What causes laptop batteries to run down when off?

[John Gray]

This should be an easy one.

Even if a laptop is properly shut down, if it is left for a significant period of time, the battery runs down, even going completely flat if you leave it long enough.

Is there anything that can be done to prevent this happening, leaving the laptop (almost) fully charged for the next use?

[HansV]

If the laptop won't be used for some time, it's best to take the battery out of the laptop and to store it with a 40%-60% charge in a refrigerator (not in a freezer, just above 0°C is ideal).
Take it out of the refrigerator an hour or two before using it again - using it straight out of the fridge causes a sharp rise in temperature.

[John Gray]

Thanks, Hans - I've come across this before, probably at Battery University!

But you haven't said what drains the battery when switched off. Anything? My satnav Li-ion doesn't seem to run down in the same manner.

[HansV]

I won't pretend knowing that...

[Leif]

But you haven't said what drains the battery when switched off. Anything?

Yes - Self-discharge.

[John Gray]

"Typically, among rechargeable batteries, lithium batteries suffer the least amount of self-discharge (around 2–3% discharge per month)"

That's considerably slower than I observe! (On two different laptops...)

[Leif]

I would guess that is based on them not being connected to anything.

Are your observations based on them detached from the laptop, or still connected? If connected, by definition there has to be some circuitry connected to the battery, if for no other purpose than to power the Turn-On logic when you press the power button.

[PJ_in_FL]

Leakage. All digital devices have a leakage spec. It's the current flow through the device when in the "OFF" state.

On hand-head devices, much care is taken to design a circuit that minimuzes leakage to very low MICRO-amps, so batteries don't discharge when the unit is "OFF".

Laptops are much less rigorous in power efficient design, so, even when executing "SHUTDOWN", some circuitry is still alive. Same with desktops. The amount and efficiency of the circuitry varies by design.

[John Gray]

Thanks, both. I was just surprised about how (comparatively) rapidly the laptop batteries seem to run down...