Charging NiMH: What happens if your varying charge voltage goes too high or too low?

Question

I'm researching NiMH batteries, and I understand that these batteries charge via CC, but what I don't understand is what happens if you have a CC source, but the voltage goes above the max battery cell voltage, or if your voltage is too low to fully charge the cell.

Example 1: I'm charging a single cell NiMH, rated for 1800mAh with 180mA of constant current with a max supply voltage of 5V. What happens when the cell reaches 1.4-1.5V? Will the power supply voltage match the cell voltage?

Example 2: I'm charging a single cell NiMH, rated for 1800mAh with 180mA of constant current this time with a max supply voltage of 1.1V. Once the cell reaches 1.1V, will the cell just be in a state of equilibrium with the power supply and basically just stop charging?

Answer 1

Cells such as NiMH have a charging curve. At 1.1 V, it would be almost completely discharged. A fully-charged cell should be kept at about 1.45 V, so double-check the specs for that charger.

Excursions over ~1.55 V cause gas generation, as water is electrolyzed to hydrogen and oxygen. Some cells have catalysts to slowly recombine 2 H2 + O2 -> 2 H2O, others don't, and must vent the gases, losing electrolyte. At higher voltage, both types are damaged.

Therefore, constant current charge can start (at about A-hour/10 or slower) until the open-circuit voltage across the cell is ~1.45 V. Note that while power is being "pumped" into the cell, the voltage might rise higher, so chargers may open the circuit and pause, then measure cell voltage.

Executive Summary:

It starts as a constant current charge, and ends with a constant voltage.