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When I charge my battery at a constant voltage and current, the energy supplied is constant throughout the charging process. If it takes 1h to charge to 80%, the next 20% takes another 1h.

Since I supplied the same amount of electrical energy during the first and second one-hours, is a significant part of energy wasted during the second one-hour?

I understand the battery internal resistance increases by the degree of charge. Does it mean we waste a significant of the electrical energy supplied in the form of heat?

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    \$\begingroup\$ How are you measuring "%age complete" and "constant voltage and current"? Lots of charging systems reduce the charge current towards the end. And "fuel gauge" systems are notoriously inaccurate. \$\endgroup\$ – pjc50 Mar 23 '18 at 13:33
  • \$\begingroup\$ @pjc50 good point indeed. Assume we are using a plain power supply with constant voltage and current. How will be the charging behavior? \$\endgroup\$ – Yoria Mar 23 '18 at 13:39
  • \$\begingroup\$ you do waste some in heat, how much is "significant" is an opinion. \$\endgroup\$ – dandavis Mar 24 '18 at 17:27
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Since I supplied the same amount of electrical energy during the first and second one-hours, is a significant part of energy wasted during the second one-hour?

Your assumptions are wrong. Take a Lithium Ion battery for instance: -

enter image description here

Picture source.

In stage 1 capacity rises linearly with time because the current is constant. When stage 2 occurs (the terminal voltage reaches a set-point) capacity rises more slowly and tails off just as the charging current tails off.

In other words in stage 1 there is roughly a constant wattage pumped into the battery but during stage 2 that wattage gradually reduces as current falls.

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