The Efficiency of Regeneration?

Not exactly published material, but Electrek had an article discussing the efficiency and factors affecting recovered electricity. Here's a clip from that article:
Tesla drivers have reported back energy contribution data using different data tracking apps. Model S drivers have reported recapturing as much as 32% of their total energy use while driving up and then back downhill.

 

I would first define a simple "regen efficiency" as how much additional energy can made available in the battery due to energy recovered from the wheels while regen "braking". No windage or tire rolling friction is considered because that brings speed into the picture. You could add up the generally-understood efficiencies of the gear reducer, motor, inverter and one-half the battery cycle loss. At best that might be (respectively) 97% x 90% x 90% x 99% = 78%.

Next is "regen effectiveness", meaning what's the practical advantage of having this feature. Clearly that brings more variables into play so you'd have to associate the answer with a specific test. A quick search returned this article.

There's no question that it's a feature worth having and I'll note that it's not easy to avoid taking advantage in the Kona because Hyundai's braking system uses regen whenever it's possible and advantageous. Their EVs are designed so that driving the car like an ICE results in good economy. A couple of other brand EVs are not quite like this and require you to drive in a one-pedal mode to get the same result.

I've tried my own tests on a 30% grade in an attempt to measure the efficiency but failed to get anything useful because the hill was too short. But I have been able to illustrate the advantage on a hilly 300 km round trip by logging energy flowing in and out of the battery. The graph horizontal axis is SoC to simplify the results, noting that the slope of the net energy used is essentially the battery capacity, 64 kWh per 100% SoC.