Electricity has 2 basic properties, voltage (electrical “pressure”) and current (electrical “flow”).
To force current into a battery to charge it, you need to apply a higher *voltage* than the battery already has in it. Otherwise electricity would want to flow *from* the battery *into* the charger, or at least stop flowing, and neither of those is helpful.
But when the battery is near the maximum voltage it can safely be exposed to… the amount of voltage you can use to force current into it is just a *tiny* bit higher than its current voltage, and the flow of electricity into it gets very slow.
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