Hymotion LED flashing error (solved)

Many Hymotion plug-in battery owners have had problems with the LED flashing 13 times. There were a lot of questions as to why this happened, what it means and how to resolve it.

This error indicates low voltage on the 12 volt battery in the Prius. The lowest value that Hymotion can accept for a successful startup is 11.6 volts. You can determine your voltage with the ScanGauge II (VLT).

You may have noticed this behavior. It recently started happening to me with the cold weather. When you first get into your Hymotion Prius after it has been sitting for a while, you will press the Start button on your Prius, then the flashing starts and the Hymotion battery provides nothing to your system. After you drive around a while, you may stop for an errand and get out of your car for 5 minutes. When you get back in to start your Prius, the Hymotion battery is working fine.

Here is what happened. When you first get into your car while it is off, the voltage is likely around 12.0 or 12.1 (see ScanGauge II). While the Prius is starting, this number drops dramatically while it is starting the various accessories in the car. I saw it drop to 11.0 in my Prius. If that value falls below 11.6, then Hymotion gets the error.

However, from a warmed up Prius, this value is much higher and stronger, so if you try to start the Prius after you have been driving around for a while, the 12 volt battery doesn't drop as much during startup.

Here is the workaround:
If you know your Prius is cold, first turn your Hymotion switch to the OFF position. Start your Prius in normal mode. You only need to wait a few seconds until all of the accessories are on. The Voltage comes back to normal right away (2 or 3 seconds). Then switch your Hymotion ON. This avoids the danger of the Hymotion failing to make the connection.

If you start getting the 13 flashes on the LED, the only way to stop it is to turn off the vehicle for about 3 to 5 minutes and allow it to timeout. With the car off, you will notice that the LED stays on for a few minutes. Once it goes off automaticaaly, then you can start again. If the car is still cold, try with the Hymotion switch in the OFF position. If the car is warm, you can leave it in the ON position.

I am now in the habit of just starting my Hymotion Prius with the switch in the OFF position.

I will post some YouTube videos of this when I get a chance.

 

I have also started turning off all accessories (radio, heater, headlights, etc) when I pull into the garage. If they are in the off position when I restart the vehicle, I believe there is less draw on the 12 volt battery during startup.