@khaase interesting getting the LiSOCL2 activated.
My 2Cents is it would be good to characterize the 350mA pulse – and isolate it from the mega1284’s high impedance LiSOCL2. That could mean that for the “high impedance” (for a battery) LiSOCL2, the current draw has to be kept below a threshold, treated as a constant current source.
Then the issue is also how to detect when the LiSOCL2 is running out of energy, and can’t take more high current pulses, can it gracefully manage the power situation. Unfortunately its not a trivial problem.