# Using constant current and constant voltage charging to fully charge a battery

I did not find a thread which properly answers this question so I hope by asking this here I can gain some understanding and it would be here for later use,

From what I understand, Constant current charging is when you fix the current supplied to a battery and the voltage would vary depending on the battery. Constant Voltage charging is when you connect a certain Voltage across the terminals of a battery, the OCV (open circuit voltage).

My Question lies in trying to use a combination of the two to try to fully charge a battery. Let me explain.

Constant Current Charging

I have been able to replicate constant current charging by connecting a 5V usb cable as the input into an SMPS. The SMPS is connected to an arduino which controls the current going into the battery. This is done by setting a reference current and then a PID controller keep the current around this reference current.

This was for a LiFePO4 battery which was charged at a constant current.

The charge cycle was controlled by the following states: Charge until 3.6V, rest for some seconds, discharge until 2.5 V, rest for some seconds and then repeat the cycle.

If you notice from the graph, after the battery is charged to 3.6V and enters a stage of rest where it is neither charge or discharging, it immediately falls to around 3.4V ( this can be seen around 3900 seconds). I have contributed this to ESR and parasitic capacitance in the battery.

My question

I have read previously that it is possible to fully charge a battery to 3.6V by switching to constant voltage charging after 3.4V and just watching it until the battery draws very small amounts of current which would mean it is fully charged. Is this true?

The reason I have not tried this is because I do not have a constant voltage charging source. If I was somehow able to implement this, what constant voltage should be chosen?

Additionally, is the reason this drop occurs due to ESR and capacitance and if this is the case wouldn't this also happen with constant voltage charging?

Battery that I am charging: https://www.ampsplus.co.uk/ampsplus-14500-3-2v-500mah-battery-button

From what I understand, Constant current charging is when you fix the current supplied to a battery and the voltage would vary depending on the battery. Constant Voltage charging is when you connect a certain Voltage across the terminals of a battery

Not quite. True 'Constant Voltage' charging would risk blowing up the battery or charger, because there is no defined current limit. Actual 'Constant Voltage' chargers are also 'Constant Current' as well, in other words they have simultaneous current and voltage limiting.

I have read previously that it is possible to fully charge a battery to 3.6V by switching to constant voltage charging after 3.4V and just watching it until the battery draws very small amounts of current which would mean it is fully charged. Is this true?

No, that makes no sense. If 'switched' to Constant Voltage at 3.4 V it would either stay at 3.4 V, or instantly jump to 3.6 V (possibly drawing a very large current) if it raised the voltage to that value.

A practical LiFePO4 charger would simply be set for a voltage output of 3.6 V, simultaneously current limited to the desired charging current. In use the battery voltage would keep going up until it reached 3.6 V, then the current would decrease exponentially until the charger cut off at eg. 10% of the current limit. During this time the charger would hold its output voltage down to 3.6 V while the internal battery voltage caught up, with current being determined by the voltage drop across the battery's internal resistance.

BTW your graph does not show a time where the voltage stops rising during charging, suggesting that it never went into the 'Constant Voltage' phase and therefore did not receive a full charge (though it may have been close to it).

I have read previously that it is possible to fully charge a battery to 3.6V by switching to constant voltage charging after 3.4V and just watching it until the battery draws very small amounts of current which would mean it is fully charged. Is this true?

Yes that is true.

If I was somehow able to implement this, what constant voltage should be chosen?

The voltage for CV charging is usually whatever the 100% charged voltage of the battery is.

Additionally, is the reason this drop occurs due to ESR and capacitance and if this is the case wouldn't this also happen with constant voltage charging?

It will happen in both modes, but since the current is lower in the CV phase, the voltage drop due to ESR is also lower, and it approaches zero as the battery is fully charged.

How does one make sure that I get close to 100% SOC, without over charging and killing the battery?

Overcharging the battery would require you to apply a voltage higher than the battery produces when fully charged. Constant current charging can potentially do this, which is why you switch to constant voltage, which does not.

It may be helpful to consider that a constant current charger, when restricted to some maximum safe voltage will naturally start out in CC mode and then switch to CV mode as the battery becomes nearly charged. CV mode is essentially just the rate of charge approaching zero and then stopping when the battery is fully charged.

• How does one make sure that I get close to 100% SOC, without over charging and killing the battery? do you just watch the current and stop charging when it becomes a small value?
– fred
Jun 2, 2021 at 2:58
• @fred See edit. Jun 2, 2021 at 3:31