I'm James Louw, Director of Engineering, at Wottz. We have invested in some pretty nice test equipment and I thought it would be a shame for me to keep it all to myself.
So as an intro into the sort of topics I want to tackle I thought I start with how much power is ‘lost’ through a standard EV charging cable.
I believe this concerns people more when investigating longer cable runs and I myself have tried to find answers but was never fully satisfied with the information available on Google.
There are many tables that give estimates for power loss however none of these tables are specifically for the new EV charging flex cable. I myself created a graph using values for volt-drop per ampere per meter from this table I found at Voltage Drop Calculations
I am sure that these values are close to real world, but that don’t include the contact resistance within the plugs themselves. Introducing our 4-Wire Kelvin tester:
If you don’t know what 4-Wire Kelvin testing is, a brief extract from Cami-Research explains it best.
Four-Wire Kelvin measurement makes it possible to accurately measure resistance values less than 0.1 Ω while eliminating the inherent resistance of the lead wires connecting the measurement instrument to the component being measured.
What is 4-Wire Measurement?
Ohm’s law defines resistance, “R”, as the ratio of voltage “V” across a component, to the current “I” passing through it: R = V/I
To measure resistance, we apply a test current to a wire and detect the voltage drop developed. From this, we easily calculate the resistance as shown in the following figure.
We measure the resistance of interest, RW, between the conductor ’s two mating pins. The entire circuit, however, includes the resistance of the lead wires, RL1 and RL2, so the voltage drop used in the calculation includes all three of these resistances. In many situations the lead wire resistance is much lower than the resistance of the conductor or component we aim to measure and therefore can be disregarded.
In some situations, however, the resistance of interest, RW, approaches the resistance value of the lead wires used to measure it resulting in an inaccurate reading. We correct this problem by moving the voltage measurement points out to the endpoints of the mating pins, thus, bypassing any voltage drop that may occur in the lead wires. Refer to the figure below:
Now that we have a better understanding on how we get such precise measurements, we can actually take a measurement.
The cable tested is rated at 32A single phase (7kW) and is 15m long.
Power loss within a cable can also be estimated from the cable current and resistance:
P = power loss, W
I = cable design current, A
R = cable resistance, Ω
n = factor depending on circuit type/number of conductors
= 2 for a.c. single phase, d.c. circuits
= 3 for a.c. three phase circuits (assumed balanced)
Therefore total power loss in this cable at 32A is (I used the average resistance of the L and N leg):
This power loss of 88W is actually 25W less than using the bog standard voltage drop formulas!
Hope this insight is useful to some people, let me know anything else you would like me to tackle in future.