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I am about to take delivery of a new Leaf. I was wondering how the 12v was charged and did a Google to find out. Imagine my dismay when I came across loads of threads complains about the problems with the Leaf 12v battery.
Is this still an issue and is there something I should be aware of not might not be covered in the manual?
Would really appreciate some feedback on this.
 

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I think the 12V battery issues were sorted out long ago so I wouldn't worry. The car has the equivalent of an alternator so charges it when you drive like a fossil does.
 

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Sort of but not quite.

Most ICE cars run the electrical system at 14v. This is high enough to charge the battery, and because the voltage is always up near 14v when the engine is running the battery is kept full.

On the LEAF (well all EVs), there is a converter which takes a supply from the traction battery and produces the power to run all the 12v in-car electronics, as well as topping up the 12v battery. However in a bid to save a bit of energy, this converter tries to be clever. It raises the voltage to 14v for a while, to charge the 12v, but then drops the voltage down to 12.8v which runs the cars systems, but essentially stops charging the 12v. This usually works well enough, many people never have issues. But the right combination of usage cycles seems to get the car into a state where the converter never runs at 14v for long enough to actually fully recharge the 12v battery, and over time it slowly gets weaker. As a secondary effect, the 12v battery becomes damaged from being kept in this low state of charge, which makes everything worse, and it snowballs eventually to a dead battery (or one thats low enough to cause "weird stuff" to happen)

There are two schools of thought, either ignore it, and hope it just works, which for the most part it will. Or apply an infrequent (perhaps monthly) charge using a standalone battery charger to ensure the battery is kept nice and full.
 

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Not 40kWh I know (not sure if there are any differences in how the 12v battery is handled) but so far I've not had any issues with my 30kWh model discharging the 12v battery. In fact in the 3 months I've had it I've tried putting it on my intelligent 12v charger twice once a month to check and it has shut off within about 20 minutes - and given a 50% discharge takes over 8 hours to replenish on that particular charger it means the charger is satisfied that the battery is more or less fully charged. So I haven't bothered to use it again and probably wont.

A few possible factors are involved in Leaf 12v batteries going flat from what I've seen. These are some I'm aware of.

1) If the car is unplugged and left parked it will charge the 12v battery for a short time (about 4 minutes) once every 24 hours, however if it is plugged in but not charging (battery full, or charging postponed by the charge timer) then it will apparently not do this once every 24 hour top up. So leaving a Leaf plugged in for weeks can lead to the 12v battery going flat ironically. The moral here is that if you have to leave the car unused for many weeks leave the traction battery between 50-80% and do not plug the car in. That way it should give the 12v battery a quick top up once every 24 hours.

2) If you turn the car on in "accessory mode" by pressing the power button twice instead of holding the brake pedal the traction battery is not engaged and the 12v battery is not being charged, despite the entire dashboard being lit up and most functions (including wipers, blower fan, headlights etc) working. If you use the car for long like this it would be easy to discharge the 12v battery. The only real clue that you're in accessory mode instead of fully on (apart from heater / A/C not producing hot/cold air) is the red battery warning light on the dash. (On a 30kWh model anyway) So don't do that. If you are going to sit and wait in the car and want the fan and/or radio on, turn the car fully on so that the 12v battery is being maintained. I suspect a lot of Leaf flat batteries are due to people sitting in accessory mode for a long time.

3) As mentioned, the Leaf will maintain the battery at only about 13 volts a lot of the time you drive the car. This is a "neutral" voltage for a Lead acid battery. It will neither allow it to discharge or charge it. However certain accessory systems will force the battery voltage up to 14.4 volts, including operating the windscreen wipers which will immediately push it up to 14.4. So in a car where the windscreen wipers are used a lot (rainy weather...) the 12v battery is going to be kept better charged than in dry weather where it lets it drop back to 13 volts most of the time.

4) Timed climate control (plugged in) or remote climate control (whether plugged in or not) will bring the 12v battery up to 14.4 volts and keep it there from what I've seen. So if you make use of timed/remote climate control each day as I do you're giving that 12v battery a bit of extra charging it normally wouldn't have.

The 12v battery maintenance logic in the Leaf is definitely a bit strange (the bit about not doing the topups if plugged in in particular) but if you avoid certain situations like using accessory mode or leaving the car plugged in for weeks then you may be fine.
 

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I did leave mine unplugged when I was away for a few weeks, I think 5 weeks was the longest stint and didn't have any issues.
 

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Amongst all the dash displays, is there anything that tell you the state of the 12v battery?
On the 24 and 30, no.

Apart from the 12v warning light which appears if the car is in accessory mode or presumably if the battery is very flat.

You'd need either Leafspy or a cigar lighter socket volt meter to measure the 12v battery voltage from inside the car.
 
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