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Potentially bad 12v battery experience

2450 Views 5 Replies 3 Participants Last post by  Vincent Rapide
We have found some experimental evidence that may be related to the
Smart car battery "bricking" issue.

Bill, a forum member, reached out reporting a phantom power drain in
his 2015 451ED. It was sometimes losing 10% charge overnight and he
was trying to chase it down. Both his high voltage and low voltage
(12V) batteries were original to the car.

After some investigation, it appears that the car was frequently
charging the LV battery from the HV battery and this was likely the
source of his phantom power loss. If left to continue unchecked, this
is also a possible source of the smart's "bricking" problem.

The generic Group 47 battery ($105 at Costco) is about an inch longer
than the old original battery, but it slid into place just fine. The
only problem is that the original hold down clamp would not go into
position. A little hacksaw work on it allowed it to do its job again.
There is a sticky near the top of the forum giving installation instructions.

More details in the next post about the data collected along the way,
which you can skip if you're not interested, but the takeaways for us

If you notice an unexplained power drain on your HV battery, you need
to find out why.

If your battery is five years old or older, you should keep an eye on

If you have a multi-meter or a battery test unit, you should check
the 12v battery every once in a while, more often if either of the
first two issues are yours.

Good luck. Hopefully this experience will help a few owners from owning
a "bricked" Smart car.

Len, Bill, and Jim
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I plan to disconnect the negative post on the 12v battery when I'm away on trips having concluded that this will save the HV battery from any inadvertent discharge.
Ideally leave the HV at about 50 to 75%. When re-connecting do so at exactly 12 noon. 8^)

No! don't so that!

There is a case in this forum of a Canadian owner in this forum removing his 12V battery and taking it indoors for the winter. When spring came, he found that the HV battery had somehow discharged itself completely, near-zero volts flat (he actually removed the battery, opened it up and measured the cell voltages). Very quickly, this led to cases of others who saw their HV batteries self-destroy when the 12V battery went flat. This led to a worknig hypotheses here that the 451 ED has some kind of bug that caused the BMS to self-destroy the HV battery if the 12 volt syatem is de-energized.

The perferred thing to do is to connect a maintenance trickle charger to the 12V battery if the car will be parked for an extended period of time.
1 - 1 of 6 Posts
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