DIY Powerwall - Part 2

Finally some time for an UPDATE! The short story…I finally processed enough 18650 cells to start the build. For the long story, keep reading and check out the video. In this post, I am going to walk through some of the bits and pieces I have had to gather and sort out prior to starting the cabinet install. More often than not when undertaking a project like this, we never think about all the small details which must be taken care of prior to a build being completed. This is no different. As we know, lithium batteries, if not treated correctly, can be somewhat volatile, thus, some extra safety precautions need to be taken before bringing the batter online.


The Shed - No chance am I putting this build inside the house!

Having a suitable location for your powerwall is probably one of the largest considerations of the whole build. As mentioned earlier, if lithium batteries are not treated correctly it could end up in a molten mess and no matter how many fire extinguishers you have, the lithium battery fire will consume everything in its path. If you do not believe me, check out some of the videos on youtube! My build takes us out to the shed of course. The shed is not far enough away from my house for my liking, but have to work with what I have.

I was able to pickup this nice B&R electrical cabinet for $150. These cabinets are very sturdy and made from steel. The orange powder coasting was not by choice, but for the price, who am I to complain. The cabinet is large enough to fit my first 14S100P setup, with room to double it at a later date. (Top and Bottom) I am also looking to get some airflow top to bottom in the cabinet at some stage.

I went ahead and mounted some structural pine to the wall. My plan is to now use some slotted C-Channel to mount the inverter and components to the wall.. This will allow me to conceal some of the cabling, whilst maintaining the structural integrity of the ply and wall. I’m yet to decide if I should paint the ply….see what happens. I also need to look at cable management, and how/where to mount all the other associated equipment.

Insulation – As we are working from a common garden shed, I had to insulate the roof of the shed. It was getting WAY too hot inside to house all these components. I ended up purchasing some foam foilboard from the local hardware store and mounted a small 200mm solar vent. It seems to keep the temperature steady. Even on 40deg C days, it seems ok inside. I do plan to hook up some temp sensors into the future. Also some more ventilation. Bring in the cool air from the bottom, vent out the top.


Sorting and Stacking Packs

From what I have read, sorting your cells into equal packs can be crucial to the whole setup. The aim is to have the same (or very similar) capacity in each pack. But how do we do that? well, there are a couple of methods. The first and possibly most accurate is using some online software called “rePackr” which is located here. With this tool, the idea is you enter in the capacity of every single 18650 you have and it tells you which pack to put them in. Pretty much sorts them out so that each pack is as close to the same as possible. The downside is that you need to type in or cut and past in the value of every cell. When your wall contains over 1400 batteries this can become a PITA.

The method I chose to use was a bit more archaic but has been proven to get the job done. I sorted all the batteries out into groups of 50mAh. For example, the cells that are at a capacity of 2050mAh to 2100mAh would all be grouped together. I did this across the whole range of my cells. I think I ended up with 20 groups of cells. From here it was then pretty easy to take one from each group and fill the packs so that they were somewhat evenly mixed. The proof will be in the pack testing. Only then will I know how close I got.

Once we know the remaining capacity of a cell, we then write it on the side of the cell for future reference and we also notate the current voltage of the cell at the time. The cells are then placed into tubs grouped by capacity and left to sit for a minimum of one week. The reason for this is that we want to identify any cells that can not hold their voltage. These are known as “Self Dischargers” we do not want a cell in our packs that cannot hold a charge/voltage. This can have significant effects on our packs once built.


Battery Management and Safety

ok, so we have our packs now and we need a few additional bits of hardware to make sure out battery packs are as safe as we can make them. The first item used is a Battery Management System (BMS). The BMS we chose was the Batrium Watchmon setup. This seems to be the ‘go-to’ BMS for DIY type powerwalls at the moment. They seem to be doing a fair bit of development on the hardware and software which is always good. If you want to check out more of their items head over to the website here. In a nutshell, the Batrium BMS controls the charge states and the balancing of the packs. It is important to control this as we do not want to over/under charge the batteries and we also want each pack to drain and recharge in a balanced fashion.

The BMS works by connecting “Longmons” to each pack. These are small bits of hardware which link each pack together and provide the feedback to the Watchmon controller. The Longmons are the workers and do the balancing, monitor temps and a few other cool bits and pieces. The BMS, with the help of some relays, can also be used to trigger a shunt trip. The Shunt trip will basically cut off any use of the batteries during a fault state. For example, if the temp rises past a set parameter, the BMS can trigger the trip and all use of the batteries will be cut. This is just one of the safety features which I intend to implement.

In line with the circuit breaker, we also have some large fuse(s). These 160A HRC type fuses in a disconnect/isolator will also be used. At $12 per fuse and $50 for the holder, you can never have too much circuit protection. So not only will each of the individual batteries be fused, but the entire pack will also be covered.


So that’s it for part 2! Whats next:

  • Bus Bars on packs.
  • Cabinet Install
  • Solar Install
  • Lots of wiring up!

