EV battery as mains power source? - Page 1
Forum section: electronics / general

author: unknownparticle
author_rank: Frequent Contributor
timestamp: January 27, 2022, 05:56:43 pm
content: OK, so I know next to nothing about EV battery and power systems but I'm trying to work out if an EV battery, from a scrapped vehicle, could be used as a battery powered mains system for a domestic property?This is in response to the epic increase in UK energy prices with more to come in April when our price cap is revised to account for market conditions.My thoughts are that an EV battery pack could be used to charge from night time, low cost, off peak mains electricity, then during the day provide mains power for the house instead of the grid. I think that the charge side is easy, because the batteries normal charger could be used, but it's the conversion side that I am concerned with. The battery output would need to go through an inverter of course, but there is the question of voltage compatibility and conflicts with the batteries control systems etc.Has anyone done this, or knows the system parameters etc?

author: bob91343
author_rank: Super Contributor
timestamp: January 27, 2022, 06:09:53 pm
content: This is a complex issue. The nub is the inverter. Depending on the load, it may need to have sinusoidal output. That is difficult to generate. For lighting it doesn't matter, especially if the lamps are incandescent. For motor loads such as refrigerators and fans and HVAC the motors may not operate well unless driven by sine waves.In addition, one must plan for starting surges such as refrigerator compressors.Take a clue from the solar panel people and see what they are doing.In addition, there are difficulties in connecting the system without conflict with the mains system.

author: unknownparticle
author_rank: Frequent Contributor
timestamp: January 27, 2022, 06:22:01 pm
content: Yeah, would have to be sine wave, which all Solar PV inverters are. I have an Excellent Solar PV inverter which would defo do the sine wave part, but it has a 300 volt minimum input voltage, obviously intended for PV panel strings. So, it depends on the battery voltage output and how much hacking of the control systems is neccessary, or not! Thing is, EV batteries have a really high KWH rating and are relatively cheap on the EV breakers market, compared to purpose designed home battery storage systems.On the mains changeover side, thats no problem as there are off the shelf solutions for that.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 27, 2022, 11:26:26 pm
content: Look up "DIY Powerwall". Easiest way would be to use a bidirectional inverter. It would operate as a charger off peak and switch to operating as a zero export inverter on peak.A Prius inverter could be modified for that purpose although the controls side is going to be complex. That's basically what I'm doing (but with added solar and a thermal storage system for HVAC), the biggest thing to note is the large inductors needed for pure sine output. If you're OK with square wave or modified sine for backup operation and don't mind a poor power factor, in principle you could have the inverter only supplying current on the peaks, with the boost converter inside the inverter following the AC waveform and the outputs switched on during the correct times of the cycle. In that case, the inductors would be much smaller and only need to limit EMI to acceptable levels.

author: Faringdon
author_rank: Super Contributor
timestamp: January 30, 2022, 04:39:13 pm
content: QuoteOne of the big players did this....using old EV batteries...it was so bad that they stopped....they go unbalanced big time and you loose loads of battery capacity (they get so unbalanced you cant rebalance them)....using EV batts is not vaiable, so they concluded, after multiple customer complaints....if you do it, you might be lucky and get that golden EV battery that lasts ages.What you propose in the top post sounds like a great idea...but the practicalities win out...and in the end , its a pretty miserable thing.......your payback time is very long....and when you start breaking even, your batteries need replacing because they have lost so much capacity....also, your inverter....goodness hope that doesnt fail, it will eventually (say its el caps go bad), because then that eats even further into your payback........then theres the size and weight of the inverter and batteries and charger.......its a big piece of hardware..........then theres the noise of the fans.........unless you buy a tesla power-wall which doesnt have fans...but they cost something like...is it £13kfor 8kwh ()..you can look this up on the web.I think powervault do the lowest cost system...and they can also do it in conjunction with eOn **()....and if you want to save money , you can agree to be a "mini power station", and give the el co occasional control of your system so they can use it to re-balance the grid.............and if you are cunning, you can opt for this, and get the discount, but then disconnect your system from the internet, so that they cant get control of your system...and you just pretend its not your fault..and that there is some problem with their management of your internet connection in your unit.....lots of customers do that..**OK my question marks came out as faces, sorry about that..But having said all the above, if the differential between night time rate and day time rate gets massive, (and stays like that ) then the battery storage starts to look good.Also, i think in the UK , there is a limit to the power of the Grid Tied Inverter that you are allowed to use on the mains....i think its 5kW..........they dont want people using more than this because otherwise the mains voltage level could get too high, and sOme people could transmit too much power to the grid, and not let others get a chance...say if it was a very sunny day...remember a GTI wont turn on if the mains is above a certain level....and loads of people transmitting loads of power to the grid raises the grid voltage.But if you want to "island" your system, then obviously you can have inverter and batteries as big as you like.........just hope you dont get the wrong batteries that go unbalanced and loose capacity...because this is a gamble....if you kop it, it will be a very expensive situation for you to replace them.I think if you go with say powervault, then for 10 years or something, they will replace any bad batteries....i think the warranty on batts is 10 years, i coudl be wrong.Remeber also that if you have a 5kW inverter.....its efficiency is pretty bad at low power levels, say a couple hundred watts.....so theres even more misery into it.Also, how are you going to make sure your solar doesnt put into your storage battery when you car battery needs charging?...because its inefficient to put solar into the storage battery, and then from there to the car battery........there is conversion efficiency losses.I could be wrong, but i think its only the tesal power wall that can be instealled outdoors?.....the others need to be indoors, or in the garage, but the fans whirring away annoys some people.....and the fans go on at night because thats when you are shovellign in the cheap rate electricity.

