Freehand 9 (because I have it and it still output clean EPS files - which the laser cutter can use.

The actual cutting was done by Graley Plastics & Laser Cutting in Petone (located on my way home) and it cost $63.25 including GST. It took 3 days for them to do it.

This was my very first laser cut project and it worked out perfect for the task.

I've since tweaked the design a tiny bit. Next time I plan to laminate two layers of corrugated cardboard from a used bike box and get that laser cut.

I like the idea of housing the recycled batteries in a recycled front panel. But I wonder if it would be significantly safer in a metal box - that I can't easily quantify.

17 March 2016 BMS options


Solar lithium charge controller and BMS:

Solar controller:

Balance wires:

Watt meter:

You Tube:

9 Mar 2016 Why?

MJLorton "What is a Battery Management System / BMS?: https://www.youtube.com/watch?v=MZyY1dpka7c

8 Mar 2016 YouTube channels of interest

These YouTube channels cover 18650's or related electronics:

Useful websites:

6 Mar 2016 Soldering Tesla-style fuses

Switched to using Jaycar 0.25mm Enamel Copper Wire after running out of the 5 Amp mains fuse wire I had been using (it was too expensive).

This should be suitable for approx. 10 Amps http://www.powerstream.com/wire-fusing-currents.htm

And more background on fuses http://powerelectronics.com/passive-components/selecting-fuses-simple-procedures-get-right-overcurrent-protection-dc-dc-converte

7 mm * 0.2 mm nickel strap roll 1 kg.

29 Feb 2016 Sorting cells into balanced groups

I've just sorted the best 168 pairs of cells into 7 groups of roughly equally capacity.

The median pair was 3402 mAh.

3402 mAh divided by 2 = 1701 mAh for a median cell

1701 mAh * 48 cells (in parallel) per group = 81,648 mAh or 81.648 Ah

81.648 Ah * 25.9 V (nominal 3.6 V * 7 groups in series) = 2.114 kWh

See also: Rinoa Super-Genius, How To Organize 18650 Cells for your Battery Pack: https://www.youtube.com/watch?v=NVJNol7jq0M

28 Feb 2016 More data on recycled laptop cells


22 Feb 2016 Making more paired cells

I've decided to solder together some of my best single cells to make good pairs. I'll add those to the pack so I can bump out the lowest capacity pairs - thus raising the overall capacity of the pack. Hopefully I can solder the cells without damaging them.

I really want to get the nominal capacity over 2 kWh.

15 Feb 2016 How long will it take?

Before you embark on a large laptop battery project it’s worth working out how long it’s going to take to complete.

For example - if you want to do a battery pack for a Powerwall or an electric car you will need as many as one thousand cells or more.

I will go through my numbers step by step. Some of these will be specific to my pack design and some will be more generic. Either way you’ll be able to see how to copy my process if you want to.

Here's my Google Spreadsheet so you can copy and reuse it.

Your typical eighteen-six-fifty laptop battery is going to have 6 cells inside. I have seen as few as 4 cells and as many as 12.

In order to simplify my process I try to only buy 6 cell batteries. That also means I’m very likely to get 3 pairs of cells, and I’m only interested in pairs of cells for my projects.

The Powerwall I’m working on at the moment is a 2 kilo-Watt-hour pack which will be made up of 336 cells, or 168 pairs. To get that many good cells I need to open 75 laptop batteries, each containing 6 cells which actually results in 450 cells. That is 25 percent more than I need. After measuring the internal resistance and capacity I can discard the weakest 25 percent, which I give back to the recycler.

So, the first major task is to open each battery, cut off the BMS and wires, measure the voltage, write that on the cell, and then put it in a sorted pile.

I’ve measured the time this took me for 25 laptop batteries, and the average time was 5 minutes 24 seconds. The quickest I could manage was 3 minutes 36 seconds and the longest was 9 minutes and 25 seconds, and that was because the plastic casing was brittle and had to be broken apart chunk by chunk. Another battery was glued so strongly that each cell to ages to pry out. And of course they didn’t stay in pairs so are less useful to me. If I was more ruthless, I’d discard any battery that took too long, or I didn’t think I could get pairs out of.

That average of 5 minutes 24 second times 75 batteries adds up to 6 hours and 43 minutes of solid battery opening. Or one easy day. So that’s not too bad.

