I know common VOOC charger have fast charging rate of 18W(9V/2A). But how it possible even current is only 2A. I mean 2A current have only stored charge 2000mAh(2A*1h). How changing voltage can charge battery faster? On other side Oneplus Dash charger supports upto 20W(5V/4A). This makes sense that higher current can charge battery faster. But again how can just 5V can charge battery to 4A rating?

Thnaks for answer/comment. Corrections and suggestions are welcome.

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    \$\begingroup\$ what kind of a battery are you talking about? ... you should really include all information in your question. \$\endgroup\$ – jsotola Dec 28 '17 at 3:37
  • \$\begingroup\$ Lithium ion batteries. \$\endgroup\$ – user3785133 Dec 28 '17 at 4:27

Li-ion cell charging voltage ranges from ~3.7V to 4.2V. 18W/4.2V = 4.29A. The charger has a DC/DC voltage converter circuit which pulses current through an inductor to 'transform' the voltage down and current up. This process is typically 85~90% efficient, so it should be able to deliver 4A for most of the charge cycle.

But again how can just 5V can charge battery to 4A rating?

20W/4.2V = 4.76A, or ~4A at 85% efficiency. The OPPO VOOC (Voltage Open Loop Multi-step Constant-Current Charging) 'Flash Charging' system claims to charge certain OPPO devices from 0 to 75% in 30 minutes, implying a capacity of ~2.7Ah.

At 4A a 4Ah cell would only get 50% charge in 30 minutes. Charging to 75% would take (4Ah x 75%) / 4A = 45 minutes.

At 75% charge the cell has probably reached 4.2V, then current must be reduced to avoid damaging the battery. Getting The last 25% into the cell may take another 20-30 minutes.

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