Nothing. Not enough power in the booster to charge the van's battery. There was still some life in the booster -- its on-board light would light, and it could charge my mobile phone -- but not enough to pump power back into the van's battery.
I called AAA and within about 30-40 minutes a truck arrived. The driver asked about my antennas and was interested in the ham radio rig. He was impressed with the console Lisa built to hold the radios and the antenna controls. (He's a ham himself, although he hasn't used his radio much. His father, like Lisa's, is into the hobby big-time, including having a 100-foot tower antenna on his property.) He jump-started my van, tested the battery to confirm that it was still good (it was), and I was on my way, about 90 minutes later than I expected. But consequently the traffic going home was much smoother than it would have been if I'd left when I originally planned.
Bringing the booster unit home and examining the instructions more carefully, I find that you're supposed to recharge the unit once a month when it hasn't been used. Phooey! I hadn't noticed that, and it's sat for months under the seat of my van since its original charge. Okay, now I'll set a monthly reminder to "recharge booster battery."