Fix uneven distribution of battery drain from motors (cataclysmbnteam…

…#1601)

Earlier, when riding a car with electric engines and multiple batteries, batteries discharged one after one.
However, they should discharge by similar percent (this is how connected electric grids work).
Made fuel consumption reuse existing code for utility discharges.

Tested by playing locally and looking at battery changes.

src/vehicle.cpp

@@ -4382,13 +4382,25 @@ std::map<itype_id, int> vehicle::fuel_usage() const
 
 double vehicle::drain_energy( const itype_id &ftype, double energy_j )
 {
+    // Consumption of battery power is done differently.
+    // From all batteries at once and doesn't change mass.
+    if( ftype == fuel_type_battery ) {
+        // Batteries stored in kilojoules
+        const int total_kj_to_drain = static_cast<int>( energy_j / 1000.0 );
+        if( total_kj_to_drain <= 0 ) {
+            return 0.0;
+        }
+        const int not_fulfilled = discharge_battery( total_kj_to_drain );
+        return static_cast<double>( total_kj_to_drain - not_fulfilled ) * 1000.0;
+    }
+
     double drained = 0.0f;
     for( auto &p : parts ) {
         if( energy_j <= 0.0f ) {
             break;
         }
 
-        double consumed = p.consume_energy( ftype, energy_j );
+        const double consumed = p.consume_energy( ftype, energy_j );
         drained += consumed;
         energy_j -= consumed;
     }

tests/vehicle_efficiency_test.cpp

@@ -431,7 +431,7 @@ TEST_CASE( "vehicle_efficiency", "[vehicle] [engine]" )
     test_vehicle( "beetle", 815669, 431300, 338700, 95610, 68060 );
     test_vehicle( "car", 1120618, 617500, 386100, 52730, 25170 );
     test_vehicle( "car_sports", 1154214, 352600, 267600, 36790, 22350 );
-    test_vehicle( "electric_car", 1126087, 132700, 72290, 8160, 3390 );
+    test_vehicle( "electric_car", 1126087, 132700, 72290, 8240, 3390 );
     test_vehicle( "suv", 1320286, 1163000, 630000, 85540, 30810 );
     test_vehicle( "motorcycle", 163085, 120300, 99920, 63320, 50810 );
     test_vehicle( "quad_bike", 265345, 116100, 116100, 46770, 46770 );