a bag is not meant to transport them... they're for charging discharging the batteries.
I use something like this:
http://www.tacklewarehouse.com/Plano...age-P3600.html
go to walmart. they're 5 bucks. leave the whole box in the car, don't carry batteries with you on your kit.
A guy here in ottawa tripped and fell on a branch, it punctured the spare lipo that he was carrying in a pouch... it went up in flames while strapped to him.
No injury, but good enough reason not to carry a battery with you on your kit... at least without a hard case/box.
If you must carry one on your kit in a utility pouch, find an appropriate hard case to put it in that will at least offer some protection.