Overtime pack, when to use?

[jeffreyb]

Got an overtime pack during the sale and now wondering what the best way to use the overtime pack would be on those long days.
Should it be used at the beginning of ride when both batteries are fresh? Mid way when the main battery is partially drained? Does it even make a difference?

 

[Sudastone]

I have mine plugged in to the main battery from the beginning that way the iwoc sees it as one massive battery.