Fast Charging Explained: Why More Power Doesn’t Mean Less Time [The Battery Cycle #3]

In 2021, Claudius Jehle, CEO of volytica diagnostics GmbH, and Sustainable Bus launched a series of articles around the “The Battery Cycle”. They shed light on the complexities of Li-Ion batteries and provided valuable insights for anyone involved in electric mobility. In 2025 and 2026, the articles were updated with real-world data and lessons learned to understand how battery chemistry affects real-world operations. (The article was first published in Juni 2021, this is a revised version.)
For questions and contributions please reach out to us via contact@volytica.com.

In our last article on state of charge of the Battery Cycle series, we learned that we can picture batteries like bulbous wine decanters filled with foamy beer, which greatly helps to understand everyday problems with SOC estimation, balancing problems and sudden standstills. But this analogy also greatly helps to understand typical questions around charging of batteries, which we will address in this article.

It’ll be the place where to explain – among many other things – how and why doubling charging power cannot cut charging time half, at least if one intends to charge until full.

Charging Batteries, State of Charge

The peculiar “glass shape”* of Li ion batteries, with the most problematic ones being LFP and LTO, gives a hard time to any electronics and software to relate the filling height (voltage) to the filling content (SOC). Additionally, a phenomenon similar to foam on liquid** makes even the very measurement of this height a complex task, especially in dynamic situations like driving. It comes naturally that SOC determination is a complex task.

But let’s take a closer look at this “foam” – it can help to explain a lot more about battery charging. Let’s start with depot charging setups, i.e. with buses or trucks with rather large batteries (>300 kWh) that are meant to serve a route all day with no or only few top-up charges. Typically, one wants the battery to be at 100% in the morning and to end with some reserve at the end of the day.

Depot Charging: a Wise Choice for Battery Charging?

But charging a battery to 100% before the day starts is not as straightforward as one might think. The charger first “negotiates” the upcoming charging process with the vehicle via some sort of dedicated communication protocol, involving an agreement on – first and foremost – the planned charging power that the charger is able to give, but also that the battery is capable of receiving. We limit ourselves to the battery part here, because one could write endless articles on this delicate and fragile communication part. In particular, we are explaining the situation for well-sized and not super-oversized batteries; for oversized batteries, the below explained CV-part typically does not affect charging so much.

Typically, charging commences with the agreed current in a constant flow – this phase is thus called constant current phase or simply CC phase. In the beer analogy, the glass constantly fills up, and the filling height increases; both the beer rises, and foam starts building – electrochemically speaking, “overpotentials” on the measurable voltage start building up (this is due to the migration of the bulky ions into the electrodes taking some effort, if you will, and this is macroscopically visible as reversible voltage build up, slowly vanishing again after charging stops, just like foam on beer…).

Once the total filling height, i.e. beer plus foam, approaches the rim of the glass, i.e. the battery voltage reaches the absolute maximum admissible voltage, charger and battery quickly start renegotiating a new, smoothly changing current profile that keeps the total filling height just constant at the top – the constant voltage or CV phase. What in essence happens is that stepwise reducing the current will let the foamy party decrease, while the liquid part can continue to rise. This process in total is often called “CCCV charging”.

Assume you would just stop charging when the voltage first hits the top – foam would settle and, somewhat surprisingly, you would find your glass only 85% full after some 10 to 30 minutes. So it is exactly like drafting a beer: CC beer flow until the foam hits the rim, then reduction of beer flow, keeping the total height constant (CV), only changing the liquid-to-foam ratio.

What are the implications? Look at the simulation results in the diagram – one straightforward consequence is that doubling charging power cannot cut charging time half, at least if one intends to charge full. As a matter of fact, the CC part is sped up, but also not by the factor 2, as the foam rises much faster, so the CV part becomes even more pronounced. In the example, there is even no CV part reached after ~900s (it would come later), but by doubling the power, CV is reached after ~570s, reducing the energy intake by 24%. Tripling the power obviously brings the system into CV right away (no CC observable), resulting in a less than double energy intake!

Needless to say, this is just an example and might not be applicable to your specific assets. Oversized batteries – and this is very true for LFP-based assets – don’t show this effect as pronounced as explained here (as the CV-part might not even be reached, i.e. you are drafting ca. 1 liter of beer into a 1.5 liter glass: you will hardly produce enough foam to hit the rim).

Learning? 3x charging power doesn’t necessarily reduce charging time by 3! It can easily be that the CV part takes just as long as the CC part. To get the first 80% in can take just as long as to get the last 20% in! Observe it in your monitoring systems: first, SOC rises linearly, then it slowly decreases until it rests at 100%.

If you cannot observe this in your data when approaching 100%, the very likely reason is that the SOC that you see is not the actual, the “real” SOC. OEMs and manufacturers distinguish between the SOC that is communicated (often “operational SOC”) and the “real”, internally calculated one (“technical SOC”), which is obviously particularly relevant for oversized batteries where the usable, operational net capacity differs significantly from the technically available capacity.

The operational SOC is adapted to e.g. show 100% when the real is only at ~90%. Without going too much into detail, there are good reasons to do this: (a) to have some reserves (to accommodate the known estimation errors, cf. our last article), (b) to save the battery (too high/low SOCs are often detrimental to health) and (c) to emulate a seemingly uniform behavior over the whole lifetime by unlocking the reserves over the lifetime. So, looking at (c), if you do not observe this charging behavior now, you might well observe it later – bear in mind that the depot charging time of the vehicle might increase in the future!

