Charge house backup battery from car

[blueshoe]

I could use some advice evaluating options for charging backup batteries (in the house) from the car (on the nearby driveway). The car, as the (trip-hazard) cable runs, is about 20 feet from the kitchen (the closest enclosed space in the house), or about 60 feet from the basement (by the service panel).

The basic idea would be to have enough battery back-up (plus a healthy reserve) in the basement to power the house's critical loads through the night. If power loss is prolonged, use the car as needed during the day to re-charge / top off the batteries (and, potentially, simultaneously power daytime critical loads).

(Long term plan is to DIY solar panels on the house; tie to the grid; buy an electric car; convert gas appliances to electric; then finally to retire to a mountain or an island somewhere and start this all over again. But first thing's first, and that's having a good power backup plan for this winter.)


So, there's plenty of advice out there on charging accessory batteries from car alternators. But what I've found looks to be geared for permanent installations which have a good location for power equipment that is located not too far from the inverter and the batteries. I'm not sure how well it applies to my desire for temporary use, without a great location for placing the equipment, located a sizable distance (for 12VDC) from the alternator.

I'm looking for to do something based on the following requirements / priorities:

• Safety first!
  • Don't shock people or small animals
  • Don't explode or start fires
  • Don't damage the car
    • (The risk of accidentally draining the car's starter battery would be great to design around, but is potentially acceptable.)
  • (I can accept the trip hazards.)
• Minimum modifications to the car
  • Preferably none!
  • (I don't necessarily know in advance what, or whose, car I'd be using this with, for one thing.)
• Can be used during bad weather (meaning snow and rain, not meaning during-a-hurricane or if the driveway is flooded)
• Tolerates a significant distance between alternator and batteries
  • (My own application is for 60 feet, but potentially apartment-dwellers and such might want 150 feet or more?)
• Reliable enough to be comfortable believing it will work when I need it
• Cheap enough to justify implementing even when living in a location where power outages are rare and brief

Here's some options I've been considering. I'd appreciate some feedback!

Approach	Details	Disadvantages	Advantages
In-car charging	Ferry batteries back and forth between the basement and the car; charge them inside the car.	Likely needs (minor?) mods to the car to bring power cables from the engine compartment to where the charging equipment and batteries are Implies purchase of extra batteries to power house's critical loads while charging Potential SOC-managing headaches when juggling multiple batteries Weight of some battery packs could be a problem Potential inefficiencies if alternator's available idle-current is sufficiently higher than charger current How to keep track of what's going on with the charging? Sufficient ventilation? Any risks to the car?	Excellent weather-resistance Good match for normal approaches for charging accessory batteries in cars Low (lowest?) cabling cost No need to solve "how do I want to get the cabling into the house" question
Direct DC	Connect DC cables "directly" (with circuit-breaker / fusing but not much else) from the car all the way out to the batteries in the house.	This is the approach everyone says not to do even for short-run cabling! First concern is there aren't enough available volts available between the output of the alternator and what the batteries need for charging in order to absorb the voltage drop from the long cabling Even in short runs, it's better to have something more intelligent than a wire (or even a diode) in between the alternator and the "accessory" battery Large cost for getting thick DC cables	Great weather resistance Low-cost for non-cabling components
AC inverter	Connect an AC inverter close to the car. Either put it in a weatherproof box / tent, or put the inverter in the car. Run AC into the house and use an AC->DC charger on the batteries in the house.	Potential need to buy / construct a n enclosure Potentially needs (minor?) mods to the car to get DC power to the inverter, and to get AC power out of the car Many inverters are modified-square-wave; potential incompatibility with the AC->DC charger? Sufficient ventilation? Any risks to the car?	Low-cost cabling Modified sine wave are cheap Would probably have the AC->DC charger anyway (For charging whatever the battery pack is while the grid is up.) Good match for normal approach using a standard generator
12VDC charger	Run 12VDC cables (with circuit-breaker / fusing but not much else) from the car to the house, but use an appropriate DC -> DC charger to charge the batteries. (This would be a charger designed for alternator-based charging, targeting RV / marine / etc. usage.)	Large cost for getting thick DC cables Even with thick DC cables, how well will the charger work with a 60 foot length between the alternator and the charger?	Great weather resistance
DC boost	Use a DC-DC converter in / near the car to boost the 12VDC to ?VDC, run ?VDC to the house, use a DC -> DC charger to charge the batteries. (Possibly a solar designed for solar-based charging?)	Potential need to buy / build a weather enclosure (If DC booster is outside of the car) Potential need to modify the car (If DC booster is inside the car, but outside the engine compartment) Any higher risk for the higher DC voltage? (How high would it make sense to boost up to, anyway?)	Potentially great weather resistance (If DC booster fits in the engine compartment) Potentially cheap weather resistance (If DC booster is outside of the car and it works out that the enclosure is cheaper (no ventilation needed?) than other approaches) Ok cost for getting thinner cabling

