An easy cure for range anxiety.

Our next car is going to be electric and the biggest decision is how much battery is enough. My daily commute is only 20 km in each direction so i technically only need a range of 50km for about 95% of my driving, but occasionally would like to take a day trip without having to stop and charge. Carrying all that battery for 5% of my trips seems like a big waste of energy. Something like a trailer battery pack seems like the perfect solution to electric car range issues doesnt it? It is a pity the Tesla X is the only el car that can tow from the factory at the moment. Come on car makers - standardize connections for power packs and lets move on from range anxiety. The only disadvantages i see are the need for bigger car parks, difficulty reversing and extra set of tires to maintain but i think not carrying all that unused capacity around 95% of the time would be enough compensation.