40D battery grip discharging

Maybe not, but at least a difference in the range of dozens of minutes compared to 20 seconds ought to mean something, even if it's designed by Canon.

Not really, natural differences in the internals of batteries mean that they can charge at different rates along their cycle so a battery at 60% may charge to 80% in 1 minute when a different but same model battery may take 10 minutes to charge from 60% to 80%.
Try a Hahnel Twin V charger, it takes a pair of batteries and has a proper power readout for each. Also works as a battery tester when not externally powered.

if it's true that the left battery is used more than the right one, it might be wise to change the batteries every once in a while...

HankScorpio wrote in post #6114381
If you are judging how discharged a battery is based on Canon's wonderful flashing lights then that's not a decent indicator.

The Grip is wired to use both batteries in parallel and it does. All batteries vary slightly from each other but if you use a decent charger like the Hahnel one I have that shows battery capacity in percentage then you'll see that the batteries are always within a percent or two of each other.

^^Basically what he said.