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Is it better to completely drain my rechargeable batteries every time?

Is better to leave devices (cellphone, laptop) with lithium-ion batteries plugged in all the time?

  • I thought you were supposed to charge them fully, then let them drain almost completely, then charge them, and so on. But this month's issue of PC World says that's only the case for (older) nickel-cadmium batteries, and that current lithium-ion batteries have "the opposite problem"—does that mean they best keep their capacity if you leave them plugged in as much of the time as possible?

  • Answer:

    That article is correct for anything with a Li-ion battery - which is ALL phones nowadays, all laptops made in the last 15 years and all MP3 players. A Li-ion battery has a limited lifetime in terms of cycles. (normally about 400-500) One full discharge/charge cycle counts as one cycle; so discharging it 1/10th of its capacity and then charging again is 1/10th of a cycle. Thus, charging it whenever you can and keeping it on charge is best. Footnote about leaving it on charge: the charger does not carry on feeding a trickle charge into the battery like with NiCd cells. It will stop when it is senses it is charged, and then monitor the charge level. When it self-discharges to about 98% of its capacity (this is called the charge threshold level), charging will resume once again until it is full. This cycle continues all the time you leave a device attached to its charger.

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Normally, I find PC World to be a hype magazine going ga-ga over all the new gadgets, useful or not. But the article is entirely correct. As an electrical engineer, I've been working with rechargeable batteries for 30 years. The old NiCd's did tend to suffer from memory charge, and only a full discharge would over come that problem. Cadmium crystals typically built up from gradual discharge, and with only partial discharges, the crystals become ever larger. Normal charging would break down and reverse the smaller crystals, but for the larger ones, it required a full discharge and massive recharge to fully break them down. NiMh batteries do not suffer such problems, nor do lithium-ion batteries. In fact, fully discharging them substantially REDUCES their life. Much better to leave them plugged in for trickle-charging when not in use. Actually, that's exactly the process that lead-sulphur batteries in your car operate, always plugged in, trickle-charging, so they'll be fully charged when you need to start the car.

Marc X

The article is correct. You will get the longest life from a Li-ION battery if you recharge it before it is half drained. However, then you only get 1/2 of the capacity. It's best to recharge Li-ION batteries whenever convenient but, if possible, before they are fully drained.

Ex Chemist

You should let the battery drained every once in a while - at least in my experience (with both cellphones and laptops) that the battery life extends slightly when I let the battery drain. I do it once a week or so. The rest of the time, I charge laptops and phones over nite

AM

if you do that with cell phones it kills the battery capcity to charge it. Laptops yeah

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