The blackening was likely due to a poor connection and arcing (sparking). I'm guessing you are not wired up using Deans Super connectors, which are a two blade connector and make a really good connection that can sustain high amperage with low resistance. The kind of plug connector that has round pins inside typically a white plug, used by Tamiya, isnt as good for high current. It can get hot from the resistance, the plastic melts, etc.
What kind of batteries do you have? Are they a stick pack, where all the cells are sealed in a package? If so, the only way to cycle them is through the connector. You drain them down to 0.9volts per cell, so six cell you drain down to 5.4 volts then stop. A good charger will have this feature. That measurement is under load, by the way, not just measuring a pack sitting there. They used to sell bulb dischargers, which you could plug into a pack and it would run until a cutoff voltage was met. Ask at your local hobby shop what's available.