I remember, whenever I got an electric device that could be powered either by battery or grid, I usually also got a transformer, that is positioned between the device an the power plug, while charging.

The transformer would be of such a way, that it would meet certain requirements set by the maker of the device. Recently I got an electric toothbrush that could be recharged, it's description said: "Charging over laptop or the mobile phone charger is fine." - that made me highly suspicious - but according to the Internet it works. But how?

The specifications set by my laptop or cellphone are different and surely different than that of the toothbrush. I am confused now why there are even so precise information what transformer etc. to use and especially what not to do. What do these differences in voltage an ampere do? How do the devices or batteries react to the wrong voltage? How does it affect the device and the charging procedure?

Thus far I found informative: How does charging a phone battery work?

Related is the following, however doesn't go into depth too much :/ Charging devices - Voltage and Amperage

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    \$\begingroup\$ There is much more than just a transformer between the mains outlet and a battery. The specifications set by my laptop or cellphone are different and surely different than that of the toothbrush. What specifications, what difference and how would any difference matter? I guess the toothbrush can charge from a USB port. These are standardized and normally supply 5 V. Your toothbrush is designed for charging from 5 V. A circuit inside takes care of proper charging no matter where the 5 V comes from. You make some wrong conclusions due to lack of understanding. \$\endgroup\$ Mar 7 '18 at 9:50
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    \$\begingroup\$ @Bimpelrekkie, yes, I lack understanding, still I try to build my own understanding of the matter, but keep in mind it might be very flawed, thus I ask others (of who I expect to be far more knowledgeable), so I can correct myself and say straight out where I am wrong. \$\endgroup\$
    – Imago
    Mar 7 '18 at 9:56
  • \$\begingroup\$ Do they all use USB ports? \$\endgroup\$
    – user253751
    Dec 5 '18 at 21:46

"What do these differences in voltage an ampere do"

Charging devices requires different voltages and current for two key reasons:

1) The cell type/chemistry...

i.e. the type of cells used in the device. Modern day household items such as mobile phone, laptops tend to be some form of Li-ion chemistry. The voltage required to charge it is dictated by the potential difference required to move charge carriers within the cells, this differs between different cell chemistries.

2)Battery structure:

Whilst a mobile phone carries a single cell, lapotops carry several cells ( a battery is a multiplle of cells). The cells can be connected in series and/or parallel configuration, in series the votlages of the cells add up, and therfore so does the votage required to charge them. In parralel the current is split between paralel cells (almost equally, if identical cells are used) therefore a higher overall current is needed to charge the cells effectively.

"How do the devices or batteries react to the wrong voltage"

1) This comes down to cell chemistry. If too high a voltage is given it forces the cell to saturate in the number of charge carriers, in the case of Li-ion this can cause metal deposits to build up in the cell to such an extent that the cell short circuits itself. (a cell is made of many layers of positive and negative plates to generate a larger surface area for charge carrier transfer). If the cell short circuits the current drawn by the cell will spike causing a massive amount of power to be drawn by the cell. This increase in power generates heat which destabilises the cell chemistr which can lead to thermal run away i.e. the device gets hotter, draws more current, gets hotter still draws more current... and could burst into flames.

"How does it affect the device and the charging procedure"

well as shown too high a voltage is bad for the device, and therefore when charging battery management systems are used to ensure that the device does not exceed the limit. Too high a current can cause thermal runaway and is also monitored and accounted for. Two schemes of charging exist in terms of li-ion constant voltage and constant current. Constant current is used initially to bring the cell to its peak voltage, and then constant voltage is used to fully charge the cell, the current will drop in this phase as the cell reaches full charge.


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