iPhone has a good run of ten years so far. Technologies such as display, camera, wireless, storage and main processor have progressed hugely. The battery pales in comparison. Inside latest iPhones is the same Lithium-ion polymer battery technology deployed since the very first original iPhone. Capacities also have changed little. Interestingly four out of ten iPhones came with a capacity of ~1400mAh same as the original iPhone. iPhone 3G, 3Gs and 6s shipped with a smaller battery than the previous generations. Apple can afford that due to improved power saving with more advanced processor technology even though the processing and graphics incredibly outperform the predecessors they replace.
Cycle life has improved somewhat over the past ten years. One cycle life is a full discharge from 100% to 0% and recharge back to 100%. Earlier models have a cycle life of 300 counts. Recent models increase to 500 counts. Apple claims when the cycle count reaches that point the battery shall still retain 80% of its original capacity (aka design capacity). Note that partial discharge and recharge is not counted as one cycle but several partial discharge and recharge will reach an equivalent of one full cycle.
Battery industry calls the amount of discharge before recharge the depth of discharge (DOD). A 100% DOD means discharge from 100% to 0% where the built-in circuitry inside the battery cuts off further discharge. Complete depletion will permanently damage a Lithium-ion cell. The circuitry stops discharge to preserve a certain level of residual charges and protects cells from any irreversible damage. So the actual capacity of a battery pack shall ideally be a bit higher than the design capacity to account for the residual charges that will never be utilised. I believe that's the case for iPhone batteries. For example, the battery in iPhone 6 has a design capacity of 1810mAh and is rated 6.91Wh. Look closely at fine prints on the back, the battery is rated 7.01Wh.
Mindful users will notice that both iPad's and Macbook's have battery of longer cycle life. Apple says so herself. Since 2010, the batteries in Macbook's last 1000 cycles (before dropping to 80% of design capacity). My first iPad was a 2nd-gen which was already qualified for 1000 cycles. Lithium-ion technology works better with higher capacity and lower discharge rate. iPhones are confined to a smaller space and hence capacity. Coupled with high discharge rate (think of activities such as gaming, video record and playback), this puts lot of stress on the battery and increases its wear and tear. Apple as good as she can possibly be has not managed to strike a desirable balance between capacity and power consumption where she can warrant 1000 cycles for the iPhone batteries.
More to come. Keep tuned.