I like to carry one of these style jump starters/air compressor/power supply with us especially when we boondock. It's nice to know if our battery
gets low I can still run my CPAP another night off one of these, and they are always good to have if you need a jump start or have to top off your tire pressure.
While not the same one I have, I noticed WalMart has these on sale for $50 starting today. This one actually seems a little better than mine, since it also has a USB port.
You can even recharge it while you drive around if it needs charging.
Stanley 'FatMax' 700-Amp Peak Jump Starter with Compressor