Hi Pirots, I would hazard a guess they are lithium batteries which need to be kept at a certain level of charge. When you stored the Bixby over lockdown was the battery pack fully charged or at least 50% charged. If you have left then with little to no charge it is possible they have gone beyond the point of shutdown/cut off and the voltage is to low for the charger to recognize it, normally below 3.4V,
This usually happens when the battery hasn’t being used for a long time. However you can revive the battery if the battery isn’t completely dead. You need to give more potential difference to do so all you need is a bigger power adapter (charger) to push the power in that battery (jump start). Or you can try and leave it on charge for a day or so depending on the size of the battery pack hopefully getting your battery to recognise the charger.
If the voltage has got to low and the cells have dried out there is nothing you can do other than refurbish your current battery pack yourself or if you know someone, or buy a new one.
Finally are you sure your charger is working as it should.
Post a picture if you still got the battery pack opened.