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My question is if lithium-ion batteries just lose capacity over time or if they also become more wasteful. From a practical perspective, can you easily get around loss of capacity in older batteries/devices by just carrying a powerpack or would an older battery also use up more power in a certain amount of time, thus draining the powerpack faster?

This is coming from a consumer perspective but I hope it's still interesting enough to be answered here.

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up vote 16 down vote accepted

The primary aging effect in a Lithium-ion battery is increased internal resistance (caused by oxidation of the plates). This doesn't affect the Ah capacity, but it does reduce voltage and waste power at high current. Since voltage also drops as the battery discharges, the increased resistance causes it to reach cutoff voltage earlier and so reduces its effective capacity.

An old lithium-ion battery which is not powerful enough to run the device it was designed for may still be useful in a lower current application. General Motors and Nissan are reusing old electric car batteries as stationary storage for homes and businesses. At the lower current drain required these 'worn out' batteries can still deliver more than 80% capacity.

Using a power pack on an appliance with an old Li-ion battery will not use any more power than normal. The power pack will simply take over from the internal battery to supply the power the device needs. This is not the case with with old Nicad and NimH batteries, which tend to become leaky as they age and require constant topping up, which does waste power.

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Thanks for your answer! You say the powerpack simply takes over, but as I understand it, in the case of for example a smartphone the powerpack still charges the internal battery and the internal battery still supplies the device. Or am I wrong and can the powerpack supply the device directly when the powerpack is plugged in while using the device? – Sebastiaan van den Broek Jan 7 at 1:53
    
Also you say it wastes power at high current. Does that not mean that not all the power provided by the powerpack is used effectively? – Sebastiaan van den Broek Jan 7 at 1:57
    
If the power pack is charging the battery then current is going into the battery, not out of it. The power pack must also supply whatever excess current the phone needs to operate. Once the battery is fully charged it will accept no more, so all the current from the powerpack goes towards running the phone. – Bruce Abbott Jan 7 at 2:00
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"Does that not mean that not all the power provided by the powerpack is used effectively?" - If the powerpack is used only to charge the internal battery then yes, the power is not being used as effectively as it would with a fresh battery (though the difference will only be significant when the the phone is drawing high current, eg. during a call, not on standby when it is drawing low current). If the phone is being operated from the power pack (which is what I assume you meant by 'carrying a powerpack') then the internal battery doesn't waste any power because it isn't being used. – Bruce Abbott Jan 7 at 2:16
    
Thanks, maybe it's worth editing that into the answer. I accepted it anyway :) – Sebastiaan van den Broek Jan 7 at 4:00

Yes, both effects take place. As Bruce mentions, because of the aging effect, the internal resistance of the batteries increases. The internal resistance is the main cause of "wasted" power (converts it to heat) and loss of effective capacity, so as it increases, more power is wasted and capacity reduced.
With respect to a power pack or power bank, think of them as just a "charger" for the battery in question. However, if the device is on, then the charger has to supply the current demanded by the device and the current demanded by the battery, until the battery is charged to the operating voltage of the device.

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