LEAF's 12V battery behaviors - and why they go bad

Well I am having second thoughts about the LifePO battery after watching Leaf Spy's voltmeter more. I watched it this morning and for a good part of my commute it was holding at 14.6v. I definitely don't want a fire so I will need to inspect the battery when it gets here, I don't even know if it has a BMS, I just recognized my favorite brand and clicked buy now. It is designed for motorcycles so I assume (hope) it has a way to deal with higher than normal LifePO4 voltage.

Also this is an interesting data point that suggests Nissan changed a few things. I don't know what model year this is but mine is a later 2013 that never had any recalls issued.
http://www.mynissanleaf.com/viewtopic.php?f=30&t=12448&start=190#p482871

VitaminJ said:

Durandal said:

That's a very good point. I certainly intend on swapping my Pb battery for a LifePO battery once the Pb battery fails. I do charge it every couple of months on a 2a charger when I think about it to desulfate the battery a bit.

Click to expand...

Mine died over the weekend, brought it back to life, but I just ordered this as a replacement:
https://www.amazon.com/gp/product/B01J9XN49M/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

Gens-Ace Tattu is about the best brand of lithium battery I've ever used. I have sold hundreds of them all over the world and haven't had a single failure.

Click to expand...

VitaminJ said:

Well I am having second thoughts about the LifePO battery after watching Leaf Spy's voltmeter more. I watched it this morning and for a good part of my commute it was holding at 14.6v. I definitely don't want a fire so I will need to inspect the battery when it gets here, I don't even know if it has a BMS, I just recognized my favorite brand and clicked buy now. It is designed for motorcycles so I assume (hope) it has a way to deal with higher than normal LifePO4 voltage.

Also this is an interesting data point that suggests Nissan changed a few things. I don't know what model year this is but mine is a later 2013 that never had any recalls issued.
http://www.mynissanleaf.com/viewtopic.php?f=30&t=12448&start=190#p482871

VitaminJ said:

Durandal said:

That's a very good point. I certainly intend on swapping my Pb battery for a LifePO battery once the Pb battery fails. I do charge it every couple of months on a 2a charger when I think about it to desulfate the battery a bit.

Click to expand...

Mine died over the weekend, brought it back to life, but I just ordered this as a replacement:
https://www.amazon.com/gp/product/B01J9XN49M/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

Gens-Ace Tattu is about the best brand of lithium battery I've ever used. I have sold hundreds of them all over the world and haven't had a single failure.

Click to expand...

Click to expand...

The features listed on Amazon include:
5. Accurate LED battery indicator and built-in charge and equalizing protection board with over-charging&over-discharging, over-current, short-circuit protection.

That's why I like Gens-Ace! :mrgreen:

Well I got the 5Ah LifePO4 that I posted. So far everything is as described. It came at 13.3v and the instructions said if it was above 13v it can be used right away. Leaf Spy showing 12v system voltages about the same as before and I will keep monitoring, especially while sitting on a CHAdeMO with all the accessories going (I believe CHAdeMO doesn't charge 12v at the same time).

The battery is much smaller and required some spacers. I used an old aluminum arduino case and a cut down block of wood. The new battery is 2lbs which represents about 18lbs savings over the lead acid battery right over the front end (and driver's side!). I also had to buy some aftermarket screw-on battery posts as it only came with a couple 10mm bolts.










e: I should note that I charged the original battery full on the bench and then put it in the back of the car along with a 10mm wrench in case I need it!

VitaminJ said:

Well I got the 5Ah LifePO4 that I posted. So far everything is as described. It came at 13.3v and the instructions said if it was above 13v it can be used right away.

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Oh that is fantastic! Looks awesome, and I'm thinking of replacing mine with that now, too. When it comes time, that's probably exactly what I'll do

Leaf Spy showing 12v system voltages about the same as before and I will keep monitoring, especially while sitting on a CHAdeMO with all the accessories going (I believe CHAdeMO doesn't charge 12v at the same time).

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Aha... but not. Literally any time the HV battery is engaged, it's running the standard charge cycle, same as driving, L2 charging, L1 charging, preheating/cooling, quick charging... um, did I miss any? But yeah, it does the same charge cycle (14.4v down to 10 amps -> 13.0v) any time.

What I'd be concerned about is that the Leaf's DC-DC actually selects a voltage, so if it goes to 14.4v, pumps charge in until it draws under 10 amps, then decreases to 13.0v, the lithium battery could begin supplying power to the car's systems (as the Leaf's DC-DC is only trying to run 13.0v, the now-higher 12v battery will begin discharging) until its standing voltage goes below 13.0v. Lead batteries just *pff* stop cold at 13.0v; their standing voltage is never over that but for a couple seconds after stopping a charge, thus that wouldn't be a problem with lead... but lithium is very linear. It could be doing a 80~100~80% sweep every time you start/drive/stop the car...

It'd be interesting to put a clamp meter on it and watch which way the power flows, and what the Leaf starts holding its voltage at...

