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I have a 2010 Prius II with 78 k miles. If I do not drive it for 4 days, the car will not start. I have taken the car to 2 different Toyota dealers service centers in California. Their diagnostics can not find any problem with the car.

I have replaced the 12 volt AGM battery (in the back of the car) twice. The current AGM battery and 200 Volt battery when tested are strong. The car has an OEM alarm. I disconnected an aftermarket alarm. I always check to make sure all lights are off and the doors are shut.

I can not determine why the car won’t start. Is something draining the 12 V / AGM battery when not car is not in use ? When I jump start the 200 volt battery under the front hood, the car starts again I can drive it & re-charge the car. But If I don’t drive it for 4 days, it is dead again.

Any suggestions on how to solve this problem ?? I welcome any questions or comments. Thank you.
 

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I have a 2010 Prius II with 78 k miles. If I do not drive it for 4 days, the car will not start. I have taken the car to 2 different Toyota dealers service centers in California. Their diagnostics can not find any problem with the car.

I have replaced the 12 volt AGM battery (in the back of the car) twice. The current AGM battery and 200 Volt battery when tested are strong. The car has an OEM alarm. I disconnected an aftermarket alarm. I always check to make sure all lights are off and the doors are shut.

I can not determine why the car won’t start. Is something draining the 12 V / AGM battery when not car is not in use ? When I jump start the 200 volt battery under the front hood, the car starts again I can drive it & re-charge the car. But If I don’t drive it for 4 days, it is dead again.

Any suggestions on how to solve this problem ??I welcome any questions or comments. Thank you.
It sounds like your car is experiencing a parasitic drain on the 12V/AGM battery, which is causing it to lose its charge and preventing the car from starting after a few days of not being driven. A parasitic drain is when some component of the car is drawing power from the battery even when the car is turned off. This can be caused by a variety of things, such as a faulty component that continues to draw power, a short circuit in the wiring, or an aftermarket accessory that is not properly installed or wired.
One way to troubleshoot this issue is to perform a parasitic drain test. To do this, you will need a multimeter to measure the current draw on the battery. First, make sure all the accessories in the car are turned off and the doors are closed. Then, connect the multimeter in series with the negative terminal of the battery. Set the multimeter to measure current in milliamps (mA). Start the car and let it idle for a few minutes to make sure everything is stabilized. Then, turn the car off and disconnect the negative terminal of the battery. The multimeter should continue to register a small amount of current draw, which is normal. This is known as the "key off" current draw, and it should be less than 50mA. If the current draw is higher than this, it could be a sign of a parasitic drain.
To further troubleshoot the issue, you can try removing the fuses one at a time and see if the current draw decreases. This will help you identify which circuit or component is causing the drain. You can also try unplugging any aftermarket accessories or disconnecting the negative terminal of the aftermarket alarm, as these can sometimes cause parasitic drains.
If you are unable to find the cause of the parasitic drain through these methods, it may be necessary to bring the car to a mechanic for further diagnosis. They may have access to more specialized equipment and expertise that can help identify the cause of the issue.
 
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