Turn your battery terminal into a quick disconnect

bobkart said:

nlspace said:

The coulomb-counting current sensor is useless on these cars and may actually be part of the problem with keeping the aux fully charged.

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An interesting suggestion.

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Although I have not tried it (and have no reason to), I suspect that unplugging the sensor will cause the DC-DC converter to charge at 14 to 14.5 volts instead of dropping back to 13 volts because there will not be feedback to show the amount of current flowing into the battery.

GerryAZ said:

Bobkart,

There is no reason not to disconnect the negative terminal--it is safer because a slip of the wrench (bumping the body or other metal) will not cause a spark since negative is already chassis ground. You will not hurt the current sensor and there is no need to unplug the sensor wiring connector. I have disconnected the battery by removing the negative battery cable from the current sensor numerous times on the 2011 and 2015 (have not needed to yet on 2019). I thought about replacing the nut with a wing nut or plastic knob with metal nut insert, but decided just to keep the correct size wrench in the glove box or console (so it is accessible without opening the hatch in case system is dead).

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Thanks again Gerry for your help.

Some have suggested that a wing nut could suffer from loosening during operation, and I can see that. Another potential disconnect location I see is where the negative cable from the 12V battery connects to the PDM. It's even more accessible than the location under discussion, and could accommodate a thumbscrew large enough to allow more tightening torque, to better avoid loosening during operation, such as this one:

https://www.mcmaster.com/65035K89/

I actually already have several of a smaller version of that thumbscrew, for connecting an inverter to the PDM without needing tools (allowed by use of a coupling nut on the positive side):

GerryAZ said:

bobkart said:

nlspace said:

The coulomb-counting current sensor is useless on these cars and may actually be part of the problem with keeping the aux fully charged.

Click to expand...

An interesting suggestion.

Click to expand...

Although I have not tried it (and have no reason to), I suspect that unplugging the sensor will cause the DC-DC converter to charge at 14 to 14.5 volts instead of dropping back to 13 volts because there will not be feedback to show the amount of current flowing into the battery.

Click to expand...

I guess with Lead-Acid chemistry, this change risks overcharging. But an LiFePO4 battery should be okay. Could be worth a try, especially if I encounter more symptoms that could be explained by undercharge of the 12V battery.

You need to disconnect at the hall effect current sensor at the negative terminal of the battery. You can get a knob like your image with a nut instead of a cap screw (which I was thinking of doing) if you want to make it easy to disconnect. If you follow the negative cable, you will find a clip that looks like it is just to hold the cable in place, but is actually a ground connection to the body a few inches from the battery so disconnecting the negative cable at the PDM will not disconnect the battery from chassis ground. Therefore, you need to disconnect at the battery (current sensor). Also, if you have intermittent 12V issues, it is a good idea to check that body ground connection (I had to clean it on both 2011 and 2015).

GerryAZ said:

You need to disconnect at the hall effect current sensor at the negative terminal of the battery.
...If you follow the negative cable, you will find a clip ...

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I see. Those two connections (battery to PDM and battery to chassis ground) are redundant (parallel) then. I wasn't clear on that. So the PDM will still have a ground reference even without the direct battery connection, preventing it from losing power.

I've actually reworked those battery connections after something of a battery relocation (lower and more inboard), the result of building a smaller/lighter battery mounting/tray (for the smaller/lighter LiFePO4 12V battery). So I knew of the 'clip' that's really a ground connection, just didn't understand that it would keep the PDM alive even with the direct connection removed.

I'll go back to trying to add a tool-less (but still reliable) disconnect method where the negative cable attaches to the current sensor. Thanks again for the advice.

nlspace said:

The 10mm wrench is a better solution than some flimsy junction that might accidentally work loose during operation--losing the return path would cause momumental ECU damage.

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I appreciate the 'working loose' possibility, and used a lockwasher on the knob I installed.

And I got a data point today on the 'ECU damage' concern, as I had a problem that caused the negative terminal clamp to come loose from the battery while driving. (It was NOT the quick-disconnect knob that came loose.)