 


DIY Powerwall - Part 1

Why has the blog been so quiet I hear you ask…..Well…I started work on a DIY Powerwall from recycled laptop batteries. Yes, you heard correctly, I have started building a Tesla style power-wall from 18650 batteries. These are commonly found in Laptop battery packs. A why would I do such a thing, I hear you ask… In a nutshell, I want to save on electricity bills by using Solar a bit smarter and storing the energy for later use. I also wanted to see if it was possible to build a reliable/equivalent power-wall at a fraction of the cost of a Tesla style power-wall.

After a small bit of research, I stumbled across two resources that have become staples in my DIY Powerwall diet. Those are the “Second Life Storage” forum and the “HBpowerwall” Youtube channel, run by Peter Matthews. Using these two resources you can find out just about everything you need to know about building your own power-wall. Check out the links. Also, check out my journey below.


First Steps - Find Laptop Packs, Pull them apart...

Some of the DIY’ers find this step one of the most difficult. Finding laptop battery packs to recycle the 18650 cells from can be tricky. I approached a few battery stores and computer stores but most seemed disinterested in selling me the old packs. I am not sure if it’s a health and safety thing, or they get more recycling them. However, after finding the right people through a few Facebook groups I was able to get a steady supply of laptop batteries through an IT recycler. At first, I purchased 20Kgs of batteries not knowing what I would get. But then went on to purchase 30kgs, 40kg and most recently 60kgs. The break-down of how many usable cells I actually got from these old laptop batteries is below.

Everyone has their own method for pulling apart the laptop packs, however, I will say that safety is paramount here. The last thing you want to do is slice yourself open on the nickel strip or even worse short/explode a cell or two. (Saying that, it’s pretty hard to do this unless you’re super careless.)

I would suggest purchasing the following items:

  • Vice Grips
  • Sharp Small Side Cutters
  • Gloves
  • Eye protection

The end state of pulling the laptop batteries apart is to get the singular 18650 batteries out. Once we have them out and ready to go we can begin to analyse them to see if they are suitable for our powerwall. Noting that these batteries did come from old laptop batteries we really do not know what state they are in, we must “process” the cells to determine the capacity of every cell. Also, try and detect the bad from the good cells.


Step two - process a heap of cells

Once you have started your journey to building a DIY powerwall, you will no doubt need to process bulk cells to weed out the good from the bad. There are many ways to achieve this outcome, however, I will give you a rundown on how I am doing it. (And a basic guide to budget required for processing.) This part is easily the most tedious part of the build. For example, if you decide you would like a 48v 10-12kwh powerwall then you are looking at requiring 1400 cells at a minimum. 1400 may not sound like many, however, after weeding out the bad cells, you soon find that it does take time.

The process that I follow to process cells is as follows. First I will check the voltage of each cell. If a cell pulled directly from a laptop pack is at 2V then it will go into the pile to be charged/discharged via the charge wall. If the cell is below two volts then I will put the cell into another pile which will require a specific charger to get them back to health (If they can be revived…)

For the cells that pass the 2v test, they will then be placed into the cell holders attached to the TP-4056 chargers. These small lithium specific chargers are very cheap and an ideal way to bring any old batteries up to full charge. You can pick these up from eBay very cheap in packs of 10-20. I went with 20.

Once the cells are charged to maximum voltage, the cells are then cycled into the Opus chargers for a discharge test. This is will give us the remaining capacity of the cells. Basically, it ensures that the cell is at 4.20v, discharges the battery to 3v, records the capacity in milliampere hours (mAh), then charges the battery back up to 4.20v ready for the next test.

Once we know the remaining capacity of a cell, we then write it on the side of the cell for future reference and we also notate the current voltage of the cell at the time. The cells are then placed into tubs grouped by capacity and left to sit for a minimum of one week. The reason for this is that we want to identify any cells that can not hold their voltage. These are known as “Self Dischargers” we do not want a cell in our packs that cannot hold a charge/voltage. This can have significant effects on our packs once built.


Step three - prepare processed cells for packs

It will depend on a lot of factors how you will proceed with building your packs. Each choice will have pro’s and con’s. Go with the method that best suits your cell count and abilities. At present, I have not 100% decided on the method I am going to use, however, I am re-wrapping the vast majority of my cells first. Once I have 1400 quality cells, I will then arrange them into 14 packs of 100 cells. (14S100P) This will give me a 48v nominal power-wall around the 10-12kWh.

So where am I at right now… Well I have processed approximately 60Kgs of recycled laptop batteries. I set my limits for the cells to go into my wall at 2000mAh. I currently have 4 packs with 100 cells in each pack. If I lower my standards to 1800mAh I could probably have a 5th pack built, but for now, I plan to stick to the magic 2000mAh in capacity for my wall.


Once I move onto the next phase of this build, I’ll post Part 2. The next post will cover off on some of the following bits and pieces:

  • Cell pack builds (Once I decide which method to use)
  • Some tips and tricks for better pack build.
  • Solar/Inverter installation and connection to battery packs.
  • cost/cell breakdown
  • Anything else I can think of that may be relevant! (Let me know in the comments what you want to know?!)