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[MN Driver]

E27006, Most NiMh chargers implement a good deal of overcharge though, I've personally watched mine and monitored each of the 6 cells both for negative delta voltage and temperature gain. My charger put in nearly 1Ah extra after all of the cells peaked and were dropping in voltage and temperature was gaining 2 degrees C every minute at 1C with the charge rate determined by the measured discharge capacity. If I let those same cells that just finished the charge at about 45C rest until they are cooled to room temperature 4 hours later and restart another charge it will burn off over 1Ah before terminating. Granted the delta V is adjustable, I hadn't adjusted it because I knew that my packs weren't balanced and the extra power going in balances those sticks for me and I didn't mind as the sticks that I have aren't suitable for use in the car anyway, each for their own different reasons.

It's hard to measure efficiency when this sort of equipment is designed to overcharge to correct balance issues and ensure a full charge every time. When I've stopped the charge at the point where I thought it was full and then discharged, I got better than 92% efficiency between the charge and discharge, in one specific example from a discharge to 5.4volts, I put in 6504 at 7amps and took out 6186, which is just a hair over 95%. The car very specifically stays away from the top and the bottom for a good reason. The top 5% or so leaves you with a fair amount of energy spent with heat as the result. The bottom and you risk reversing cells and when there is less energy leftover in the cells and you continue to pull high amperage from them, they tend to shed more heat as they just don't have the same availability of power leftover so efficiency takes a hit there, especially when the voltage drops down and you do the math of volts*amps=watts The current you continue to pull gives you less watts but still pulls down your state of charge as your voltage continues to sag lower.

Efficiency should match what the car is doing but that is hard to measure. It would take some sort of test like taking the known capacity, emptying the cell, charging it with 80% of its measured discharge capacity, discharging it 80% and repeating the process until the battery is measured at empty. This would take time and probably has enough flaws to make it not worth the effort though.