ownerVoltage[msg.sender] = 100;
function testLoseVoltageFromBattery() public {
  address player = _ownerAddress;
  uint8 voltageSpendAmount = 10;
  _mintGameItemForReceiver(player);
  uint256 currentVoltage = _voltageManagerContract.ownerVoltage(player);
  assertEq(currentVoltage, 0);

  // battery gives full 100 voltage
  _voltageManagerContract.useVoltageBattery();
  _voltageManagerContract.spendVoltage(player, voltageSpendAmount);
  assertEq(_voltageManagerContract.ownerVoltage(player), 100 - voltageSpendAmount); // 10 voltage just spent

  // Call use battery again, for whatever reason
  _voltageManagerContract.useVoltageBattery();
  // battery gave only 10 voltage
  assertEq(_voltageManagerContract.ownerVoltage(player), 100);
}
function useVoltageBattery() public {
...
- ownerVoltage[msg.sender] = 100;
+ ownerVoltage[msg.sender] += 100;
...
}