Some of those options could be tweaked by locating some of the components in the kitchen (20 feet from the car) instead of the basement (60 feet from the car).

Any thoughts? Thanks!

 

[jasonhc73]

Reconsider converting gas to electric, if you already have gas.
I can get a $1200 hybrid water heater, 10 warranty, or just another gas water heater for $479 for 6 years, or $599 for 12 years. In this case, the water heater swap makes zero sense.

You will not be able to do anything cheap for a long while.
There are no inexpensive V2H or V2G options yet. The only VTH option there is that you can actually buy uses Chademo, and only the Nissan Leaf is setup for bi-directional use through the connection. A first-gen Kia Soul EV might work also.

If you are a cash flow guy, like me. I realize the benefit the very next month with a reduction of cash spent. You will never have a positive cash flow experience in most states. There might be a couple left that actually buy what you give them, not just credit your account and then absorb your credit once a year.

 

[Hedges]

Don't forget "carbon monoxide kills occupants" and "someone steals car with engine running.

Main concern with charging house batteries from car alternator is alternator can't deliver its rated watts for long without failing. Consider at DC to DC charger with limits current to a sustainable amount.

Efficiency will be poor (extracting 300W from a 150 HP engine at idle), but could be useful to protect frozen food.

In-car charging with portable battery means carrying weight, but big benefit is it occurs while driving. With a commute of a few hours per day, could easily power a refrigerator. Less than several hours would mean more than 300W, have to see what is feasible.

Now that I can buy a 300W panel for $60, spending money to get power from car becomes less appealing. I can also buy a cheapie 900W generator for about $100.

 

[blueshoe]

Don't forget "carbon monoxide kills occupants" and "someone steals car with engine running.

Are you concerned that a car's exhaust is more dangerous than a generator's exhaust is?

Fortunately I don't have a garage, so I won't be tempted to run the car inside one. (In the long run the car in question might be an EV anyway.)

For theft concerns I'd consider getting a anti-theft locking parking boot. At less than $100 it could potentially be the cheapest part of the whole setup!

Main concern with charging house batteries from car alternator is alternator can't deliver its rated watts for long without failing. Consider at DC to DC charger with limits current to a sustainable amount.

Yes, absolutely. I could have been clearer that I really saw the "Direct DC" option as the worst of the "options", and essentially a non-option.

The "12VDC Charger" option would specifically be for a charger targeted for alternator-based charging, including current limiting. I'm concerned at how well the charger would do at the other end of 60' of cabling. Any thoughts?

(Or maybe I'm saying I'm concerned about the expense of paying for the gauge-as-yet-undetermined-but-presumably-quite-thick 60' of cabling. It's one thing to pay I'm-not-sure-how-many hundreds of dollars for cabling for a permanent RV or solar installation, another for a backup-to-the-backup that I'll use maybe twice or maybe never.)

Concerns about problematic voltage drop (and ripple?) and / or cabling pricing are really what drive the "AC Inverter" and "DC Boost" options.

Efficiency will be poor (extracting 300W from a 150 HP engine at idle), but could be useful to protect frozen food.

Efficiency isn't quite the point, but it certainly needs to be efficient enough to basically work! Upgrading the alternator is potentially on the table.

In-car charging with portable battery means carrying weight, but big benefit is it occurs while driving. With a commute of a few hours per day, could easily power a refrigerator. Less than several hours would mean more than 300W, have to see what is feasible.

