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Charger specs

Charger type : USB Charger

Voltage : 5V

Amp : 2 Amp

Freq : 60 Hz

Battery specs Type : Li-ion

Power : 7.4 Wh

Current : 2 Ah

Discharge Voltage : 3.7 V

Charging limit voltage : 4.2 V

Charging limit current : Not specified

Execution Criteria : GB/T18287-2000

Situation :

My present charger deliver 0.750 Amps and it takes 4 hours to charge a mobile phone.

I plan to buy a tablet charger which is fullly compatible with my mobile phone, which delivers 2 Amp.

I plan to do this expecting that the charging time will be 2.66 faster (2 / 0.75) times faster. So, I expect the mobile to be charged in 2 hour 40 mins.

Known issue :

Most electronic devices has a circuit which limits the maximum current that is allowed into the battery. (I am not aware if my phone has this)

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    \$\begingroup\$ Most USB charged devices will not pull more current than the USB spec limit until they decide they are connected to a high current charger, rather than a 500ma limit legacy USB port. How they do that varies - often bias resistors on the data lines, but a number of tablets have used custom connectors with extra pins. \$\endgroup\$
    – Chris Stratton
    Commented Sep 29, 2013 at 15:36

2 Answers 2

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Frankly, current is PULLED, not PUSHED. If your 2A supply is connected to a battery that charges at 0.7A, it will still charge at 0.7A, regardless of the power supply capacity.

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    \$\begingroup\$ Well said Passerby, this is the confusion with most of the engineers +1 \$\endgroup\$
    – AKR
    Commented Sep 29, 2013 at 9:51
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If a system with a LiIon battery is charged from 5V then the device will control the charging current (as %V is above what is needed or safe so there MUST be an additional charger circuit internal to the device.

If the charger outputs 4.2V when lightly loaded it is probably a charger in its own right - using a larger one may lead to more charge current - regardless of whether the drbvice can tolerate more current.

Usually LiIon are specified at 1A max charge rate per 1000 Mhh capacity.
Some few are rated at less - maybe 500 mAh and a few at higher.

Modern "USB" phone chargers which have a USB plug as their output and which plug into a device's USB port are chargers in their own right. A charger capable of charging a phone at 5V, 2A will almost certainly also charge a tablet OK at 5V, 2A. Some devices have non standard ways of 'talking' to their chargers and may need special attention to get them to work - usually just a few resistors on the interface.

As above, a system with a 2Ah battery will usually allow 2A max charge current. If your power supply supports up to 2A charging rate then at lower max charge rates time to charge will typically increase by a factor of LESS THAN Imax_possible/Imax_charger.
eg a 750 mA charger will take LESS than 2000/750 = <= 2.66 x 2A rate.
The reason that it will be less is that the initial part of the charge cycle is constant current and time is proportional to current, but the final part if at constant voltage with declining voltage, and the lower current charger may continue charging at its full rate during some or even most of this CV period.
Example: Assume a device with a 2 Ah battery that charges at 2A until Vbat = 4.2V then at a constant 4.2V until I charge drops to 25% of Imax = 500 mA
If a 500 mA supply is used instead then it may charge at full current until the current starts to fall, when charging is complete.

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