There is also another idea out there that states that recharging when the battery isn’t fully discharged damages the accumulator. This idea is linked to the phenomenon called “the memory effect”. A couple of years ago, the manufacturers of Ni/Cd and Ni/Mh batteries tried to give one another the reputation of having the memory effect in their products in order to fabricate bad publicity for each other. This resulted in a mass confusion for the public. So, let us briefly clear up the issue.

A fake problem

The “memory effect” says that a battery which is put on recharge before it has been totally drained will remember the point at which it was put on charge and assume that this level is its total amount of charge. For example, if you put the battery on charge while it has 25% charge remaining, the battery will “forget” that it has this amount of energy, and tell you in future that it is drained when, in fact, it still has 25% power remaining.

This phenomenon was initially discovered by NASA. Its satellites, put in orbit around the earth, charged during the day time and discharged at night time.

The charging/discharging times stayed the same. After several charges/discharges, NASA realised that the battery didn’t give out any more energy after having reached the point where it was used to being discharged. The battery had kept a “memory” of the usual threshold and had refused to go any further.

Day to day, it is almost impossible to suffer from the memory effect on Ni/Cd batteries. The charging times are rarely (even never) scrupulously identical during several successive cycles. The customer would have to try to reproduce the memory effect, which would still have a minute probability of being reproduced. To tell you the truth, this is still only a hypothesis as the reproduction is extremely hard even under laboratory conditions, so you really have nothing to fear.

The real fake-memory effect

Nevertheless, the manufacturers of Ni/Cd and Ni/Mh batteries have attributed the memory effect to another phenomenon: “the depression of tension”, that we today call the “fake memory effect”. The popular belief became that, contrary to the arguments of the time, the memory effect affected the accumulators of Ni/Cd as much as Ni/Mh, even if the first suffered more than the latter in reality. When a battery is plugged into its charger when it’s full (on average for more than a day after the battery was fully discharged), the current from the charger will generate a deterioration of the accumulator’s structure which will only be able to deliver a pressure inferior (1.08 V/element) to the Ni/Cd accumulator’s normal pressure(1.2 V/element).

We then find ourselves with some accumulators that deliver the expected tension and some that deliver an inferior pressure. Although, if the device using the battery requires the expected tension, the deteriorated accumulators would not be able to deliver it and it would seem that we are dealing with the “memory effect”. This phenomenon can be reversed with NI/Cd batteries by discharging them down to their critical threshold and fully charging it again.

Lithium-ion has no memory

We could ask ourselves whether or not this problem also applies to Li-ion batteries, especially those that stay in a laptop that is constantly plugged into the mains. We could have the impression that in this case the battery could suffer from the same tension depression as it is constantly being charged. The truth however is this problem isn’t present in Lithium-ion batteries because once the battery is charged the electric circuit cuts off the power going to the battery and devotes itself entirely to powering the computer. As it is Li-ion batteries take longer to charge when the computer is on, as the mains power is split between charging the battery and powering the machine itself. This ability to separate battery and computer power means that once the battery is fully charged it is smart enough to stop accepting power from the mains, thus any tension depression cannot happen.

Thankfully, Li-ion batteries hate surcharges (a surcharge is produced from hydrogen and increases the pressure, generating an explosion). This is why it is necessary to use the chargers provided with electronic devices and to charge the battery according to the instructions of the manufacturer. All electronic products going from the laptop to the MP3 player now come with a charger, and it is essential to use it and use no other when recharging your battery. A non-adapted charger could cause an explosion. It is possible to obtain a charger that is more powerful than the provided one on condition that it does not go past the tension of the original charger, however.

Charge tension

On average, the charge tension for laptop batteries is 4.2 V/cell, and if that increased by even 0.1V/cell the circuit is supposed to cut the charge. This limit used to be lower, but this barrier was overcome thanks by the addition of a chemical agent in the battery. The problem is that the charge tension could increase the capacity of the accumulator, but would greatly reduce its lifetime. However, if we take the charger of our test computer, we can see that the tension is labelled as being 20 V. How is this explained? Our computer’s battery’s accumulators have a 4S2P structure. This means that 4 cells are mounted in series and the charge delivered by the charger will be divided by 4 (like we multiply the tension of the accumulators by 4 to determine the total tension delivered by the battery).

2 to 3 volts of this are allocated for the regulation of the tension or electronic losses, leaving us with 17 V, or 4.25 V/cell, which is still close to the value mentioned above. So, in case we hadn’t said it enough yet, use the bloody charger you get with the damn device, or you’ll get teh blown up. Thank you. […Okay -Ed]

The batteries in our test machine are Li-ion polymers but are still representative of what can happen with Li-ion batteries. We would also like to point out that super chargers that supposedly rejuvenate your Li-ion batteries are to be avoided. A dead cell cannot be brought back to life, similarly to how a blown up leg can be quite difficult to replace.