DIY V2L using 12V system

[LMF5000]

As you might have guessed from the title, I'm looking to use my Leafs (a 2022 40kWh and a 2016 24kWh) as a backup power source for when there are powercuts, because they are a lot more silent and less smelly than my backup petrol generator.

From reading these forums, it's clear that a CHaDeMo-based solution is years away and very, very expensive, so the next oft-suggested solution is to use a little inverter connected to the 12V battery.

I'm very familiar with the inverter side of things, having used a DIY solution using firstly a sealed lead-acid battery and eventually a LiFePO4 battery to power my home office during the pandemic. Where I need some advice is on the Leaf side of things.

Here are my questions:
1. What is the power output of the DC-DC converter on the old and new leafs? Various forum posts tend to err on the side of ~1500W continuous

2. What happens if the inverter draws too much power and the circuit is overloaded? Say I have a large inverter and connect 4000W of loads. I know the leaf's 12V battery will try to pick up some (or all) of the slack, but what about the DC-DC converter? Does it simply lower output voltage gracefully until the 12V battery is doing all the heavy lifting? Does a fuse blow somewhere in the system? Does it try to supply the load regardless until something expensive within the DC-DC power module burns out? Does the voltage of the 12V bus sag enough that the whole car turns off suddenly when it can no longer keep the high-tension contactor switched on?

3. For anyone who's done this and drawn significant currents (over 1000W) from the setup - any suggestions? I've seen mentioned to use the leaf to charge a large, external 12V battery as a buffer, and then connect the inverter to the external 12V battery. This sounds like a good idea (I have a 100Ah AGM starting battery and a pair of heavy gauge jump-leads that would do the job). Any other ideas?

4. Any limitations on how far you can discharge the main battery and whether there are any power or discharge limits beyond which it will trigger some DTCs or errors?

 

[knightmb]

As you might have guessed from the title, I'm looking to use my Leafs (a 2022 40kWh and a 2016 24kWh) as a backup power source for when there are powercuts, because they are a lot more silent and less smelly than my backup petrol generator.

From reading these forums, it's clear that a CHaDeMo-based solution is years away and very, very expensive, so the next oft-suggested solution is to use a little inverter connected to the 12V battery.

I'm very familiar with the inverter side of things, having used a DIY solution using firstly a sealed lead-acid battery and eventually a LiFePO4 battery to power my home office during the pandemic. Where I need some advice is on the Leaf side of things.

Here are my questions:
1. What is the power output of the DC-DC converter on the old and new leafs? Various forum posts tend to err on the side of ~1500W continuous

2000 watts max on both the Gen 1 and Gen 2 Leafs, but you have to reserve about 200 watts from the Leaf needing that power just to be "on" when not doing anything. They recommend 1500 watts so don't exceed the max by accident. You can technically still exceed with 1500 watts due to surge loads that start up, like a Refrigerator or a window AC unit.

2. What happens if the inverter draws too much power and the circuit is overloaded? Say I have a large inverter and connect 4000W of loads. I know the leaf's 12V battery will try to pick up some (or all) of the slack, but what about the DC-DC converter? Does it simply lower output voltage gracefully until the 12V battery is doing all the heavy lifting? Does a fuse blow somewhere in the system? Does it try to supply the load regardless until something expensive within the DC-DC power module burns out? Does the voltage of the 12V bus sag enough that the whole car turns off suddenly when it can no longer keep the high-tension contactor switched on?

The Leaf will shutdown into safety mode, basically, you won't be able to turn it back on or do anything else, it will appear to be "dead". You will have to disconnect the 12V battery wait about 5 minutes, then reconnect it to reboot and hope everything comes back up properly.

3. For anyone who's done this and drawn significant currents (over 1000W) from the setup - any suggestions? I've seen mentioned to use the leaf to charge a large, external 12V battery as a buffer, and then connect the inverter to the external 12V battery. This sounds like a good idea (I have a 100Ah AGM starting battery and a pair of heavy gauge jump-leads that would do the job). Any other ideas?

Mainly, take some time to learn about power (more than just knowing that some voltage and amps are in the lingo), research what inverter you will use, how much surge it will provide, is it using a sine wave or square wave (sine wave recommended) and then research high power quick connectors, wire gauge size for the amount of power trying to use, and ask lots of questions just in case.

