How to - Export 12v DC to 120vAC power from LVB connection points

[dtbaker61]

It is possible to use the 12v battery 'connection points' to EXPORT power... either to 'give a jump' to another vehicle, or run a (small-ish) pure-sine inverter that can be had for $300 or so to run a few AC loads, up to about 2000watts while camping, charging cordless tools at a job site, or even emergency power for your home if and when the grid goes down to run your 'critical loads' for as long as your HV battery holds out above 20% state-of-charge (SOC).

I've tested at least the simplest configuration, and it seems to work fine... as long as you understand the limitations of our MME and the systems that keep the 12v battery charged up from the big high voltage (HV) battery. The MME has plenty of Energy on board, but is a little limited in how much power can be exported via the 12v connection points.... and there are some 'tricks' to get things running and keep things running.

Here are the major considerations I know about so far (updated 2/18/23):

============

1. Access to the LVB connection points is a real PIA unless you cut an access hatch, or remove the driver's side frunk 'beauty cover'. at a minimum, you can cut (2x) 1-7/8" holes, and use jumper cables. Holes can be cut, and plugged when not in use per the DIY thread I wrote up a while back; https://www.macheforum.com/site/threads/how-to-add-access-ports-for-lvb-connection-points.4107/ I would suggest a third hole over the HV disconnect relay as well since that *should* be easier to access.

2. Power export limit - 1500 to 2000watts MAX - As far as I have found, the on board dc-dc that pulls energy from the HV battery to keep the LV battery charged is limited to a TOTAL of 160amps for most trim levels to all 12v loads including LVB 'charging/connection points' . If the MME is 'sleeping' and the LVB is depleted and voltage drops below 11.9, it should signal the dcdc to 'wake up' and recharge it. The dc-dc will then send energy from HV to run loads and recharge LVB back to 'full'..... UNLESS/UNTIL remaining charge on the HVB is less than 15%; at which point the HVB stops trying to maintain the LVB.

3. Because the LVB is so small; it is best to plan on turning the MME 'on' and disabling the 'auto-off timer' setting so the connection point voltage is maintained at 14.4 by the dc-dc rather than pulling Energy out of the LVB and forcing it to ask for support when it gets low.

4. Interestingly, you also need to close all the doors to enable the DC-DC to energize the 12v connection points. If doors are open, Energy is extracted directly from the LVB rather than from the HV battery. Note that if you are Exporting 1500watts of Power, the cables connected to the 12v connection points need to be big enough to carry 120amps of current or so.... I'd suggest 2AWG at a minimum. Regular jumper cables will work, but they will heat up significantly if you draw heavy load for very long.

5. minimum equipment needed
- 12vDC to 120vAC inverter - I'd suggest a 'pure sine wave' type which generates smoother power which is easier on your AC loads. Available for under $300.
- jumper cables for light loads... or more 'semi-permanent' heavy gauge (2 awg) cables with insulated connectors such as an 'Anderson connector' for better/safer connections

6. Tips

- When MME is 'on' you have the best chance of the dcdc system maintaining the connection point voltage at 14+v. If the voltage sags too much under heavy load, your inverter will turn itself off, or beep, or do some other type of alarm.

- if the MME is 'off', or sleeping, I don't know how long it takes for the dcdc to wake up to support heavy loads that pull the resting voltage down from 12.4. It looks like if the LVB sags down to 11.9-12.1 it will wake a 'sleeping' MME, but response from the dc-dc will be better if you leave it 'on' for full power to accessories.

- non-GT trims have a dc-dc converter that specs support for 160a. GT documentation shows a different part number for the dc-dc supporting up to 220a.... But I haven't verified the max or the expected duty cycle.

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[timbop]

Back when I had a conversion van we had an inverter that plugged into the cigarette lighter. They probably still make those, but the real question is how much it will let you draw

 

[phidauex]

Back when I had a conversion van we had an inverter that plugged into the cigarette lighter. They probably still make those, but the real question is how much it will let you draw

The 12V outlets typically are fused at 15A, which means a max of ~180W, or more like 140W continuous.

These days with LED lighting and computers being fairly efficient, I think most people who want a little on-the-road or camping power would probably be happy with 500W or less. I'm a big fan of Morningstar's little SureSine inverter - 300W (but a REAL 300W, not a fake peak number you see on most marketing material, and they surge to 600W for starting fans and motors), full sine wave, and super reliable. Not the cheapest things, but I've seen them running continously since they were released a little over 10 years ago with no replacements. https://www.morningstarcorp.com/products/suresine/

They are nice and small, too, I'm thinking about mounting one inside the frunk compartment behind one of the plastic fascias and running a small receptacle into the side of the frunk wall.

