EV fleet workshops are busy places. And, of course, the work usually comes in waves and rushes, most often just as your technicians are relaxing and looking forward to a weekend off. Typical.
So, what if you could predict those battery problems earlier? That’s where a comprehensive battery maintenance schedule comes in. And the more you know about the series and parallel components of your vehicle’s high-voltage battery pack, the better.
So, how do the various series and parallel connections that make up your battery pack actually affect the performance of your electric vehicles?
Series vs parallel in EV batteries
Cells in series have their voltages added. So, if you wire 100 lithium-ion cells (at 3.6v nominal each) in series, you get 360v. That’s enough to drive a light- to medium-duty EV. However, the current stays the same as one cell. This reduces resistive (I²R) power losses across the wiring, which is why manufacturers favour high-voltage systems. It’s much simpler to make a high-voltage system using cells connected in series.
On the other hand, cells in parallel share the same voltage, but their amp-hour capacities stack. So, if you wire four 3.6v, 5Ah cells in parallel, you still get a total of 3.6v, but the pack can now deliver 20Ah. The more cells you put in parallel, the greater the instantaneous current draw the pack supports.
Instantaneous current draw determines how much power the battery can supply at any given moment, such as during acceleration, towing or rapid load changes. In high-demand situations, such as climbing a hill or overtaking, the drivetrain demands a surge of electrical current. Parallel connections reduce strain on individual cells by distributing that demand, which lowers the internal resistance and heat generation. This also helps prevent voltage sag, which can trip safety cut-offs or reduce performance.
EV packs use a combination of series and parallel. Most EV battery packs use a fixed number of cells in series to reach the voltage the motor needs. Each of those series points, or ‘nodes’, is made up of several cells connected in parallel. This increases how much energy the pack can store and how much current it can safely deliver. For example, the 96S46P layout in the Tesla Model 3 Long Range AWD has 96 cells in series, and each one of those is made up of 46 cells in parallel (making 4,416 cells in total).
Degradation patterns and failure points
The best example to understand a series circuit’s potential problems is those older-style Christmas tree lights. You know the ones where if one little bulb breaks, none of them light up? That’s because the whole circuit is one long string. So, if there’s any break in that ‘string’, the power can’t reach any of the components.
The same goes for the battery packs in EVs. In a series circuit, all the cells must work together and maintain a roughly similar state of charge (SoC). But over time, slight differences in cell quality, temperature or ageing can cause some cells to charge or discharge faster than others. These are called unbalanced cells. They are cells in the same string that no longer hold or deliver energy at the same rate as the rest.
When this happens, the weaker cells reach full charge or empty earlier than the others. The vehicle’s on-board battery management system (BMS) then has to limit charging or discharging based on the weakest cell, even if the rest still have usable capacity. This increases internal resistance in the affected cells, which generates heat and speeds up degradation. In a series layout, that one weaker cell can limit the performance of the whole pack, often causing a sudden drop in range or triggering a fault code.
Parallel strings handle failures differently. A low-performing cell has a less immediate impact on the whole module because the load is shared across multiple paths. However, if one cell develops very low internal resistance, and the cell-level protective measures (e.g. fuses) also fail, the other cells in parallel dump current into it. This creates thermal runaway risk, especially if fuses or thermal cutouts aren’t present. The failure modes are different in series vs parallel circuits:
- Series faults – undervoltage, string dropout, overcurrent cut-off and localised overheating
- Parallel faults – thermal runaway, bypass current, cell venting and cascading failure if not isolated
The most common issue in fleet EVs is partial degradation in series strings, combined with passive balancing systems that can’t correct for drift.
Which configuration lasts longer, series vs parallel?
The simple, theoretical answer is parallel. But the real-life answer is more complex.
Parallel-heavy designs distribute load more evenly. That lowers per-cell current, reducing resistive heat and stress on electrodes. Over time, this leads to slower capacity fade. But design alone doesn’t dictate lifespan. After all, you still need a high voltage to power the wheels.
So, ‘which configuration lasts longer?’ is more of a holistic systems question than a throwaway thought. Some of the influencing factors include:
- Charge/discharge C-rates – high current draw stresses series strings. Fast chargers often hit packs at between 1.5°C and 2.0°C. Cells with higher internal resistance heat up quickly, reducing life expectancy.
- Thermal regulation – temperature differentials across a pack (especially in series) accelerate degradation. A 5°C to 10°C delta can cut lifespan by 15% to 25%.
