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Old 10-11-2014, 03:18 PM
Eric H's Avatar
Eric H Eric H is offline
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Join Date: Jun 2002
Location: So. Calif
Posts: 11,565
A dead 12V battery shouldn't affect the traction battery at all but the 12v battery is required to start the car since it runs all the electronics.

Leaving the rear hatch ajar is the most common cause of Prius no starts at my work, we have to jump start them often for this reason but it's never caused any other issues.

I don't know why Toyota doesn't have a Battery Saver relay to prevent this problem, or some way to use the traction battery to give the 12v battery a boost.

I recently had to fix a couple Ford Escape Hybrids at work, they had sat for eight months so naturally the 12v batteries were dead, in addition the traction batteries had discharged to the point they wouldn't start the engine.

How to fix that? Turns out the Ford (at least on the older models) had installed an emergency button that would charge the traction battery from the 12v battery enough to start the engine.
Press the button (with a good 12v battery) and eight minutes later the vehicle starts, provided of course the traction battery hasn't been discharged to the point of ruin.
Eight months was cutting it pretty close, they were down to less than 30% charge at that point but they recovered.
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