The good news is that the car ran fine to the point of me turning it off, then of course it would not start back up until I reconnected the clamp to the battery terminal. 12V (nominal) continued to be available via the DC-DC converter. All that went wrong was the 12V battery wasn't being topped up for a portion of that drive, and of course all the trip-meter data and dash settings were lost upon powering off.

GerryAZ said:

...
Although I have not tried it (and have no reason to), I suspect that unplugging the sensor will cause the DC-DC converter to charge at 14 to 14.5 volts instead of dropping back to 13 volts because there will not be feedback to show the amount of current flowing into the battery.

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So i tried this and it works great. With the sensor plugged in the voltage drops from 14.5 down to 13.1 within about 1-2 minutes after starting. Unplugged the voltage holds at 14.0 after starting and while driving, outside temperature was 45F.

The plug is very difficult to release the catch, so i had to remove the nut (1/2" or 13mm) to gain access to the bottom of the sensor/back of the connector, then clipped the 2 plastic hinges that were nearly impossible to compress. i've taped over the plug so i can easily reverse this if necessary and the release is much easier to operate.

Great find nlspace!

This could be much of the answer to the Leaf charging algorithm shortcomings.

My only concern would be the possibility of cooking the 12 volt battery in Summer heat, on long urban drives. Still, it's worth trying.

This might be OK for a 30 minute drive, but it will likely overcharge the 12V battery during traction battery charging. Long term charging at 14 volts will definitely shorten the life of an AGM battery and probably cause excessive gassing and water loss from flooded cell batteries (which will also cause corrosion in the vicinity of the battery).

Yeah, that's what I was thinking - although I didn't think of overcharging during traction battery charging. Here is what might work, in a half-assed way: find a way to switch that sensor off and on from the cabin, and you'll have a 'manual charge mode' that can be used to top up the 12 volt battery as you near home. The best version would be idiot-resistant, by employing a 15 minute or so automatic off timer. I'm also sure that anyone who can design and build circuits could build a regulator/charge circuit that would switch the sensor off and on to maintain a voltage of 12.8 volts or so, thus augmenting Nissan's poorly implemented charging algorithm.

In the meantime, I've been using a near-daily short drive I was taking for months to see if there is a way to keep that voltage up, based on what we know. I've made sure to run the wipers once or twice per 10-15 minute drive, for just a cycle or two, and during the brief stop I made I left the car On. I also leave the headlights on Auto, run the climate control with heat on in Eco mode (usually) and the blower on 3 or 4. The result? Yesterday, after quite a few weeks of this routine, the rest voltage of the accessory battery was 12.6 volts - acceptable.

Generally outgassing occurs at 14.4V depending upon temperature. i'm not worried about 14.0 especially during the typical 30-minute drivetime.

i'll monitor it during a few charge cycles but expect it will be fine such as when connected to a trickle chargger for long periods--when the battery voltage reaches the chargge voltage, then current flow drops toward zero.

The 13.1V set point with the current sensor connected may be to allow use of AGM or FLA, but it is not really adequate for FLA.

IDK what everyone is on about with keeping a car battery charging at 14V, gas cars don't turn off their alternators when the battery is full, they just run at 14-15v all the time, and once the battery is full, it stops taking a charge. You don't need to do anything. Traditional cars and their batteries are fine running at 14v or so non-stop for hours/days at a time without damaging the battery.

My 1982 Chevy C10 doesn't monitor the 12v battery current, nor is there anything between the alternator and the battery other than a big fuse, and that battery was fine for a drive from Virginia to Texas straight through, and I wouldn't expect anything else. Do people here just not know how traditional 12v car systems work?

An alternator puts out 13.9-14.8V constantly, and that's considered just fine for lead-acid batteries. The 14.4v the leaf DC-DC system puts out isn't going to damage a regular car battery.

mcmancuso said:

Do people here just not know how traditional 12v car systems work?

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Very few people understand this both here and on every car forum out there.

But folks on EV forums don't understand the extra importance of a strong battery, and the expensive damage that can result if the system voltage drops or gets interrupted while driving or charging due to an old, weak or worn out battery.

EV vs ICE battery have different load profiles and the capacity loss over time is different. But a lead acid battery must be kept fully charged at all times to get the longest service life.