Measuring the resistance of the battery would be useful.
In thinking about this, I have the following theory:
In another thread you indicated you have two group 24 batteries. Any battery with good CCA (including starting batteries, marine/hybrid, any any AGM I have run across including true deep cycle AGMs) compared to its amp hours will tend to jump voltage back up after a load I believe, especially a moderate or higher load.
Flooded deep cycles tend to acid starve at the plates and recover voltage more slowly when the load is removed.
I'm thinking the voltage spike is just normal recovery for your battery type and capacity from a load that size. True deep cycles in a similar amp hour bank might jump the voltage up less with the sudden removal of a load.
Jim
