def col_variables(datatype):
    """This function provides a key for column names of the two
    most widely used battery data collection instruments, Arbin and
    MACCOR"""
    assert datatype == 'ARBIN' or datatype == 'MACCOR'
    if datatype == 'ARBIN':
        cycle_ind_col = 'Cycle_Index'
        data_point_col = 'Data_Point'
        volt_col = 'Voltage(V)'
        curr_col = 'Current(A)'
        dis_cap_col = 'Discharge_Capacity(Ah)'
        char_cap_col = 'Charge_Capacity(Ah)'
        charge_or_discharge = 'Step_Index'

    elif datatype == 'MACCOR':
        cycle_ind_col = 'Cycle_Index'
        data_point_col = 'Rec'
        volt_col = 'Voltage(V)'
        curr_col = 'Current(A)'
        dis_cap_col = 'Cap(Ah)'
        char_cap_col = 'Cap(Ah)'
        charge_or_discharge = 'Md'
    else:
        return None
    return(cycle_ind_col, data_point_col, volt_col,
           curr_col, dis_cap_col, char_cap_col, charge_or_discharge)