How does a charger know Ni-MH/Ni-CD or Li-ion batteries get fully charged?

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How does a charger know Ni-MH/Ni-CD or Li-ion batteries get fully charged?

The nominal voltage of Ni-MH battery is 1.2V. One Li-ion cell is about 3.7V (4.2V full). Ni-MH batteries are somewhat more tolerant of exceeding their ‘maximum’ applied voltage during charging; While, lithium is not tolerant of over-voltage at all. As these cells’ different characteristics, there are different ways of a charger tells whether a Ni-MH/Ni-CD battery or a Li-ion battery is fully charged.

Because Ni-MH/Ni-CD battery doesn’t have a specific full-charged voltage, you can’t use a fixed voltage to judge whether the battery is charged fully or not. And some people use the thermal detection, as the Ni-MH/Ni-CD battery will quickly heat up when it’s fully charged. But detecting the end of charge of a NiMH cell by cell temperature rising also poses difficulties. The cell will often feel warm when it is being overcharged, sometimes the temperature rise can be difficult to assess accurately as the centre of the cell will be much hotter than the exterior.

The favoured method for detecting end of charge of a Ni-MH/Ni-CD cell is to use the 0ΔV and -ΔV method. This method detects a drop in voltage that appears as the cell becomes full charged, and is not affected by other factors such as temperature and charging current. When the battery voltage drops slightly (just fully charged), the charger immediately stops charging. Such as xtar VC8 uses this method to judge the fully charge of Ni-MH/Ni-CD batteries.

As for the Li-ion battery, it has accurate cut-off voltage 4.2V (+-0.05V). Based on its characteristics, the better charging way for a charger is to adopt 3-stage charging methods: TC wake-up, CC fast-charge and CV fully charge. When the battery is almost fully charged, the charger starts CV charging to make sure the battery is fully charged and less damaged. Also take xtar VC8 for example, when it charges Li-ion batteries, it won’t make over-charge with 3-stage charging.

In short, a professional charger indicates a fully-charged Ni-MH/Ni-CD battery via 0ΔV and -ΔV method, and a fully-charged Li-ion battery with the constant voltage 4.2V (+-0.05V). For some XTAR chargers supporting both Li-ion and Ni-MH/Ni-CD batteries, a few customers told that the charger shows 100% charged volume when charging Ni-MH batteries, but in fact the batteries were not full. They mistakely thought this charger was broken. As explained above, there are different methods for the charger to judge the fully charge of Ni-MH/Ni-CD and Li-ion batteries, when charging Ni-MH/Ni-CD batteries, the percentage data display on screen is a reference value based on the voltage, not the actual battery capacity. As our charger uses different way to judge the fully charging of Li-ion and Ni-MH batteries, so please don’t remove the Ni-MH battery until you see “FULL” on the screen. That means the battery got fully charged.