- Balancing strategy – passive balancing burns excess energy as heat. Active systems (still very much in R&D and not yet released on any mainstream EV) reduce the delta in state of charge, extending pack life.
- Software and firmware – cell detection thresholds, charging curves and thermal cut-in points vary by manufacturer. Poorly tuned BMS algorithms can shorten the battery’s service life.
- Vehicle duty cycle – stop-start urban driving with frequent charging cycles creates different wear patterns compared to steady rural or long-distance operation.
An EV pack’s lifespan is often defined not just by capacity fade, but by when the first module hits the BMS threshold. If one module drops below usable limits, the whole pack is flagged, even if the rest still performs. In series-heavy packs with no balancing, that happens sooner.
It’s not series vs parallel – it’s system vs system
And so, the ultimate answer to the question posed by this article is, it depends. You could make the argument that parallel-focused packs are safer or that series-oriented packs are more efficient. But in practice, it’s not that simple.
Every EV manufacturer designs its battery pack to fit a larger system (motor power needs, available cooling, charging strategy, weight distribution, repair logistics, etc.). The battery pack circuit layout is just one part of a much larger equation. For example, Tesla uses many cells connected in series and parallel. Their cylindrical cells and strong pack design help with efficiency and cooling. Of course, Tesla was (and, in many ways, remains) the EV trailblazer.
Lucid uses completely unique, very high-voltage systems, packing cells tightly to get more energy in less space, allowing it to hit ~900 Volts.
And the Volkswagen Group uses modular battery platforms (like MEB, which has prismatic cells, around 400 Volts, and adjusts pack size for different car models).
BYD’s strategy is also unique. It employs large, flat ‘blade’ cells in modular packs that are safe and thermally stable, often using medium-voltage systems.
So the layout alone, whether series or parallel, doesn’t tell you which pack is better. What matters is how well that layout integrates with the vehicle’s cooling system, inverter, drivetrain and BMS. Because you’re not dealing with a battery in isolation. You’re maintaining a tightly engineered energy system.
Still, there’s one rule that applies across every pack design: all battery systems need balancing. Whether the pack uses 96 cells or 4,000, in series or parallel, over time, the cells will drift. Temperatures, use patterns and charge rates vary across the pack. Without balancing, that drift leads to degradation, uneven performance, and ultimately, failure. That’s where diagnostics come in.
How to interpret the data
High-voltage battery packs tend to wear out slowly, and issues often begin with just a few cells. To understand your test data, you need to know how the pack is built (whether it’s more series- or parallel-focused) and what that means for the numbers you see.
Let’s start with voltage delta (the difference in voltage between modules). In a 96S pack, an 80mV difference could mean something is wrong. In a 12S pack, that same difference might be negligible. In general, note that more cells in series means less room for error. Here, even the tiniest changes can spell trouble.
Parallel-heavy packs work differently. When cells are in parallel, you get more amp-hours. That means more current flow is possible without damage (provided the pack stays cool). But on the other hand, this can also hide problems. For instance, if one cell gets weak, the others take the load, so you won’t always notice until the pack is already wearing out.
Then there’s internal resistance (IR), which increases as cells age. When IR goes up, the voltage drops faster during use. The pack also gets hotter. However, IR doesn’t appear on normal voltage or SoC readings. To spot it, you need to test how the pack behaves under load. So, you need to test the battery under load, ideally after a short rest, using reliable equipment. Most BMS systems only report average values across entire modules or packs, which can mask early signs of failure. That’s why a module might show ‘fully charged’ while hiding cells with dangerously high IR.
In short, layout affects how you interpret data. However, only detailed testing using trusted machines reveals what’s really going on.
How Rotronics helps you manage your EV fleet
Rotronics provides advanced battery testing tools specifically designed for this type of work. The Midtronics XMB-9640 is safe, accurate and quick, and unlike many battery testers, it’s explicitly for the high-voltage propulsion packs found on EVs. It even uses advanced algorithms to test right down to the module and string level. With this tool, fleet technicians have the means to detect early cell imbalance, identify resistance changes and assess the true health of a battery system, long before it drops below spec or shuts down unexpectedly.
If you’re running EVs across multiple manufacturers or seeing pack issues emerge before end-of-life, it’s time to look at layout, load and longevity together. Talk to us about implementing high-voltage diagnostics across your operation. Let’s make sure your packs last as long as your vans do.
And for your non-EV vehicles, we’ve got all the 12-volt and 24-volt diagnostic testers and chargers you could possibly need. Reach out to us today for more information on any of our products and a free chat on how we’ll help you optimise your battery maintenance schedule.
