1
\$\begingroup\$

As much as I know if your li-ion cell is providing 4.20V then it is fully charged one. But when I bought a pair of new ones they were at 3.9V each and then I charged both using TP4056 module and they both started showing fully charged after a few hours and then I measured the voltage and found to be 4.11V and 4.16V but when I plugged them again in the module for further charging they are not charging and the module also indicating they are fully charged.

So is it possible that a fully charged li-ion can give these voltages or are they not fully charged?

\$\endgroup\$
1
  • 1
    \$\begingroup\$ That is a component tolerance issue you are experiencing and TP4056 was never designed for accuracy. If you take a look at a li-ion battery charge curve, you will notice there is very little energy stored between 4.11 and 4.2 V. Don't sweat it. \$\endgroup\$ – winny Feb 3 at 7:50
1
\$\begingroup\$

There are three main reasons why a 'fully charged' cell does not stay at 4.2 V after charging. Firstly the cell has internal resistance, so any current going into it will raise the terminal voltage, while any current drawn from it will lower the voltage. Secondly, most chargers do not put in a full charge. Thirdly, voltage will gradually drop after charging as the charge 'settles' into the cell.

Here is the actual charging profile of a typical Li-ion cell:-

enter image description here

The voltage (red line) increases faster in the latter part of the 'constant current' phase, until it hits 4.2 V. Then the current (green line) must be reduced to prevent the voltage rising any further (the 'constant voltage' phase).

When current drops to ~10% of the 'constant current' rate, the charger shuts off (yellow line). At this point the battery is not quite fully charged (had the charger not cut off it could have pushed a few more mA into the cell). The voltage immediately drops slightly because there is no longer any current flowing through the cell's internal resistance.

Over the next hour the voltage drops slowly as charge 'settles' inside the cell. After a few hours the voltage should stabilize and only go down very slowly due to internal leakage.

How close the cell gets to full charge depends on the charger settings (charging current, cutoff %, peak voltage) and the internal resistance of the cell. 'High capacity' cells usually have higher resistance and so have to be charged at a lower current to avoid excessive voltage drop. As a li-ion cell ages its internal resistance increases, causing greater voltage drop and less charge put in before cut off - unless a lower charging current is set to compensate for the aging.

New cells typically have a 'resting' voltage after charging of ~ 4.16-4.18 V. A lower than expected resting voltage could indicate an old or faulty cell, excessive charging current, or a charger that is cutting off too early or not putting out a full 4.2 V.

\$\endgroup\$
1
\$\begingroup\$

You must take into account the charging (and recharging) profile and charging cycle. For instance in the following profile given on Digi-Key's site:

enter image description here

the hysteresis is clear where you can see the lower threshold. So it will wait to discharge till 3.9v (for this particular profile) before recharging.

\$\endgroup\$
1
\$\begingroup\$

Voltage is not a very accurate way to determine how full a battery is, especially a flat discharge curve battery like a Li-Ion. The most accurate ways are super inconvenient though. Chargers also don't use voltage as the sole method of determining when a battery is full. They use a combination of methods.

\$\endgroup\$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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