Browsed by
Month: June 2018

Deciding thresholds for cell categories

Deciding thresholds for cell categories

One decision that I initially postponed was the threshold to use for labelling a cell as “good”. I wanted to keep the storage ability as high as possible by using only cells with a high capacity, but I also wanted to be realistic and avoid “disqualifying” a big majority of the reclaimed cells. I decided to wait a bit and see what the data suggested.

After testing 20 cells I had an almost perfect dichotomy between dead cells (assigned a nominal capacity <1 mAh) and cells with capacities over 1900 mAh. The following histogram illustrates this situation, with the dead cells omitted as they are not interesting.

The histogram looked nice with 200 mAh bins, and that suggested a threshold for “good” of 1800 mAh. I still had plenty of cells to test, but adopted this as my draft categorisation to start counting how many good cells I was accumulating. Another nice thing about this threshold is that having an average cell capacity of 1930 mAh means that a 7s20p battery is almost exactly 1 kWh of storage (1.93 Ah × 3.7 V ×  7 series × 20 parallel).

By the time I had tested 100 cells, the picture was surprisingly similar. There were no longer any empty bins in the histogram, but the cluster above 1800 mAh remained. The chart below shows that a second smaller cluster had started to appear above a threshold of 1000 mAh.

I was noticing cells turning up with capacity between 1000 mAh and 1800 mAh which were otherwise perfectly passing all my tests, and it seemed a shame to throw them away with the dead cells. The appearance of this second cluster led me to introduce a third category of “poor” cells, and I can think about what to do with them later.

The clustering in the cell capacity distribution is fascinating and caught me by surprise. It is still clearly visible in my histogram (currently 172 cells have been tested).

Cell-testing and data collection

Cell-testing and data collection

Perhaps the biggest problem with waste laptop batteries is that their usage history is unknown. It is important to run a series of tests to ensure that each cell is safe and useful. I have developed a production-line approach, and mark each cell with a “serial number” (YYMMDD## format) as I extract them from battery packs. This lets me record a range of data for each cell across each testing stage.

I read quite a lot of good ideas and put together the following test cycle:

  1. 20180606_202327_cropWhen I open a battery and extract the cells, I measure the “voltage at extraction”. Li-Ion cells have a normal voltage range of 2.7 – 4.2 V (extremes), but many waste laptop batteries have been sitting on shelves or in boxes for a long time. It is quite common to find cells that are significantly under-voltage, and some are even at 0 V.
  2. Any cells that are below 3 V get a careful “restoration” charge at just 50 mA to try and recover voltage. I use the NiMH setting on my smart charger for this step, and it is always supervised. Charging an under-voltage Li-Ion cell can cause shunts to form between the electrodes, creating an internal short-circuit (very bad). I understand that using a very low current helps prevent this.
  3. Cells are charged at 1.0 A on the LiPo setting of my hobby (RC planes) smart chargers, up to 4.2 V. Any cells that heat up while charging are instantly disqualified from further testing. I have not yet properly quantified “heat up”, but it is very obvious (they get hot to the touch). I record the date/time each cell finishes charging.
  4. One of my chargers gives an internal resistance reading. I am recording this for the cells that get charged on that device.
  5. The next major test is for self-discharge. I leave the cells sitting on the bench (in egg cartons) and measure their voltage after 24 hours, after 7 days, and after 14 days. I need cells which can hold most of their charge over this 2 week period.
  6. The final (and perhaps most important) thing to test is capacity. I want a standard test for this, and so typically give the cells a top-up charge to let them all start from “full”. I discharge them at 1 A on my smart charger, which logs how many mAh it draws from the cell. The cut-off is set to 3.2 V. I record the capacity.  1 A is a higher discharge current than most laptop cells would experience, but it is a nice round number and lets the discharge test be a bit quicker than if I used a lower current. It also serves as an upper-bound for the design of my home battery.

20180410_215002_crop

For improved safety, these tests are always conducted outside on a compressed fibre-cement sheet. Cells are never left charging while I am away from the house.

Making a start

Making a start

20170512_084633_v1It’s just over a year since I took the first steps towards this project. After thinking and reading about battery reclamation for a while, I happened upon two laptop batteries in an e-waste bin at work. Finally I had an opportunity to get hands-on!

Gingerly opening the laptop batteries gave me 10 cells, and 9 of them were at about 3.7 V. This is the nominal LiPo voltage and is also a good long-term storage voltage, so I was optimistic that they could still be in decent condition. Frustratingly my electronics things were still in packing boxes from moving house, and so I couldn’t do serious testing on these cells straight away.
20170512_121233_v1

It took another 6 months for me to really bite into this as a serious project, and in that time I’d moved house again and misplaced these initial cells in one of the packing boxes. I only re-found them this week, and was delighted to find that they had dropped a mere 0.01 V in the intervening 12 months. That’s definitely a “pass” for the self-discharge test 🙂

A project brief

A project brief

In short…

I want to build a home battery from reclaimed lithium ion cells. By using this battery to store solar energy, I aim to demonstrate off-grid electrical self-sufficiency.

Motivation

363px-recycling_li-ion-svgElectricity is expensive in Australia, and the Australian government is woefully unexcited about the renewable-energy future. For a country with so much sunlight, it is hard to believe how reticent we’ve been to embrace solar power. On top of this, lithium batteries are shipped overseas for non-trivial recycling. Extending their utility lifespan just makes sense!

Goals

The ultimate goal is to demonstrate off-grid self sufficiency, but this home-brewed project will have to start small and pull itself up by its bootstraps. Some of the important steps include:

  1. Build a modular home battery from cells reclaimed from waste laptop batteries.
  2. Usefully power some appliances in the house from the battery.
  3. Charge the battery using electricity from on-site roof-top solar panels.

Other less tangible goals include keeping costs at a minimum and learning more about the various technologies required for an off-grid solar-powered home.

Feasibility

The sun delivers about 1000W per square metre to the Earth’s surface. My house has a roof area of about 100 square metres, and Sydney gets about 6 effective solar hours per day.  This means that my house is delivered 600 kWh of energy per day!

It’s not so easy to capture all of this energy. Let’s assume that only a quarter of my roof is angled appropriately (towards North), and that I could cover this area with solar panels which are about 15% efficient. I would produce 22.5 kWh of electricity per day.  My household consumption over the last 12 months has averaged about 13.5 kWh per day, and so it should be possible for my small suburban home to be electrically self-sufficient.

Clouds and nighttime make it necessary to store energy, and batteries do this for us all the time (phones, laptops, watches, etc). Lithium ion chemistry has become widespread due to its high energy density, and it is this density that is important in many applications – we want portable devices to be small and light! As batteries age they lose some of their capacity, and they usually lose utility before they are fully “dead”. A home battery is stationary and has space, so does not require high density. It is the ideal way to make further use of aged cells.  Laptops are ubiquitous, and discarded laptop batteries must be plentiful.