How much heat is generated by a Li-ion phosphate battery during discharge
Either your battery is 10 kWh or 10 kAh but not normally referred to as 10 kVAh (a term we might use in AC circuits due to power-factor).
If your battery's internal resistance is 320 mΩ then the maximum current you could draw into a dead short (not recommended) would be \$ I = \frac V R = \frac {50}{0.33} = 150 \ \text A \$ but you would have zero volts at the terminals so no power.
I suggest that your internal resistance calculation is wrong or your battery can only supply about 15 A with a 10% voltage drop.