I have covered some battery chargers in past articles. Here is my new article on the topic of how to exactly charge NiMh or NiCd batteries without any protection circuitry. I will let you know the exact profile and charging mechanism in the CC/CV mode because the charging mechanism is a bit different. here is the picture of discharged one with setup:

Usually, these batteries do not pose a risk of explosion or leakage due to their chemistry and potential. The batteries can be charged without protection in the slow charging mode at 0.1C for 10-12 hours. But it is way time-consuming, and we need something fast, that’s why I built a fast charger based on the CN3085 IC. This is a good fast charger that comes with built-in current monitoring and can charge the battery at a rate of 1A. But what if we say these batteries can be charged with a much simpler approach, just using a power supply that supports CC/CV mode?

How the Battery Charging Module Works:

There will be two dominating modes: Constant current and constant voltage charging. These two modes are common for almost every kind of chemistry. CC is nothing but a limited-current mode, and similarly, CV is a limited-voltage mode. Either we can control voltage or current at a time, but not both simultaneously, unless limited by the power supply. First let's see the state of charge of a NiCd/NiMh battery.

• Precharge(Conditioning): If battery voltage is <1V, a low current trickle charge is applied. In this case, only 1/10th of the constant current is applied to the battery for safety purposes. So that battery can be pulled back to the state of charge.
• Constant Current (CC) Mode: When battery voltage rises above 1.2V, the programmed charge current is applied. Constant current mode is simply a current limiter; we limit the maximum charging current to the IC. It will be discussed further in more detail.
• Maintenance (CV) Mode: As the battery approaches 1.65V, the charging current tapers off. This is basically filling the battery from 90% to 100%. This is important for good battery health, but we can also plug out the battery after CC mode is done.

Say if a battery can charge at a max of 5A. The charger is connected with a constant voltage with no current limit. Then as the voltage increases, it drops the current to 5-4-3-2… down to 0A. This is a good method becasue here the current is not fixed 0.1C rating of battery. Bit this is also an old method and is not used much because it requires high currents, which can not be handled by small-scale integrated circuits. Instead of this, all the battery charge limits the current and charges as per the above-given profile.

Advance Method of Charging:

A discharged battery is charged in CC mode, it gets constant current, and the voltage starts increasing. As the voltage increases, the charging current decreases. Say if it is demanding a max of 5A, but we only provide a maximum of 1A. So it charges 5x slowly but at a constant rate of 1A. And when the charging current drops to 1A, it shifts from CC to CV mode.

In the constant current mode, the power supply voltage can be shifted to higher values, say 1.8 to 2V. It will not damage the battery because the voltage is not constant, hence it drops to the same potential as the battery as it gets connected. Once the CC mode is complete, decrease the power supply voltage to 1.6V and charge in CV mode. Now we can see the current going from 1A to 0A in a linear fashion.

So if we charge the battery in CV profile it will take large current and the old 0.1C charging is way slower method. If we know when to terminate the charge we can charge the batteries with CC mode with higher current than 0.1C.

How to Determine a Full Charge:

1. (ΔT/dt) Temperature Termination: 

During charging, NiMH and NiCd batteries convert excess energy into heat once they approach full charge. This causes a rapid rise in temperature. In rate-of-temperature-rise termination, the charger continuously...