It is a common recommendation to charge an electric car up to about 80% instead of fully charging the battery to 100%. There are several reasons why this is recommended:

1. Battery life and lifespan: Charging a battery to 100% can reduce the battery's lifespan. Batteries usually last longer if they are not constantly charged to the maximum level. By limiting the battery to about 80% charge, the battery can last longer before noticeably degrading.

2. Fast charging capability: Charging a battery from 80% to 100% often takes longer than charging the battery from 0% to 80%. This is because charging the last 20% of the battery usually goes slower to protect the battery. If you want to charge quickly, it is more efficient to charge the battery up to 80% and then leave.

3. Energy efficiency: Charging a battery from 80% to 100% costs more energy in relation to the amount of added driving range. This is due to the way the battery charge is managed and the properties of battery chemistry. By limiting the charge to 80%, you can make more efficient use of the available electricity and reduce costs and the load on the power grid.

It is important to note that these recommendations may vary depending on the specific model and manufacturer's advice. Some electric vehicles have features such as adaptive charging systems that optimize charging and better manage the battery. Always consult the manual or manufacturer's information for specific guidelines regarding the charging of your electric car's battery.