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post #1 of 7 (permalink) Old 01-23-2009, 06:24 AM Thread Starter
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Default Charging of 12v aux battery

Hi!

My 12v aux battery is dead and I need to charge it. Since it seems to be really tricky to remove the 12v battery from it's location in the trunk I was wondering if it is possible to use a "smart charger" (C-Tek) to charge the battery using the jump start temrinal under the hood. This method should be possible to use for conventional vehicles, but since the Prius is a bit "special" I feel a bit hesitant. I've been searching like crazy for an answer on this question but all I've found are a bunch of guesses...

Thanks in advance!
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post #2 of 7 (permalink) Old 01-23-2009, 11:24 AM
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Default Re: Charging of 12v aux battery

The best thing to do is to jump start the car using the terminal under the hood. Once you are able to get in the Ready mode, drive the car for at least 1/2 hour. Be very careful to get the polarity correct when jump starting, as serious damage ($$$) will be done if the polarity is reversed. The 12v battery is small and the charging rate is low. That's why I recommend the procedure above. Once the car is in the ready mode, the 12v battery will be charged from the traction battery.

You don't say how old the battery is. If the battery is 3-4 years old and is completely depleted, you should replace it as it may not hold a charge.
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post #3 of 7 (permalink) Old 01-23-2009, 11:34 AM Thread Starter
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Default Re: Charging of 12v aux battery

Hi!

Quote:
Originally Posted by firepa63
The best thing to do is to jump start the car using the terminal under the hood. Once you are able to get in the Ready mode, drive the car for at least 1/2 hour. Be very careful to get the polarity correct when jump starting, as serious damage ($$$) will be done if the polarity is reversed. The 12v battery is small and the charging rate is low. That's why I recommend the procedure above. Once the car is in the ready mode, the 12v battery will be charged from the traction battery.
I did this once but I didn't have time to drive more than about 15 minutes and the charge did not hold for enough time for me to go out for another drive. That's why I wanted to give the battery a long time charge with a charger to make sure it's really fully charge.
Quote:
Originally Posted by firepa63
You don't say how old the battery is. If the battery is 3-4 years old and is completely depleted, you should replace it as it may not hold a charge.
The car is just over 2 years old but I bought it about 6 months ago. A new battery seem to be a good idea but first I need to be able to drive off and buy one
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post #4 of 7 (permalink) Old 01-23-2009, 02:00 PM
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Default Re: Charging of 12v aux battery

Yup, you need a new battery. If it didn't hold a charge after 15 minutes, the battery is probably damaged. You need to determine why the battery went dead. Were interior lights left on, headlights, etc.?
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post #5 of 7 (permalink) Old 01-23-2009, 04:15 PM
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Default Re: Charging of 12v aux battery

If the battery is completely dead, it make take more current and a longer charge time than is practical while installed in the car. Before you invest in a new battery, take the battery out of the car and invest in a regular battery charger (3/5/7 amp, or so), and charge it at least overnight. If it is completely dead, it may take several hours before it will even start to take a charge.

You can get a regular charger at Auto Zone, Checker, etc.; or Sears routinely has sales on them. It will probably be around $50, or much less (up to half off) on sale. Some of the larger ones will even crank a regular car with a dead battery, though not for long (30 seconds?).

By the way, I wouldn't try to charge the battery while hooked up to your Prius. The voltage from this type of charger isn't very clean (pulsating DC, with lots of line noise), and I'm not sure how the Prius computers would react. Its a remote possibility, but it could damage the electronics. (Auto batteries aren't sensitive to this kind of noise, so manufacturers don't put in any circuitry to clean up the signal.) Better to be safe, than sorry.
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post #6 of 7 (permalink) Old 01-23-2009, 05:06 PM
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Default Re: Charging of 12v aux battery

The 12V auxiliary battery has a label warning against using a charging current in excess of ~4A, so if a charger is used I suggest that it should be set to a low current position. Also if the battery is quite dead, the charger may refuse to provide power to the battery for safety reasons (not knowing if it is connected to a battery or not). In that case it would be necessary to use jumper cables (observing correct battery polarity of course) or else install a new 12V battery.
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I see the advice to remove the 12V to charge it with a charger. However, I don't know how to open the trunk (Gen II) when the system won't start. Trying to open trunk, fob doesn't work, switch on driver side door doesn't work. FWIW, I think the battery is dead because of car not being used for 7 months. Don't know if it will be "all dead" or only "mostly dead."
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