Now that I can buy a 300W panel for $60, spending money to get power from car becomes less appealing. I can also buy a cheapie 900W generator for about $100.

There's no question that if I'm considering going to all of this trouble then I should get at least one solar panel. (This is the DIY solar forum, after all.)

But I don't want to _just_ get a solar panel, I want to have some fuel-powered backup capacity as well. Getting a generator is the obvious solution, but adds some challenges that a car already has solved.

 

[Samsonite801]

Any thoughts? Thanks!

I can add in another way to do this that can apply in some cases like mine. If you own a hybrid vehicle one can tap into the 'hybrid traction battery'...

In my case, on my 2018 Toyota Prius, the traction battery is a 207.2v DC lithium battery, and I was able to find a used datacenter takeout UPS on eBay (APC SURT5000XLT) which normally operates using 16s, 12v SLA batteries (2s cartridges of 8x each) which is 201.6v nominal (the battery input range of the UPS is about 170-250v)...

So I made a tap into the Prius hybrid battery (inside the main relay box by the battery, under the back seat), fused it near the tap, ran some 8 awg marine cable to the trunk, and connect it with an Anderson connector directly to the UPS external battery connector (unplugging the internal SLA battery harness), so the car lithium hybrid battery is the battery for the UPS, and manages the charging when needed, just as it normally does anyways. I haven't measured for myself yet, but others would say they burn about 1.6 gallons of gas used in 12 hours, about 0.13 gallons per hour under moderate load.





Turn on the APC UPS in 'cold start' mode (since no shore power connection exists), meaning hold down power button until hearing second long beep, let off button during second beep. Unit will power on, and the SURT5000XLT (pictured on right side in trunk) puts out 3.5kW continuous (5.2kW surge). I use this UPS with a SURT003 split transformer (pictured on left side in trunk) to convert the 240v to allow 2 legs of 120v as well. They also have the SURT6000XLT which provides 4.2kW continuous.

When the car is 'turned on', it only starts up every few minutes long enough to charge, and shuts off for a few minutes to drain, providing constant power for whatever 240v/120v AC loads running.

Some people would mount these UPS's inside their house, by the breaker panel, and just run the 207.2v cable from the car to the UPS for temporary connection, while in my case, for now, I am leaving the whole system in the trunk of my car (yes it is heavy), so I can use it as a portable generator, since I have a storage unit where I go and sometimes need to run an air compressor or a drill press or something. It could also be used with an AC charger to charge some home battery banks as well, or as a generator input on an all-in-one inverter.

Another guy with a Prius, went so far as to use a Midnight Classic MPPT charge controller, and input the hybrid traction battery voltage on the MPPT input, using a wind generator custom profile, and directly channeled the output of it to his 48v home battery bank, so there are other options available as well...




When it's all said and done, it was a worthwhile mod to make for me. This concept of tapping into hybrids and especially Prius's became very popular after hurricane Sandy era with wide-spread extended power outages, but lately it has become a bit of a forgotten art.

There is a company which had been strong during that time which made inverters for Prius's back then, is making a comeback this year, and they claim they will offer more model inverters for more hybrids, based on demand from pre-signups: https://plugoutpower.com/

Anyways, just thought I'd share that as another viable option for people with hybrids...

 

[blueshoe]

Reconsider converting gas to electric, if you already have gas.
I can get a $1200 hybrid water heater, 10 warranty, or just another gas water heater for $479 for 6 years, or $599 for 12 years. In this case, the water heater swap makes zero sense.

You will not be able to do anything cheap for a long while.

Yes, good points. I am talking my long-term plans; I'd likely wait until a natural replacement point for replacing most of the gas stuff. (And even then, some of it just might not make financial sense to replace with electric, if I don't already have spare electrical solar capacity, and if I do I may have unwisely over-provisioned my array!.)

The boiler is different, though. It's ready to outlast me. (But probably not the pipes to the radiators.) I suspect it was originally coal-fired. But at...maybe... 60% efficiency (not including how parts of the house get way too hot while other parts remain too cold) it's not doing my budget any favors. If we finally swap out our window AC units with mini-split or some such, I suspect it will make sense to get units which also have heating capability.

