The battery must be stored with a 40% charge. This threshold was defined as a precaution, as the lower the charge is when in storage, the more you take the risk of depleting the battery totally and of permanently damaging it. On the other hand a fully charged battery in storage for a prolonged amount of time is extremely bad. The reason is that the internal tension is stronger when the battery is full; this increases stress and wear. 40% was established by experts as the best compromise.

It is also preferable to reach the 40% by charging the battery rather than by discharging it. The reason for this is that the operating system measures the level of charge corresponding to the present activity of the computer, so that for example if the system is fully charged, it will calculate the battery time left according to the amount of power being used per minute. This can quickly change if the processor’s frequency changes or the screen brightness changes during the process. Therefore it is preferable to get a more accurate measurement by charging the battery up from 5% to 40% on the mains rather than letting it go from 100% to 40% on the computer, which is not a consistent drain on the battery.

Heating problems

Heat directly impacts the accumulators. So it is crucial to keep your battery cool and dry. The best place to store your battery is the fridge. We estimate that a Lithium-ion battery, charged up to 40% and kept at 0 degrees C will retain 98% of its capacity over a year. At 25 degrees C (average ambient temperature), it goes down to 96% of its capacity, at 40 degrees C down to 85% and at 60oC down to 75%. As we have seen, a battery loses approximately 10% of its charge per month. Comparatively, a fully charged battery stored for a year at a temperature of 0oC will retain 94% of its charge. At 25 degrees C, it is 80% versus 65% at 40 degrees C.

When freezing the battery be aware that the electrolyte present in the accumulators freezes below -40oC (according to L.M Cristo and T. B Atwater in the book Characteristics and Behaviour of 1M LiPF6 1EC:1DMC Electrolyte at Low Temperatures. Fort Monmouth, NJ: U.S. Army Research).

Although this value is debated, with some saying that this happens at temperatures of -75 degrees C, others say -30 degrees C. We chose the one that seemed to be the safest. However, we have not found any sources to advise us against putting your battery in your fridge and to approach 0 degrees C as much as possible, the optimal storage temperature.

The marks of time

No matter what way you shake it, how carefully you calibrate it, no matter how cold a place you store it or how much voodoo you perform on a Friday night before clocking off work, a Lithium-ion battery will lose some of its capacity over time. In fact, a battery starts aging as soon as it is manufactured. On average, a well preserved and well maintained battery lasts for around two to three years after its date of fabrication. This is a result of the oxidation of the cells, a normal phenomenon that occurs whether the battery is used or just sits on a shelf. So it is important to know the date of manufacture before or at the moment of purchase. If you ordered a battery via the internet, don’t hesitate to send it back if you realise it has been sitting on the self in a warehouse for six months. Those are your six months.

Finally, what to do once your battery is dead? The environmental risks are low as these batteries are manufactured to be leak-proof and are safer than the cadmium ones for example. However, it is clear that incineration will give out toxic gases, so your best bet is to find a battery recycling program when disposing of your worn-out stock.