We have recently built our own LiFePO4 Battery to power all of our house usage as well as to power our electric motor. The switch from AGM or Lead Acid to Lithium is other-worldly!
When facing the conversion, there are two main choices you can take to make it happen. You can either buy store bought batteries that are assembled and perfect, or you can build them yourself from various components.
How do you get from a cell to a battery pole that you can hook your boat up to? Simple!
You will need: Lithium Cells, a BMS for your size of battery, and some wire! That’s it. (And then you will need a bunch of smaller items that will tie it all together)
First you will need some cells. Cells come in all different shapes and sizes, but the two main categories are cylindrical and prismatic. Cylindrical cells are, as their name implies, little cylinders. They live inside a steel casing so they have all the structural support they could need and tend to have higher discharge ratings. This means that you can such more juice more quickly out of the little cylindrical cell than you could out of a prismatic cell.
The discharge rating is denoted by the letter C. 1C is the equivalent of 1x the battery capacity. In our case with the cells shown, they are 6ah cells, so 1C would mean I could pull 6 amps out of the cell in an instant. This is where cylindrical cells shine, as they can normally operate at ratings of 3C, so this 6ah cell can give up 18 amps at a time! A prismatic cell usually maxes out at 1C, but some can go up to 2C.
Prismatic cells are normally rectangular, and in a box (but some can be in a pouch). Their case is not always as strong as it needs to be so some need to be mounted inside a box that can compress them, otherwise they can bulge and fail.
As you can see, these prismatic cells are much larger. Being a larger cell means that they can hold more power in them and you wont need as many. A common size for prismatic cells is around 100ah, and at a 1C rating, you can pull 100 amps from them at any given point! 18 amps from a cylindrical cell seems pretty dim compared to 100 amps from a prismatic cell, but that’s where cell layout comes into play. By grouping many cells, you can up the power of the pack and have a tremendous battery that can give up a lot more energy in a short amount of time. This is especially handy for electric drives that will need lots of power in an instant while docking and maneuvering.
A 100ah battery pack in prismatic cells can give up 1C, so 100 amps; the same pack in cylindrical cells with a rating of 3C can give up 300 amps!
We chose to go with cylindrical cells because they were a little cheaper than prismatic cells and the small shape meant that we can organize them however we needed to fit them into our existing battery boxes.
To hold all of these cells together, we needed a lot of cell holders! The cylindrical cells and cell holders we used can be purchased at Battery Hookup and you can save 5% with the promo code RIGGING5 .
Once the cell holders are assembled, the cells need to be tightly inserted in the correct orientation so that they can be wired up into a massive battery.
The cells are wired up with very thin Nickle strips that are spot welded onto the ends of the batteries. This is a very tedious process that involves the constant repetition of a very simple task. Nothing about it was hard, it just involved a lot of doing the same thing over and over and over again!
We used a spot welder that we bought from Amazon as well as the spool of Ni strip which we cut to length to connect the batteries together. Links to the ones I purchased are right over there —>
For our battery build, we decided to make a 480ah battery divided into 5 packs of 96ah each. This meant that each pack contains 256 cells that all need to be interconnected and held together by 512 cell holders. We then did this 5 times!
Once everything was spot welded together, we covered the packs in Kapton Tape, which is a special tape used to prevent short circuits and helps isolate the electrical parts. It also makes it look cool with its golden-bronze color, as well as hold all the wires and stuff together. Once again, this is the exact one that I used. It’s 2 inches wide so it is big enough to cover two rows of cells in a single pass, but also small enough to be manageable! Imagine trying to use a 6 inch wide roll! It would cover the pack quickly but it would be a challenge! The same holds true for the 1/2 tape, way to small. 2 inches was the exact distance between the cell holders on the top and bottom, letting me slide the tape in there perfectly and hold all the wires in place.
The BMS or Battery Management System is a crucial part of the battery build, and honestly the biggest reason why people choose to buy a built Lithium battery instead of building their own. All those wires look pretty intimidating! The fact is, it’s really simple. I built a 48v battery out of 3.2v cells. To get to my desired voltage, I linked 16 cells in series (know as a 16s battery). The BMS has 17 wires that come out of it to hook up to the battery to check the cells and balance them if needed. Why 17 instead of 16? Because they connect to the negative side of the battery and then to each positive part on the way across the battery. In other words, start with the negative and then put the next wire on the positive all the way across the whole battery. I bought the BMS’s from Overkill Solar. They also have incredibly simple to follow instructions that will make the installation a simple procedure.
Kapton tape is great and all, but I’m a fan of added security. Being how I’m stacking the batteries in their battery box, there is a lot of potential for the tape to get chafed as we sail and short one battery to the other. To prevent this catastrophe and also to keep the batteries from sliding around much, I placed a sheet of 1/16” rubber to further isolate the two battery packs. I put one under the bottom pack as well just to act as a bit of cushion between the pack and the bottom of the box.
In the end, we built a huge battery pack which took a ton of time (2 weeks to be exact) but saved us thousands of dollars in the process! Building our batteries cost $3,100 in cells, and $5,000 for the entire project, including all the extra parts and tools we needed to purchase to make it come to life.
Lets compare to some other pre-made batteries and see what the cost savings came out to be:
Our battery build was 480ah at 48V, this translates down to 1920ah at 12V. For math simplicity, lets call it 1900ah at 12V, or nineteen 12V 100ah batteries.
Our cost was $5,000 for 19 batteries, or $263 per battery.
If we built our battery bank with Battle Born Batteries, it would have cost us $18,050.
$13,050 savings!
Renogy batteries are a little cheaper, but 19 of them would still cost us $15,200.
$10,200 in savings!
Building your own batteries is a time consuming process, not a difficult one. You simply have to weigh out the value of your time. If you can earn the savings amount and more by working your normal job for the time it takes to build the battery, its better to buy them outright. If you live a cheaper life, you can afford the luxuries of a lithium battery pack without the cost barrier.
