A large-scale analysis by German automobile club ADAC has provided new real-world insight into the ageing of plug-in hybrid vehicle (PHEV) batteries, confirming a pattern already observed in international studies: smaller batteries, such as those in plug-in hybrids, tend to degrade faster than the larger packs used in fully electric vehicles (BEVs).
The ADAC study, conducted in collaboration with battery-diagnostics partner Aviloo, analyzed data from more than 28,000 plug-in hybrids across a range of makes and models. On average, battery degradation was moderate and within the expected limits for a typical vehicle lifespan.
Usage and model make a difference
In other words, battery aging is on par with the vehicle’s degradation, but usage and the model make a difference. The typical state of health (SoH) values were around 92% after 50,000 kilometres, 88% after 100,000 kilometres, and roughly 80% after 200,000 kilometres.
The analysis also revealed that differences between models can be significant: Mercedes-Benz vehicles maintained relatively stable SoH levels even above 200,000 kilometres, while some Mitsubishi models showed earlier declines. More importantly, the data confirmed a strong link between usage behavior and battery ageing.
Cars driven more frequently in electric mode showed faster degradation, while those that relied more on their combustion engines tended to preserve their batteries better. The physics of battery operation easily explains this finding.
Smaller batteries depleted more often
PHEVs are equipped with relatively small batteries, typically 8-20 kilowatt-hours, while BEVs carry much larger packs, ranging from 50 to 100 kilowatt-hours or more. The difference in capacity means that a PHEV battery is cycled much more frequently and more deeply.
A driver who covers 40 or 50 kilometres a day electrically will almost fully discharge and recharge the pack every day. By contrast, a BEV performing the same daily distance will use only 10 to 30 percent of its capacity before recharging. Over the lifetime of the vehicle, the PHEV therefore undergoes many more full equivalent cycles, and each kilowatt-hour of its capacity is ‘worked harder.’
This cycling intensity leads to faster chemical wear in the battery’s cells. The smaller packs also operate at higher C-rates — the amount of current drawn or charged per unit of capacity — which increases heat generation and internal resistance.
Heat and high current density are both known accelerants of lithium-ion degradation. Moreover, PHEVs often rely on simpler thermal management systems than BEVs, particularly in earlier generations, which means they are more exposed to temperature-related ageing. Taken together, these factors create a higher average stress level per cell in a PHEV battery, leading to a faster decline in capacity over time.
Supported by international research
These conclusions are supported by international research. The European Commission’s Joint Research Centre (JRC) reported that PHEV batteries tend to lose usable capacity more quickly than BEV batteries, primarily because of their smaller size and higher cycling frequency.
A long-term comparison by the U.S. Idaho National Laboratory found that a Toyota Prius plug-in hybrid reached its degradation threshold sooner than a Nissan Leaf, despite the Leaf having processed more total energy.
Laboratory studies from major cell manufacturers such as CATL and LG Energy Solution confirm the same relationship: cycle life decreases exponentially as depth of discharge and C-rate increase, both of which are inherently higher in smaller packs.
For operators, the practical implications are straightforward. PHEV batteries remain reliable for normal service life, but their health is more dependent on user behaviour than BEV batteries.
Frequent charging to 100 percent, parking at a high state of charge, or exposing the battery to high temperatures can accelerate wear. Conversely, keeping the state of charge within moderate limits and avoiding unnecessary full cycles can significantly extend the lifespan. From a cost perspective, smaller PHEV batteries are cheaper to replace, and most will continue to function adequately for eight to ten years of everyday use.
Bottom line: what should used car buyers pay attention to now? Based on ADAC’s evaluations and experience, the SoH (cyclical health in %) should be as follows for a used PHEV, according to the experts.
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at 50,000 km: at least 92%
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at 100,000 km: at least 88%
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at 150,000 km: at least 84%
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at 200,000 km: at least 80%
ADAC adds: “If the value of the battery check is significantly lower, this could be an indication of an excessively fast-aged battery or individual weakening battery cells. A battery check to determine the current SoH value provides transparency and safety – and is a recommendation for prospective buyers who have envisaged a used plug-in hybrid.”
Narrowing the gap
For manufacturers, the message is clear: advances in cell chemistry, cooling, and software-based charge management are essential to narrowing the gap with EVs. For fleet managers and professional users, consistent charging discipline and regular battery health monitoring will help maximise operational life and residual value.
From an engineering and fleet-management perspective, extended-range EVs (EREVs) represent a more battery-friendly compromise between PHEVs and BEVs. Their larger packs and controlled operation lead to significantly slower ageing, often comparable to that of pure EVs.
Where PHEVs suffer from their small pack size and frequent deep cycling, EREVs mitigate these issues through increased capacity, lower per-cell stress, and active charge management.
They are therefore a technically robust solution for operators seeking electric-dominant driving without full reliance on public charging infrastructure — and, crucially, they avoid most of the accelerated degradation that affects smaller plug-in hybrid batteries.
More fire risks while aging?
While the risk of PHEV battery fire can rise modestly as packs age, it remains statistically very low and far below that of conventional fuel systems when vehicles are adequately maintained.
The main difference between PHEVs and BEVs is not the intrinsic stability of the cells but the complex operating environment — proximity to an engine, vibration, and more frequent cycling.
However, PHEVs inherit the same explosion risks as combustion vehicles, since their petrol systems can feed or intensify a fire once it starts — regardless of whether the ignition originated in the engine or the battery.
A battery-electric vehicle cannot ‘explode’ like a plug-in hybrid because it has no fuel vapour source and no combustible liquid to ignite in volume. Even in catastrophic failures, EV batteries vent and burn rather than detonate.