4. Any limitations on how far you can discharge the main battery and whether there are any power or discharge limits beyond which it will trigger some DTCs or errors?

Unless you have an actual pack problem, you can run it down until shutdown, and then you run down the 12V battery until it is dead (but most inverters will shutdown before that to save the 12V battery)

 

[coulomb]

2. What happens if the inverter draws too much power and the circuit is overloaded?

The Leaf will shutdown into safety mode, basically, you won't be able to turn it back on or do anything else,

Are you sure? I don't know for sure, but it it seems reasonable to me that the DC-DC simply pushes all it can into the 12V system, so nothing breaks, and if there is a power deficit, Kirchoff's law says that the auxiliary battery provides the difference. The DC-DC converter knows its limits, and doesn't exceed them.

Of course, then the auxiliary battery will slowly (or not so slowly, depending on the deficit) lose SoC, so the battery voltage will decline, until eventually the battery voltage goes below the inverter's cut-off voltage and the inverter stops. Even at that point, the auxiliary battery should have enough charge to boot the system back up again, and/or remain in ready or aux mode.

Maybe the car calls it quits before the inverter does, in that case there may be some fault codes, though I would hope that the car would just turn off to save the auxiliary battery. It might need a short charge (when power is available again) to reboot in those circumstances. Maybe this is the circumstance where the Leaf enters this "safety mode".

But if you're vigilant, you should be able to stop using excessive power before the car goes into this mode. Not exactly plug and play like V2L is supposed to be.

 

[LeftieBiker]

Of course, then the auxiliary battery will slowly (or not so slowly, depending on the deficit) lose SoC, so the battery voltage will decline, until eventually the battery voltage goes below the inverter's cut-off voltage and the inverter stops. Even at that point, the auxiliary battery should have enough charge to boot the system back up again, and/or remain in ready or aux mode.

This depends on the inverter having a high enough fixed or maximum selectable cutoff voltage. My experience with them is that they tend to shut down at around 10-11 volts. (I've always found that very annoying: they will allow battery damage, THEN shut down to "protect the battery.") That wouldn't leave the accessory battery with enough juice to restart the Leaf. That's one reason I recommend a buffer battery - to take abuse from the inverter and (usually) save the car's 12 volt battery from it.

 

[knightmb]

2. What happens if the inverter draws too much power and the circuit is overloaded?

The Leaf will shutdown into safety mode, basically, you won't be able to turn it back on or do anything else,

Are you sure? I don't know for sure, but it it seems reasonable to me that the DC-DC simply pushes all it can into the 12V system, so nothing breaks, and if there is a power deficit, Kirchoff's law says that the auxiliary battery provides the difference. The DC-DC converter knows its limits, and doesn't exceed them.

Yes, I've already had relatives do this by accident. :lol:

They were using a 2000 watt (4000 watt surge) inverter when it happened both times. :?

 

[LMF5000]

Thank you all for your replies. I forgot that the inverter itself will have its own low-voltage cutoff - this could supply some good protection, however when a large-enough inverter (say one with a capacity of over 2000W) is used it would be good to have some clarity about how the DC/DC converter will react to the overload.

Imagine a scenario where for example I try to run a combination of household loads that gradually take the power consumption over 2000W.

We know that the DC/DC converter ("DDC" in short) will try and maintain a given voltage setpoint on the "12V" bus. On my 2016 leaf with a newly replaced flooded lead-acid battery, it's 14.3V across the battery terminals when the DDC is active. When the inverter starts drawing current from the 12V rail, the DDC will try and maintain the 14.3V setpoint by pumping more current into the 12V battery rail. At some point, the power draw from the inverter will get so large that the DDC will be supplying full power and it still won't be able to keep up.

What will happen at this point? One possibility is that the car senses something is wrong (since in its intended application of powering just the car's 12V loads, it should never experience a 2000W+ power draw on the 12V rail that maxes out the DDC's capacity unless something goes seriously haywire... so at that point it might start throwing errors for the 12V bus). Or else the DDC could handle it "gracefully" - meaning the bus voltage will be allowed to fall low enough that the 12V battery itself starts supplying the missing power. Based on what I know of lead-acid batteries, this voltage will be somewhere in the region of 10.5-12.6V depending on actual power draw, battery health (internal resistance) and charge level.