 

[dtbaker61]

I want to know what the max is so I can run a 1500w teapot, or inductive cooktop, or backfeed critical circuits to my house if grid goes down.

68 kwhr in the mme could power my house for days, and run all critical loads if I know what the upper limit is.

the first step is to hone in on the max wattage, then
1. simple use with 12v->120vAC inverter
2. tips to keep the MME 'on' to export for hours if need be
3. advanced use with inverter+ auto-former for 12vDC -> 240vAC to backfeed thru home NEMA 14-50 outlet as a 'generator' for critical load backup

 

[dtbaker61]

Back when I had a conversion van we had an inverter that plugged into the cigarette lighter. They probably still make those, but the real question is how much it will let you draw

not enough

 

[dsalad]

Quite a bit of info about this on the chevybolt forums (I know, I know). I sold my Bolt, but there are plenty of people who tap into their LVB and let the HVB charge it at ~1500/~2000 watts. The Bolt folks asked the lead engineer if this was ok and he basically said yes, but wouldn't endorse it for fear of lawsuits. In fact, there's a guy who 3d printed some connectors. The trick was figuring out how to keep the car on for as long as you needed the power... https://www.chevybolt.org/threads/installing-anderson-power-socket-12v-inverter-etc.37416/

 

[dtbaker61]

Quite a bit of info about this on the chevybolt forums (I know, I know). I sold my Bolt, but there are plenty of people who tap into their LVB and let the HVB charge it at ~1500/~2000 watts. The Bolt folks asked the lead engineer if this was ok and he basically said yes, but wouldn't endorse it for fear of lawsuits. In fact, there's a guy who 3d printed some connectors. The trick was figuring out how to keep the car on for as long as you needed the power... https://www.chevybolt.org/threads/installing-anderson-power-socket-12v-inverter-etc.37416/

yup, there are also a couple companies that made special connectors for several versions of Prius...

I've tested with temp connection made with jumper cables.... and plan to explore better/safe connectors for the MME in the next few months. As far as I've tested so far, it will be best to leave the MME 'on' (and disable the auto-off setting) when exporting energy to an inverter to be sure the MME is maintaining the connection points at 14+v rather than allowing the LVB to drain down and get fed only 'periodically'.

 

[swingdj]

It is possible to use the 12v battery 'connection points' to EXPORT power... either to 'give a jump' to another vehicle, or run a (small) inverter to run a few AC loads while camping, charging cordless tools at a job site, or even emergency power for your home if and when the grid goes down to run your 'critical loads' for as long as your battery holds out.

I'm still digging into the details, but I've tested at least the simplest configuration, and it seems to work fine... as long as you understand the limitations of our MME and the systems that keep the 12v battery charged up from the big high voltage (HV) battery. The MME has plenty of Energy on board, but is pretty limited in how much power can be exported via the 12v connection points.... and there are some 'tricks' to get things running and keep things running.

My plan is to update this top post as I learn details, or ya'll contribute facts to the thread so new readers don't have to go thru pages and pages of posts to extract all the info. So, here are the major considerations I know about so far:

============

1. Access to the LVB connection points is a real PIA unless you cut an access hatch, or remove the driver's side frunk 'beauty cover'. at a minimum, you can cut 2x 2" holes, and use jumper cables. Holes can be cut, and plugged when not in use per the DIY thread I wrote up a while back; https://www.macheforum.com/site/threads/how-to-add-access-ports-for-lvb-connection-points.4107/

2. Power export limit - 1500 to 2000watts MAX - As far as I have found, the on board dc-dc that pulls energy from the HV battery to keep the LV battery charged is limited to a TOTAL of 160amps to all 12v accessory loads including LVB 'charging' . The design is: as the LVB is depleted and voltage drops below (???), it should signal the dcdc to 'wake up' and recharge it. The dc-dc will then send energy from HV to run loads and recharge LVB back to 'full'..... UNLESS/UNTIL remaining range on the HVB guessometer is 30 miles; at which point the HVB stops trying to maintain the LVB. (this info per Ford Engr I managed to get a few answers from.)

