The last time I replaced my battery, the memory was erased. The next time I have to replace I wonder if I could plug the charger into the cigarette lighter to keep the memory from erasing until I am able to put the new battery in.
I just hooked a 12 volt power brick from some random device I had into a male cigarette lighter and plugged it in while I did it. The output was 12 volts and around 1 amp. Be mindful that there will still be voltage running through the system so watch the positive battery cable when you're wrestling that new battery in there so it doesnt ground on the chassis. Also watch opening doors, etc. anything that would trigger something electrical like a courtesy light or the egear pump. Either unplug the frunk light or wait till it goes out on its own from the timer before unhooking the battery and running off the adapter.