author: free_electron
author_rank: Super Contributor
timestamp: January 30, 2022, 05:04:57 pm
content: the biggest issue is not the inverter but the gateway.the gateway takes the mains power, battery power , solar power ( if you want this ) and orchestrates the power flow from battery and grid to the house and to and from the batteryIt is not a hard switchover. the charger is modulated so that the combined , instantaneous, draw from the grid ( house + battery charge) does not overload your feed from the grid.at the same time , when running on battery , it allows for the grid to feed extra energy if demand requires it.To do this the gateway sets current limits on the charger and modulates the output voltage of the inverter. the inverter runs in sync with the grid and needs ot be able to take over if the grid drops out. when the grid comes back it needs to resynchronize the output frequencyThere are a bunch of safety mechanisms needed to prevent backfeeding the grid or to reconnect out of phase.i have a system ( 56kwh storage , 48 solar panels , two inverters.) i can run the house and charge the battery fully at daytime and run the house at night off battery. in wintertime there is not enough solar to fill the battery completely but i can still run 4 hours at night of battery.The gateway orchestrates all powerflow. there is not so much as a single lost line period when the grid falls away.each battery has its own inverter/charger built in and all four need to remain synced to produce output.

author: Faringdon
author_rank: Super Contributor
timestamp: January 30, 2022, 05:21:31 pm
content: ..yes with some systems, when the grid fails, your inverter has to turn off, in case you electrocute linesmen.If you wanted to get electricity from your batts when the grid fails, then this isnt straightforward, your system needs to be able to manage all the whys and wherefores of doing that....as described kindly above.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 31, 2022, 03:13:59 am
content: Quote from: Faringdon on January 30, 2022, 04:39:13 pmNot an issue if you get a BMS that actually works.That said, if most of the load is thermal in nature as in HVAC and hot water, thermal storage would be a lot cheaper and has no inherent limit on lifespan.Quote from: Faringdon on January 30, 2022, 05:21:31 pmEasily done with a transfer switch, getting one that switches very fast could be a challenge. Might be easier to have the most sensitive loads on an inverter that runs full time.

author: f4eru
author_rank: Super Contributor
timestamp: January 31, 2022, 07:51:58 am
content: People still try to use Li batteries without balancing and BMS ?

author: unknownparticle
author_rank: Frequent Contributor
timestamp: January 31, 2022, 07:12:13 pm
content: Hmmm, some surprising info there! Why do EV batteries go unbalanced? Is due to running at low load? I wouldn't be using solar, so no issue there. As mentioned, I'd also be using an transfer switch, which is common practice. Cheap rate electricity is under a third of full rate, so the savings are huge, whether this stays so when the cap is revised is an issue. As for the battery itself, I can get large capacity units at around £500 or so, payback is therefore pretty quick! Need to do something as the prospect of £1000+ and increasing electricity bills is not welcome or affordable.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 31, 2022, 07:37:02 pm
content: Quote from: unknownparticle on January 31, 2022, 07:12:13 pmThey don't, ignore Faringdon's drunk writing above.Of course with human-made systems, mistakes happen, and gross pack unbalance is caused either by failed cells, failed BMS, or combination thereof. This isn't a normal condition though, and reasons are not in usage patterns, but manufacturing and/or design errors which could be anything.Physical reasons for unbalancing are, manufacturing variations, temperature differences during storage/operation of cells, and poorly designed external circuits like broken-by-design BMS's. Normal level of unbalancing is like 1-2%/year. Can be even lower, can be a bit more. In any case, any operational (properly designed) BMS easily takes care of that. Even packs with no balancing usually tend to be in balance after years of use, but with expensive packs like in EV's, at least minimum level of electronics is to be expected.The real question is, if you are getting a large li-ion pack for one tenth of its original/actual value, are you sure about its condition? Sounds risky to me. Expect to test and measure cells, remove dead cells, etc.