Battery Charging: What About Opportunity Charging?

With opportunity charging along the route, there are other challenges. One typically does not charge to 100% (only in the morning, as above) but repeatedly top up a smaller battery (<200kWh). The foam hardly hits the rim, so no tenacious CV phase, only CC. More like topping up a glass of water, or red wine, now and then.

For speedy charging interruptions, the power is typically quite high. So although there is no CV part that could “choke” the fast charging, the high pulses can result in resistance-induced voltage spikes (here the glass & liquid analogy fails a bit – maybe think of sparkling wine: you pour it in, the foam extremely quickly rises very high and decays in virtually only seconds). As the resistance increases due to degradation, these spikes increase likewise. Take-away message: Opportunity charging has beneficial characteristics, at least charging timewise, but can suffer from degradation induced charging time increases.

Finally: Inductive Charging

One remark on inductive charging, where the physical plugging for a galvanic connection is replaced by wireless over-the-air energy transmission. It’s a fabulous idea, as there would be virtually no positioning and manual handling efforts anymore. One could even charge while driving. But physics kick in: The ratio of energy that reaches the vehicle vs. what is lost due to peculiar induction inefficiencies – plainly: that is converted into heating the environment – chiefly depends on (a) the distance between the receiver and the sender and (b) the frequency of the alternating electro-magnetic induction field.

For inductive charging not to become uneconomically unbearable, you either have to (a) reduce the distance between sender/receiver to only millimeters and/or (b) increase the frequency and thus coil size and thus weight. Ideally, a (several hundred kilograms!) copper coil should be located millimeters away from the sender. This proved, and proves, to be technologically so challenging that until today, no economically viable and scalable solution was found, and close to 100% of all batteries are charged with direct connections.

A final-final “aha moment” on overhead catenary charging like in trolley buses or some e-truck application ideas: Next to the obvious infrastructural challenges of maintaining a catenary network, a real challenge lies in the electrical setup of the vehicles: Most road vehicles have rubber tires. They don’t conduct electrical current. Hence rubber-tire vehicles with catenary need 2 overhead wires (adding to the infrastructure complexity vs. metal-tire trains).

The real “but” however is safety: In a metal wheel-setup, an electric fault (such as: a live wire touching the vehicle frame and setting it under high voltage) would be immediately diverted to the ground via the wheels; the danger of metal frames or vehicle body being “under voltage” is very low. But in rubber tire assets, an electric fault might set the whole vehicle under voltage, and a passenger touching the body might get an electric shock. The result is that the electrical setup of such vehicles adds complexity and costs, making it more challenging economically.

Battery Charging Takeaways at a Glance

• Li ion batteries exhibit a curious effect: the amount of energy chargeable in a certain time depends on the charging power, e.g. doubling charging power does not necessarily cut the charging time in half.
• The SOC you see (“operational/ net SOC”) is most often not the real SOC, for good reason.
• In depot charging applications or anywhere, where a battery is charged to 100% (really 100%), and where the battery is not heavily oversized, some form of CCCV charging is employed.
• The peculiar pattern of CCCV leads to the behavior that doubling or tripling charging power does not cut the time half or by a third.
• Opportunity charging is mostly CC, so it is not susceptible to this pattern, but is susceptible to rising resistances due to degradation.
• Inductive charging is nice but is technologically extremely challenging.

We encourage everybody to openly discuss the options and implications of different charging philosophies with their OEM partners. It is very helpful to ask for maximum transparency in the basic battery & charger signals and even for periodic information about the degradation, as we already learned how degradation affects every aspect of a battery.

* i.e. OCV characteristics, see Battery Cycle #3
** i.e. the overpotential/dynamic voltages
*** Simulation (LFP, SOC0=50%, 30°C env) results from: Bunzel, A.; Morawietz, L.; Ufert, M.: Technologische und ökonomische Bewertung der Elektrifizierung von ÖPNV-Busflotten im Werkstatt- und Betriebshofbereich. Fachtagung „Werkstatt- und Betriebshofkonzepte für Elektrobusse“, Dresden, 11.-12.10.2018.

Looking Ahead in the Series

In the next article on cell balance, we will look at balancing inhomogeneities, or why a LFP bus sometimes stops.

All knowledge articles of the battery cycle:
Intro – The Battery Cycle – opening the black box
1 – NMC, LFP, LTO: What’s the Difference in Battery Chemistry? – energy density, safety, lifetime, cost
2 – State of Charge: Why It’s Harder to Measure Than You Think – about really knowing how full your battery is
3 – Fast Charging Explained: Why More Power Doesn’t Mean Less Time – how to keep a battery healthy
4 – Why an LFP Bus Can Suddenly Stop: The Battery’s Weakest Cell – why imbalances define the limit
5 – Battery SoH: The Number That Doesn’t Tell the Whole Story – State of Health is mostly misunderstood
6 – Stress Level: The Key Drivers of Battery Degradation – what really hurts a battery
7 – Battery Data: Are You Seeing the Full Picture? – use and interpret your data correctly