Battery packs: the heart and soul of an electric motorcycle
To better understand what goes into designing an electric motorcycle, a better understanding of the “heart” of the vehicle needs to take place. During the course of building Evoke Electric Motorcycles, Nathan Siy and his team had the opportunity to consult and help out other companies getting into the field, and the biggest area that they seem to think they understand the most, yet in actually understand the least, is the battery pack itself. When discussing lithium-based battery packs, many vehicle designers seem to think that the standard voltage and capacity are the only 2 factors to consider when choosing a pack supplier or preparing to build a back for their own application.
In this second article in the series Nathan takes a look at what it takes to design and manufacture battery packs for electric motorcycle applications; high C discharge, wide temperature applications, and stability in the pack to classify it as a true electric motorcycle power source.
The standard voltage / capacity are usually written a 72v / 20ah configuration, denoting that the nominal voltage of the pack is 72v (which technically isn’t true, and I’ll explain why in a bit) and that the capacity of the cells internally make up 20 ah, which would allow 20A to be discharged for a period of 1 hour before fully depleted.
To cover a bit of basics when dealing with lithium-based battery packs, we need to look at the inherent make up of a final pack. Cells are electrically connected in a series and parallel connection, bringing a grouping of them together to meet the right voltage and the right capacity for the application. The issue with lithium-based battery packs is that don’t adhere to the standard 12v “blocks” that we’re used to. You’ll usually hear automotive applications adhering to a 12v starter battery or a 24v electrical system. When building electric vehicles utilizing lead acid batteries in the past, one would also follow this type of 12v “block” system. Most lead acid batteries are made up of 6 cells with a nominal (resting or average voltage) of 2.1v. Connecting them integrally provides a nominal 12.6v lead acid battery; so even by classification a 12v battery isn’t 12v exactly, and this moves up in voltage to 14.4v when charging at maximum and drops down to 10.0v when empty.
When it comes to equivalent lithium-based batteries, they’re also build utilizing cells with an individual nominal voltage of, usually, 3.2v, 3.6v or 3.7v depending on the chemical makeup of the cell. Bring them together in series will essentially increase your voltage, while connecting them in parallel will then increase capacity. The fundamentals of battery pack design are somewhat basic, but like all industries and products, the devil lies within the details.
When dealing with a 3rd party pack assembler, or choosing to build a li-ion battery pack for yourself, there are 3 major areas of interest that will determine the stability, range and power output of your battery pack. Those areas of interest are:
- Quality of cells
- Cell interconnects
- Cell consistency of the manufacturing batch
In regards to the quality of cells for a specific application, you’ll need to analyze their data sheets and explore what is available within an existing or custom supply chain. Different chemical makeups and manufacturing methods can yield cells that are more geared towards quick discharges, higher capacities or a balanced setup. Quick discharge cells will usually have a smaller capacity from the high capacity cells, but will be able to discharge 3x to 10x (known as a C rating) of their stored energy in a short period of time without a massive spike in cell heat. Higher capacity cells will sacrifice speed of discharge for a denser array of lithium ions in the cell. Typically, they can discharge 1x to 3x of their stored energy without significant stress on the cell. Understanding the chemical makeup and how it applies to your application provides a great starting point for developing the perfect pack for the application.
Beyond selecting the type of cell, attention should be paid on the internal resistance, which will factor in to the amount of heat produced for a certain scenario. Selecting the incorrect cell will place the cells under stress under the proposed load, creating heat within the pack itself, introducing the potential for catastrophic thermal runaway and fire within the pack or reducing the lifespan of the cells.
The 2nd area of interest is on the cell interconnects, whether it be nickel strips, copper bus bars, printed circuit board, wire bonding, careful consideration must be made to carry the proposed current through the entire system.
This is an excerpt from a whitepaper discussing thermal and heat management in a li-ion battery pack:
Exploring the thermal efficiencies of parallel and series connections between the individual cells is an interesting one. As we look at the thermal image, we see most of the heat being generated at the series connection while the parallel connections between the cells stay relatively cool. This is a property of the electrical connection in which cell is parallel share the distributed current, where each cell would see 1/x of the total power being delivered to the load. The series connection leaves a single path for all electrons to flow through, therefore you’ll notice the entire current load flowing through the series connection strips.
When designing a pack, careful consideration must be addressed on the total amperage of the system, in peak and RMS (continuous) measurements. While a certain connector may take a peak current burst without damage, current over time causes connectors and terminals to increase in heat and sometime glow due to the current passing through it. Running a constant current (CC) test on a single 8.0mm x 0.1mm thick nickel strip at 40A, we can see that the strip exhibits a 46°C to 323°C increase in a matter of 5 seconds. In mobility usage cases, accelerating at wide open throttle (WOT) can draw in excess of a few hundred amps on a full speed electric motorcycle and can cause serious heat issues if not properly designed.
Finally, the cell consistency in manufacturing processes has a significant effect on the total effect of the pack in terms of total range and capacity of the pack. The way that most li-ion cells are manufactured is in a batch process, where theoretically, cells of the same batch should have an almost exact replication in the manufacturing process, but in reality, the substrates and coatings used are done in large sheets to make up cylindrical, prismatic or hard body cells, and any abnormalities within the sheet may lead to an abnormality in only a single cell. But that single abnormality in the production process would mean that cell may still be good, but not the exact same as the rest of the batch. Because we learnt earlier that li-ion battery packs are made up series and parallel connections, that 1 cell will be the weak link in the entire pack, thereby limiting the total capacity of the pack to the ability of that weak cell.
Sifting and sorting through the weaker or abnormal cells is a somewhat time consuming effort at scale, and the level of balance in the pack will depend either on your cell manufacture doing the sorting, the pack assembler doing the sorting, or potentially both.
At scale, utilizing an IR sorting machine is a good 1st step to finding out abnormal cells in the batch and also grouping similar cells together to form parallel banks. The IR sorting machines are not 100% accurate though since they work very quickly and only take a snapshot of the cells IR at the point it enters the machine, and due to chemical processes, heat, uniformity, and subsequent charges, that condition of the cell may change.
The more thorough way is to run an entire cycle test on each of the cells before they’re connected into a total pack. This allows an analysis of the full chemical makeup and electrical properties that the cell will undergo through its life. This will also show clearly weak or abnormal cells through the test.
As a basic starting point, understanding the cells and the cell properties going into your pack are going to give you great insight on how the pack will fit your application. Moving to the assembly process, make sure the pack designer has a solid understanding of your application and has the capability of producing for the application with the right interconnects. Saving a money on the interconnects is very common in the industry and makes up a large amount of cost on pack assembly, so saving here goes along way for the pack assembler, but creates a dangerous situation with whomever is using the pack. Finally, make sure that there’s a suitable process in manufacturing to sort, group or QC the cells before going in. The worst thing is finding out your 25.0Ah pack only discharges to 19.7Ah because there is 1 string too low and won’t balance. You then effectively pay more per kWh since your usable is far below what is advertised.
If you’re interested in learning more on thermally managing a li-ion battery pack, there’s a whitepaper titled: Thermal Management of Cylindrical Cells for Mobility Applications. Link can be found here: http://www.evokemotorcycles.com/batteries