I have this Noco. Charges cellphone while at the park/beach.
Starts the boat, motorcycle, jetski, generator, lawn tractor...
I consider it a must have when going fishing.
I also love the look on someone's face when they ask for jumper cables.
http://nocousa.com
Powerall, Junopower, Matco, Motobatt, StarkPower, Wagan, .... have similar products.
http://www.wagan.com/index.php/7501-ionboost-v8.html
http://www.thepowerall.com/product.html
http://junopower.com/collections/automobile
http://www.matcotools.com/catalog/product/MINIJUMP/VERSAPOWER-MICRO-POWER-SOURCE-AND-JUMP-START http://store.starkpower.com/Jump-Box-Batteries_c_37.html
My gripe with the above...no 12v cigarette lighter adapter socket for non-usb devices(like my laptop or small inverter).
I will be ordering the Junopower soon since I don't have any larger engines and don't need the flashlight feature.
The Noco Genius, or equivalent, is a must have!
I do fear lithium batteries left inside vehicles parked in the sun. Keep it with your spare tire, or in the trunk, bed toolbox... Best to keep lithium thermal incidents away from passengers.