3. equipment needed
- 12vDC to 120vAC inverter - I'd suggest a 'pure sine wave' type which generates smoother power which is easier on your AC loads.
- jumper cables... or more 'semi-permanent' heavy gauge (#6 or #4) cables with insulated connectors such as an 'Anderson connector'

4. Tips
- When MME is 'on' you have the best chance of the dcdc system watching the LVB and 'recharging' as needed. The system *should* wake up and do it automatically, but if you are running a heavy load you will need to turn MME 'on' to prevent voltage 'sag' when the load draws from the little LVB. If the voltage sags too much under load, your inverter will turn itself off, or beep, or do some other type of alarm.

QUESTIONS:
- I'm not sure what 'low' voltage on the LVB connections will signal the dcdc to wake up, or if it will be automatic, or how long it takes for the dcdc to wake up
- I'm also not sure how much energy the dcdc will 'send' to recharge the LVB... 2000 watts continuous is just a guess at this point.

Dan, I cut the two access ports and I have a 1500w inverter. Have you figured out a better way to connect to the (+) and (-) LVB connections other than alligators?
Did you ground the inverter? If so, to a sidewall?

 

[dtbaker61]

Dan, I cut the two access ports and I have a 1500w inverter. Have you figured out a better way to connect to the (+) and (-) LVB connections other than alligators?
Did you ground the inverter? If so, to a sidewall?

I have not permanently mounted an inverter yet, but will probably use a 3000watt pure-sine to handle the momentary loads from inductive motors on small tools, compressors, and pumps. I plan to limit continuous loads to no more than 2000watts so the onboard dc-dc can 'keep up'.

Alligator clips do not have enough contact for decent amounts of current... I'll probably use big fat 'Anderson' connectors and 1/0 wire.

and no I would not ground the inverter to frame.... that is not ground. Remember that goes back to the 12v-, and might create some odd unintentional ground loops.

 

[swingdj]

I have not permanently mounted an inverter yet, but will probably use a 3000watt pure-sine to handle the momentary loads from inductive motors on small tools, compressors, and pumps. I plan to limit continuous loads to no more than 2000watts so the onboard dc-dc can 'keep up'.

Alligator clips do not have enough contact for decent amounts of current... I'll probably use big fat 'Anderson' connectors and 1/0 wire.

and no I would not ground the inverter to frame.... that is not ground. Remember that goes back to the 12v-, and might create some odd unintentional ground loops.

Thanks for the details. I’m not an electrical guy. Someone on FB was questioning the ability to do this:
“Sorry, but do you plan on connecting the small 12v battery to 1500w inverter? If so you'll just kill your battery.. small car batteries are not made for giving up 125 amps for more than a few seconds. You'll need a large battery bank like that on a yacht to so that.”
I’ve seen numerous others posting about using inverters this way so I’m thinking this guy is missing something.

 

[dtbaker61]

Thanks for the details. I’m not an electrical guy. Someone on FB was questioning the ability to do this:
“Sorry, but do you plan on connecting the small 12v battery to 1500w inverter? If so you'll just kill your battery.. small car batteries are not made for giving up 125 amps for more than a few seconds. You'll need a large battery bank like that on a yacht to so that.”
I’ve seen numerous others posting about using inverters this way so I’m thinking this guy is missing something.

the (very little) 12v battery in the MME essentially has a GIANT battery bank backing it up when the MME is 'on'.... the entire MME high-voltage battery! The limiting factor is how good a job the onboard dc-dc converter will do extracting energy from the HV battery to keep the LV battery voltage 'up'.

The working theory is that when the MME is 'on', the dcdc tries to keep the LVB at 14+ volts.... and is capable of holding continuous 160amps export at 12-14 volts (about 2000 watts).

Some initial testing I did confirmed that if a big load, like a 1500 watt teapot, fires up when the MME is 'off'.... the dcdc takes a bit to wake up, and the little LVB is on its own for a bit which allows the voltage to sag under load and kicks off the inverter. If MME is 'on', then the LVB doesn't have to take the load since the dcdc pulls from the HV battery to keep the voltage up at the connection points.

so.... while this is less power than the Lightening will be able to handle (7kw), I am hopeful that the MME can be used as a very reasonable emergency power source, car camping, and job site power as long as the continuous load is <2kw.

