Why is charging with Lithium batteries with a small load dangerous?

Question

I'm well aware of the best practices for charging lithium chemistry batteries, and how the charges themselves work.

I've never had a water tight explanation on why having a load on a battery during the CC phase is dangerous.

My understanding is that battery charging works as follows:

1. Battery is charged at constant current, variable voltage
2. When current flow reduces to 10% of capacity, charger switches to constant voltage (full voltage) to top the battery off

I know that if a load is present on the battery during the CC phase, the charger might not neccissarily detect that 10% threshold, my question is, why is this a problem.

Unless I'm mistaken, the charger achieves the constant current by increasing the charging voltage to the max charge voltage - e.g for 4S that would be 16.8V, it doesn't increase it beyond this.

So, you can't actually over charge the battery?

The battery voltage and charger voltage could be slightly out if there was a load on it, but it still wouldn't be over the max voltage as the charger (to my mind) does not do this.

Answer 1

If your charger outputs 1 A in CC mode, and has a 500 mA constant current load connected, then 500 mA goes to battery and 500 mA to the load. So the battery will also charge slower because only half the current goes into it.

When a battery has charged to 4.2 V per cell and the charger switches to CV mode, the battery current starts to drop, but the load will still take 500 mA.

So the charger never sees that the current has dropped to the 100 mA threshold to terminate charging, because there is always a 500 mA load connected.

The battery current may become zero, and it will be floated indefinitely at 4.2 V unless the charger has a safety timeout feature to cut off after certain time if the current has not dropped to a level when to stop charging.

So the load prevents the charger from working as intended.

To recap, battery is not full when CC mode changes to CV mode with 4.2 V on battery. It still takes in current. Battery is full when there is 4.2 V on battery and battery current has dropped to 10% of current and charging should stop here. If charging is not stopped, there still is current to battery, and this will overcharge and damage the battery. And overcharged and damaged lithium batteries are not safe, they can explode or burst into flames.

So if charger should stop when battery current has dropped to 100 mA, any extra current drawn from charger will make the charger to overcharge and damage the battery. If you draw even only 50mA of extra current, it means battery charging is not stopped when battery current is 100mA, battery will be float charged at 4.2V until battery charging current has dropped to 50mA if extra load draws 50mA.

Answer 2

Basically you want to charge the battery until the end of charge condition is reached (voltage >= Vthreshold and current <= Ithreshold) and then end the charge and let the battery rest.

The end of charge voltage is a compromise. It should be as high as possible to stuff as much energy into the battery as possible, but higher voltage makes it age quicker, and past a certain threshold it becomes dangerous. 4.2V is ideal for portable devices because you do want to store as much energy as possible. However, keeping it at 4.2V for an extended period of time will sorten its life: this happens with laptops that are constantly plugged ; after a year or two when you unplug it, battery life will be significantly shortened even though it has had few cycles.

As Justme said the extra load can prevent the charger's end of charge detection from stopping the charge, so it will float the battery at the end of charge voltage for a long time, which will shorten its life.

If you want to use a LiIon battery like a lead-acid battery or a capacitor, and float it to a constant voltage indefinitely while a load is drawing current, simply use lower voltage. So for a battery that reaches end of charge at 4.2V, you can set it to 4.05V. You will lose a bit of capacity, but it will last much longer at this voltage.

Answer 3

When current flow reduces to 10% of capacity, charger switches to constant voltage (full voltage) to top the battery off

That's not right. The charger limits voltage to the battery's peak rating (typically 4.2 V per cell). As a result the current automatically reduces due to resistances in the circuit (including internal cell resistances). During this time ~20% more charge is put into the battery as its internal voltage rises to meet the regulated charger voltage.

Li-ion battery manufacturers generally rate the voltage for maximum safe capacity, which is just below the point of 'overcharge' where it will be damaged. But if held at this voltage for long it will degrade much faster, particuarly at high temperature. Therefore chargers which might be connected continuously often cut off when the current falls to ~10% of the 'constant' charge current, allowing the battery voltage to settle and go down as it is discharged by the device.

Answer 4

The real simple explanation is that in the early days of lithium ion batteries it was discovered empirically that floating the cells at 4.2 V indefinitely sometimes led to cell failure which could include venting and ignition. It then became a best practice to avoid floating at that voltage.

Any set of circumstances which leads to the cell being kept at 4.2 V indefinitely is outside of best practices.

Lowering the float voltage to 4.0 V or 4.1 V probably reduces the hazard (although I have never seen a cell datasheet that endorsed this practice).