1. Buy Used EV Battery Modules
51Vdc 10 kWh - QTY 28 modules
51Vdc 15 kWh - QTY 42 modules
51Vdc 20 kWh - QTY 56 modules
You need to purchase a BMS as well. This is the battery cells only.
I have seen a lot of people on the internet repurposing old laptop batteries and batteries from electric vehicles. This is a great way to acquire low cost, high quality batteries, but it does require a lot of work to disassemble, test, and sort them. I think this will become a common method in the future because a battery that doesn’t work well for an electric vehicle might still be a good fit for a battery energy storage system (BESS), since the charge and discharge rates are so much lower in a BESS compared to an EV. Typically a BESS uses much lower power compared to an EV that needs to accelerate quickly.
Building a lithium battery pack from used cells is a great way to save money and get more life out of something that would otherwise be discarded. Do not underestimate how useful a BMS can be to monitor and protect you from an unsafe condition, especially when using used cells. Please note that since a BMS set up incorrectly will not protect your pack from thermal runaway, you need to contact an expert to ensure the BMS is properly configured to protect the specific cells you are using.
Every battery cell is as unique as a snowflake, with microscopic variations on the cathode and anode material that result in slight functional variations. When manufacturers build new packs they grade and sort cells so that packs have nearly identical characteristics. They consider energy capacity, internal resistance, and manufacturing date. When you are building your pack from used cells, I recommend you do the same. If you want to build a 16 cell pack, you should buy more than 16 cells and “cherry pick” the best to match. If you use a high quality BMS, you can manage the variations and rebalance the pack after each cycle.
When batteries age their available amp-hour capacity decreases. Sometimes the internal resistance will increase too, which means the voltage drops proportionally to the current. If the voltage drops too low, then the system loses power. You can imagine this like a clogged pipe where the narrowing pipeline allows less flow than before.
Building a battery from used cells requires a strong foundation in battery and electrical knowledge and I only recommend it for experts.
2. Assemble Modules into a Pack and attach copper busbars
Nissan Leaf has a very unusually (and sometimes frustrating) way of stacking all of the modules together. There are holes in which you push a long rod through and fasten down on an end plate. If you get one piece backwards, then you have to start all over again, hence the frustration.
Stack a P-group of modules together usually 2–6 modules using the module type “A” with the positive terminals on one side, then alternate with module type “B”. This makes it easier when you are attaching bus bars in the series connections.
You will need to cut copper busbars to size and drill holes. This is the only step in which you really need to have the tools, such as a drill press and maybe a bandsaw. More detailed plans to follow.
3. Buy a BMS and stack switch gear to protect the modules
Why is a Battery Management System (BMS) so Important? Lithium-ion batteries always require some electronics to protect the cells from extreme voltage, current, or temperatures. In many cases, a proprietary Battery Management System (BMS) comes with a battery pack to equalize and protect the individual battery cells. But you can also build a battery pack by assembling cells and adding a BMS. Most batteries other than lithium-ion do not require a BMS for safe usage. Lithium batteries are unique in this way because they can easily catch on fire if the voltage, peak current, or temperature for an individual cell is not kept under control.
A BMS monitors each cell and ensures each remains in a safe voltage range. Some mistakenly think they can ensure safety by simply keeping the overall pack voltage below a safe limit, ignoring the individual cell voltage. The problem with this approach is that it assumes that all the cells are in exact balance. In reality all battery cells have unique variations and they rarely have the same voltage and internal resistance. This causes each cell to drift on their state of charge and specific voltage. As a battery pack is charged and discharged many times the cells can become out of balance.
For example, if a pack voltage is measured at 28.8V for 8 cells in series, you might think that all cells are at 3.6V by taking the average of the pack voltage per cell.
28.8V / 8 cells = 3.6 average V per cell
If the cells are in perfect balance, then each cell would be at 3.6V. But all cells do not function the same. Every battery cell (lithium, lead-acid, etc.) is a unique snowflake. Its capacity, resistance characteristics, and aging patterns are all slightly different.
The magic of a BMS is that it can help you with these variations between cells. It can actively rebalance or discharge the highest cells so the average cell voltage and each individual cell’s voltages are close. It can also disconnect the pack if any one cell gets into an unsafe condition.
In the example above with a pack voltage of 28.8V, without a BMS there is no way to tell if any one cell has reached its maximum voltage limit. Below is an example of two packs with the same pack voltage but with very different cell voltages. Without a BMS there is no way to stop Cell #5 from overcharging.
Some people on the internet recommend managing the peak voltage on a cell by “top balancing” each cell before assembling the pack. This refers to manually charging all cells up to a peak voltage so that they all match. After they “top balance” the cells, they only monitor the pack voltage. What they fail to consider is that over time, all cells will naturally drift from each other and cell voltages will eventually not match at the top of the charge. As I described above, measuring exclusively pack voltage is a dangerous practice, as top balancing only reduces but does not eliminate the chance of thermal runaway.
4. Get the Right Inverter for the job!
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Inverters convert direct current (DC) to alternating current (AC), to render solar or battery power into energy usable for your home. But in some circumstances, it is useful, also, to be able to convert AC to DC as well (for example, to use a generator to charge batteries on cloudy winter days). AC to DC conversion is called rectifying; a rectifier changes AC to DC.
A Bi-Directional Inverter, also called a Hybrid or Battery Inverter, converts between both DC and AC. It can rectify (convert from AC to DC) and invert the power (convert from DC to AC). All that means is that the inverter also has a battery charger built into it. Some of these solar battery inverters also have the ability to convert high voltage DC from the solar array to a lower voltage for the battery. In this case it is simply an inverter with a battery charger and a charge controller in one box. It could be beneficial to buy all of these components in one box rather than have three separate components.
Read more from Joe O’Connor, author of Off Grid Solar: A handbook for Photovoltaics with Lead-Acid or Lithium-Ion batteries.