How is it that the charger has a 1A power supply, yet the manufacturer of the charger guarantees charging 4 batteries also at a current of 1A – after all, that totals 4A?!

However, not necessarily, as the output current is a value that can only be compared in situations where the output voltage of a given power source is the same (or very similar). The value that we can always compare between different power sources is power - based on it, we can say that one power supply is stronger than another. 

First, let's recall what the formula for the power of a device looks like:

P (power) = U (voltage) * I (current)

Why is this so important? By comparing only the values of current, one can arrive at very erroneous/abstract conclusions. For example – let's take a hammer drill with a power of 500W powered from a 230V network. Such a drill requires just over 2A of current to operate. 2A of current is relatively little – a typical Ni-MH AA battery – e.g. the popular Eneloop battery can easily deliver around 5A of output current.

Does this mean that we can power a hammer drill with one small battery? – after all, the current “matches”.

Of course, we cannot power a device with a power of 500W from one such battery - completely ignoring the different output voltage of such a battery, the capabilities of a given power source are not determined by the current, but by the mentioned power, which is the product of voltage and output current.

We will explain this using our example with the charger:

The charger is supposed to charge 4 AA/AAA Ni-MH 1.2V batteries with a charging current of 1A, and it comes with a 1A power supply with a voltage of 12V. Is such a power supply sufficient to charge each of the 4 cells with a current of up to 1A?

The answer is – it is sufficient and with a considerable margin.

The 12V / 1A power supply has a power of 12V x 1A = 12W.

The charging voltage of a typical Ni-MH battery usually does not exceed 1.6V.

Therefore, to charge 1 piece of battery with a current of 1A, we need power: 1.6V x 1A = 1.6W. To charge 4 pieces simultaneously, our demand will increase 4-fold – we will need: 1.6W x 4 = 6.4W.

Thus summarizing – the charger power supply has a power of 12W, while to charge 4 batteries at maximum efficiency requires ~6.4W. Of course, the charging process is not perfectly efficient and there are some losses during the operation of the charger, however, in such a situation, nearly 2 times the power margin of the power supply is completely sufficient for any good charger.

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