If even more current is drawn, beyond what the battery itself is able to supply, then presumably the low-voltage cutoff of the inverter is hit (typically 10.5V) and it should protectively shut down to protect what it thinks is "the battery" from overdischarge.

Another scenario is possible - the inverter's low-voltage cutoff fails, or a dumb DC load is applied that indiscriminately draws well over 2000W from the 12V rail with no voltage-based protection. In that case it's conceivable that the 12V rail's voltage is dragged down to a very low level - maybe 9V or 8V. If that happens I would expect the car to be very unhappy about it and potentially enter some kind of protective shutdown that's hard to get out of.

 

[LeftieBiker]

Install a DC circuit breaker at the inverter connection. Use one that will trip when the draw exceeds 1500 watts.

Plenty of people have actually done this. It would be helpful if one or more of them would post here.

 

[jinneth]

From reading these forums, it's clear that a CHaDeMo-based solution is years away and very, very expensive..

Not years away.
Expensive yes.

https://m.alibaba.com/product/60719352779/CHAdeMO-Vehicle-to-X-V2X-and.html

http://www.electway.net/product/Electway_V2H_solution.html

 

[moapys]

The under-hood/non-traction battery is ill-suited for any invertor support (E.E. w/Leaf-Experience here...): You're much better served by looking into the better developed Float+Solar+AGM/Lithium home powering solutions out there...

You can study up, determine your power requirements, and spend what is necessary for your anticipated current draw and duration - e.g. resulting "KW Hours" - and spare yourself the frustration of working with the lawn-tractor-sized Leaf aux battery

 

[LMF5000]

The under-hood/non-traction battery is ill-suited for any invertor support (E.E. w/Leaf-Experience here...): You're much better served by looking into the better developed Float+Solar+AGM/Lithium home powering solutions out there...

You can study up, determine your power requirements, and spend what is necessary for your anticipated current draw and duration - e.g. resulting "KW Hours" - and spare yourself the frustration of working with the lawn-tractor-sized Leaf aux battery

Well, the aux battery isn't really supplying much because the leaf is kept on (in ready-to-drive mode) so it's taking power from the 400V traction battery and feeding the 12V bus via the onboard DC-DC converter. That way the inverter is indirectly feeding from the 24kWh of the traction battery. Unfortunately the car itself draws about 100-300W from the traction battery just to stay in ready to drive mode, which is about the same as the amount of consumption the essential loads in my actual house have, so it cuts my potential runtime in half.

I've currently put in a order for a 12V 280Ah LiFePO4 battery pack to use for inverter power. Hopefully that'll arrive in due course and work as a good power backup

 

[moapys]

Well, the aux battery isn't really supplying much because the leaf is kept on (in ready-to-drive mode) so it's taking power from the 400V traction battery and feeding the 12V bus via the onboard DC-DC converter. That way the inverter is indirectly feeding from the 24kWh of the traction battery. Unfortunately the car itself draws about 100-300W from the traction battery just to stay in ready to drive mode, which is about the same as the amount of consumption the essential loads in my actual house have, so it cuts my potential runtime in half.

I've currently put in a order for a 12V 280Ah LiFePO4 battery pack to use for inverter power. Hopefully that'll arrive in due course and work as a good power backup

Looking forward to your update reporting on your satisfaction with the lithium-based aux-system. I'm still at the "proof of concept" stage (all very positive so far), with the anticipated A-h limitation of AGM - which I had already for another project: My UPS/array keeps me toasty for about 45min, easily enough for back and forth to work; your Li pack (per the rating) ought to triple that of mine.

Also, as noted in a previous post, inverters' low-voltage-cutoff protection circuits should be much happier with the LiFePO4 discharge curve - staying "up" until longer until essentially depleted: Lead-Acid has the gradual decline curve which usually sees the inverter cutting out at around only 30-40% depletion...so the next battery upgrade for me will definitely be Li+.