 

[ZuleMME]

Of course another question is what duty cycle the built in dc/dc inverter is capable of maintaining. It's likely not intended to do much other than compensate for the stock 12v draw + same margin for charging. Similar to a UPS battery system if you run the inverter too long it overheats and dies. Don't wanna do this to the car!

 

[dtbaker61]

Of course another question is what duty cycle the built in dc/dc inverter is capable of maintaining. It's likely not intended to do much other than compensate for the stock 12v draw + same margin for charging. Similar to a UPS battery system if you run the inverter too long it overheats and dies. Don't wanna do this to the car!

my hope is that the dcdc is intended for 100% duty cycle up to 160amps..... as it would likely be designed to run all the 12v 'stuff' that is likely to be 'on' at the same time like lights, radio, power ports, and the biggy being the cabin and battery enviro heaters/coolers/fans/etc.

The wires and circuit breakers in the MME are designed for continuous use, so I'd be disappointed if the dcdc wasn't designed solid enough to support all the loads supported by the Breaker on the circuit.

That being said, I can't imagine consuming 2kw continuously for very long... even backing up household 'critical loads' in an emergency situation. Background load in my house is almost always under 300 watts for lights, circ pumps, regrigerator, and might bump up to 1500-2000 watts for the coffee maker or microwave, but just for minutes at a time.

 

[Mach-Lee]

Of course another question is what duty cycle the built in dc/dc inverter is capable of maintaining. It's likely not intended to do much other than compensate for the stock 12v draw + same margin for charging. Similar to a UPS battery system if you run the inverter too long it overheats and dies. Don't wanna do this to the car!

Yes THIS! 160A is the PEAK/MAXIMUM not for continuous use!

my hope is that the dcdc is intended for 100% duty cycle up to 160amps..... as it would likely be designed to run all the 12v 'stuff' that is likely to be 'on' at the same time like lights, radio, power ports, and the biggy being the cabin and battery enviro heaters/coolers/fans/etc.

The wires and circuit breakers in the MME are designed for continuous use, so I'd be disappointed if the dcdc wasn't designed solid enough to support all the loads supported by the Breaker on the circuit.

That being said, I can't imagine consuming 2kw continuously for very long... even backing up household 'critical loads' in an emergency situation. Background load in my house is almost always under 300 watts for lights, circ pumps, regrigerator, and might bump up to 1500-2000 watts for the coffee maker or microwave, but just for minutes at a time.

160A is not for 100% duty cycle! If you look at engineering data sheets for power components, they will typically show the maximum output at room temperature, but the output decreases significantly as the ambient temp goes up. At max component temp the output might only be half (so like 80A).

You're also making the assumption that the car will draw 160A, I argue that it will be almost impossible to get it above 100A with everything on. It probably runs around 50A or less most of the time. Measure it if you don't believe me. So they've already included a healthy margin of it outputting more than the car would ever need, they didn't design it to run your kitchen appliances. This is certainly not covered under warranty because it's an improper use.

I'll quote the service manual here: "Depending on the vehicle and environmental conditions, the DCDC is capable of outputting up to 160 amps to the 12-volt battery."
To me that says 160A is for a very short time (seconds) under optimal conditions, but less than that the rest of the time, especially if things heat up. So if you want 100% duty cycle I would limit it to 100A max, or whatever the car actually draws with everything on.

Last, fuse ratings are designed for 75% operation (not 100%) plus any temp derate. Automotive applications typically might use 100ºC ambient for under-hood conditions which would apply another 89% derate to the fuse. Combine those and you get 66% total derate. So if you see a 40A fused circuit, it's only designed for about 26A continuous.

 

[Mach-Lee]

I’ll also add that you need to consider inverter efficiency because that will increase input current draw. At peak load a good inverter might only be 80% efficient, so if it’s outputting 1000W it will be drawing 1250W. If it’s only 60% efficient it will be drawing 1666W from the battery to output the same 1000W. You need to account for this inefficiency so you don’t overload your source.

Based on all this, I’m really going to recommend that folks use a 1200W inverter or less. Here is the model I recommend if you want to do this: https://samlexamerica.com/products/1000-watt-pure-sine-wave-inverter-ntx-1000-12/

Max continuous input is 100A, which is inline with what the DC/DC can output with a safe margin for peak loads up to 2000W. Look at the inverter specs and make sure the max rated input amp draw is less than 120A (75% of 160A) on whatever model you choose. Good design principles would dictate that the inverter should shut down on overload before ever reaching 160A input.

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