These battery pack are hard to find now, a few years ago they were many around. Sovereigns do need 12 volt supply and most say they are 12 volts that are out there but are only 11 to 11.7 volts and close to the low battery alert. Many tried using the alkaline pack with the 8 rechargeable battery, but the rechargeable are only 1.2 and 8 would be 9.6 volts as you need 10 rechargeable battery to get the 12 volts needed. if you want to run 8 alkaline they are 1.5 volts which 8 would work in the pack.
I just bought another Sovereign a while back and he said it was a new rechargeable pack which are available, but these pack have 3 of the 3.7 volt cell in it and that is 11.3 volts, but notice it wasnt running smooth, so I built a alkaline pack with 8 of the 1.5 AA batteries and now the Sovereign runs smooth now.. Some will say that 8 rechargeable in the alkaline packs will work, but I have never seen that myself..
Now getting back to the drop in packs is most didn't stay tight with some pushing the cover open and you would lose connection, soo many of us would change it to the pug in pack so you had a a wire that you would plug the pack into, so you had good connection and the battery cover stayed on. I used to do this on those xs2 and xs2a that I had in my shop and not hard to do. Now the plugin pack were easy to get, but not anymore, so I am going built a alkaline pack for now with 8 of the 1.5 volt batteries. until I can find someone that build a 10 cell pack that are rechargeable and fit in the Sovereign as there is not a lot of room to work with, or take it to a place like battery place that can make a pack. Did one for a XLT at Interstate battery a few year ago that worked good, not sure they still can do it.
The alkaline pack that was made for the Sovereign from the factory was 2 pack of 4 battery's that look like they were 9 volt battery clips and connected in parallel by 2 of the packs that had 1 wire that connected to the wire in the battery compartment that powered the Sovereign. Think I am having a hard time explaining it right now, but that is what I just did on the used Sovereign I picked up as it will use this untill I can find a rechargeable set up.