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Keep a dying 12V battery going as long as possible

Posted: Mon Jan 24, 2022 5:03 pm
by knightmb
I started a new topic for this because I found this Leaf quirk by accident as I was doing some research and experimentation for another topic I started here, but didn't really fit in with it. If anything, this seems to be some good info for those struggling with a dying 12V battery in winter time more than anything.

Some background, being Leaf EV owner since the 2013 model and in all that time, I just assumed that when you press the power button to shut off the Leaf, it shut down most of the devices on the 12V bus right away. Like, before it shuts off the DC to DC converter. I found it, it doesn't actually go in that order. Well at least not on my 2020. I haven't visited any friends yet to check this on earlier models or my wife's 2018 just yet... ;)

This is how I stumbled upon this, interesting quirk. I am in the process of getting a real time power monitor setup inline with my current 12V battery with the aim to measure how much power it uses while sitting idle for hours, overnight, while driving, etc. So I take readings, look at spreadsheets, etc. Now, this is power going in and out of the 12V battery, no the 12V bus as a whole, from the DC to DC converter for example. So last week, while looking over the data, I noticed these weird power spikes. It's not coming from the Leaf DC to DC, but from the battery itself. So further investigation, some massive power spikes were being discharged from the 12V battery in short burst. Enough to make even my Lithium battery fall into the 10.9V range sometimes, which it just never happens with it. At first, I figured it's a really old 12V Lithium and it's age is just showing, but I finally figured out what was happening and what was causing this massive drain.

When I shut off my Leaf, the DC to DC converter shuts down *before* all the other high drain devices on the 12V bus are shut off automatically. I'm referring to things such as the rear window defroster, heated seats, heated steering wheel, radiator fans, cabin ventilation fan, etc. Sometimes there is only a second delay, but sometimes I've seen a few seconds delay. You can watch it happen in LeafSpy sometimes if you are on the screen to monitor the 12V battery current. If it happens to be Night time for example, cold outside, and you have just about everything running on the 12V system (lights included), shutting down the Leaf can cause some power draws in excess of +700 watts sometimes from the battery for a few seconds. Now, I know that good battery can probably handle this brief spike, but I wonder if people that have failing 12V batteries, if this can be a catalyst for depleting the battery enough to fail quickly and be left with a dead Leaf at any random time in the future (day or night).

I'll do more research, but if my hunch is correct and you are one of the unlucky people that need to make your 12V battery last just a little longer until a new one comes in for replacement, the best way to avoid this draw spike is to shut down everything before you turn off the Leaf. So turn off your climate control, heated seats/wheel, rear defroster, lights, etc. Doing this I noticed eased that power draw spike to much, much less power.

Re: Keep a dying 12V battery going as long as possible

Posted: Mon Jan 24, 2022 6:55 pm
by LeftieBiker
Good info - thanks! I always did that with my ICE cars, and I tell others to do it in Winter. I also tell them to run the engine slightly above idle for a few seconds, let it fall to idle (to keep raw gas out of the cat converter), then turn the car off. In the case of the Leaf, something like this may be in order:

1. Turn off all 12 volt accessories.

2. "Blip" the front windshield wipers to produce one wipe cycle.

3. WAIT 30 SECONDS, for the increased 12 volt system voltage to add some charge to the 12 volt battery.

4. Turn Leaf off.

That should help to at least reduce the overall loss from that drain produced by the DC-DC converter shutting down.

Again, good work.

Re: Keep a dying 12V battery going as long as possible

Posted: Tue Jan 25, 2022 8:02 am
by jlv
This makes sense as long as you are talking about doing this only while you wait for the replacement 12V battery (presumably 51R AGM) that you ordered to arrive, and it's not something you are suggesting people do all the time.

Re: Keep a dying 12V battery going as long as possible

Posted: Tue Jan 25, 2022 8:51 am
by mjb750j
great info.....my 2016SL still has it's original 12V battery which failed on me. I havent driven the car since 12/22 ( I broke my shoulder ) and with everything else going on I forgot to hook up my battery tender also being near 0 at night do not stack the odds in the batteries favor. have the battery in my brothers shop on a slow charge and then will do a load test in a couple of days to get a better read on the overall health.
i will definately follow all the advice you gave when it comes time to power down my leaf
thanks for sharing

Re: Keep a dying 12V battery going as long as possible

Posted: Tue Jan 25, 2022 9:16 am
by Stanton
jlv wrote: Tue Jan 25, 2022 8:02 am This makes sense as long as you are talking about doing this only while you wait for the replacement 12V battery (presumably 51R AGM) that you ordered to arrive, and it's not something you are suggesting people do all the time.
Agreed, because sometimes I think we have the "tail wagging the dog" with some of these 12v battery suggestions.
Not saying it isn't interesting info, but if you are that worried about being stranded, just replace it every couple of years (or go with Lithium).

