12V Battery - Flooded Lead Acid vs. AGM vs. Lithium

[lorenfb]

Ok, some interesting data. Last night, I had a unique opportunity (for my area anyway) in which the temperatures fell to 19F. I knew this was coming, so I just left the hood up on my Leaf to let as much cold area get into the front where the 12V battery as possible. I also put a glass of water on top of my 12V battery to get an idea that it was really below freezing at that point. I came back out to the Leaf around midnight and the battery was covered in frost, the glass of water was a solid at that point, it was easily well below freezing. So, with everything turned off, I got a LeafSpy capture of what the Leaf was reading as battery voltage and current draw.

As you will notice from the first screen-shot, me sitting in the Leaf with the computer booted up in ACC mode is drawing about 3.54 amps or about (12.88 x 3.54) = 45.6 watts of power. So, make sure everything was off, no headlights, air, fan, unplug all power from the front console, and finally turn the Leaf on and just let it sit idle so I could watch how much power it would put back into the 12 volt battery.
At first, it started at about 10 amps, then quickly ticked down to roughly 0.6 amps and stayed there. I wanted to see if it would change or go back up but it was too cold to be sitting in the Leaf with no heat, so I just walked back into the house as my phone can reach that far with the Bluetooth with Leafspy. Watched the readings for about 15 minutes and it had not changed, it was still doing a "trickle" charge on the 12V battery. Doing the math, it was only feeding about (14.56V x 0.59A) = 8.6 watts of power into the battery. If this was a Flooded Lead Acid battery, I'm not sure if that would be enough to keep it topped off with a proper charge or not given how cold it was, but for a LFP battery getting below freezing means you can only charge at a very, very low rate. According to research papers, the allowable charge rate at –30°C (–22°F) is 0.02C at maximum. The warmer it gets, the amount ticks up a little until you finally reach the freezing point again. That means at -30C/-22F, my battery would max at (256 * 0.02) = 5 watts of safe charging current. Since LeafSpy was reading the motor at 26F, I'll use that for the battery too since it has a lot less thermal mass. I couldn't find a convenient online calculator for charge rate to temperature for the LiFePo4 12V battery. So I just used a research paper that had a -5C (23F) value instead where they tested to see what was the safe charge rate. It listed 0.05C for that temperature. It was a little lower than what I was seeing, but for this thought experiment, should be sufficient.
That means my 12V LFP battery should be able to safely charge at around (256 * 0.05) = 12.8 watts of power at that temperature.

It's interesting that the Leaf appears to be doing a low power trickle charge on purpose, well within the safety limits of my LFP. I'm curious now if that is just a normal behavior for all Leaf and charging the 12V battery or if it is actually lowering the current on purpose because it detects a below freezing outside temperature?
Either way, that may explain why my own and Stanton's LFP batteries have lasted as long as they have. Even when it's below freezing, the Leaf is not damaging our 12V batteries by trying to blast large amounts of current into it when it's below freezing. Either by accident or on purpose. :lol:

Based on your 12V battery charging current measurements, the initial 10 amp value seems excessive for a typical 51R AGM battery rated at 40 Ah, right?
Furthermore, you've not done a worst case analysis of the charging current over other conditions, e.g. temperatures. Without doing a worst case analysis,
one really doesn't know how the Leaf's 12V battery charging modes will affect an AGM battery life, being recommended as a replacement Leaf battery.

 

[knightmb]

Based on your 12V battery charging current measurements, the initial 10 amp value seems excessive for a typical 51R AGM battery rated at 40 Ah, right?
Furthermore, you've not done a worst case analysis of the charging current over other conditions, e.g. temperatures. Without doing a worst case analysis,
one really doesn't know how the Leaf's 12V battery charging modes will affect an AGM battery life, being recommended as a replacement Leaf battery.

You are right, I haven't done a worst case yet. It would be easy enough with Dry Ice and a box, but I think the Leaf still needs to know what the outside temperature is as well and trying to Dry Ice an entire Leaf would be a beast on it's own without a lab setup.

