Look at dc to dc converters for EVs. Yes it should output 13.5-14V. The input voltage is rather high because they are designed to be fed by EV pack voltage.
Making a high voltage small capacity pack is easy but for charging it back up you would have to find an acceptable method. ( tapping into the pack by charging leads and use multiple chargers, or get the appropriate high voltage charger etc.)
Here is the first one I found on an ebay search
EV DC to DC Converter Electric Car Voltage Reduce 144V volt to 13.8V 35 Amp 480W | eBay
There was also talk about diy converters on this forum too but I did not search for that.
Barna