The answer seems obvious: a rechargeable battery is necessarily more economical, since it can be recharged hundreds of times. For the same reason, we say that it is necessarily less bad for the environment than a battery that we would throw away after its first use. But it would be wrong to stick to the obvious: in many cases, it will prove to be a bad choice for the budget, and, even more often, for the planet.

Let’s start the math at the wallet level: Rechargeable batteries are about three times more expensive than disposable batteries. Price to which must be added the cost of the charger, or rather a fraction of its price, since it is generally used to fill several sets of batteries. So in general a set of rechargeable batteries only becomes profitable after having used them at least once a year until completely discharged, for five years, which is the average lifespan of nickel-hydride batteries. metallic (NiMh), the most common.

Their cost will be easily amortized when they are used to power a radio or a camera, which consume energy, but they risk being more expensive than conventional batteries if they are used with a remote control or a clock, devices which can often operate for more than a year with the same batteries. For many others, like flashlights or toys, it will all depend on how often you use them.

Eco-friendly after dozens of charging cycles

Rechargeable batteries are therefore far from always being profitable. And they’re not always the most eco-friendly option either. They certainly produce less waste to recycle, but their manufacturing is more polluting because their design is “more complex”, insists Frédéric Hédouin, general director of Corepile, one of the eco-organizations responsible for their collection.

In addition, the calculation of their ecological footprint should also include the impact of the extraction of materials and the associated depletion of environmental resources, as well as the emissions linked to their transport, the acidification of the oceans, the impact on ozone layer, human and environmental toxicity, etc.

All these factors are taken into account in a comparative study of the life cycle of rechargeable and disposable batteries, carried out by researchers from the Polytechnic of Milan. Their conclusions? At least fifty cycles are necessary for NiMh rechargeables to become environmentally more attractive than alkalines, the most common disposable technology. That is, considering that their lifespan is five years, approximately one recharge per month.

The study may be more recent than other comparable work and based on more credible usage scenarios, but it is not without flaws. It “dates from 2016, the data which was used for the calculations is outdated, like the calculation methods themselves”, warns Camilla Tua, one of its authors. She then only focuses on the Italian case, “a country which uses more coal-fired power plants than France,” adds Louise Aubet, head of research and development at the Resilio consultancy firm.

Electric recharging is therefore less polluting in France than in Italy, which mechanically reduces the number of recharges necessary before a rechargeable battery becomes more ecological than a disposable battery. Without, however, reversing the equation: even in France, disposable batteries remain preferable to NiMh which one would not plan to recharge several times a year.

Over the entire lifespan of rechargeable batteries, i.e. five years on average, an adjusted threshold of twenty recharges seems reasonable to both Camilla Tua and Louise Aubet. Or at least four complete recharges per year to cushion their ecological impact.

Cold and storage

This rule has three exceptions to keep in mind. The first two work against disposable alkaline batteries. These are, in fact, unsuitable for devices that consume a lot of electricity, such as powerful flashlights, cameras or radio-controlled cars with large engines. In these devices, they last two to five times less than a rechargeable battery.

Same thing in very cold weather, since the alkalines lose a lot of their capacity, halved at 0°C and by eight at − 20°C.

The third exception works to the disadvantage of NiMh rechargeable batteries. They don’t like being stored: “We have to keep them alive,” confirms Frédéric Hédouin. Avoid, for example, leaving them lying around in a device that is not used for months. Also avoid keeping a spare set or two in a drawer, unless you take great care of them by recharging them every six months.

Alkaline batteries are much more suitable for storage: “Good quality models easily last ten years before losing 20% ​​of their capacity,” according to Mr. Hédouin. Provided they are stored at room temperature in a dry place.