Leaf Service EV System Adventures with P0AA6-1A and a Suspect 12V Battery

[CaptLarry]

I try not to argue with the opinions here when it is subjective. I will present counter-arguments in the presence of evidence though. The first claim that the Leaf 12V charging system is setup to only work with FLA batteries is probably not correct. No one that I am aware of has the engineering documents for the Leaf, so none of us know exactly what Nissan intends to do exactly, but we can infer based on the evidence of what we can measure and observe ourselves.

For starters, if the Leaf was intended to only work for FLA batteries, it would not be floating the 12V battery at 13.0V To properly take care of the FLA, the voltage would need to float between 13.5V and 13.62V Most Manufacturers recommend 2.25V to 2.27V per cell when the battery is "warm", like room temperature warm. When the battery is colder, the float voltage has to be higher to protect it's capacity. When the battery is hot, the float voltage has to be lowered, again to protect capacity. AGM batteries can float at a lower voltage, usually the recommended range is 13.2V to 13.6V. But, again the Leaf is not doing this. So, the Leaf fails on that compared to a typical ICE vehicle alternator that is keeping a constant +14V going on the battery at all times.

Next the Leaf never fully charges a FLA battery because it is charging based on a combination of voltage and power readings made by the battery current sensor. That is the logical way to protect the battery from being over-charged because the Leaf open voltage is 15V. This works fine when a FLA battery is brand new. As the FLA battery ages, the amount of power it can recharge with decreases. This means there is a way to get both a high charge voltage and small charge current that the system can not account for, instead it makes a best guess on the state of charge; of the 12V battery. Because there is no system counting power in and out of the 12V system, the Leaf has no idea if the 12V battery is actually fully charged. It's very easy to blast a FLA battery to 14.4V with 100 amps of power but there is a point where the amount of power going in will have to be decreased to prevent the voltage from climbing too high and causing capacity damage. Again, the Leaf is guessing, it does not know exactly how much power was in the FLA battery and it does not track how much it is putting in. It is working only on the voltage and current. The Leaf does not try to keep a 12V battery topped off like a battery maintainer would.

Given the above statements, the Leaf wasn't designed to only work with FLA or AGM batteries, it was designed to be compatible with them. Otherwise, why go through all the coding to make such a complicated charging system? Why not just blast +14V and be done with it? The Leaf could easily do that and it would be a much simpler system. The reason is safety related. Most vehicle alternators max at 100 amps. More of an average, you can find plenty that do less or more I'm sure. They are tuned to +14V and have the added benefit that the battery itself is the current limit along with the physical load on the alternator pushing back. The whole system has over a century of research and development to match up FLA batteries to ICE vehicle usage quite well.

Let's think about an engineering perspective on the Leaf. It's 200x and the Leaf needs a 12V power source to run everything except the drive motor and HVAC. The only viable technology that has years of testing and mass production to boot is FLA batteries. Seems like an easy choice, you only need it to switch on the main relays and then the HV battery takes over from there. Sure, now how do you charge the 12V battery back up? Well, use some power from that fancy 12V system you created. Ok, but how do this in a safe way? You can't just blast in +150 amps into the battery everytime. What if the battery was freezing? What if the battery has capacity damage from age or manufacturing issues? You need a way to control how much power you are forcing in. You also need to run all your 12V accessories without having them fail from voltage that is too high or too low. Nissan didn't built two "12 Volt" systems, they are using the same one to do both. That makes it literally impossible to have something that is designed around a certain battery chemistry and yet also runs the accessories at the same time with a regulated and predictable power level. That's why the whole system is a compromise at 13.0V. It keeps the accessories at a consistent power level and it is close to, but not quite there for keeping a 12V battery at the maximum state of charge. Nissan uses some workarounds like trying to get the 12V battery to 14.4V as fast as it can before leveling off at 13.0V but it's usually not enough time to keep any FLA or AGM battery at the maximum state of charge. Just like it's bad to leave a Lithium battery at 100% SOC for long periods of time, the opposite is true for FLA / AGM batteries, they need to be at 100% SOC all the time.

