FalconFour
Well-known member
I was a little stuck between posting this in "technical discussion" forum or the heavier-traffic General forums. This seems to be a topic very relevant to troubleshooting, less about benign technical details. Here, I'll explain how the Leaf's 12V system behaves, and why those of us without 12V solar panels on the back are screwed so bad.
https://www.youtube.com/watch?v=9VuTDCjMzRw
The problem here is that the 12V battery is maintained at a 13.0V trickle charge with a very brief 14.4V boost charge. First of all: 14.4V is the typical charging voltage for lead-acid batteries. Lower than that, they either charge very slowly, or they don't charge at all. There's a threshold voltage that has to be exceeded in order to actively charge the battery. It's possible that the 14.4V charge is continued until the battery is only absorbing a low number of amps, before it drops down to 13.0V.
The Leaf's DC-DC system (which acts as an "alternator" for converting high-voltage ~380v DC into low-voltage 12v DC) is entirely controlled by the computers in the Leaf - with very precise control. The computer uses a single PWM signal to tell the DC-DC unit exactly what voltage to provide to the 12V system - I think between 10V and 15V DC. See here: https://www.youtube.com/watch?v=BMhcHkOg-Mk
The problem is that, while 13.0V will make all the systems run off the DC-DC power (instead of discharging the 12V battery constantly), it will not actively charge the 12V battery. It'll just maintain the charge level it was last left at after the 14.4V charging phase ramps down.
This is also contrary to what the Nissan EV techs (at the hotline) have told people in the past, as well as what the manual seems to indicate - that charging the 12V battery is only done while in "Ready" mode, and that regular L2 charging, quick charging, and remote climate control won't engage charging. That's a complete load of nonsense. It appears to execute the same charging behavior any time the HV battery is involved for any reason - charging, climate control, and driving alike. It may also perform a maintenance charge every couple weeks, as I've heard, but I don't feel like waiting around a couple weeks with a camera and multimeter to find out
So, while the Leaf does this quick little burst to 14.4V, it seems like it maintains only 60-70% SOC in the lead 12V battery, which subjects it to sulfation and an early need for replacement. Additionally, little bugs in the Leaf's software tend to make it occasionally, and randomly, not go completely to "sleep" after 2 minutes - as seen in the video here. With such a limited 12V capacity available, that lack of deep-sleep makes the 12V battery completely die, and you're then left with a car that won't start (or you may think you need a new 12V battery at that point), when really all you need is a full, proper 12V battery charge.
Leaving the 12V battery at ~60-70% SOC is great for those Leaf owners with a solar panel in the rear, but it's a recipe for disaster for those without. Nissan should've provided a different algorithm (13.5V at least, 14~14.4V optimally) for models without a solar panel, so that the 12V battery is properly maintained and continuously left at a full charge as lead-acid batteries prefer. At voltages like those seen in my video, sulfation is occurring constantly in the battery.
Thoughts?
(edit: OH MY GOD this forum has GOT to stop chopping-up partial words and turning them into links... now I look like a Volt advert - aargh!)
https://www.youtube.com/watch?v=9VuTDCjMzRw
The problem here is that the 12V battery is maintained at a 13.0V trickle charge with a very brief 14.4V boost charge. First of all: 14.4V is the typical charging voltage for lead-acid batteries. Lower than that, they either charge very slowly, or they don't charge at all. There's a threshold voltage that has to be exceeded in order to actively charge the battery. It's possible that the 14.4V charge is continued until the battery is only absorbing a low number of amps, before it drops down to 13.0V.
The Leaf's DC-DC system (which acts as an "alternator" for converting high-voltage ~380v DC into low-voltage 12v DC) is entirely controlled by the computers in the Leaf - with very precise control. The computer uses a single PWM signal to tell the DC-DC unit exactly what voltage to provide to the 12V system - I think between 10V and 15V DC. See here: https://www.youtube.com/watch?v=BMhcHkOg-Mk
The problem is that, while 13.0V will make all the systems run off the DC-DC power (instead of discharging the 12V battery constantly), it will not actively charge the 12V battery. It'll just maintain the charge level it was last left at after the 14.4V charging phase ramps down.
This is also contrary to what the Nissan EV techs (at the hotline) have told people in the past, as well as what the manual seems to indicate - that charging the 12V battery is only done while in "Ready" mode, and that regular L2 charging, quick charging, and remote climate control won't engage charging. That's a complete load of nonsense. It appears to execute the same charging behavior any time the HV battery is involved for any reason - charging, climate control, and driving alike. It may also perform a maintenance charge every couple weeks, as I've heard, but I don't feel like waiting around a couple weeks with a camera and multimeter to find out
So, while the Leaf does this quick little burst to 14.4V, it seems like it maintains only 60-70% SOC in the lead 12V battery, which subjects it to sulfation and an early need for replacement. Additionally, little bugs in the Leaf's software tend to make it occasionally, and randomly, not go completely to "sleep" after 2 minutes - as seen in the video here. With such a limited 12V capacity available, that lack of deep-sleep makes the 12V battery completely die, and you're then left with a car that won't start (or you may think you need a new 12V battery at that point), when really all you need is a full, proper 12V battery charge.
Leaving the 12V battery at ~60-70% SOC is great for those Leaf owners with a solar panel in the rear, but it's a recipe for disaster for those without. Nissan should've provided a different algorithm (13.5V at least, 14~14.4V optimally) for models without a solar panel, so that the 12V battery is properly maintained and continuously left at a full charge as lead-acid batteries prefer. At voltages like those seen in my video, sulfation is occurring constantly in the battery.
Thoughts?
(edit: OH MY GOD this forum has GOT to stop chopping-up partial words and turning them into links... now I look like a Volt advert - aargh!)