I have noticed when the DC converter changes voltages to a lower voltage, it will "hang" for a bit. Before it would go from 14.4v to ~13v almost instantly, now it will slowly count down from 14.4 and will linger in the mid 13 range. I'm almost positive that this is what you're describing. Is this harmful to the battery? I don't think so. Also I have been paranoid and constantly popping my hood and feeling the battery, but it's always ambient, never warm.

Really it's hard to tell if it'll be bad... it's quite an unusual charge/discharge pattern for a lithium battery. It'll sit most of its life at around 80% charge, but with a heavy hammer to 100%* then immediately getting sucked down to 80% (as it briefly powers all the car's 12v systems during that ramp-down). It might not really be harmful at all, especially given that low depth of discharge and that it spends most of its life at a comfortable below-100% level. Even while charging for a long time or driving for a long time, the car will bump it to 100% then level off at 80% for the rest of the time.
* - unlike lead batteries, which slowly absorb the charge while sitting at 14.4v, a lithium battery just eats energy as its voltage increases. So if the battery ever goes totally dead, the Leaf will really ram a charge into that thing FAST! Limited only by the resistance on the lines. It'd be interesting to see how it responds...

80% may not be quite right, either, just a guesstimate from voltage levels -- round-about 3.275v/cell in a 4-cell LiFePO4 pack at 13.0v. 14.4v puts it at 3.6v/cell which is ideal full-charge voltage, and there's a bit of a top-off curve to those cells (between 3.3v and 3.6v is a steep spike based on SOC%). I think 2.0v is fully discharged, and between about 2.9v and 2.0v is another steep curve.

Er, guess I could just Google a graph...


^^ yea yea, that.

So it's probably not doing any damage... hell, in some regards, it might be doing some good

Well I'm about 3 weeks in with my 5Ah LifePO4 battery and I've had no issues whatsoever with it. In fact I've noticed a big improvement in brake feel at low speeds, the brake booster is electrically actuated, not hydraulically by your foot pressure like most other cars so any change in voltage will cause a change in braking pressure. I had noticed jerky brakes, especially at slow speeds where I would keep constant pressure on the pedal but the braking force would increase and decrease suddenly. This is different from the braking/regen mixing. Since I've changed to the Life the braking has been super consistent. I'm chalking that up to the lithium battery's ability to delivery high current and have very low voltage sag under load compared to the lead acid, especially a weak lead acid.

I have been regularly checking the voltage while driving and after parking the car without driving over the weekend and the lowest I've seen it when it was sitting outside for over 24 hours at about 10F and it was 13.2v. Most of the time at rest it is 13.4v or higher, especially if I have been driving it. Heat, A/C, accessories, etc all function normally. My Carwings has worked 100% for the last 3 weeks as well.

Disconnecting and reconnecting the 12V battery usually improves low speed braking feel for a while if they've gotten grabby. It has nothing to do with the actual battery, the DC-DC converter can supply plenty of power for all 12V systems.

drees said:

It has nothing to do with the actual battery, the DC-DC converter can supply plenty of power for all 12V systems.

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Then why does the 12v battery exist in the first place? Why does the 12v system voltage drop momentarily when load is applied like AC or heater? That's where the jerkyness comes from; press on the brakes, 12v system voltage dips for a moment until DC converter increases power, voltage climbs back up, and the brakes grab harder than they originally did.

VitaminJ said:

drees said:

It has nothing to do with the actual battery, the DC-DC converter can supply plenty of power for all 12V systems.

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Then why does the 12v battery exist in the first place? Why does the 12v system voltage drop momentarily when load is applied like AC or heater? That's where the jerkyness comes from; press on the brakes, 12v system voltage dips for a moment until DC converter increases power, voltage climbs back up, and the brakes grab harder than they originally did.

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12V battery is there only for the period when DC-DC converter is switched off.
It does absolutely nothing when vehicle is fully operational (except it accepts charge if it has been discharged, less than 5% load for DC-DC converter).
Devices on 12V system can not feel anything if you just randomly disconnect 12V battery. I bet Leaf will not notice :lol:
Don't do it.

arnis said:

12V battery is there only for the period when DC-DC converter is switched off.
It does absolutely nothing when vehicle is fully operational (except it accepts charge if it has been discharged, less than 5% load for DC-DC converter).
Devices on 12V system can not feel anything if you just randomly disconnect 12V battery. I bet Leaf will not notice :lol:
Don't do it.

Click to expand...

https://www.youtube.com/watch?v=bO5EdEbnkLw

If DC converter is off then 12V system voltages goes below 12.6V (resting voltage of the 12V lead acid battery while being around 80% charged).
DC converter has two modes, 13V mode and 14.5V mode (approximates, depends on ambient temperature also).

Actually it is already new that you were able to engage 14.5V mode without wipers. But yes, usually Leaf keeps DC-converter at 13V setting.

