300W power inverter + travel charger= BAD?

[bigtuffguy]

in the car i have a 300W power inverter and i plugged my travel charger into it, then into betty. everything was running fine. i was looking up an address when all of a sudden Betty's screen started to fade, then she turned off. ( my batt was at full even before i plugged her in) it required a soft reset to get her going again..

i finished the trip on betty's battery.

any ideas why this happened?