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The FastCharge 2.0 standard has these charging specifications in the exact same charger (a VoltCraft car charger):

  • 5 V up to 2.0 A (10 W max.)
  • 7 V up to 2.5 A (17.5 W max.)
  • 9 V up to 2.0 A (18 W max.)
  • 12 V up to 1.5 A (18 W max.)

So the 7 V, 9 V and 12 V modes deliver pretty much the same power. The phone automatically adjusts to the supplied voltage. Some smartphones such as UlePhone Armor2 support 9V2A and 12V×1.5A.

For cables with higher resistance (i.e. long cables), the phone can 'request an increase of power' from the charger to compensate for the increased cable losses. So it's, "I want 1.66A, please increase the voltage until 1.66A is reached, I will deal with resistance control".

My question is:

When is utilizing 7 V × 2.5 A, when 9 V × 2 A and when 12 V × 1.5 A more suitable? The latter two deliver the same Wattage.

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    \$\begingroup\$ different battery voltages \$\endgroup\$ – jsotola Jan 17 '18 at 7:52
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    \$\begingroup\$ @jsotola unlikely, this is mostly used in the domain of phones, which as far as I know use a single cell. \$\endgroup\$ – Arsenal Jan 17 '18 at 7:58
  • \$\begingroup\$ @jsotola The UlePhone Armor 2 supports both 9V2A and 12V×1.5A. When does it use 9V, when 12V? Both equal 18W. \$\endgroup\$ – neverMind9 Jan 17 '18 at 8:00
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    \$\begingroup\$ I guess it could switch to 12V and 1.5A if the voltage drops too much at 9V and 2A (because of resistance), but I'm only guessing. \$\endgroup\$ – Arsenal Jan 17 '18 at 8:03
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    \$\begingroup\$ Does every fast charger have to support every combination of voltage and current? Maybe some power sources are not capable of delivering 2A at 9V, but they are capable of delivering 1.5A at 12V. Or the other way around. So the optimal thing for the power consumer is to support as many configurations as possible. \$\endgroup\$ – mkeith Jan 17 '18 at 8:21
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So it's, "I want 1.66A, please increase the voltage until 1.66A is reached, I will deal with resistance control".

No, it doesn't work like that. To be able to efficiently (fast) charge a single cell (3.6 - 4.2 V) battery from more than 5 V, a switching charging controller is needed.

A switching charging controller is basically a switching converter (a DCDC converter) where voltage and current are controlled using feedback and a microcontroller to charge the battery.

Only if that switching charging controller finds that it is unable to provide a certain charging current to the battery, due to the input voltage being too low, then it will request a higher input voltage.

Due to the switching nature of a switching controller, the current at the input side (coming from the adapter) and the current charging the battery, are not the same. A 100% efficient switching charger can for example use 9 V at 1 A and using that charge a battery at 4 V at 2.25 A. Both equate to 9 Watt of power. Note how "only" 1A flows through the cable while charging happens with 2.25 A !

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  • \$\begingroup\$ Thanks. But why do some phones support both then? When do they use 9V and when 12V? (question text updated) \$\endgroup\$ – neverMind9 Jan 17 '18 at 11:18
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    \$\begingroup\$ I do not know why that is so. My guess is that it allows for some flexibility regarding the electronics in the phone, if a manufacturer does not want to support 12 V (yet) they can limit the voltage to 9 V and still support QuickCharge 2.0. Supporting 12 V will require a circuit supporting that. \$\endgroup\$ – Bimpelrekkie Jan 17 '18 at 11:26

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