I have about 60 odd NP-FZ100 Sony batteries so no offence but your talking nonsense. What I suggested doing is exactly what Sony told me to do when I queried why newish batteries weren't charging to 100% and has been working for me for years.
You do you though.
Charger used makes no difference by the way as have seen people mention that. A good quality third party charger will do just as good a job as the oem Sony chargers, the cheap tat might be different but I don't have any of those. The 4 Sony chargers I have work exactly the same as the 3 third party chargers I have and that includes the newer BC-ZD1 and the pro Sony NPA-MQZ1K.
This is one of those cases where you're both right, but in different contexts. There two different things going on here.
Over time, the amount of total charge that Lithium batteries will hold will fall off slowly. It's built into their chemistry, every charge cycle reduces that total a little, as will simply sitting on the shelf.
An NP50 lithium battery leaves the factory with a nominal total capacity of 1020 milliamp hours (for each individual battery it possibly won't be
exactly that, but that's the spec). The battery has circuitry to keep track of how much total charge the battery can hold, so on day 1 it knows that 100% is 1020 mAh.
Six months later, that battery may only be able to hold 1000 mAh of charge, so when you charge the battery, it will only get to 98% before stopping.
Sony's advice to run it down to 0% and then charge up to 100% triggers the built in monitoring circuit to recalibrate what 100% is from 1020 mAh down to 1000 mAh, and the battery then again reports it's charging to 100% capacity to any device that asks it, such as your camera.
This recalibration mechanism (run to zero and charge back to full) is pretty common for a multitude of devices that use lithium batteries, such as Apple laptops that I have been using and supporting for over 30 years.
Several years down the line, and several recalibrations later, your battery may be reporting it gets to 100% charge, but 100% for that battery is now only 816 mAh. If you'd never recalibrated, then it would max out at 80%. You may notice that you are getting fewer shots out of it and that the % indicator drops faster than it does with another battery you bought more recently, which can hold more total charge.
The other advice against running down to 0% is
also true, though that does not apply in these circumstances.
There's another feature of batteries known as self-discharge, where they (very slowly) lose some of the charge that they have, which is why if you charge a battery to 90% and leave it on the shelf at 90% for several months, you may find it is down to 85% the next time you pick it up.
If you run a lithium cell down to 0% charge, it
can pretty much kill it by permanently altering the internal chemistry; the total charge it can carry may be severely reduced as a result, even to the point that it won't charge at all. Manufacturers know this and when the battery's circuit reports 0% charge, it keeps a
tiny bit of charge in reserve so that the cells don't actually get to zero. However, if you let it get to 0% and then leave the battery uncharged for a long period, that reserve will be exhausted through self-discharge, and when you go to charge it up, it won't take any charge and you have to send it for recycling.
Suffice to say that it's fine to let a battery get to 0% occasionally so that it can recalibrate, but if you do, certainly don't then leave a battery lying around at 0% charge for a long time.
For various other reasons, leaving a battery charged to 100% for a long period also has negative impact on long-term charge capacity, which is why they usually ship from the factory charged to about 75%.
However, the optimum range for maintaining longevity of lithium cells is between 20% and 80%. In an ideal world, you'd use the battery down 20% and then recharge it only up to 80%.
I'll update it later.