Where does LiFePO4 fit into all of this? It has the advantage of a higher operating voltage nominal voltage of 12.8V instead of 12.0V for the FLA / AGM family. That means it takes a higher voltage to damage the battery capacity, +14.7V or higher to be specific for most manufactuers. The Leaf doesn't have an issue with over-charging the 12V battery though, its problem seems to be not keeping a high enough state of charge. Again, LiFePO4 to the rescue, as part of the Lithium battery family, it also does not need to be kept at 100% state of charge all the time, it's operating range is 20% on the low end to 80% on the high end for the state of charge, that is where it functions best. It also has thousands more charge cycles than the Lead battery family, thus extending its life much longer. It also has a lot less self-discharge than the Lead family of batteries. Finally, most modern drop in 12V starter battery replacments using LiFePO4 have a BMS protection board that limits how much charging or discharging the battery can take to further preserve life and to act as a safety backup.

To claim that using a LiFePO4 is going to cause you issues with your Leaf or leave you stranded flies in the face of actual real world evidence from it's users both here on this forum and elsewhere around the world. The opposite is actually true, Leaf owners using a LiFePO4 have reported reliability that puts the FLA / AGM batteries to shame. It seems obvious that the design of the 12V system in the Leaf wasn't created specifically for 12V LiFePO4 batteries, but they just happen to operate very well in a Leaf because of its 12V charging limitations that disproportionally affect the Lead Battery family more than the Lithium battery family.

Using a LiFePO4 isn't a cure-all for the strangness that is the Leaf 12V system, but it seems to be able to operate best in that environment. That doesn't mean crappy LiFePO4 don't exist, it just means that with anything else in life, you get what you pay for. If I buy a cheap FLA or AGM it might only last a few years while a high quality version is able to live on for much longer. I know there are places that sell Lead Family batteries that can beat the OEM Nissan battery to death in reliability and lifespan just like the 12V LiFePO4 battery I chose +9 years ago continues to work today across (2) generations of Leaf syles and has never let me down in the coldest weather we get where I live 0F (-17.7C).

Finally, despite all the talk about the limitations of LiFePO4 in terms of performance and temperature, what often seems to be ignored is that FLA and AGM are also limited in cold weather performance. Most manufactures don't recommend charging them if the battery temperature is below 32F (0 C) because of (surprise ), capacity damage. At least the Leaf will reduce charging current to a trickle when it detects freezing outdoors temperatures, do you think a car alternator does that?

this is an interesting and well reasoned post. Most of the statements about Lead Acid batteries are correct. I think I did suggest that an AGM designed for deep cycle applications would be a better fit than a typical starting type automotive FLA. However, it is quite true that lead acid batteries do want to see 13.6v charging ... however deep cycles can and do cycle many 1000's of times between 10.5 and 13 volts... with 11 being a better minimum. The problem with LiPo's is the huge number of pedlers of dangerous junk. I am glad to have read your post... as I made assumptions about the charging ..Larry

 

[GreaterInfinities]

Does the '18 have an "electronic parking brake"? I know the early ones did, the mid ones like my 2015 have the foot pedal I have no idea about the later ones.
Don't over look battery post connections , dirty connections that can't handle current can still effect Leafs just like a conventional car.
I think the important take a way is do the work, don't guess.
On a side note, I was trying to read up on the P0AA6 code because it is a bad one if the condition exists for repair, not cheap and easy. Dala's video shows his, and while he didn't list costs they weren't small, and that doesn't include his labor.
Anyway in my reading, and I'm not 100% sure on this yet. A true P0AA6 code isn't only for the HV battery, but could be anywhere in the HV system. Unfortunately, just about any componant in the HV system is going to be costly. So PDM/inverter, Charger, AC compressor and drive motor all could have a HV leak and set the code. I also looked to see if it said how much of a "leak" had to be present to toss the code, and so far haven't found that info.
Please follow up with all you find out.

