# Charging Metrics - KW & Miles Logistics - Wally E. Rippel

Here is a suggestion which I have made to the EV industry concerning charging metrics:

I believe that charging time is in fact a poor and often confusing metric. Let me explain. Suppose you have two vehicles - one with a high capacity battery (say 20 kWh) which provides a high range (say 100 miles). Suppose we compare this with a second vehicle which has a cheap, small battery (say 5 kWh) which provides a range of 25 miles. (Note that in both cases, the range per unit energy is 5 miles per kWh.) Now suppose both vehicles have the same charger (assume that the charge rate is 2 kW). The first vehicle will have a recharge time (full recharge) of 20 kWh/2kW = 10 hr. The second vehicle will have a recharge time (again, full recharge) of 5 kWh/2 kW = 2.5 hr.

Based on the above, it might appear that the second vehicle has superior recharge (2.5 hr versus 10 hr). In both cases, however, the range gained per hour of recharge is the same - namely 10 miles of added range per hour of recharge. This is the number which counts. How long does one have to charge to provide a given amount of driving. The metric of use is therefore "miles per hour" - miles of driving per hour of recharge. If this concept makes sense to you, I suggest that you consider using it.

This product was added to our catalog on Tuesday 25 May, 2010.