I believe that Nissan has actually already done all the hard work and research though. The more research papers I read about AGM, Flooded, Lithium batteries and cold weather charging, the more it makes sense why the Leaf would reduce charging current when it gets below freezing. All of them suffer from low charging current when below freezing, just some more than others. Flooded Lead Acid being the best to take the most current (though, still very small in comparison), followed by AGM and finally Lithium in last place.

Nissan seems to have gone the safety route and appears to reduce charging current to a very low minimum when the outside temperature is below freezing. This precaution basically makes it safe to charge any (3) of the battery technologies without any worry about cold damage. Going this route though seems to have an unintended side-effect for Flooded Lead Acid batteries. Having charge current this low means the battery is never fully charged and probably is actually losing charge in the long run if a capacity is being used quicker than it can be replenished. This might explain why AGM & Lithium do better, they can tolerate the lower charge level better than the Flooded Lead Acid until warm weather returns and the Leaf will charge at a much higher power level. Repeat this process year after year and the Flooded Lead Acid battery just doesn't live very long.

 

[knightmb]

To add a thought experiment to this, if it takes 175 watts for 2 seconds to start a Leaf and the temperature is below freezing, the Leaf is only going to charge the 12V battery with 8 watts of power. So for every-time you start the Leaf, it will take (175 / 8 x 2 =) 44 seconds to replenish the power used to start it. That doesn't seem too bad, you need only drive it for 1 minute and the battery should be at 100% again. Well, when you turn off the Leaf and leave it sitting over-night for example, that small power load for CPU sleep will slowly whittle away the charge level and then the cold weather itself is eating away at some of that capacity. All those vampire drains are slowly eating power, so that by the next day, when you drive you Leaf again, the battery might be 95% charge. The only issue is, if you don't drive it long enough before you turn it off again, it might only charge up to 98%, then 95%, 93%, etc. The driving pattern of short trips might not be enough to hold back the gradual discharge of the battery into a low state of charge where capacity damage starts to happen.
The only thing that should stop that pattern though is the Leaf doing it's own thing of powering up the DC to DC converter and charging the 12V battery while just sitting idle on occasion. I wonder if the Nissan programmers had that charging mode based on time and not on capacity. Meaning, they programmed to Leaf to power up, check the 12V battery, then say charge for 15 minutes and go back to sleep. Maybe they forgot to account for cold weather reducing the charging power to near nothing and 15 minutes (or whatever they picked) isn't long enough to give any real charging benefit?
Yeah, lots of "what if's" in this one from me. :mrgreen:

 

[LeftieBiker]

The above could well be true. It may also be true that the original plan was to have a lithium battery in the Leaf, that this was nixed because of cost and/or because it was determined that in some climates that it would be unwise, and the FLA battery was substituted after perfunctory testing showed that it would work. It's not as if the first iteration of the Leaf didn't lack engineering gaffes - like the cabin heating system. They did, after all, have PTC to air heaters available in 2008...

 

[nlspace]

Here is another experiment for ya--disconnect the connector to the current sensor on the negative terminal of the starter battery, then see what the voltage and current do. If LS still reports the aux current, then that sensor must not be in the main CAN loop.

i did that on my car about a year ago and it seemed to keep the battery charged better. i think there is a temperature sensor in that device also.

 

[lorenfb]

Based on your 12V battery charging current measurements, the initial 10 amp value seems excessive for a typical 51R AGM battery rated at 40 Ah, right?
Furthermore, you've not done a worst case analysis of the charging current over other conditions, e.g. temperatures. Without doing a worst case analysis,
one really doesn't know how the Leaf's 12V battery charging modes will affect an AGM battery life, being recommended as a replacement Leaf battery.

You are right, I haven't done a worst case yet. It would be easy enough with Dry Ice and a box, but I think the Leaf still needs to know what the outside temperature is as well and trying to Dry Ice an entire Leaf would be a beast on it's own without a lab setup.

So why are you just focusing on the low end of the temperature extreme?

