Charging lithium ion batteries.

[steinar96]

I was wondering if during the CC phase they raised the voltage of the lithium cells above 4.2V to keep the current constant. Or is 4.2V the absolute maximum voltage across them. Info on this is very vaque.

---------- Post added at 16:19 ---------- Previous post was at 15:59 ----------

I suspect that during the CC they put whatever voltage is required to get the current sought after since the cells clamp the output voltage anyway to the sum of their internal voltages so in effect the voltage across them is never higher then 4.2V unless they have started charging beyond specifications.

 

[RCinFLA]

LiIon polymer cells should not be taken above 4.2 vdc. LiIon charger is constant current with voltage limiter to cap the voltage at 4.2 vdc per cell. There is also a minimum voltage of about 2.8-3.0 vdc before full constant current charge is applied.

If battery is less then 1.5vdc, it is bad and should not be charged. Between 1.5vdc and 2.8vdc the charge current is limited to about 0.05 C rate.

LiIon batteries do not take overcharge which creates problems with series connected cells. As cell state of charge diverge over time between series connected cells, some cell may reach 4.2 vdc point before other cells in the series stack. Series battery packs have bleeder circuits that help keep individual cells from getting overcharged before other cells in the battery pack have achieved full charge.

Charge is terminated when cell has reached 4.2 vdc and current has tappered off to about 0.02 C to 0.05C current level. Cell charge current should not be greater then 0.5C to 0.7C rate. It takes about 2.5 hours to fully charge a LiIon polymer battery. You can achieve about 85% charge within an hour.

Cells should not be allowed to be discharged below 2.1 vdc to 2.8 vdc, depending on discharge current rate, to prevent damage to cell. Cellphone and laptop battery packs have a series MOSFET to disconnect battery when voltage drops below this level to avoid permanent damage to cells.

 

[steinar96]

I've inspected the charging instruction for these batteries. When trying it out the recommended constant current cannot be achieved without exceeding 4.2V per cell during the early charging stages. Are you sure about this ?. During charging, even when the voltage applied across it is greater then 4.2V the actually voltage measured across it is only the "state voltage" since the cell clamps the charging voltage to it's own. The only effect of increasing the charging voltage is higher charging current.

I am however well aware of that eventually the cell state voltage will rise above 4.2V if the charging voltage is above 4.2V for long enough.

But in light of this i would suspect that the charging voltage can exceed 4.2 per cell to CC charge the cell about 70% since the measured voltage across them never exceeds 4.2V. After which normal CV charging would take over to finish the charging process in order not to overcharge the cell

 

[Audioguru]

Rechargeable lithium battery cells are never charged above 4.20V. The cell does not limit the voltage, maybe your cells have a protection circuit that limits the voltage.
Read about it here:

 

[steinar96]

I'm not talking about charging them above 4.2V. I'm talking about applying a voltage higher then 4.2V per cell during the initial constant current phase in order to charge them at the rated maximum charge current. That is what i'm talking about, and what most articles don't seem to clarify.

In order to reach 0.7C or even 1C constant current a voltage higher then 4.2V per cell needs to be applied. I am ofcourse well aware of that charging them above 4.2V in terms of their own voltage is harmful to the cells.

 

[Audioguru]

Your constat current charging circuit must never allow the Lithium rechargeable cell's voltage to exceed 4.20V.
It is as simple as that.

 

[steinar96]

Ok, thanks for clarifying that. I assume you are meaning that at no time during the charging process, even temperoraly the voltage applied across the cell cannot exceed 4.2V even though the internal voltage of the cell has not yet exceeded 4.2V

 

[Audioguru]

Ok, thanks for clarifying that. I assume you are meaning that at no time during the charging process, even temperoraly the voltage applied across the cell cannot exceed 4.2V even though the internal voltage of the cell has not yet exceeded 4.2V

The internal resistance of a rechargeable lithium cell is extremely low if it is healthy. But when it is worn then its internal resistance rises so its charging and max discharging currents become less. Since the internal resistance is so low then the internal and external terminal voltages are the same. So do not exceed 4.20V when charging a cell.