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I replaced my 12V battery in March, but all these posts about dead cars due to dead 12V batteries scare me. I recently came across this gadget, the Victron Energy Smart Battery Sense. $39 on Amazon and it lets you check your battery voltage via bluetooth (if you're within range). They claim the current drain when not in use is something like .3mA, which is pretty tiny. It comes with large ring connectors on the attachment cables which should have been dead simple to attach, but the obvious nuts on the smart battery loosen but don't actually remove (for me at least). I wound up replacing one ring connector with a spade lug and attaching both near the battery but not on the battery itself. The monitor can also report battery temperature and it comes with a sticky back so I stuck it on the battery.
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I was shocked to find my battery voltage was 11.73 volts, but I checked it against a regular voltmeter while I had access to the battery and sure enough, that was correct.
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After a drive it came back up to 12.66 volts. This gadget is apparently part of a suite of Victron charging gadgets, but it works fine standalone for this purpose.
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How old is the 12v battery in your car? If it was mine and reading that low, I think I'd be replacing it pretty quickly. You can test both your HV and 12v batteries using one of the test units that are available. Here is one example -


Len
 

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Just a quick refresher course. To check the HV battery, hook the unit up, turn the key and read the data on your laptop. This test will give you an inflated reading for the 12v battery, because the HV battery is charging the 12v battery. To get a more accurate reading on your 12v battery, don't use the key at all, hook everything up and press the lock/unlock button on the dash. That keeps the HV battery out of the picture and gives you a much more accurate reading of your 12v battery. It's probably advisable to do the 12v battery test after your car has been sitting awhile and not driven and not just coming off the charger.

Len
 

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I replaced my 12V battery in March, but all these posts about dead cars due to dead 12V batteries scare me.

I was shocked to find my battery voltage was 11.73 volts, but I checked it against a regular voltmeter while I had access to the battery and sure enough, that was correct.

After a drive it came back up to 12.66 volts.
How often are you driving, garaged? Trickle charge or EVSE when not in use?

For a new in March battery, those readings (11.73V/12.66V) could be cause for concern - especially if outside in the chill?
 

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Just installed the Victron smart sense gizmo. Some quick findings: on a 2017 453 ED, using a L2 EFB 60Ah 510A battery, with an initial voltage of 12.86v @ 52degs F.

Opening and then closing the door starts a drop of about 0.5 volts for about 5 minutes, until all lights and indicators go out. No contacts sounded on or off or fans heard. The voltage has returned to 12.85 after an hour.

The app also shows a graph of change in voltage and temp over time, which might be savable with a record function. So far it seems to reset every time I start the app, so I may have to resort to screen grabs to keep track of any occasions.

This device is intended to be used with a Victron solar panel system, using a proprietary network (which can be ignored) with their other products (controllers, converters, etc.), so you won't accidently overcharge a hot battery or battery set. But it seems very accurate, and Bluetooth range is very good. I am now a floor above the garage, it reads the battery voltage quickly and easily. There is a slightly cheaper (and probably older) model that has a much shorter range too. But with this one I can just check my battery from bed at 3 am on a cold subzero February night to make sure all is good!
 
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