battery charging

Not Mopar specific. I have switched 12V power recepticals in my work van that turn off after 10 minutes when key is removed. This does not fare well with my tablet as it almost needs to be charged on a constant basis due to the battery being very weak. My question is: can i safely install a 12V sealed alarm type battery between the tablet and the switched receptical? I understand the battery will charge while i am driving and will power my tablet for a while when i am parked when the switched outlet goes dark..Or will the small battery (8A/hrs) overcharge and blow up...? It will never charge for more than 40 minutes at a time.