Basically all I've been able to read and find out until now is that the capacity of lithium ion batteries goes down with cycles. I'm currently programming an arduino to accurately estimate the State of Charge of a 58V+ Li-Ion battery. I've managed to obtain plots of cell capacity against cycles as well as discharge rates at different temperatures and current draws. I am wondering how that capacity reduction with cycles impacts the cells' discharge profile. Is it decreased by a factor so that it still is bounded by the same voltages (4.2V and 2.5V) but discharges more slowly. Or does the discharge rate stay relatively the same but the bounding voltages reduce in range? Or both?
Thank you for your help!
Electronic – Lithium ion battery impact of cycles on discharge rate
batteriescapacitydischargelithium ionvoltage
Related Solutions
The reasoning about the internal resistance is correct. To illustrate it, I found this plot for different discharge rates at low temperatures.
Energizer L91 Lithium Battery Capacity and Temperature Curve. Source.
There is another discussion about temperature and discharge rate here.
Yes, it is normal for the resting voltage to drop below the charging voltage.
When charging a Lithium-Iron-Phosphate cell the voltage will gradually rise as the stored charge increases, then rapidly climb to 3.6V and over as it reaches full charge. This rapid voltage rise is a result of the battery running out of places to store charge, and will not be sustained when it is resting.
In a series pack one cell may reach full charge sooner than the others, and then its voltage could rise to over 4V before the battery reaches peak voltage. To prevent this you should put a 'balancer' circuit across each cell to bypass the current when the cell reaches 3.6V. When all cells reach 3.6V you can stop charging because the pack will be almost full. Alternatively you can gradually reduce charging current to zero while staying at peak voltage, which might get an extra 10% or so into the battery.
In the example plot below the green line is charging current, the red line is voltage across the cell, and the blue line is absorbed charge. You can see that very little charge is accepted after reaching 3.6V, and the resting voltage soon drops to 3.4V after charging current is removed.
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Best Answer
Unless you are short circuiting the battery discharge rate is dictated by the load connected rather than the battery cells themselves.
When a cells capacity has reduced the full and empty voltages remain the same, what changes is the maximum amount of charge stored within the cells. They will take less charge to reach the maximum charging voltage and so have less charge in them to discharge.
Also keep in mind that for Li-ions voltage is significantly impacted by temperature.