If I wanted to build an 8 kilo-Watt-hour pack with 1 thousand 3 hundred and 44 cells that would require 3 and a half days to open 299 laptop batteries.

The second major task is charging, and measuring the internal resistance, of all the cells. In my situation I have an eye-Charger ten-ten which can balance charge 10 cells in series at a time. And, given that I’m using pairs of cells, that is 20 cells at a time.

My i-Charger take on average 11 hours and 9 minutes to complete a balance charge of 10 pairs at 1 Amp. It can take as long as 15 hours or as little as 7 hours. I can often do 2 loads per day.

I do not recommend trying to charge these random cells fast. You do not know what condition they are in, so it is better to charge slowly this first time.

It takes about 5 minutes of handling time per load, which is not too much hassle. If I average one point five loads per day it’ll take me 15 days to charge up all 225 pairs of cells. Typically I start a load in the morning and reload in the evening.

The third major task is discharging and measuring the Amp-hour capacity of all the cells. This is the largest time cost. This is where you’ll need to use multiple dischargers to get through as many batteries in a reasonable time.

My i-Max B6 clones take on average 3 hours 47 minutes to discharge a pair of cells down to 3 volts at 1.5 Amps. The longest time was an amazing 6 hours and 40 minutes. Weak cells will discharge much faster.

Again, I do not recommend discharging recycled cells harder that 1.5 Aaamps per pair. You don’t yet know how much you can trust them.

I typically do the discharging at work because they need attention intermittently all day.

I currently have 4 i-Max’s running at a time to speed this process along. But that means I have to do a reload at random times throughout the day. Which is a hassle.

If I had 8 i-Maxes it would double the number of random times I would have to check for a finished run. Even more hassle. This task-process could be improved.

My current process will take about 19 days on this task, to discharge all 225 cell pairs. An 8 kilo-Watt-hour pack would take 75 days to complete this task.

The fourth major task is 3D printing cell-pair holders. As you can not yet buy battery holders for pairs of cells from e-Bay or Ali-Express you will have to 3D print them.

My 2 kilo-Watt-hour pack requires 42 of my 2-by-4 pair battery holders. That involves about 1 and a half minutes of handling time per print and 57 minutes for my little Printrbot Junior to print. That adds up to roughly 42 hours of printing, but only 1 hour total handling time.

The first four major tasks can be done in parallel to some extent.

The fifth, sixth and seventh major tasks are; soldering, testing and packaging for which I estimate about 2 days all up. More if you factor in package design time.

So to complete my 2 kilo-Watt-hour pack I’m expecting about 21 8-hour days worth of time on my part.

An 8 kilo-Watt-hour Laptop Powerwall, built with the equipment I currently have, would take about 77 days, or over 16 40-hour weeks.

It might be cheaper to buy a real Tesla Powerwall. But not nearly as fun.

7 Feb 2016 Printing 18650 battery holders

If you'd like to 3D print my 18650-pairs battery holder here is the STL file: http://www.thingiverse.com/thing:1324840

It requires 3.44 meters of 1.75mm dia. PLA to print one of my battery holders, which holds 16 cells (8 pairs).

A 1kg roll of 1.75mm diameter PLA has about 300 meters and costs around US$30-60 depending on the source. So for a US$50 roll, that 3.4 meters would be US$0.57 per holder - but you need two - top and bottom. My 2kWh pack needed 42 in total. So for the whole pack it was about US$24 of PLA.

Pretty cheap... as long as you don't factor in time, and the cost of the 3D printer. My little Printrbot Jr was US$400 plus shipping and taxes.

#20160201 1 Feb 2016 How long will it take?

How long does it take me to open a laptop battery, separate it into pairs and measure the voltages? On average: 5 minutes 27 seconds.

That includes opening the laptop battery case, clipping off all the unwanted bits, measuring the voltage and then writing the voltage on the cell(s).

If I'm going to be doing a lot of this - it's a good idea to budget the time each task takes. So today I timed myself opening 26 laptop batteries over the last two days:

Attach:laptop-battery-opening-times.jpg Δ

It ranges from 3 minutes 54 seconds, up to 9 minutes 3 seconds. Mostly the difference is due to how easy the plastic case comes apart. Some batteries are really stubborn and come apart in small pieces. Others just take a chomp at one corner with the wires cutters, then running the screw driver along the back seam, after which the bottom pops off with ease.

