A mobile phone battery charged to indicated 100%, which is not being charged at the moment, might have the same terminal voltage as if it is being charged at 2A while being at an indicated capacity of 70%.

A deep discharge (terminal voltage below 2.5V) is said to cause a harmful chemical reaction in a lithium-ion battery.

If a battery is shorted out in a way the voltage drops below 2.5V, does that have the same harmful effect?

And does a battery wear out because of terminal voltage or capacity if fully charged?

If it is because of terminal voltage, fast charging stresses the battery in the same way an ordinary full charge does.

  • 1
    \$\begingroup\$ 1.yes 2.yes when charge is full it must float at 0A 3. Fast charge raises temp which accelerates aging \$\endgroup\$ Sep 18, 2018 at 19:37
  • \$\begingroup\$ @TonyEErocketscientist Does aging only accelerate solely because of temperature? \$\endgroup\$
    – neverMind9
    Sep 18, 2018 at 22:09
  • 2
    \$\begingroup\$ of course not. Low voltage duration and extent <3V, and time spent charging above 4V. But 4.2 gives extra capacity. Try the battery university site and please learn. da basics. \$\endgroup\$ Sep 18, 2018 at 22:29
  • \$\begingroup\$ @TonyEErocketscientist I already read some of their articles. Excellently detailed. \$\endgroup\$
    – neverMind9
    Sep 18, 2018 at 22:53

1 Answer 1


Indeed a discharge below 2.5 V will cause stress in many Li-Ion based batteries. Not all Li-Ion based batteries are the same though, some handle a deep discharge better than others.

But discharges below 2.5 V should be avoided and that's why all nearly all batteries used in electronic consumer devices have a battery protection circuit which will simply disconnect the battery when its voltage drops too low.

Usually the same circuit will also protect against overcharging and drawing too much current from the battery.

If a battery is shorted out in a way the voltage drops below 2.5V, does that have the same harmful effect?

1) You should NEVER short a charged Li-Ion based battery.

2) Obviously shorting the battery terminals makes the voltage across them equal to zero. Then the only resistance that limits the current flowing is the internal resistance of the battery. This internal resistance is quite low in most Li-Ion batteries. For sure the current that will flow will be larger than the maximum current that the battery can safely deliver. The current and voltage across this internal resistance means that this internal resistance needs to dissipate a lot of heat. And since the internal resistance is part of the battery's structure it "sits" inside the battery. So where does the heat go: inside the battery. So the battery will get hot. Heat is bad for batteries, even the engineers at Boeing underestimated that. What happens when the battery gets hot: more energy is released.

My point: you should not even be thinking about a shorted Li-Ion battery because that is outside their normal range of operation. Any condition outside the normal range of operation causes extra stress and thus a shorter battery lifetime.

Li-Ion batteries always wear out, it's like a continuous process. To slow this process down as much as possible it is recommended to (dis)charge the battery to about 40 % to 60 % charge level and then disconnect the battery (if possible).

The terminal voltage and battery voltage will be the same when no current is flowing. And that is the best condition to measure the battery voltage to estimate the charge level, when no current is flowing.

Your point about terminal voltage makes no sense. If the charging current causes the terminal voltage to be much higher than the battery's internal voltage then you're probably charging the battery too fast and you should change to a lower charging current.

Fast charging always causes more stress than slow charging but that has nothing to do with terminal voltage. Fast charging is always a compromise between how fast the battery is charged and the lifetime you want from the battery. Also fast charging ideally should only be allowed between charge levels of around 30% to 80 %. Above and below that increased the stress on the battery.

  • \$\begingroup\$ “1) You should NEVER short a charged Li-Ion based battery.” I know that. I just asked hypothetically. \$\endgroup\$
    – neverMind9
    Sep 18, 2018 at 22:05
  • 1
    \$\begingroup\$ Does charging (70%) at 4.3V At 70% charge level the battery voltage is not 4.3 V. You should forget about "terminal voltage", what matters is the battery voltage. Under normal circumstances both voltages are the same. If they're not you're not using the battery properly. An ageing battery will only have a decreased capacity (some if it was lost) and an increased internal resistance. fast charging to 70% equals charging to 100% for battery lifespan That statement makes no sense to me. \$\endgroup\$ Sep 19, 2018 at 6:27
  • 1
    \$\begingroup\$ While charging at high speeds while at 70%, the voltage can be 4.3VOK, so then charging should stop as 4.3 V means that the battery is full. Do you agree? I disagree, what would be my reasoning for disagreeing? \$\endgroup\$ Sep 19, 2018 at 9:18
  • 1
    \$\begingroup\$ Why should charging continue? Because the battery isn't full yet. If you fast charge and that increases the voltage at the battery connections to 4.3 V (then you're doing something wrong but anyway) then you cannot rely on that 4.3 V. So charging should stop for a moment and then the battery voltage should be measured. If it is lower than 4.3 V then charging can continue. \$\endgroup\$ Sep 19, 2018 at 19:23
  • 1
    \$\begingroup\$ does fastcharging to 70% affect the battery life like slowcharging to 100%? There is no clear "do X and get battery life Y" relations, so your question is unanswerable. You would have to test and measure it under controlled conditions. If you want longest battery life: charge slow but often. Keep the charge level between 40% and 80%. But it also depends on many other factors, some of which are chosen for you by the manufacturers of phones and batteries. \$\endgroup\$ Sep 19, 2018 at 19:29

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.