Well, this gets into the land of speculation. Note, while I've found a bit about batteries and such, fundamentally I am a software guy, so take what I say with appropriate grains of salt.
I've shot with an E-m5 mark I since 2014 and with the E-m1 mark I since 2016. Both of these cameras use the BLN-1 battery. In 2020 I've moved on to E-m5 mark III and in 2021 I bought the E-m1 mark II. The E-m5 mark III uses the BLS-50 battery and the E-m1 mark II uses the BLH-1 battery.
I've used a combination of Olympus batteries and third party batteries (mostly Watson). I've had all brands swell up and have to be replaced over time, including at least one Olympus battery.
My experience was the Wasabi batteries I used to use as my goto battery swelled up within a year. A lot of other forum members also had Wasabi batteries swell in the 2014-2016 time period. I don't know if Wasabi just had a bad batch, or fundamentally their BLN-1 batteries were flawed, but since then, I have not bought new Wasabi batteries. My Watson batteries seem to last about 3 years before swelling. I forget how long the Olympus battery lasted before it swelled, but figure 3-4 years.
There was this old thread (
DPreview thread about BLN from 2016) about the quality of many of third party BLN-1 batteries that alerted me to the issue.
As I understand it, BLN-1 batteries use a slightly different chemistry from the normal 2 cell battery. It has a nominal voltage of 7.6 volts, while the current generic battery has a nominal voltage of 7.4 volts (and the older batteries were 7.2 volts). This means that chargers meant for one nominal voltage might not properly top off the battery with the others. A secondary effect is the battery with the lower nominal voltage will appear empty sooner since the camera was designed for the higher nominal voltage (the camera uses the voltage to just how much capacity remains).
I now use the charger with the same branding as the battery, i.e. I use Watson chargers for the Watson batteries, and the Olympus charger for the Olympus batteries. I use Watson batteries since that thread mentioned that of the third party batteries Airmel looked at, the Watsons were the best, and many of the others were junk. For my secondary batteries for the E-m1 mark II, I went with Kastar batteries (Watson did not have BLH-1 batteries at the time or they were sold out).
So a lot of people say they only use official Olympus batteries. Unfortunately we get into market realities. The E-m5 mark II was the last camera Olympus made that used BLN-1 batteries. It has been replaced with the E-m5 mark III that uses the BLS-50 battery. Similarly, the E-m1 mark I camera has been replaced with the E-m1 mark II/III, and E-m1x cameras that use the BLH-1 battery (and the new OM-1 uses the BLX-1 battery).
In addition, to it being awhile since the last BLN-1 camera was released, in that time period, Olympus closed its factory in China and moved operations to Vietnam, and then Olympus sold the camera division to OM Systems. Both of these incidents might make the company officials re-evaluate what older products should be dropped.
My speculation is Olympus stopped producing new BLN-1 batteries a few years ago. A similar thread within the last month (on a different forum) had people nothing that the 'new' Olympus BLN-1 batteries they recently bought were built in 2019. A lot of the FAQs for lithium-ion batteries say that you should start replacing these batteries around 5 years after the battery was made (it doesn't matter as much how you use the battery, but when it was made). This means even if you buy a bunch of 'new' BLN-1 batteries now, likely you will need to start thinking of replacing them in 2-3 years time. This means picking the best clone battery.
You should also start monitoring your batteries, and if it gets harder to put the battery into the camera and take it out, properly dispose of the battery and replace it ASAP.
