Why does a charging IC keep a low voltage while connected to a used battery?

Question

I have a set of devices using Li-Ion batteries and featuring a charging IC (LTH7). Charging works normally on recent batteries, but does not on used ones.

Knowing that chargers initially try to maintain a constant current (CC phase), I was assuming to have a very low or no charging current for those used batteries, and thus expecting the charger to respond like if no battery were connected: increasing Vbat until the max value (4.2V).

Instead of that, I observe that Vbat remains constant around 0.1V. Any idea why the charging IC does not increase the battery voltage ?

Answer 1

It is unsafe to rapidly charge a deeply discharged li-ion battery, and so many li-ion charging chips will use a pre-set trickle charge if the battery is below a certain voltage. In the datasheet this is usually indicated by V_TRIKL and I_TRIKL.

As another safety precaution, most charging ICs will refuse to charge a battery which is lower than a certain voltage. This is called under voltage lockout. I wasn't able to find what that is for the LTH7 but I would suspect it is probably around 2.0V. In any case, I definitely would regard a 0.1V battery voltage to trigger the lockout.

One way that usually works to "revive" deeply discharged cells is to charge them using a constant-current power supply at a very low current. Set your power supply to 3V and the current limit to a low value like 10mA (you need a current limiting power supply). A lot of times the voltage on the cell will rapidly rise to a voltage around 3V at which point you can put it on a normal charger.