HCH-II Recalibrations (Recal). What are they ?

We often hear about recals from Honda Hybrid owners and we learn to fear them based on the reports of an impending traction battery failure. As the HCH-II ages, these chats about recals are likely to become more regular and often send other owners into a panic mode. Is this a reason to worry? Probably not.

What is a recal (recalibration) on the HCH-II ?

The HCH recalibration is an automatic process that is purely instrumented. In other words, there is a sub-system (BCM) in your HCH-II that is responsible for monitoring and managing the charge level of the hybrid battery pack and because of the errors incurred in measuring not only the pack's charge level but also how much energy enters and leaves the battery pack, in time this measurement becomes less precise.

To better explain this relationship let's depict a typical charging and discharging scenario:

1. When you depress the brake pedal your car will capture a given amount of energy which is then sent to the battery pack. The car's system measures that amount of energy and assumes that (let's say) 1600 watts-hour worth of energy was send to battery pack. But because of the energy losses due to converting electrical energy to chemical (with heat as a by-product) and the approximated method of measuring this energy, the real amount of energy ends us being a little lower than measured... perhaps just 1460 watts-hour actually made it into the battery pack. Yet the system does not necessarily see it that way. The SoC is incremented as a function of this estimate of energy that "was" added to the pack.
2. When you depress the gas pedal and you get electric assist to move forward, the system will obtain energy from the battery pack and will attempt to measure how much of this energy is flowing into the power-train. This measurement is also an approximation and the charge level of the battery pack will be adjusted to reflect the energy that was consumed.
3. Finally, the measurement of the battery pack's charge level is in itself an approximation not only because it is affected by the estimate of how much energy was stored in it but also how much energy was obtained from the pack.
   

You can see that over time, the battery charge level will be inaccurate and the car's governing system may evoke a recalibration. Basically, a recalibration simply invalidates the current charge level by "setting' it to a low enough charge level that is enough to trigger a forced regeneration process. Often we will see the charge level dipping rapidly towards an SoC reading 1 or 2 bars only which is equivalent to the lowest safe voltage permitted by the Battery Condition Monitor (BCM).
Because of this induced and fake low charge level, a forced regeneration (charging) will be triggered and will stay in effect until the car's SoC reaches 7 or 8 bars. When the highest safe voltage is detected (SoC at 7 or 8 bars) the BCM will request the cancellation of the forced regeneration.

Now, if these recals are very frequent then these events could indicate a possible condition caused by any of the following:

• If the battery pack is too cold or too hot, the system expects measurement errors to be magnified and the chance of a recal increases. Very cold weather or very hot weather with AC use are typical scenarios. This is normal.
• A problem with one or more cells in the battery pack can also cause a recal. A single faulty cell can cause precocious fluctuations in the battery pack's SoC and these fluctuations in turn, can cause the detection of either extreme of the permissible charge level which inevitably leads to a recal. This is not normal and will likely be a symptom of a fault condition. In this situation you may see an IMA warning light appear on the dash and when it does you should schedule an appointment with your dealer for a detailed assessment of the problem.

What if we are getting frequent recals, especially in moderate weather but no IMA light appears? You may try any of the following:
- Perform a power reset procedure.
- Check with your dealer for any software updates for your car.
- Maintain a detailed summary of these incidents and report them to your dealer is they persist.

In summary:

Just because we a get a recalibration it does not mean that our battery pack is going downhill. Keep an eye on it and ask online (www.CleanMPG.com) to be sure. :)