The best I can suggest is what is known as a jump-pack. It is a battery and a self contained charger that can be used to jump start your car. They have an on-board charger for the pack so you charge it in your apartment and carry it to your car for a jump. Most have a charge port (cigarette lighter socket) and a double ended cable that allows you to plug it into the car’s power port while it is running to charge the jump pack. When the car is not running, charge goes the other way if the battery is running down. They look like this;
But if your battery is going dead, find out why.