Battery issues

[caguero]

I've been testing a few different aspects of batteries and here's a list of things that don't behave as expected:

1. Battery load: The battery drains at a higher rate than it should be (100x). Approximately, it should be:

time = (<capacity> * <voltage>)/(<power_load>)

2. The percentage field is a bit confusing to me because is set to 1 when the battery is fully charged. Either we multiply it by 100 to be a percentage or we should rename the label.

header {
  stamp {
    sec: 147
    nsec: 644000000
  }
}
voltage: 4.2
charge: 1.25
capacity: 2.5
percentage: 1
power_supply_status: FULL

3. Incorrect Initial state of charge when <initial_charge> is set: Let's see an example using:

<initial_charge>1.25</initial_charge>
<capacity>2.5 </capacity>

Initially, we should observe a value of 50% as state of charge, however we get 100%.

header {
  stamp {
    sec: 227
    nsec: 261000000
  }
}
voltage: 3.1944770399030378
current: 0.078866614536290638
charge: 1.2499971286507221
capacity: 2.5
percentage: 0.99992326032470913
power_supply_status: DISCHARGING

header {
  stamp {
    sec: 227
    nsec: 281000000
  }
}

4. Charging time doesn't behave as expected: charging_time is the number of hours required to fully charged a battery. It takes way less time than the expected value to recharge the battery. It might be related to (1).

5. If <power_load> is high enough, voltage becomes 0 before the state of charge gets to 0. Example with this sdf:

<plugin filename="libignition-gazebo-linearbatteryplugin-system.so"
        name="ignition::gazebo::systems::LinearBatteryPlugin">
        <!--Li-ion battery spec from LIR18650 datasheet-->
        <battery_name>linear_battery</battery_name>
        <voltage>4.2</voltage>
        <open_circuit_voltage_constant_coef>4.2</open_circuit_voltage_constant_coef>
        <open_circuit_voltage_linear_coef>-2.0</open_circuit_voltage_linear_coef>
        <initial_charge>0.25</initial_charge>
        <capacity>0.25 </capacity>
        <resistance>0.07</resistance>
        <smooth_current_tau>2.0</smooth_current_tau>
        <enable_recharge>true</enable_recharge>
        <charging_time>0.16</charging_time>
        <power_load>200.1</power_load>
        <start_on_motion>false</start_on_motion>
        <recharge_by_topic>true</recharge_by_topic>
      </plugin>

After a few iterations, the battery status stays at:

header {
  stamp {
    sec: 55
    nsec: 404000000
  }
}
current: 59.703274818691881
charge: 0.24093013988070575
capacity: 0.25
percentage: 96.3720559522823
power_supply_status: DISCHARGING

Battery status doesn't change because voltage is 0.

[caguero]

Fixed in #230