As far as the car goes...the 2002 Civic is...mostly... going strong. Replacing it with an EV doesn't necessarily make strict financial sense. But we're probably one-big-repair away from moving on.

There are no inexpensive V2H or V2G options yet. The only VTH option there is that you can actually buy uses Chademo, and only the Nissan Leaf is setup for bi-directional use through the connection. A first-gen Kia Soul EV might work also.

Doesn't Setec's inverter also work with the Mitsubishi Outlander PHEV? But that does help the Leaf's status as a strong contender for putative Civic-replacement.

If you are a cash flow guy, like me. I realize the benefit the very next month with a reduction of cash spent. You will never have a positive cash flow experience in most states. There might be a couple left that actually buy what you give them, not just credit your account and then absorb your credit once a year.

Getting my electric down to zero, or even just close to it, is a reasonable goal for me. Predicting my future electric use for the next 25? years, will be tricky...since my plans are pretty vague for how and when my electric usage will change. But the roof's not that large, I can't get it too wrong!

 

[jasonhc73]

I forgot the Mitsubishi, it will probably work also. Japanese brands were required to be bi directional because of the Fukushima Daiichi nuclear disaster.

Going for mini splits is a very good idea as well. I have been researching them also. The key take away that I have discovered is to get a unit that is actually smaller than you need. The longer the mini split runs the more efficient it is and the longer it lasts! It turns out that the start up and shut down is where all the damage happens.

I am shopping right now for a water heater. Turns out the 6 year water heater I have is 13 years old. I expect a 12 year one will fully last 20 years. My only concern with the hybrid water heaters is their reliability, still mixed on the certainty of their longevity. It is just hard to justify 2.5 to 3x the price, since I already have gas there. Electric and gas are super simple and have decades of experience. Even brand new all electric units are far more effecient than ones from 20 years ago. ?

 

[Hedges]

I have the same Kenmore gas water heater which came with the house 20 years ago.
Gas bill is $8/month.
I wouldn't foresee any payback for instant or heat-pump water heater.
Just the downside of cost and maintenance (presently none. Once replaced a leaky relief valve.)

Off-grid I could see having electric, enabled only when battery full.

 

[blueshoe]

I can add in another way to do this that can apply in some cases like mine. If you own a hybrid vehicle one can tap into the 'hybrid traction battery'...

...

So I made a tap into the Prius hybrid battery (inside the main relay box by the battery, under the back seat), fused it near the tap, ran some 8 awg marine cable to the trunk, and connect it with an Anderson connector directly to the UPS external battery connector (unplugging the internal SLA battery harness), so the car lithium hybrid battery is the battery for the UPS, and manages the charging when needed, just as it normally does anyways. I haven't measured for myself yet, but others would say they burn about 1.6 gallons of gas used in 12 hours, about 0.13 gallons per hour under moderate load.

Nice! Looks pretty slick.

When it's all said and done, it was a worthwhile mod to make for me. This concept of tapping into hybrids and especially Prius's became very popular after hurricane Sandy era with wide-spread extended power outages, but lately it has become a bit of a forgotten art.

There is a company which had been strong during that time which made inverters for Prius's back then, is making a comeback this year, and they claim they will offer more model inverters for more hybrids, based on demand from pre-signups: https://plugoutpower.com/

This is good to know; I hope they're successful! I'll have to check in with them when we get to the point of upgrading our car.

I have to say, V2G leaves me a little bit cold as an individual person. Maybe it makes sense at scale, maybe it doesn't. But V2H is a different story. When I get to the point of purchasing an EV (potentially a plug-in hybrid, but more likely a straight EV) then selecting a model which I can use for V2H will be pretty important. Sounds like these guys are planning on making that decision a lot easier!

 

[Hedges]

Going for mini splits is a very good idea as well. I have been researching them also. The key take away that I have discovered is to get a unit that is actually smaller than you need. The longer the mini split runs the more efficient it is and the longer it lasts! It turns out that the start up and shut down is where all the damage happens.

