Do you have the Interstate 27s or 27Bs? What setting is the Terrova on when running at your trolling speed?

Your motor can draw 56A max, but that's going to depend on speed, wind and boat load. If you go worst case, you need to first figure out how fast you are drawing power. At 50% setting, you'd be drawing the equivalent of 50% X 56A or 23A. (It's more complicated than that, but it works to estimate). Since battery RC is calculated at 25A (normally), you need to divide your power use (23A) by the 25A to get a scaling factor for the battery RC. 23A/25A=0.92. Divide the battery RC by that number. The Interstate 27 has 160 minute RC, so 160 minutes divided by 0.92 is about 174 minute run time. For the 27B, you have 180 minute RC so it would be 195.6 minute run time.

If you are able to measure what your TM draw at 100% in typical fishing conditions, you can get a much better calculation. This is just an estimation, and the battery capacity will get smaller with each use.