New user , how do you get Agate2/battery to charge over 5kwh ?

We just did a new install here. No solar. I have a Genmax 13500 generator that runs at 11500 continuous. Generator performance parms are set to 11.5 KWh with best power duty at 70%. All the Franklin doc says that these batteries are supposed to be able to charge at 8K but the best charge we are getting is 5K. Any ideas on how to get this thing up to 8K charge rate ? Thanks !