But if they are variable speed, wouldn't you want to size them so they almost reach full speed on hottest day, running slower on cooler days?
If actually smaller than your cooling load, they would run 100% and temperature would rise.

 

[jasonhc73]

But if they are variable speed, wouldn't you want to size them so they almost reach full speed on hottest day, running slower on cooler days?
If actually smaller than your cooling load, they would run 100% and temperature would rise.

Just get one size smaller than what you "need". If your room says you need a 9k, get a 6k. If your room is calculated to need a 12k, get the 9k.

 

[Samsonite801]

Nice! Looks pretty slick.




This is good to know; I hope they're successful! I'll have to check in with them when we get to the point of upgrading our car.

I have to say, V2G leaves me a little bit cold as an individual person. Maybe it makes sense at scale, maybe it doesn't. But V2H is a different story. When I get to the point of purchasing an EV (potentially a plug-in hybrid, but more likely a straight EV) then selecting a model which I can use for V2H will be pretty important. Sounds like these guys are planning on making that decision a lot easier!

Yeah that company Plugout Power LLC is coming along, they send me newsletters every once in while (after I filled out the information form on their site awhile back), and they talk about other models coming down the path, including a 3kW and a 5kW inverter for Prius, the Prius Prime, and a couple Lexus models. I'll attach his newsletter so you can see what cars they're looking at... They're also looking for guinea pigs to volunteer on testing new cars in the area.

As far as the used datacenter inverter option, my friend has a Lexus which uses a 384v hybrid battery, and so I did some research and figured out he could use a Vertiv Liebert GXT4-10000RT208 datacenter UPS, which operates at his hybrid DC voltage. So the point is, if you find some hybrid you like, it's just a matter of researching what voltage the battery operates at, and then go find some inverter which is similar input voltage. Or you can ping Plugout Power and ask them if they have any plans with that model coming down the pipe. Or you can try the MPPT solar charger way of doing it (that's unchartered territory for me so I say nothing about how it works)...

Yeah, I like the V2H idea, a way to multipurpose your car to be a generator and help supplement home energy requirements. I also like how quiet the car is, not like a loud generator, the Prius just starts up and basically idles there for a few minutes and shuts off for a few minutes, and starts again after while...

 

[jasonhc73]

I have to say, V2G leaves me a little bit cold as an individual person. Maybe it makes sense at scale, maybe it doesn't. But V2H is a different story. When I get to the point of purchasing an EV (potentially a plug-in hybrid, but more likely a straight EV) then selecting a model which I can use for V2H will be pretty important. Sounds like these guys are planning on making that decision a lot easier!

There is really no difference between V2G and V2H. It is very easy to make one device. If off-grid then no need to install columb thingy on the grid connection. If on-grid, then you have to have the columb thingy if you don't have a net meter. This will allow you to TOU shift, and in some cases send energy to the grid when energy prices are high, and recharge when prices are low.

On the macro scale of things, California is suffering from an invented problem because of too much solar, The Duck Curve. The midday, supply is way way up, but not demand, so vast amounts of back-fed energy is up on the lines getting sent all over the surrounding states. Well, with a V2G/V2H thingamajig you would just feed that excess energy to the car, instead of the grid. This is basically how a Tesla Gateway 2.0 works. Sol-Ark has this feature in there inverter.

 

[Hedges]

There is really no difference between V2G and V2H. It is very easy to make one device. If off-grid then no need to install columb thingy on the grid connection. If on-grid, then you have to have the columb thingy if you don't have a net meter. This will allow you to TOU shift, and in some cases send energy to the grid when energy prices are high, and recharge when prices are low.

On the macro scale of things, California is suffering from an invented problem because of too much solar, The Duck Curve. The midday, supply is way way up, but not demand, so vast amounts of back-fed energy is up on the lines getting sent all over the surrounding states. Well, with a V2G/V2H thingamajig you would just feed that excess energy to the car, instead of the grid. This is basically how a Tesla Gateway 2.0 works. Sol-Ark has this feature in there inverter.

More important, can you disconnect from grid during a failure and then do V2H? (your "if off-grid")
Does it do UL1741? Is it interlocked to a main disconnect switch? That would make it more than just a grid-tie inverter.

