# Actual lifetime of 18650 cells

For a project, we are currently looking at how long we could use battery cells, such as the LG 18650MJ1 at maximum discharge and maximum charge in 25°C ambient temperatures.

Basically, in this case, how many cycles can we expect and past these cycles, how long will the battery actually live and be useful, forever? Few 1000s of cycles?

Is there any approximative way to calculate this? Also practical figures to apply to those numbers to estimate by how much life increase if we are at 70% of max discharge and/or charge for example?

Any help or info with respect to how we could have estimate these figures would be very welcome :)

Thank you!

• Read the datasheets. They often specify a minimum number of cycles. – Hearth Mar 15 at 17:09
• The datasheet states 400 cycles under these conditions to 80%. I guess my question is what happens after that, for how long will they work as we don't care much about capacity but more performance in the long term. Also is there anyway to estimate this performance? – Eliott Mar 15 at 17:18
• Sometimes, datasheets will have graphs of cycles vs capacity loss, or depth of discharge vs cycles to a given capacity loss, etc. If this is something you care about, seek out products that specify this. – Hearth Mar 15 at 17:19
• If the datasheet says 400 cycles, then you should design with the assumption that they will last no more than that. Even better, assume 300 cycles. Good engineers don't make assumptions about what will happen when a component's specified life is exceeded, and they don't make design decisions based on anecdotes they got from some internet site, even this one. – Elliot Alderson Mar 15 at 17:28
• how long will the battery actually live and be useful Then you must define what useful means to you. The cells loose capacity as they wear out. Is a cell that has 50% of its original capacity still "useful" in your application? If not, what percentage is "useful"? This is not black/white and often not easy to answer either. That's why there can rarely be a clear answer to battery lifetime questions. – Bimpelrekkie Mar 15 at 18:32

## 1 Answer

Besides Battery voltage outside rated Vmax.~Vmin. range, the biggest ageing factor on LiPO4 is thermal stress.

For example from datasheet specs below.

Cycle life : 400 cyc, charge 1.5A, discharge 4A (80%) ESR=80 mOhms {new}

Look at Power dissipation, Pd, when new. and maybe 2x to 3x when getting old and >10x when dead.
$$\P_D=I^2\cdot ESR [\Omega]\$$ = 4²*(0.08~0.24)Ω=225~675mW to 2.2W when dying under CC. If you can imagine the temp rise of a part this size dissipating this power, LG are very conservative on temp rise and ageing effects are a runaway condition where the weakest cell dies 1st.

It is difficult to compute your answer but you can try but notice DoD< 50% makes the biggest effect. i.e. preventing Vbat < ~3.4V (my est.)

Now review my charts of Batt.Univ. research.

REF My chart uses data from Batt. University tables of research on new LiPO4 cells

If you want a rated lifetime capacity 600 cycles using 4.2V at some Ah rating is a compromise of total charge cycles and capacity for a new LiPo4 cell. Some cells might be rated at higher lifespan or higher capacity and specs for CV and CC and C rates used in the specs must be compared to get meaningful results.

If you wanted max lifetime charge Ah*N cycles you keep the cells at mid- charge levels (SoC= 60%) and every 10 to 15 cycles charge to 4.2 to replenish the soaking capacity then resume to mid-charge voltages. Then using only 20% DoD from the initial charge capacity or SoC = 60% to 40% using Coulomb counting calibration methods you might achieve 14k cycles/600 *90% duty cycle rated = 21x the lifetime capacity.

If you used a CV of 3.9V vs 4.2V you gain 3200 cycle/900rated=3.5x rated cycles but lose 35%= 100%-65% capacity unless you periodically use 4.2V every 10~15 cycles which restored some "memory capacitance". otherwise the net gain is 3.5x 65% approx. doubles the life capacity.

You cannot combine the 20% DoD at mid charge with periodic 100% charges @ 4.2V every 10 cycles and use 3.9V CV because the life span from self-discharge chemistry away from mid-charge is not charted but is known to reduce life span.

You cannot expect 3.5 x 2= 7x 900 cycles of some Ah*Vavg/3600 cell Joule energy discharge but 350% x 600 cycles is certainly worth considering.

This means you might need 5x as many batteries using only 20% of the rated capacity. Also charging them 5x more frequently is a PITA, unless you have some convenient way with backup storage and a transfer switch but then they all last 21x longer.

Although these LG cells may have a minium rating of 3400mA this independent test showed less capacity than rated.

Official specifications:
Nominal Capacity: 3500mAh
Minimum Capacity: 3400mAh
Nominal voltage: 3.635V
Standard charge: 0.5C (1700mA) 4.2V, cut-off 50mA
Max. charge voltage: 4.2V +/- 0.05V
Max. charge current: 1C (3400mA)
Standard discharge: 0.2C (680mA), cut-off 2.5V
Max. discharge current: 10A
Weight: Max. 49.0g
Cycle life : 400 cycles, charge 1.5A, discharge 4A (80%)
Operating temperature: Charge: 0° ~ 45°C, Discharge: -20°C ~ 60°C
Storage temperature: 1 month: -20°C ~ 60°C, 3 months: -20°C ~ 45°C, 1 year: -20°C ~ 20°C

• Thank you very much for this! Is there a way to determine what would be the cycle life to another capacity level. For example instead of looking at the number of cycles to 70% or 60% of original capacity, I would be interested to look at the number of cycles to let's say 30% of original capacity or another arbitrary number. – Eliott Mar 18 at 10:36
• Eliott the whole idea of plots is pick any number and see the effect – Sunnyskyguy EE75 Mar 18 at 15:24
• If you find out the conditions for another battery are the same but higher capacity you might normalizing with ratios e.g. 5000mAh / 3400mAh =1.5 but these may be different chemistry so we cannot assume the same characteristics, but it may trend the same – Sunnyskyguy EE75 Mar 18 at 15:31
• also see other related answer , you judge with your own battery specs, and BMS what is best. electronics.stackexchange.com/questions/427809/… Note your battery capacity is rated only for 0.2C discharge so using more reduces capacity and life cycles. – Sunnyskyguy EE75 Mar 18 at 19:02