On a regular basis someone posts a question about how much they should charge their battery, does it hurt to charge to xx%?, etc. I've seen the graph below posted several times, usually without much explanation and the graph itself actually raises some questions.
Well, I finally found the source an read though the entire article. For reference, the source is here. The general some high points of the article were:
Well, I finally found the source an read though the entire article. For reference, the source is here. The general some high points of the article were:
- The depth of discharge determines the cycle count of the battery. The smaller the discharge, the longer the battery will last.If at all possible, avoid full discharges and charge the battery more often between uses.
- 80% DOD will degrade 30% after 300 cycles, but 40% DoD will last 1000 cycles.
- Li-Ion suffers from stress when exposed to heat, so does keeping a cell at a high charge voltage.
- At 25ºC, a battery kept at 40% will degrade 4% after one year, a battery kept at 100% will degrade 20%.
- Most Li ions charge to 4.2V. Every reduction in peak charge voltage of 0.1V doubles the life cycle. Every 70mV reduction lowers the overall capacity by 10% (≈15% for 0.1 mV)
- In terms of longevity, the optimal charge voltage is 3.92V (=60%)
- Increasing the cycle depth also raises the internal resistance of the Li-ion cell. The resistance increase is permanent.
- The cycle count on discharge stress test (the graph above) differs with the battery type, charge time, loading protocol and operating temperature so comparisons are difficult.