How to prolong a lithium battery life mathematically?

Question

How to use a lithium battery so that it gives as much energy as possible in total during all battery discharge events from the moment of purchase until the moment when its performance drops to some unacceptable level?

What percentage of discharge is most comfortable for a battery in heavy duty conditions? Are there similar metrics like C- and E-notation but for life? I mean, discharging at 1C gives significantly less harm at a charge level of 90% than 10%. I can easily imagine a device that measures the amperage of a battery and shows me the C number, because 1C means the battery will charge/discharge in 1 hour if the amperage is always the same.

But I need a device that tells how fast the battery softens, depending not on the C notation, but on the C notation regarding the battery level.

Answer 1

This is a quick 'almost midnight and things to do' post.
Ask after a week+ if you don't get better answers and I my add to it. ALL % answers are out of head guesstimates but you can follow these up once you see the principles involved.

Max continuous C rate and peak rate and related discharge time are stated by good manufacturers. SOME may give equivalent Ah at different discharge rates but these are seldom focused on whole of life performance. You may find sites that provide information on lifetime prolongation by reduction in charge and discharge rates with declining cell health but, other than Apple's infamous phone-slowdown approach I've not heard of any.
Battery University are always a good place to check.

Charging at a lower than the usual 1C rate in CCCV charging will certainly assist cycle lifetime. It will also tend to increase per cycle capacity if the same CCCV termination rules are used (which is bad) so a higher CV current termination threshold is in order.

A perhaps more useful approach is to

Stage 1: Stop charging at the end of the CC phase and do not allow CV charging. This will remove about 10% from the per cycle capacity and also increase WOLAh (whole of life delivered Ah)

Stage 2. As above plus limit Vdischarge_min to 3.2 or 3.3 V. Very little capacity is lost but the WOLAh will improve.

Stage 3: As above but limit Vcharge_ max.
For a 4.2V max rated cell, charging to 4.1V is liable to decrease per cycle capacity by maybe 10% over careful full CCCV charging (varies) BUT whole of life delivered capacity will increase by a factor of 2+. Charging to only 4.0V will substantially increase whole of life delivered Ah and also further reduce per cycle capacity.

Floating at 4.2V will lead to rapid cell destruction (flames and more optional). You CAN float at voltages lower than 4.2V but it's wiser not to unless you're always well clear of 4.2 V in all conditions.

By only limiting Vmax and Vmin and observing 'sensible' charge and discharge rates the OLPC project achieved 2000 battery cycles using standard commercial cells (from LG). The software and other project information is open source and the algorithm's can be deduced from the software and documentation. (I did that long ago).

Finally - for super long whole of life cycles charging to somewhere around 3.7V max greatly reduced per cycle capacity (50% range) BUT allows up to 8000 cycle with top quality cells. I base this on a Mars Rover report where they did that and achieved that result.