author: Faringdon
author_rank: Super Contributor
timestamp: January 31, 2022, 07:39:16 pm
content: QuoteIts just the tolerance of them. A battery consists of a number of cells in series (maybe series and parallel, but there is series in there).....and when cells are in series...then as soon as the lowest capacity one gets fully charged, then the entire battery must cease being charged.Some brand new batteries with brand new cells will go unbalanced quickly, others wont....its hard to tell....if you coudl tell, youd never make a battery out of them....but thats it, you cant tell....this is why companies like CD02 exist etc......and the older a battery pack is, the more likely it is to go unbalanced......the trade call it "rogue cell".......the battery storage vendors feedback all your cell data , and can see all the cell voltages in the pack, and can see a "rogue cell" straight away, because its terminal voltage, when loaded, is significantly different to the others' cell voltages.Theres more to it than that, and you can search for youtube videos on "cell balancing" etc.Also, in discharge, as soon as any single cell gets fully discharged, then the entire battery pack must cease discharging, otherwise the fully discjharged cell may get damaged and go short circuit or whatever.

author: langwadt
author_rank: Super Contributor
timestamp: January 31, 2022, 07:40:42 pm
content: Quote from: Faringdon on January 30, 2022, 04:39:13 pmthey must have been using crappy old worn out batteries or a broken balancing system, why shouldEV batteries suddenly stop working just because they are not driving down the street on four wheels?https://youtu.be/NDYhihUbovM

author: Faringdon
author_rank: Super Contributor
timestamp: January 31, 2022, 07:45:35 pm
content: QuoteThats right, will it stay liek that for years and years?...surely if fual costs get higher, then eventually day rate and night rate leccy will even out in cost.......and also, they want people to buy battery storage systems , because it kind of evens out the grid, balances it, so they sre doing this day/nite thing to get people to buy battery storage...but when loads of people have it...wont they bin the night rate?Payback time for batt storage is generally considered to be 10 years.I think its limited aswell...because i think a grid tied inverter for a household is limited to be no more than 5kW....at least in UK.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 31, 2022, 07:47:09 pm
content: In any case, second-life is hard, because there is a reason why the packs are removed from the vehicles!Sometimes you are lucky and the reason is the vehicle being totaled with the pack somehow luckily escaping any damage. But if you have to do second-life in large numbers, you are going to see large number of failures, too, unless you come up with some ground-breaking quality control process.For an individual doing it once for DIY, it's all about luck, really.

author: Faringdon
author_rank: Super Contributor
timestamp: January 31, 2022, 07:52:54 pm
content: I mean, say if you have an 8kwh battery, and you put that in at night, then use it in the day, and night rate = 5p/kwh and day rate is 15p/kwh, then you save 80p per day........so thats a saving of £292 per year....and thats considering 100% efficiency which you wont get.But then again theres solar storage too....so maybe its not so bad in that case.Also, when i was in the trade, there was talk of a variable rate of leccy, which was varying wildly on a 15 minute basis, through the day and night....and you had to have an interactive storage system, so you coudl start and stop it accordingly.

author: unknownparticle
author_rank: Frequent Contributor
timestamp: January 31, 2022, 10:04:13 pm
content: Quote from: Faringdon on January 31, 2022, 07:45:35 pmI wouldn't be going grid tied.

author: unknownparticle
author_rank: Frequent Contributor
timestamp: January 31, 2022, 10:09:47 pm
content: Quote from: Siwastaja on January 31, 2022, 07:47:09 pmAgreed there is risk of a damaged pack. I have seen a number of low mile packs on ebay though, from accident damaged right off vehicles, with no obvious physical damage. It seems that EV's are vulnerable to high cost drive line accident damage which makes them uneconomical to repair, due to the cost of motors etc. Also, repacing the battery packs on some of these vehicles is very labour intensive, which limits the value of used battery packs.

author: langwadt
author_rank: Super Contributor
timestamp: January 31, 2022, 11:28:11 pm
content: Quote from: unknownparticle on January 31, 2022, 10:09:47 pmI think it is also a case of manufacturers/dealers refusing to do any kind of "improvised" repair, something that is done all the time on ICE vehiclesexample with a Tesla,https://youtu.be/n8-OkfCcRAo?t=38

author: richnormand
author_rank: Unknown
timestamp: February 01, 2022, 12:24:54 am
content: I was looking at an EV but Tesla is off my list. That video referred above is a perfect example. Tesla is like Apple in philosophy.Fortunately several other EV options are coming to the market in the next few years.

author: jonpaul
author_rank: Super Contributor
timestamp: February 01, 2022, 12:01:17 pm
content: Use EV auto, add connectors and UPS inverter:See Frichard Factor's site form 12 years agohttp://www.priups.com/riddle/answer-1.htmJon