I'm experience a P0AA6 and restart inhibition each time I try to start my car after charging. There has been a steady decline of State of Charge percentage that my car will charge up to. Initially, my car would charge well into the 80s% and now it is in the low 70s% before it stops charging and throws the codes. My car works like normal except for after charging. My question to you is if you know anything about troubleshooting this problem and how to isolate which part of the HV system is causing the problem. Failing that, if you could recommend a technician I can reach out to. Thanks in advance.

 

[cornbinder89]

Dala did a coupla video's on how he can find if the problem is in the battery. This test will not help if the isolation problem is outside the battery.
Let see if I can link to them.

 

[nlspace]

As usual those are great videos, but what was the root culprit?

The voltage leak to chassis seemed to disappear when he relieved the clamping force holding that module. Too bad he didn't do the megger test while it was still "broken".

Also from the chassis voltage readings to the positive and negative terminals he could pinpoint which Cell unit in the module was "leaking". Maybe something sharp internally punctured into the pouch enough to make contact with the exterior shell.

Since Dala replaced the module he still has the old one and could do a teardown to investigate the Cell.

 

[cornbinder89]

The case that battery was in had a dent in the bottom, as it came from a wreck. I'm sure he could have broken it down and tried to solve the problem, but the work to take the battery down and disassemble, if it comes back, wasn't worth the chance, so he replaced.
One of those things: How much is your time worth and are you prepared to do the job over?

 

[nlspace]

oh i agree in the interest in getting the car fixed quickly and avoiding a come-back, the best solution was to swap out the bad parts.

But my comment was geared toward finding the root cause afterwards if possible, especially since a nissan tech told him they had swapped out several modules and it was always the one in that same location.

There is another thread of a car in the Bahamas with this DTC--might it have an issue with this same module?

The megger test was not conclusive to me since it was reading off the scale on the high side (>500 M). i carry a 20 Meg 1% resistor in my megger kit in order to do a quick calibration and sanity check. It's hard to prove anything with a negative result or lack of a reading, especially since there was a direct voltage reading to chassis earlier.

 

[cornbinder89]

We have too little information to make any accurate guess. In Dala's case, if I were he, I wouldn't be sending the offending stack to a recycler, but keeping it around, may be disassembling to 2 cel packs, and either using to good, non leakers for power storage or to repair other stacks.
Again it only makes sense if you make a business of working with these batteries, otherwise replace the offending pack and put the car together once.

 

[GreaterInfinities]

oh i agree in the interest in getting the car fixed quickly and avoiding a come-back, the best solution was to swap out the bad parts.

But my comment was geared toward finding the root cause afterwards if possible, especially since a nissan tech told him they had swapped out several modules and it was always the one in that same location.

There is another thread of a car in the Bahamas with this DTC--might it have an issue with this same module?

The megger test was not conclusive to me since it was reading off the scale on the high side (>500 M). i carry a 20 Meg 1% resistor in my megger kit in order to do a quick calibration and sanity check. It's hard to prove anything with a negative result or lack of a reading, especially since there was a direct voltage reading to chassis earlier.

The other thread about the car in The Bahamas is mine I will have a look at Dala's videos to see if anything resonates but I'm putting feelers out for a tech that would be willing to help diagnose remotely or traveling here. Thanks all.

 

[cornbinder89]

The other thread about the car in The Bahamas is mine I will have a look at Dala's videos to see if anything resonates but I'm putting feelers out for a tech that would be willing to help diagnose remotely or traveling here. Thanks all.

I am not that person! I rarely venture more than 15 miles from home, and can't do 1/4 of what I used to.
As I stated in another post, there is a real business opportunity for some younger fellow to set up EV repair shops. There are a few, and some seam real good and dedicated, but not enough of them.
It really can't be someone who works on "conventional" vehicles also, the learning curve for EV's is a steep one and you can be master of all trades.

 

[GreaterInfinities]

I am not that person! I rarely venture more than 15 miles from home, and can't do 1/4 of what I used to.
As I stated in another post, there is a real business opportunity for some younger fellow to set up EV repair shops. There are a few, and some seam real good and dedicated, but not enough of them.
It really can't be someone who works on "conventional" vehicles also, the learning curve for EV's is a steep one and you can be master of all trades.

Thanks for the advice, much appreciated.