 

[jasonhc73]

More important, can you disconnect from grid during a failure and then do V2H? (your "if off-grid")
Does it do UL1741? Is it interlocked to a main disconnect switch? That would make it more than just a grid-tie inverter.

Read what Sol-Ark is doing. From what I am seeing they are saying is that they do not actually physically disconnect from the grid during an outage. They figured out how to not let any energy go out on the grid by only outputting enough energy to meet demand inside the house and no more.

 

[Hedges]

Read what Sol-Ark is doing. From what I am seeing they are saying is that they do not actually physically disconnect from the grid during an outage. They figured out how to not let any energy go out on the grid by only outputting enough energy to meet demand inside the house and no more.

Can't be done. If you put 120/240V on your wires, without isolating from the grid (which is down), it backflows to the grid.

What can be done while the grid is up is "zero export". Measure current and voltage at service entrance, and produce all the power you can, or just enough to drop watts imported to zero, whichever is less.

To power the house while grid is down, electrical connection to the grid has to be isolated.
Or, you can have an important loads panel which is isolated from the rest of the house, only power that.

 

[jasonhc73]

And yet they are doing it.

 

[Hedges]

And yet they are doing it.

No they aren't, not providing backup for house without disconnecting from grid.

https://www.sol-ark.com/wp-content/uploads/2020/07/12K-brochure-combined-7-22-2020.pdf

Sol-Arc has AC in, AC out, transfer time 4ms.
That's just a relay inside, works like a transfer switch. Only the downstream loads are protected, like any other UPS.
50A limit on pass-through current.

If you have V2H, it can't just be a cable plugged into the house wiring.
It needs a disconnect switch to isolate grid.
When using a generator you need a transfer switch (DPDT) to move both hot legs from grid to generator.
A UPS like Sol-Arc has DPST because it's inverter section can backfeed the grid, UL1741.

Your V2H solution would have a switch to disconnect from grid, and a signal to vehicle telling it to send AC to house. Could be a vehicle charger with battery inverter, getting its DC from vehicle.
An ideal implementation would communicate with household loads and GT inverters, managing them.

 

[ianganderton]

Cant see any maths done on this yet

Have you done a power audit? What is your expected energy requirement? i.e. when using the batteries how much power are you going to want to use and for how long?

How big are your batteries? What chemistry are they?

What altenator do you have? (Normally a rating in amps). As a rule of thumb I expect to be able to use 1/3 of the altenator's rating for charging

Based on how much power you are planning to use each day you can work out how long it will take to charge by running the car. Does the maths work out ok?

One of the problems to bear in mind for car OEM altenators is they are specifically designed to power a car's electrical system very efficently. Use them our of that parameter and you can run into problems.

for instance. A car starter battery has a very high load for a few seconds only, this then needs recharging for a short period after starting. The altenator in modern cars will then significant reduce its power output so it reduces its load on the engine and there fore reduces fuel consumption. This means smart altenators dont nescessarily produce consistant amps

Secondly altenators need airflow to cool them. They have internal fans that work ok at higher revs but problems can occur when there are high charging loads at low revs. This causes heat to build up and can burn out the wiring in the altenator.

Other altenators can be engineered differently. Marine altenators for instance are designed for charging batteries at low revs. Many industrial vans have additional mounts on the engine for adding a second altenator for charging house batteries and powing workshop tooling or winches etc

An inverter generator might be a better option, depends on the maths.

 

[A.Justice]

....12VDC charger....DC boost

Any thoughts? Thanks!

12v DC charger- I have seen solar batteries charged through a cheap 10-15 amp PWM controller, plugged in the car lighter outlet, or just to the battery itself. It wasn't the fastest or most efficient thing, but if you can get your vehicle close to your batteries, it would be possible (with proper circuit protection, like you said) to charge the batteries, albeit slowly. I actually tested it out myself, but didn't run it for long, so I really cant say if its safe, only that it IS possible.

DC Boost- Will probably work, but will have a lot of loss. It would be easier to just get bigger cables, or get the vehicle closer to the batteries.