The list of advantages of cylindrical cells over prismatic cells is pretty small. Cylindrical cells do not rely on the box to provide compression to the cell, they can charge and discharge faster, and they are smaller allowing them to be built to fit custom shaped boxes.
This last point comes in really handy when fitting lithium batteries in your sailboat!
The concept of connecting the cells together is simple. Connect the same ends of the cells together to form your parallel groups, and then connect the parallel group to the opposite charge parallel group.
In other words, hook all the positive cells together in a row making one big patch of positive connections. Then connect all the negative cells together in a row making one big negative patch of connections. Now you have a big positive and a big negative group set up in parallel, when you connect them to each other, they will now be connected in series.
In the top picture, we were making the battery in a square shape, so the positive and negative strings are easy to visualize, as are the series connections linking the positive and negative groups together. This if fine if you are building a square battery because you have plenty of space, and if you have plenty of space, you are probably also considering a preassembled battery which will cost more than the raw components but remove all the effort of building your battery!
When you are building a battery to fit into a strangely shaped box is when the cylindrical cells shine as they allow you to fit them into unique designs.
Think of prismatic cells as a large solid rock! They are their shape and will forever be that shape, regardless of the container that they are placed in. Now think of cylindrical cells as small pebbles. Each one has its shape and will maintain that shape but since they are smaller they can pack in there much more tightly and conform to the container that they are placed in more easily.
In the square battery, the positive and negative strings are straight; running the entire length of the battery. In the strangely shaped battery, the positive and negative strings are uniquely interlaced as we fit the cells into the space allowed as best we could.
If you look closely, you will see that the square battery where all the positive and negative cells are in a line, there is only one Ni strip connecting between one positive and one negative. On the second image, the battery is not square, but will still be forced to supply the same amount of power, and this means that the same number of interconnects between the positive and negative groups needs to exist.
In the square battery, there were 16 interconnects between the positive and negative. Each interconnect can flow 5 amps giving it the ability to charge and discharge at a rate of 80 amps. The uniquely shaped battery has a bit of an issue, there are far fewer than 16 spots where a positive can connect to the neighboring negative, meaning that the Ni strips will not be able to flow the full 80 amps! To circumvent this caveat, we simply doubled up the Ni strips to bring the total number of interconnects up to 16, allowing the same 80 amps to flow across between the two parallel groups.
If you look closely, you will see some extra Ni strips that bridge the gap and add ampacity to the system, allowing us to use this uniquely shaped battery under the same parameters as the square battery.
Building the battery is only part of the process, you then need to control the battery to avoid any dangerous and explosive events that can occur when a Lithium cell becomes over or under charged. To do this, you will need a BMS or Battery Management System which protects the cells by shutting them down before they get to dangerous levels.
Monitoring and balancing your cells is an important part of Lithium batteries, but not as important as building a safe battery. Imagine if you didn’t take into consideration the ampacity of the Ni strips and ended up building a fuse rather than an interconnect. As the amps start to be drawn from the battery, the interconnects would heat up and that would place a lot of heat right over a Lithium cell. The first thing you should know about Lithium cells is that you can never expose them to heat because they could ignite!
Knowing what you are going to do with your batteries lets you build a battery that will suit those needs and as a result, allow you to build a battery that will safely provide you the power that you need and be safely managed while it provides this power.
