If you've had issues with the 12V battery in your leaf failing prematurely, go out and unplug the low voltage connector on the negative terminal.

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If it will help I understand Tesla has also had problems utilizing 12 volt lead/acid batteries; their approach to address the problems is to implement a very small lfp battery to 'keep the wakeup circuits active and to use the DC/DC converter to charge the smaller battery as necessary.

They are also moving to 48v for their accessories circuits.
Of course this won't help us with our present crop of Leafs; but, might help Nissan design thinking on future offerings.
After replacing three batteries in my 2011, including an AGM and a fourth replacement in my 2019, I switched to a 12v LFP about a year and a half ago. So Far, So Good..
 
The connector is present on all generations of the LEAF from what I can tell from a cursory search. I have not confirmed if the behavior is similar for the earlier years, but based on the issues with the 12V battery being consistent through all generations, it seems likely that it would be have similarly.

Nope, no errors on the car or Check Engine Light. Leafspy 12V amperage display shows -128A as that data is apparently pulled from this sensor, but I don't think loss of that info is a problem for most users. The 12V voltage is still displayed correctly.
My 2012 Leaf does not have any of the connectors or small wires shown in your picture. The 12V battery typically lasts about 3 years.
 
My 2012 Leaf does not have any of the connectors or small wires shown in your picture. The 12V battery typically lasts about 3 years.
Ah, that's unfortunate. It does appear to be present on at least some pre-2018 models.

EDIT: Here's a 2013 engine bay, so 2011/2012 do not have this connector, but 2013 and newer should.
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What is being unplugged? I see a plug next to the blue wire connected to a 10mm bolt. Just unplug that?

do you also unplug the blue wire?
 
I am assuming the thin blue wires are the battery monitor that is being used to plot the data used in the opening post.......It it the bigger black connecter that would have plugged in next to the negative battery terminal that has been unplugged.
 
I have had 3 LEAFs (2013,2018,2018). Each of them had 12V batteries that lasted around 5 years before either throwing error codes or going dead and requiring a jump. Should these batteries be expected to last longer? The lives of these batteries appear to be similar to my previous ICE vehicles. If the batteries in the examples above are only lasting 2 years then I suspect its more of a manufacturing defect in the 12V battery of the battery instead of charging logic OR some small load that is incorrectly kept on all the time. Example of a small load: one 2018 MY I still have...had its 12V replaced as I left a obd2 reader plugged in for over a week. From what I know about lead acid batteries even small charging cycles over a few years will eventually cause the battery to stop taking charge.
 
I have had 3 LEAFs (2013,2018,2018). Each of them had 12V batteries that lasted around 5 years before either throwing error codes or going dead and requiring a jump. Should these batteries be expected to last longer?
It's hard to say if they should last longer than in an ICE car as we just don't have the decades of data across dozens of brands and thousands of models like we do for gas cars. The use of the 12V battery is significantly different than it is in a car with a starter load, but I can't tell you if that will shorten, lengthen, or have no change on battery life. Lead-acid battery design is based around their use in ICE engine bays, but I can't say the features that prolong life in those conditions negatively or positively influence it's life in an EV. Though generally, with the decreased peak temperature it experiences (no 900F exhaust system inches away in some cases) I would generally expect it to be similar or longer.
The lives of these batteries appear to be similar to my previous ICE vehicles. If the batteries in the examples above are only lasting 2 years then I suspect its more of a manufacturing defect in the 12V battery of the battery instead of charging logic OR some small load that is incorrectly kept on all the time.
After seeing the same behavior in 3 batteries on my car, I don't think the defect is in the battery itself. Charging logic is suspect, though I can't tell if it is the logic itself, or the data coming from the voltage and current sensors that are potentially faulty (sensors failing or being defective).

Example of a small load: one 2018 MY I still have...had its 12V replaced as I left a obd2 reader plugged in for over a week. From what I know about lead acid batteries even small charging cycles over a few years will eventually cause the battery to stop taking charge.
Considering that I drive and charge my car daily and still saw the decrease in charge of the battery over time, it's not the draw that's the issue, it's the fact that the car never tops off the battery, or even fully replenishes the charge to the state from the previous day (see the graphs in the first post).

I disconnected this plug at the beginning of February and have left it unplugged since that day, and I've had zero further issues with the battery charge/voltage being maintained where you'd expect it to be for a lead-acid battery.

Until Nissan decides to evaluate the ongoing failure of the 12V system in many of these cars, I'll keep mine unplugged. As they keep saying they're going to be discontinuing the model, I don't expect them to do anything. Considering we've seen this issue on the LEAF for many years now with just the excuse that the battery needs to be replaced (my first battery lasted less than 2 years, but since the car was over 36K miles, they refused to replace it under warranty), I don't expect Nissan to ever issue a TSB or recall to address whatever it is, whether that be the programming, sensors, or other components that are the root cause.
 
