Robert chose to use a safety spring in all his REO's. What does this mean? If your REO has a electrical short the spring will collapse into itself. The collapsing spring drops the battery away from the contact and the circuit is broken. This action prevents further damage to your REO.
If the recommended batteries are used in your REO you will notice that you can push down on the top of the battery and compact the spring. While doing this notice how much play there is in the spring.
When batteries that are longer are used the spring is compressed too much. Sometimes so much that if you press on the top of the battery you can not compress the spring further. This choice of a longer battery will not allow the spring to move in case of a short. Your REO might face further damage.
I have bought a few REO's from the classy's that came with a battery. Upon arrival I would insert the battery and check for play in the spring. There have not been any besides the recommended battery that gave me a feeling of safety if a short would occur.
So, how much will the spring drop? Below are some pictures that show a fully functional spring and a collapsed spring in a REO.
I will add that pulling the spring back up after a collapse is not a good idea. Your spring will not function as a safety feature and collapse if you have another short.
Each of us make the decision on which battery to use. The question remains, how far will you go? What amount of room for the spring to collapse makes you feel safe?