In most every car the alternator is the main source of recharging the battery and it really isn't a very efficient battery charger. It simply impresses its output voltage across the battery and the establishes a charge based on this voltage. There is no feedback from the battery, and this results in a less than fully charged battery, or having too much voltage can damage the battery. A battery charged to 85% will start your car just fine, and once this three second operation is over, your alternator takes 'charge' and you are none the wiser.
If the state of charge of your battery is critical, say as an emergency source of power, where the difference between 85% charged and 100% charged could be vital, you need a dedicated battery charger. I worked in the marine industry and occasionally got involved in the USCG testing of the emergency battery system, and we had to demonstrate the proper operation of the battery charger in each of its modes.
A simple DC power supply will keep your battery charged enough to start your car, the real value of the more intelligent chargers, like the Battery Tender, is the extension of battery life by use of the temperature sensing and desulfation mode.