1. Electricity from an outlet goes through laptop batteries even when the battery is fully charged (to the best of my knowledge).
  2. This degrades the capacity of the battery (or so I've been told).
  3. It wouldn't be too hard to route the power past the battery when the battery is charged (at least, it seems to me that it wouldn't be too hard).
  4. However, if there are laptops that do that, there aren't many of them (that I know of).

If 1-3 were true, then I would expect 4 to be false; however, to the best of my knowledge, 4 is true. Accordingly, I suspect that at least one of statements 1-4 is false.

Are any of those statements falsse; if so, which are false?

If (1) is false, how does the circuit route the power past the battery if it's fully charged.

If (3) is false, why is it difficult to do so?

Thank you,


  • 1
    \$\begingroup\$ My Lenovo (and I strongly suspect others) has "battery saver mode" where even when plugged in will disconnect the battery when it gets to 100%. It then slowly falls due to self-discharge and will only re-charge back to 100% when it falls below 95%. So it is doing #3. \$\endgroup\$ – Austin May 2 '15 at 16:48
  • \$\begingroup\$ Over-charging lithium ion batteries can cause them to catch fire or even explode, so all battery chargers will stop charging at a certain cell voltage to prevent this. Also, storing a fully charged lithium ion battery for an extended period of time will degrade its capacity. \$\endgroup\$ – alex.forencich May 3 '15 at 8:43
  • \$\begingroup\$ Just curious, why the down vote? Is it not sufficiently related to electrical engineering (not rhetorical)? \$\endgroup\$ – Hal May 3 '15 at 15:19

(1) & (2) sound like consumer mumbo-jumbo. (3) is common practise. (4) How would you know this, other than from people sprouting (1) & (2)? ;)

Phones, laptops & many other battery-powered devices have carefully thought out integration of the power source, battery, and what's being powered. It's all pretty much a 'solved problem'.

Most of these types of applications are Lithium-based batteries, which do NOT like to be 'float/trickle' charged (unlike lead-acid chemistries, for example, which can and should be left on float charge). As a result, Li-based charge management chips DO stop charging the battery once it's full, and don't start a new recharge cycle until capacity drops several %. Rinse & repeat.

This all happens whether or not the device is actually being used - if the laptop is in operation, all power for the laptop will come from the power-source, but the battery charge management will ALSO still be doing its thing, topping up the battery as required. Only when you disconnect from the power-source will the battery be powering the device.

To your first q1: This switching of power-input to both battery-charger & operation of the actual device is literally switched (with MOSFETs) by the charge controller system. These differ in complexity, and depend, for example, on whether the power-source has the capacity to power the device AND charge the battery at the maximum rate possible ("dynamic power control", e.g TI.com's bq24610 Li-Ion charge controller, & many others like it).

To your q2: This does happen, routinely. It's not "hard", you just have to "intelligently" shunt power around with MOSFET switches appropriately. But, everything has its price, and you'll sometimes see a gadget that can't operate AND be recharged at the same time. There's no inherent reason why not, other than a design decision to reduce complexity or cost or space requirements.

You can see this effect in some gadgets - you put them on charge, charge completes, you leave them on charge for "some time", then take it off, and either (a) you're quickly down to 99% or less, or (b) other times it stays at 100% for much longer - this is the few% battery capacity variance between the battery charge controller waiting until capacity has dropped a bit before starting a new recharge cycle, or if it just happened to complete that recharge cycle when you take it off charge, respectively.

  • \$\begingroup\$ Good point. Come to think of it, you're right: I believed (4) because I believed both (1) and (2). Thanks for the facts. \$\endgroup\$ – Hal May 3 '15 at 15:20
  • \$\begingroup\$ @Techydude In my laptop, i have an app BatteryBar installed. It says right now my system is only on A/C. Charging was complete about 10 minutes before. Does this mean that the power is being taken directly from A/C and not battery. If yes, am happy my battery cycles are reduced. \$\endgroup\$ – Rahul Apr 22 '18 at 21:44

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