So we have a charger outputting V volts and I Amperes and two rechargable batteries of the same capacity Q coulombs but different emf's V1&V2.

Which battery would charge faster?


So since the charge stored in both the batteries is Q C, and the battery provides a current of I, which is ICsec so the time taken should be the same in both the cases. (QIsec).

However the power provided by the adapter in both the cases is the same and the energy stored in the battery is different in both the cases so this answer is not possible.

Where did I go wrong?

  • \$\begingroup\$ Are you sure power provided by the adaptor is the same in both cases? When charging batteries, charging current tends to reduce gradually base on the cell voltage and charge voltage. \$\endgroup\$
    – Puffafish
    Commented Oct 17, 2018 at 13:59
  • \$\begingroup\$ So the adapter current output varies with different batteries? \$\endgroup\$ Commented Oct 17, 2018 at 14:05
  • \$\begingroup\$ No, the current varies with charged state of the battery. Search for "constant current charging" and lots of websites will go into lots of detail for you. If I get the chance, I'll try and do an answer for you later. \$\endgroup\$
    – Puffafish
    Commented Oct 17, 2018 at 14:07

1 Answer 1


The power provided is not the same, V1 and V2 are different, so P=I.V1 is not equal to P=I.V2.

  • \$\begingroup\$ No, the power provided by the adaptor is it's output current * output voltage. \$\endgroup\$ Commented Oct 17, 2018 at 14:06
  • \$\begingroup\$ Force the battery terminal voltage to the same value for two different EMF batteries, and the current won't be the same. one parameter is a function of the other, depending on which you control. \$\endgroup\$
    – Phil G
    Commented Oct 17, 2018 at 14:12

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