BTW, this voltage reading with the app, it is not the fastest reading. True reading should be taken with a multimeter on the battery terminals. Possibly data is from a module, not from the battery (therefore some voltage drop possible).

Switch the vehicle off while keeping leafspy active. Then you can observe real battery voltage. Also switch on headlights while vehicle off. This will increase load. Do not charge the vehicle, this will also engage DC converter at 13V or 14.5V mode.

arnis said:

It does absolutely nothing when vehicle is fully operational (except it accepts charge if it has been discharged, less than 5% load for DC-DC converter).
Devices on 12V system can not feel anything if you just randomly disconnect 12V battery. I bet Leaf will not notice :lol:

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arnis said:

Possibly data is from a module, not from the battery (therefore some voltage drop possible).

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:mrgreen:

Turn on your wipers, voltage will climb to 14V, but there will be no change in braking performance.

The 12V battery is just there to boot the car up. It just sits there once the car is started and the DC-DC tops it off.

VitaminJ said:

arnis said:

It does absolutely nothing when vehicle is fully operational (except it accepts charge if it has been discharged, less than 5% load for DC-DC converter).
Devices on 12V system can not feel anything if you just randomly disconnect 12V battery. I bet Leaf will not notice :lol:

Click to expand...

arnis said:

Possibly data is from a module, not from the battery (therefore some voltage drop possible).

Click to expand...

:mrgreen:

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Correct. No voltage drop due to charged battery being there or not. Voltage drop is due to wiring and load on that wiring. When you switch on consumers voltage drops. Has nothing to do with the battery in case of Leaf when DC-converter is active (13 or 14 volts).

arnis said:

Correct. No voltage drop due to charged battery being there or not. Voltage drop is due to wiring and load on that wiring. When you switch on consumers voltage drops. Has nothing to do with the battery in case of Leaf when DC-converter is active (13 or 14 volts).

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I just proved you were wrong that the voltage drop happens. I know you're trying to save face, but come on... If the voltage of the 12v system drops below the resting voltage of the 12v battery, then the battery is taking up some of that slack until the DC-DC converter ramps up. I proved there was a voltage drop, why don't you yank your 12v battery and prove that everything is just fine?

That voltage spike might just be a reading, not a real voltage spike at battery.
You NEED to use multimeter as a minimum.

I've measured current at the battery terminal with DC-DC clamps.

DC-DC converter is not that slow. At least on 2014 Leafs (never seen negative spike).
But we need voltmeter data on the battery terminals (cigarette lighter socket voltmeter
is not good enough due to wiring between battery and the socket).

I will throw out there again that Arnis European car is again different from our US cars (if what he is saying is correct). There is a lag in MY car between a load coming on and the DC-DC converter picking up the load. I was using a Fluke 87 connected to the battery and was testing at the time for my inverter setup. While the DC-DC is pretty quick when I would turn on a 750w load on the inverter I would see a dip to the mid 11's for about a second or so and then I could see the voltage ramp back up to the 14v in a few seconds time. With a 1500w load it dropped in to the high 10's for a second. Granted these are not loads the car would normally see, but the battery is certainly needed even while in ready mode as the DC-DC converter isn't instantaneous. Which totally makes since and behaves the way I would expect, I don't see this as a flaw or something not being right.

BrockWI said:

I will throw out there again that Arnis European car is again different from our US cars (if what he is saying is correct). There is a lag in MY car between a load coming on and the DC-DC converter picking up the load. I was using a Fluke 87 connected to the battery and was testing at the time for my inverter setup. While the DC-DC is pretty quick when I would turn on a 750w load on the inverter I would see a dip to the mid 11's for about a second or so and then I could see the voltage ramp back up to the 14v in a few seconds time. With a 1500w load it dropped in to the high 10's for a second. Granted these are not loads the car would normally see, but the battery is certainly needed even while in ready mode as the DC-DC converter isn't instantaneous. Which totally makes since and behaves the way I would expect, I don't see this as a flaw or something not being right.

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Right. The 12V battery voltage drop, i.e. without the converter, is a function of its output impedance (battery
size, temp, & condition) and the load placed on the battery. All batteries exhibit this, e.g. even my Leaf's 24kWh
battery exhibits a voltage drop under loads based on its output impedance:

Lowest Temp (11 degrees C) - 103 mohms - 15K miles (delta @ 250 amps = - 26 volts)
Highest Temp (32 degrees C) - 56 mohms - 18K miles (delta @ 250 amps = - 14 volts)

Determined by LeafDD under full load (max motor power) from a standing start.

So given the response time of the converter, 12V battery, and loads, one would expect a drop in the 12V battery
voltage as function of switching loads. So based on that, it's possible that under certain intermittent loads
and the condition of the 12V battery some Leaf ECUs could see a battery voltage that results in their being
placed in an unknown state requiring a disconnecting & reconnecting of the 12V battery to allow a normal
boot-up process, e.g. the rise time of the 12V battery from zero volts (ground).