To discourage people from slow charging to 100% at a DC charger, I think an idle fee (or “top charge fee”) should start at 80% when charging rate typically slows dramatically. Combined with regular idle fees, I think this could significantly enhance charger utilization at busy sites by discouraging charging above 80%.

For example, let’s say the regular no-charging idle fee is $1.00/min. Once a vehicle reaches 80%, a reduced fee of $0.50/min begins (after a 5-minute grace period) while still charging, in addition to the normal per kWh rate. If charging stops (at 100% presumably), then the fee jumps to the regular $1.00/min.

In a more prohibitive scheme, the fee above 80% could be inversely proportional to the charging rate. In other words, the slower you charge, the more expensive the fee gets per minute. Such as (100 - current kW rate)/100*$1.00 = Fee.

Obviously this would suit a station that charges per kWh better than a station that charges by the minute (you’re already getting penalized from charging slowly at a per minute station). Specifics could be worked out such as if the 80% fee is charged all the time or only when the station is busy, exceptions, dynamic pricing, etc.

To pair with this, I think automakers that offer free charging plans (ahem…VW) should automatically stop charging at 80% and charge idle fees. If the customer wants to charge more than 80%, then they would have to start another session and pay for it themselves.

In all these schemes, the idea is to modify behavior through financial disincentives for the greater good. It still allows charging past 80% if you needed the extra range, but you would have to pay extra for the privilege of staying and occupying a charger. The point is to improve overall throughput of a busy charging station.

Thoughts? Good or bad idea?