18650 CHARGE CYCLE QUESTIONS

[m3vuv]

Hi all,when manufactures quote lion battery cell life in charge cycles,does a charge from say 50% to 75% count as a charge cycle,ie am i better off charging my batterys often to top them up or use them untill the lvc then charge?

[Kleinstein]

The charge cycle are usually from "empty" to full, so with essentially the nominal capacity at the start and than with fixed votlage limits. At least this is how it should be.

As far As I understand it the cell suffer from getting very full and getting very empty. They kind of adjust the upper voltage limit to still get the wanted number of cycle. Cycling from 50% to 75% is likely much less stress, but only using 75% to 100% or 0-25% cycles may be nearly as bad as doing full cycles, at least for the parts effected by the extremes.
As much as I understand it is better to no regularly charge / discharge all the way to the extremes. So more like aim for a 20% to 90% cycle, though at the low end one usually does not have so much control as one may need a nearly full charge at some points. So one may have to recharge  sometimes even if still at 50% or 70%.

[Siwastaja]

Cycle life is regarding full cycles between 100% and 0%, using the standard charging and discharging conditions like current, stopping condition etc. specified in the datasheet.

Partial cycles have the tendency of wearing out the cell much less than expected from the amount of charge transferred. For example, if a cell is rated for 500 cycles 100%-0%, using it only between 70%-20% (i.e.: depth of discharge is 50% instead of 100%) increases number of such cycles way beyond the expected 1000 cycles, can be 5000-10000 cycles for example.

There are other factors too, for maximum cycle life avoid charging at low temperatures (optimum would be somewhere around 30degC), and limit charging current near full. For example, if you have to fast charge at 0.8C, instead of running at 0.8C all the time, charge at 0.9C until 4.0V then at 0.6C. Small optimizations like that can have big difference.

But all the exact details depend on the actual cell pretty much. Even given the same cathode chemistry, different manufacturers have their own different secret sauces which behave differently.

[wraper]

To maximize life, it would be preferable to not discharge below 20% and not charge above 80%. Charging above 85-90% is worse than discharging to 0 (if you don't leave it in discharged state for long).