Good question. I asked the manufacturer (UPG: www.upgi.com), and tech rep Elaine Kortkamp told me on 12/3/10 that the unit can only be charged via the AC connection. If you need that function, I think some of the Schumacher models have it.
By Michael R. Natal on December 3, 2010
I honestly wouldn't highly recommend this product the battery died completely after only having it for 3 months and it barely could put air in one SUV tire without havig to be re-charged .amazon was nice enough to credit me for the purchase price.i would look at other brands and read the reviews carefully .hope this helps
By J. Connor on December 15, 2013
I left it in my car for 4 months and forgot to take it out for the top off ever 2 months once. I checked out of curiosity and it had 83% of it's charge remaining. And that was parking it in the open all day at work too. Pretty great retention on this thing.
By A Christy on March 8, 2013