My advice - be careful! You need a regulated supply, these are not common. Typical 12V Li based systems will drop to below 12V well be fore exhaustion. SLA fully charged are 13.8v and 12.5V when dead (but deep cycle models are heavy). Have a look at this thread:
http://www.iceinspace.com.au/forum/s...d.php?t=189535
I have recently been made aware of this product which seems useful for use with unregulated powerbanks:
https://goalzero.com.au/shop/cables-...gulated-cable/