I believe that Nissan has actually already done all the hard work and research though. The more research papers I read about AGM, Flooded, Lithium batteries and cold weather charging, the more it makes sense why the Leaf would reduce charging current when it gets below freezing. All of them suffer from low charging current when below freezing, just some more than others. Flooded Lead Acid being the best to take the most current (though, still very small in comparison), followed by AGM and finally Lithium in last place Nissan seems to have gone the safety route and appears to reduce charging current to a very low minimum when the outside temperature is below freezing. This precaution basically makes it safe to charge any (3) of the battery technologies without any worry about cold damage. Going this route though seems to have an unintended side-effect for Flooded Lead Acid batteries. Having charge current this low means the battery is never fully charged and probably is actually losing charge in the long run if a capacity is being used quicker than it can be replenished. This might explain why AGM & Lithium do better, they can tolerate the lower charge level better than the Flooded Lead Acid until warm weather returns and the Leaf will charge at a much higher power level. Repeat this process year after year and the Flooded Lead Acid battery just doesn't live very long.

Again, using a single data point, at the low end of a temperature range, is not a basis for concluding that an AGM battery is a viable
replacement for the Leaf's FLA battery.

After attempting to correlate the LeafSpy current measurements for the 12V battery charging current using a clamp-on amp-meter,
one finds that the LeafSpy values are not accurate. The LeafSpy 12V current value being measured is the main 12V electrical system
current being supplied mainly by the DC to DC power converter, once the Leaf starts (powers-up). LeafSpy doesn't provide an accurate
only 12V battery charging current. So before concluding that using an AGM battery, as a Leaf replacement battery, without any long term
problematic effects on an AGM battery's performance is highly questionable!

Where are the supporting test data to conclude that an AGM battery is a long term viable Leaf FLA replacement battery?

 

[Learjet]

To add a thought experiment to this, if it takes 175 watts for 2 seconds to start a Leaf and the temperature is below freezing, the Leaf is only going to charge the 12V battery with 8 watts of power.

Where did you get the 8 watts of power? did you read my post about amps going into the battery? The voltage is constant...amps depends on the resistance of the battery. With a lead acid battery that resistance changes due to capacity lost over time.

 

[goldbrick]

Where are the supporting test data to conclude that an AGM battery is a long term viable Leaf FLA replacement battery?

While it may not be rigorously scientific, I put an AGM battery into my Leaf based on the experience of several posters here. So far, so good (it's been 4 months )

However, the original FLA battery in my Leaf lasted only 4 years which is much worse than any other battery in any car I've had the last 37 years at my present location.

 

[GerryAZ]

After attempting to correlate the LeafSpy current measurements for the 12V battery charging current using a clamp-on amp-meter,
one finds that the LeafSpy values are not accurate. The LeafSpy 12V current value being measured is the main 12V electrical system
current being supplied mainly by the DC to DC power converter, once the Leaf starts (powers-up). LeafSpy doesn't provide an accurate
only 12V battery charging current. So before concluding that using an AGM battery, as a Leaf replacement battery, without any long term
problematic effects on an AGM battery's performance is highly questionable!

I am not sure how you are measuring the battery charging current, but Leaf Spy shows the current actually flowing into (positive number) or out (negative number) of the 12V battery on my 2019 (as it did with my 2015). The early version of LEAF Spy that was available when I was driving the 2011 did not have 12V battery monitoring. The charging algorithm has changed over the years, but the LEAF charges at about 14 volts until the current absorbed by the battery drops below a threshold and then it drops to float voltage of about 13 volts (both levels are temperature compensated so they are slightly higher at low ambient temperatures and slightly lower at high ambient temperatures. The 2011 dropped to float voltage at about 6 amperes (as measured by a clamp-on ammeter), the 2015 dropped to float voltage at about 3 amperes, and the 2019 drops to float voltage once the charging current drops below 2 amperes. The current supplied by the DC-DC converter is higher because it is supplying the current to run all of the 12V systems in addition to charging the battery.

 

[knightmb]

So why are you just focusing on the low end of the temperature extreme?

It's Winter, easy to do that since the weather helps. I thought Lithium was the only technology that had issues with charging below freezing, but it turns out from the research papers I've been reading, it affects all of them quite a bit.

Again, using a single data point, at the low end of a temperature range, is not a basis for concluding that an AGM battery is a viable
replacement for the Leaf's FLA battery.

