There is a problem when transferring power from GT6 to OpenComputers. Machines in OC have limits on the rate at which they receive power from other mods. For some reason GT6 can’t work properly with this limit.
If IC2 Classic is installed and CoFHLib is not installed, the OC Machines cannot receive power from GT6 Batteries if their power exceeds the limit. Because of this, it is difficult to directly power Computer Cases, which have a very small limit. This problem does not appear if IC2 Batteries are used.
With CoFHLib installed, Computer Cases can draw power from the powerful GT6 Batteries, but there is a lot of power wasted when charging the Cases internal batteries. I don’t know how electricity works in Minecraft, but apparently when Case gets an energy pack, it takes as much from that pack as the limit allows and discards the rest. This problem does not appear if IC2 Batteries are used.
These problems can be circumvented by increasing these limits. This can be done in OpenComputers.cfg, the required section starts on line 1345.