What is the difference between AC & DC ?

The main difference between AC and DC chargers is that AC chargers are supplied with alternating current, that is, the current switches back and forth. This is the main reason why we have AC current, in the electricity lines, at our homes. On the contrary, DC chargers are supplied with direct current, that is, the current does not shift or switch back and forth. 

In DC chargers, It is simply the positive and negative type of current. So, precisely, AC current chargers are a bit complicated than DC current chargers. The built in charger in most cars are AC and limited to around 11kWh, while using a DC charger, it will bypass the built in charger and charge at a much higher speed.

What is Vehicle to Grid (V2G) ?

Vehicle-to-grid (V2G), also known as Vehicle-to-home (V2H) or Vehicle-to-load (V2L) describes a system in which plug-in electric vehicles (PEV) sell demand response services to the grid. Demand services are either delivering electricity or by reducing their charging rate. Demand services reduce pressure on the grid, which might otherwise experience disruption from load variations.

Plug-in electric vehicles include battery electric vehicles (BEV), plug-in hybrids (PHEV), and hydrogen vehicles. They share the ability to generate electricity. That electricity is typically used to power the vehicle. However, at any given time 95% of cars are parked, while their energy sits unused. V2G envisions sending some of the stored power to the grid (or reducing charge rates to pull less power from the grid). A 2015 report found that vehicle owners could receive significant payments.

Batteries have a finite number of charging cycles, as well as a shelf-life, therefore V2G can impact battery longevity. Battery capacity is a complex function of battery chemistry, charge/discharge rates, temperature, state of charge and age, and evolves with improving technology. Most studies using slow discharge rates show only a few percent of additional degradation while one study suggested that using vehicles for grid storage could improve longevity.

Hydrogen fuel cell vehicles (FCV) with tanks containing 5.6 kg of hydrogen can deliver more than 90 kWh of electricity. Vehicle batteries may hold 100 kWh or more. Reducing charge rates, termed uni-directional V2G, is technically simpler than delivering power which many PEVs are not equipped to do. UV2G can be extended by throttling other activities such as air heating/cooling.