Agreed

After attempting to correlate the LeafSpy current measurements for the 12V battery charging current using a clamp-on amp-meter,
one finds that the LeafSpy values are not accurate. The LeafSpy 12V current value being measured is the main 12V electrical system
current being supplied mainly by the DC to DC power converter, once the Leaf starts (powers-up). LeafSpy doesn't provide an accurate
only 12V battery charging current. So before concluding that using an AGM battery, as a Leaf replacement battery, without any long term
problematic effects on an AGM battery's performance is highly questionable!

Where are the supporting test data to conclude that an AGM battery is a long term viable Leaf FLA replacement battery?

Call me old school, but I'm dubious about how accurate the DC clamp on meters are at low power levels. I've got a meter that can handle loads up to 100 amps that can directly intercept the 12V wires (both positive and negative from the battery) and records power usage both ways with ability to reset it to keep records for hourly, daily, etc. usage. That's what I plan to get setup in the near future so I can see how accurate/inaccurate LeafSpy is at showing the battery current along with how much is actually going in and out of the battery. I still have my FLA battery in storage on a maintainer, so I can swap it in and get some more readings to compare the two.

I have no long term data for AGM, Lithium or even FLA at this point, I'm just making gut guesses based on what I've observed about how the system behaves and what other's on this forum have written about.

 

[LeftieBiker]

The "long term test data", of sorts, could be found in EV forums for e-bikes and e-motorcycles. It is no secret that SLA batteries have the lowest usable capacity and the shortest cycling life. However, like many stone axe technologies, they are the safest batteries to use in both Summer and Winter. Your lead batteries may freeze and/or die in Winter, but unless you try to force an enormous current through a dead one, they aren't going to blow up or catch fire on you. Personally, I rode an Oxygen Lepton scooter for about 10 years. It had four AGM batteries. They were still there when I sold the scooter after roughly eleven years, with about 60% capacity remaining. So given the combination of "reliable", "safe", and long performance if not abused, why would a $200 AGM battery be a poor choice over a $400+ lithium of the same capacity? You can argue the points on both, but as far as known safety record goes, the AGM wins.

 

[lorenfb]

After attempting to correlate the LeafSpy current measurements for the 12V battery charging current using a clamp-on amp-meter,
one finds that the LeafSpy values are not accurate. The LeafSpy 12V current value being measured is the main 12V electrical system
current being supplied mainly by the DC to DC power converter, once the Leaf starts (powers-up). LeafSpy doesn't provide an accurate
only 12V battery charging current. So before concluding that using an AGM battery, as a Leaf replacement battery, without any long term
problematic effects on an AGM battery's performance is highly questionable!

I am not sure how you are measuring the battery charging current, but Leaf Spy shows the current actually flowing into (positive number) or out (negative number) of the 12V battery on my 2019 (as it did with my 2015). The early version of LEAF Spy that was available when I was driving the 2011 did not have 12V battery monitoring. The charging algorithm has changed over the years, but the LEAF charges at about 14 volts until the current absorbed by the battery drops below a threshold and then it drops to float voltage of about 13 volts (both levels are temperature compensated so they are slightly higher at low ambient temperatures and slightly lower at high ambient temperatures. The 2011 dropped to float voltage at about 6 amperes (as measured by a clamp-on ammeter), the 2015 dropped to float voltage at about 3 amperes, and the 2019 drops to float voltage once the charging current drops below 2 amperes. The current supplied by the DC-DC converter is higher because it is supplying the current to run all of the 12V systems in addition to charging the battery.

You need to actually monitor the 12V battery current with an inductive current probe and compare it to LeafSpy. LeafSpy and a current
probe don't correlate. They should both be showing a positive current flow when charging and a negative current when discharging,
e.g. a standby discharge current of 50-100ma with the Leaf in its sleep mode, when the probe is around the negative battery lead.
The point being made is that one should use a current probe when analyzing the Leaf's battery charging current to determine its effect
on different battery types, e.g. AGM, versus relying on LeafSpy. Do a thorough analysis before recommending a replace battery for the Leaf!