Tesla’s Powerwall brought much needed innovation to the way we think about energy management, and they have made some exciting improvements with the 2016 model which now includes a built-in inverter and 14 kwh of output instead of the original 10 kwh. Thankfully, the powerwall energy storage concept has reignited America’s spirit of ingenuity, and DIY hobbyists have been hard at work developing affordable an DIY powerwall that can help bring alternative energy into reach for all.
The following DIY powerwall guide aims to provide homeowners with the basic knowledge required create their own powerwall. The DIY process is fairly straightforward and allows a much greater degree of customization so homeowners are able to create a powerwall sized precisely to their needs at a fraction of the cost of a one-size-fits-all commercial system
What Is a DIY Powerwall?
A powerwall is an in-home battery bank that stores and safely delivers the energy collected from an intermittent power supply such as solar or wind turbine power. It is generally created from 18650 Rechargeable Lithium Ion Batteries, which are the same batteries used to power electric cars, drones, laptops, and medical equipment. Improved Lithium Ion battery construction specifically engineered for the larger loads of powerwall use are anticipated in the next couple years but for the time being the 18650 Li-Ion cell remains the go to battery for powerwall builds.
How Do I Begin Building a DIY Powerwall?
Before you can get started building your new Powerwall, you are going to need to establish a project plan, which should address each of the points in the "Project Planning" section below.
Make a List
List all devices, appliances, and computers that you intend to power using the powerwall. Be sure to include as much detail as you can, including energy star ratings, age, size, and condition. This list will form the basis for determining your household’s overall power demands and the minimum size powerwall you will need to build to safely meet those demands.
Decide whether you are going to maintain a link between your home system and your existing public energy provider or create a self-contained off-grid system completely independent of public power.
Research the regulations and restrictions that are applicable to your property so there are no unpleasant legal surprises down the road.
Choose a Location
Decide where your powerwall system will be located. A powerwall capable of powering a whole house will require a fairly large space.. Many off-grid homeowners opt to locate their power sources in a small shed or outbuilding separate from the main house for an added layer of safety.
Once you have planned out the basic parameters of your project you are ready to move on the next step, sourcing your power supply and the items to build your powerwall.
18650 (so named for their 18mm x 65mm size) Li-Ion batteries are easy to find and are rechargeable. 18650 Li-Ion cells can be purchased online for about $2-$5 per battery. Recently a new avenue has emerged ,and many DIY-ers are now designing powerwalls around used electric vehicle batteries sourced from crashed electric vehicles.
The EV modules are pre-wired and have built-in cooling water connectors. For example a 24v Lithium Nickel Cobalt (LiPo) battery module salvaged from a Tesla S is resold at a cost of around $1200 or $120/kwh. There are still very few of the EV modules available, but that is likely to change in the near future.
Fortunately there is a second-hand option for sourcing the original Li-Ion cells, as well. You will want to contact computer recyclers or check the online auctions for used laptop batteries, as these will contain 4-8 cells each, most of which are still perfectly viable. Recovering the cells from the laptop battery case must be done carefully because Li-Ion batteries are prone to explosions if punctured.
Testing And Balancing
Lithium Ion batteries recovered from laptop batteries or other sources where there is any question of their integrity must be tested and balanced before use.
Safety is a top concern when working with Li-Ion batteries. Cells that have been compromised in any way must be discarded. Good cells must be separated from bad to ensure even charging and discharging of energy loads.
Testing and balancing requires the use of an inexpensive Li-Ion battery charging and testing unit. These units are available online in a range of sizes capable of charging, discharging, and testing just one or multiple batteries at once. Ideally you will want a multi-function device capable of charging and discharging batteries as well as providing alerts and readings about the health of each cell. The unit should have a shut-off mechanism so there is no risk of overcharging the batteries during the long process.
You will need to test each battery by using the testing unit to cycle each battery, through a full charge, complete discharge, and final full charge. Check the testing unit at each phase for any alerts or out of range readings and discard the offending cells if found.
As an added safety precaution you should physically feel each battery for excessive heat build-up during the cycling process, as this is a strong indication that the battery cell is compromised and should not be used.
Li-ion cells are engineered with a voltage discharge range between 3.0 to 4.3 Volts. It is imperative to maintain all Li-Ion batteries within this specified range at all times including during the testing and balancing process.
Assembling Your DIY Powerwall
For the assembly you are going to need plastic cell spacer 18650 battery holders. These are essentially plastic trays sized to hold 18650 Li-Ion battery cells immobile in a perfectly spaced configuration so that they can be wired together to form a battery pack. The trays come in many sizes and can be adjusted to fit whatever configuration you choose.
The trays can now be loaded with the tested and balanced Li-Ion cells in the following pattern.
The first row of cells are placed in the tray in parallel, meaning all polarities in the row are aligned. The next row will also be run in parallel alignment; however, the polarities will be placed in the opposite orientation from the preceding row. The third row should be placed the same as the first, and the fourth should match the second. Continue this pattern until the tray is completely filled. When you view the battery pack from above, you should be seeing alternating rows of all positive, all negative, all positive and so on, repeating from one edge of the pack to the other.
Attach Bus Strip Connections
Thin metal strips (usually nickel or copper) called bus strips are added on top of the battery packs with one long strip running the length of one row and overhanging each edge slightly. These strips will be carefully spot welded into place attaching each cell to the bus strip. You should now have a long strip of attached metal running down each of the parallel rows.
Next smaller tab sized bus strips will be spot welded in place, connecting the first cell in one parallel row to the first cell in the next parallel row, repeating the process for the remaining cells in the same pattern.
Both sides of the battery pack will need to be connected in this fashion. When all cells are attached and spot welded, you can move on to the next pack, repeating the entire process until you have the desired number of completed packs to create your powerwall.
How Do I Use a DIY Powerwall?
Once completed, you will need a Battery Management System (BMC) that monitors battery health and has an auto-shut off in the event that any one cell starts to over charge and overheat.
If you will be using solar or wind turbine power with your new powerwall, you will also need a solar or wind converter which will turn the energy generated by your solar panels or turbines into electricity which can be safely stored in your powerwall battery bank.
Finally you will need a power inverter to convert the stored energy from your powerwall into usable AC or DC current that your household appliances and devices can safely use.
While building a DIY powerwall is still an ambitious labor of love at present, incredible strides have been made in recent years that have vastly expanded the possibilities for future DIY powerwall builds and other alternative means of self-reliance. Don’t hesitate to ask any and all questions you may have on the forums, they were created to share knowledge and lessons learned so that those who came after could succeed. Remember, even the seasoned pros bounce their ideas off of the forums before undertaking a new project.