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Went away for seven weeks. When I came back car was dead and would not accept charging to the main battery. Finally had to get a heavy duty charger for the 12 volt which runs the computer systems. After boosting the 12v the car would finally accept charging to the main battery. Since the big battery usually feeds the 12v I was stuck until I got the 12v boosted. Why did the 12v and the big battery both go to zero in 7 weeks? Is leakage that bad?
 

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Automotive 12v batteries will almost always discharge after a few weeks of non-use. The car used the charge in the main traction battery in an attempt to prevent this, which is why it was discharged as well.

Get a new 12v battery as yours is probably damaged as a result of this deep discharge.

The reason your car wouldn't charge is the computers, including the one in the vehicle charger, are run off the 12V battery - no 12V battery no computers.
 

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How much charge was on the HV battery when you left and how much was on it when you returned? The HV battery normally will not discharge while sitting, at least not very much. And either way even at 0 EV miles the HV battery is far from being fully discharged, the system just won't use it anymore for propulsion until it is charged again.

The HV battery only charges the 12V battery when the car is in READY mode or while the car is being charged. The 12V battery meanwhile is just a standard automotive battery and as obermd said it's not unusual for a 12V battery to be dead after several weeks, especially if the 12V battery is a few years old or if it went through a full discharge sometime in the past. Fortunately nowadays they have those small portable jump start batteries that you can keep in your car, some are so small they fit in the glovebox. They work especially well with hybrids like the Clarity because the portable battery only has to supply enough power to get the 12V system running, then the HV battery will turn the gas engine whenever it needs to be started. And they are safer to use than jumper cables. Even when connected correctly jumper cables can damage the electronics on modern cars.
 

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I had my Clarity PHEV in storage over last winter for about 6 months. Like any car that goes without use for more than a few weeks, I put a maintainer on the 12V battery. When I pulled it out this spring, the main drive battery was still about 2/3 full, which is about where it was when I parked it 6 months before.
 

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Deep discharging a car battery will certainly kill it very quickly.

If you are leaving a car for an extended period, it's wise to have it trickle charged / maintained to avoid having to pay for a new battery out of your pocket.
Plus it avoids having to not have a dead car...

Here's a $25 battery tender:
 
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