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It's interesting that the Leaf Spy reports a voltage almost 2 volts lower than what I measure at the battery terminals...🤔
Interesting, have you had issues with the 12v battery prematurely failing? I wonder if an inaccurate sensor (sending a lower voltage value to the ECU), is perhaps causing it to charge differently than when it's reading correctly.
 
Thank You Sir! I am not a battery god but I do have 40+ years riding motorcycles with small, frequently under charged batteries. My fix was to make a mod to the rectifier output and run the positive output direct to the pos on the battery. Same with the negative. No more weak charges.
I have been worried about the early drop off, too.
 
Fortunately, no problems. I was just interested in the difference.
Great, that's a good data point. I do wonder if the sensor misreporting the voltage (causing the ECU to think that the battery is lower voltage than actual) is causing the car to keep it in a higher charge state (increasing the charge voltage because the circuit thinks the battery needs more charge due to the low apparent voltage). I wonder how widespread the misreporting from the sensor is?

If anyone else has a GOOD battery (no apparent issues), could check the actual voltage on the battery (via voltmeter/multimeter), and compare that to what leafspy reports from the ECU and post it here, it would be appreciated!

I'll reinstall my OBD reader and check leafspy vs actual voltage, I haven't checked in a while but I believe mine was reporting accurately, will update here.

It would be interesting if faulty sensors are leading to correct charging, and good sensors are leading to incomplete charging.
 
If it will help I understand Tesla has also had problems utilizing 12 volt lead/acid batteries; their approach to address the problems is to implement a very small lfp battery to 'keep the wakeup circuits active and to use the DC/DC converter to charge the smaller battery as necessary.

They are also moving to 48v for their accessories circuits.
Of course this won't help us with our present crop of Leafs; but, might help Nissan design thinking on future offerings.
After replacing three batteries in my 2011, including an AGM and a fourth replacement in my 2019, I switched to a 12v LFP about a year and a half ago. So Far, So Good..

This would be great, if there were a secondary power source monitoring the 12V system that could close the HV contactor to energize the inverter and power the 12V system to charge the battery and potentially let the car start in the event of main 12v battery failure (along with posting a CEL about the 12V battery failing). It would also mean the car could sit in storage for many months and would still be ready to drive without having to recharge the 12V battery to get it started.

IIRC, people have found a way to energize the 12V system independently of the main ECU, so it would be possible to design a small circuit to monitor the 12V system and energize the inverter manually as needed to top off the low voltage battery, but it would have been nice if Nissan had included that feature.
It's not exactly a complex addition and would add significant security to the car in emergency situations, as well as preventing the confusing sequence of errors the dying 12V battery causes (none of which are to check the 12v battery, an error that cars even without ECUs have been able to self-diagnose on the dash for decades)
 
Data with some labels to make it easier to correlate the events to the graphs:
View attachment 4489
View attachment 4491
Looking at that first graph -- those 14.5V bursts when the sensor is connected are far too short. It's as if the sensor is reading near-zero amps, so the car's logic immediately switches to 13 V float charge because it "thinks" the battery is fully charged (because accepting less than 1.5 A at the higher charge voltage).

(1) My first check would be that the current sensor is not unintentionally bypassed by connecting the negative pole of the battery directly to chassis ground. It's critical that all the battery current goes through that sensor.

(2) The second check is that the sensor itself could be faulty (or connections reversed?). Possibly test this by using Leaf Spy to get a readout of 12V battery current (as reported by that sensor) and an independent ammeter with about a 10A to 50A scale; compare the two values. Lacking an ammeter you could connect a known load such as a headlamp bulb across the battery while the car's keeping it charged - when Leaf Spy should show a few amps increase in the charge current (which is actually then not charging the battery, but feeding the test load). A 60W headlamp bulb should draw about 5A, for example.

If measuring is too difficult, there's always the N*ssan dealer technique of replacing the sensor regardless, and charging the customer for 2 hours' work and a 250% mark-up on the wholesale price of the part.

I'd disagree with the claim that Leaf 12V battery management is worse than most other EVs. The stand-out make for dreadful 12V issues in Britain seems to be some models of Kia and Hyundai. Leaf does seem to chew up 12V batteries every 3--5 years but that's typical for modern vehicles with lots of power-hungry electronics, whatever power system they use.
 
My 2012 requires a new battery about every 3 years. My 2012 Tesla S Lithium ion 12V's last about 5 years. Batteries in my ICE vehicles last as long as 20, even if weeks elapse between drives. The Leaf battery is happiest when daily driving keeps it topped off. If I'm going out of town for an extended period, I usually disconnect the battery (I replaced the nut that holds the power cable to the post clamp with a wing nut so it is very easy to disconnect, and I can leave the post clamp itself clamped on the battery post.) But this means that I lose some of the settings (like charging limit set to 70%).

If I'm not planning to drive in the next 72 hours I connect a trickle charger/desulfator to the battery. But battery life is still suboptimal, even with these hacks.

I do wonder how much power is consumed by the TCU (which is now essentially worthless due to no cellular connection) and have considered unplugging it, but I don't know whether any other systems would be affected.
 
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