My 2 kWh pack requires 336 cells, which will come from about 75 laptop batteries, will take about 6 hours and 45 minutes to open, separate and measure the voltages.

My main rationale for keeping the cells in pairs is to reduce the overall time it take to build a pack. I don't have any comparative data to say if that is true yet.

A second benefit is that most (but not all) laptop batteries with 18650 cells are wired as three pairs of cells which are then connected in series. This meas that if a single cell in a pair fades it forces it's twin to take more charge. One cell is undercharged and it's twin is over charged - leaving the pair in bad shape. My voltage, ESR (resistance) and capacity measurements should find those bad pairs and they'll be left out of the pack.

If I separate all the cells I'm liable to find the weak cells, but also be over-confident in the stronger of the twins. Keeping the cells in their pairs avoids that problem.

It also provides an easy tab for me to solder the cells together - so I avoid having to directly solder on to the cells - thus avoiding heat stressing the cell.

31 Jan 2016 18650 balance battery holder

Parts list for a multi-18650 balance battery holder:

And some other parts:

22 Jan 2016 Printing battery holders

I'm busy printing battery cell holders.

As that's happening I'm trying to decide whether to build two 1 kWh packs (and connect those together) or one 2 kWh pack. Either option would include it's own 40 Amp breaker/switch (should probably swap that for a 20 Amp breaker) and cheap 20 Amp balancing BMS with thermal cutoff, and 45 Amp Anderson connectors.

My current design is either 1 kWh 24P7S (24 in parallel, then 7 of those in series) or 2kWh 48P7S.

The long term plan is to build more packs and work my way up to 4 kWh or more.

Pros and cons:

  • 1 kWh pack is easier to handle/move because it weighs only 7.6 kg (plus Dell PC case comes to 9.4 kg) whereas the 2 kWh pack would weigh 15.2 kg (plus a beefy box would bring it closer to 20 kg).
  • 1 kWh pack has 24 cells in parallel which give a 1C rating of around 24 Amps(see note) - which fits better with the breaker, BMS and connectors I'm currently using. My current 2 kWh design has 48 cells in parallel which gives it the potential to delivery 48 Amps quite easily.
  • 2 kWh pack allows me to build up capacity faster with fewer ancillary parts.
  • 1 kWh pack delivering 20 Amps at 24 V would drive a 480 Watt load.
  • 2 kWh pack that was designed to deliver 48 Amps at 24 V would allow me to drive a 960 Watt load.
  • If I ever wanted to go above 480 Watts load I should probably switch to a 48 volt charger and inverter

I'm leaning towards building the 2 kWh pack because it'll be faster and has more potential for driving larger loads in future.

Note: I'm assuming the 1C rating for these random batteries is 1 Amp each. My Enviromower pack has a peaking draw of 94 Amps! or 7.8 Amps per cell (which is probably dangerous and bad for the batteries) - so my ball-park 1C rating of 1 Amp per cell above is very conservative.

18 Jan 2016 Nickel strap

I'll be using the same 0.2 * 7 mm nickel plate/strap that I used on the go-kart battery pack.

It has a cross-section of 1.4 mm2 which provides a current capacity of around 20 Amps (source).

17 Jan 2016 Plan coming together

I've got the solar charge controller working on my 1kW 24V 24P7S go-kart pack - which I'm currently testing as a small power wall.

I've chosen to set it to charge up to 28V (4.0V per cell group). This is about 75-80% full charge - which should mean the pack will last a lot more charge cycles. But it also means the capacity is limited to about 70-80% it's theoretical full charge. (The "full charge" is based on the capacity readings I measured on each cell pair.)

I've set the low voltage cut-off at 23V (3.3V per cell group).

21 Dec 2015 Let's give this a go

My current solar power setup uses 4 lead acid deep cycle batteries.

I'm wondering if I could replace those with a lithium battery pack made from a lot of 18650 sourced from post-consumer recycled laptop batteries.

To start with a might use the pack I built for my 24V electric go-kart.

I have a solar charge controller that can manage lithium batteries on order. The pack already has a balancing BMS.

I donated my inverter to a worthy cause so am waiting for another one to arrive.


  • web server
  • router
  • house lights
  • garage lights

Once that's all working I could build another pack identical to the electric go-kart pack and run them in parallel. Or build up a larger pack just for the main solar setup - which could be designed in the style of a https://www.teslamotors.com/powerwallTesla power wall.

Worth a look:

Cheers, Paul

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