As I posted before, I used a portable jump box on my '87 300D that was sitting for 5 months and it started right up after only about 5 seconds glowing and another 5 seconds cranking. It's a Solar ES 5000 model.

If you need to let it charge the battery for 5-10 minutes before it will start as some have said, then the jumper cables or battery cables are bad or you have a bad connection. You should even be able to start the car with the battery removed and just the jumpers attached to the battery cables.

The suggestion to hook up a jump box and give it a while to charge up your battery before trying to start it is wrong. When you hook up the jump box to a bad battery, the bad battery will draw away the current from the jump box so that after 10 minutes you won't be able to start anything. The jump box will have the most power right when you attach it. Connect the jump box, glow, and start as fast as possible.