[JesseK]

We use a cyle of 3 batteries during Quals, and haven't ever seen a [newer] battery take longer than 45 minutes to hit its peak charge. We have 2-3 in reserve for Elims that are used once Thurs/Fri (for 'difficult' matches) and then not again until Elims Saturday. Thus for Elims (6-9 matches max), we have 5-6 batteries available. We also use our oldest battery as a 'test' battery (autonomous, pre-match flight test, etc) and it stays on the robot cart. Unless we had an automated charging station, having more would be a nightmare for us.

For Einstein, you'd have 9 Elim matches (at most) in the span of 2 hours for Division Elims, and then 6 Elim matches (at most) after that. Einstein also has many long break in between the matches -- especially when Dean Kamen speaks. New batteries take ~30 minutes to [mostly] charge back up from the drain of a 2 minute match. So you may be able to shift your question to more of a "why are our batteries being drained so much during a single match?" type of perspective if you really feel you must have 8 batteries available for an event.

I'll add that year-over-year (now in our 7th year), we only have to purchase 2-3 new batteries because of our setup, and we've never come across a 'bad' battery in the bunch of batteries that are less than 4 years old. It works for us, though I admit we've never made it to Einstein.

Another tidbit is that you may want an extra battery or two for your alliance partners at some point. Yet again, we've never had an issue were we had less than 2 batteries that weren't at their prime during Elims, so 6 batteries still work for us. Maybe we haven't been around enough...