Re: Keep a dying 12V battery going as long as possible

Posted: Tue Jan 25, 2022 11:27 am
by knightmb
jlv wrote: Tue Jan 25, 2022 8:02 am This makes sense as long as you are talking about doing this only while you wait for the replacement 12V battery (presumably 51R AGM) that you ordered to arrive, and it's not something you are suggesting people do all the time.
Not myself, but I have read plenty of "my 12V died" stories here and a lot of them had a situation where the user was driving the Leaf just fine to the store or work and came back to it dead only after a short amount of time. After testing my wife's 2018 battery before replacement, it also worked fine on small loads (cabin lights, checking battery charge via EV connect app, etc.) but barely enough power to start a Leaf and stay above 10.5 volts with only a +180 watt load for a few seconds. If I had put a +700 watt load on that same battery, it would easily knock the voltage down into the 9v or less area where it causes DTC codes and weird Leaf behavior or just doesn't start at all even though I could probably charge it back up and it have enough to power up the Leaf at least once more again. Those that are accustomed to driving an ICE usually expect the battery to gradually die, as you can hear it when they start the engine and the RPM of the starter motor is less and less until the "click" death of the battery. The Leaf on the other hand doesn't give you much "warning" about your dying 12V battery. Some people see it manifest as weird behavior while running but most others just get a binary, "Leaf was working just fine" to "Leaf is dead now" without much in the way of any warning before hand. :(

My theory is part of those "situations" might be because of the order in which the Leaf shutdowns all the systems when the 12V battery is getting weak or near death. It might even be a cumulative effect that hurts FLA batteries more than AGM or Lithium for example. I still have my wife's battery from her 2018, I might try some easy 700 watt load burst tests on her old FLA and see how it handles it just for comparison.

Re: Keep a dying 12V battery going as long as possible

Posted: Tue Jan 25, 2022 11:31 am
by alozzy
I still have my wife's battery from her 2018, I might try some easy 700 watt load burst tests on her old FLA and see how it handles it just for comparison.
Many shops will load test a lead acid battery for free. It takes less than a minute to do a load test to determine the FLA battery health.

Re: Keep a dying 12V battery going as long as possible

Posted: Tue Jan 25, 2022 12:13 pm
by knightmb
alozzy wrote: Tue Jan 25, 2022 11:31 am
I still have my wife's battery from her 2018, I might try some easy 700 watt load burst tests on her old FLA and see how it handles it just for comparison.
Many shops will load test a lead acid battery for free. It takes less than a minute to do a load test to determine the FLA battery health.
Agreed, the load test is easy for them, but they won't be able to do a capacity test. You have to run the FLA down at a set load for 20 hours to determine that. So a FLA can pass a load test but fail a capacity test relative to it's rating. My wife's 2018 battery is that way. It can pass a load test at the auto store but it doesn't have enough capacity to run 12V accessories for any significant length of time. :(

Re: Keep a dying 12V battery going as long as possible

Posted: Tue Jan 25, 2022 12:16 pm
by Stanton
Like I said in the other thread, buy an Anti-Gravity Battery Tracker (https://antigravitybatteries.com/produc ... lead-acid/) if you really want to track the health (and test) your 12v battery. These things are worth every penny (especially in a Leaf).

Re: Keep a dying 12V battery going as long as possible

Posted: Tue Jan 25, 2022 12:26 pm
by alozzy
It can pass a load test at the auto store but it doesn't have enough capacity to run 12V accessories for any significant length of time. :(
Like you said then, the only explanation would seem to be a vampire draw that's loading the 12V battery when the car is powered off. An ammeter clamped on the battery lead would confirm if there's a load when the car is powered off.

There are Leaf owners who use a low capacity LiFePO4 battery as a starter battery with no issues. In theory, the starter battery shouldn't need much capacity if the DC-DC converter is doing its job properly and there are no vampire draws when the car is powered off.

You mentioned a draw of 700W for a few seconds after the car is powered off...

700W delivered over 5 seconds is only 1 wH of energy, that's next to nothing. There has to be some kind of vampire draw to explain larger capacity loss.

I think some of the explanation for 12V battery issues is people who keep their EVSE plugged in all the time, as that definitely sets up a vampire drain.