Anybody has this in use with Universal?

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http://www.boxwave.com/products/batt...ex.htm#details

i made my own battery box with a USB output plug. I guess its pretty much the same with what they're selling.
I'm a bit nervous though, as when used with 4 AA batteries:
- If 4 Alkaline batteries are used, they will produce ~ 6.4V DC
- If 4 NiMH rechargarable batteries are used, they will produce ~ 4.8V DC.
My experience with NiMH was not really good, the JJ though was charging, but it took forever to get it fully charged.
With Alkaline, its sligthly faster, but I'm wondering if its really okay to charge the JJ with over 6V DC?

That is why I wanted to see if someone has any experience with this, in the first place...