2003 Prius 12v Battery Disconnect

There's no harm in disconnecting the battery. It would be a safer practice for you to disconnect the negative battery terminal. That is easy to do if you unbolt the negative cable where it attaches to the body, then put the end of the cable in a plastic baggie to insulate it from the body.

However, the 12V battery should last longer than six days considering that it is relatively new - unless it had not been fully charged up prior to your vacation. If you do not drive your car much, then the battery may be partially discharged, and that would explain why it could not provide enough power to boot up the ECUs after a six-day period.

 

Do you have Smart Key? If so, it's generally recommended to disable it if you're going to leave the car for a long period. It may seem odd, but devices that aren't in contact use much more power than those in close range.

The cars equipped with the advanced alarm have interior sensors, which can also be disabled to reduce the current drawn, while leaving the main alarm on.

With these features off the car is supposed to only draw about 11mA, which should allow the 28Ah aux battery to last about 100 days. Source: Hobbit Tech Training pt 3.

If you do disconnect the 12V battery, you'll lose your stored MPG figure and have to reinitialize the driver's power window so auto up/down works properly. See TSB PD010-03 for the procedure.

 

Hi Mike,

In the US, a 2003 by definition will be NHW11 and will not have SE/SS. When I measured quiescent current draw on my 2001, it was around 30-35 mA, fluctuating as the LED in the security system flashed.

For purposes of calculation, lets say .035A current draw. 24 hours/day x .035A = 0.84AH / day.

As you pointed out, the NHW11 battery has a rated capacity of 28AH; however for the sake of longevity one should not draw the battery down below 50%.

Assuming that the battery was fully charged to start (which may or may not be a reasonable assumption) then you could store an NHW11 for up to 17 days without a problem (17 days x 0.84AH / day = 14.3AH)

Toyota's TSB on battery maintenance suggests making the car READY for 30 minutes every two weeks, so their advice is pretty close to the above.

 

Install a battery brain on the battery so that battery never gets below 11 volts!

Battery Brain Bronze 12 volt manual reconnect [bronze] - $65.00 : Batteries Are Us

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