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Hi all,

I am thinking of purchasing a complete BEV battery from ebay and rebuild it to fit a hybrid PV inverter, with a view to take advantage of Octopus Flux tariff, ie charge up cheap at night rate (or free from solar) and discharge to grid at peak rate. This will generate some income and would pay the cost of equipment within 5 years or less.

I am looking for advice and opinions whether there are regulatory pitfalls that would prevent me from achieving the end goal legally. The key is that I want to have a huge battery at a fraction of the cost. I had a good look on the forum but have not seen discussions about this in the last year or so. Maybe I missed or maybe it is a straight no no and discussion is a moot point.

I am East Midlands based, already have a functioning 8KW residential PV (roof) system, grid-tie only (no battery capability), MCS compliant and with DNO permission. Just looking to add a beefy battery (of course new inverter will also be needed)

Thank you very much!


ps: Having looked at some ebay prices and 30KWh worth of newish LifePO4 BEV cells cost less than a 5KWh(ish) branded battery packs. Also would I be right in assuming BEV cells are likely to be much more resilient electrically and mechanically compared to purpose made home storage batteries?

pps: I know, I could blow up my house! Questionable quality cells or BMS is going to be a big risk factor here so I have thoughts for additional layers of safeties, which will be an added cost but just a fraction.

Welcome to the forum - sounds a superb project!
Quite a few on here with battery expertise so I'm sure they'll be along soon.

Welcome to the forum!

Sounds like a really great plan! Many BEV batteries can be disassembled into 3s or 4s packs and reconfigured into a (nominally) 48V arrangement for connection to a commonly available grid-tie battery-inverter via their convenient M10 (or whatever) power connections. You would also need to provide a balancer, but really great (way better than BEVs are fitted with) active balancers are available for £50-ish and the packs usually have BMS pins that give access to individual cell voltages for balancing.

The only real drawback is the price per kWh of these scrap batteries has been so very close to commodity cells imported from China that you might as well just go for new! When I installed my Pylontech batteries they were priced so close to commodity cells that it wasn't worth building your own and ex-BEV cells were more expensive!

BEV cells aren't "more robust" they just come with very sophisticated active heating/cooling systems. However in a domestic install your C rates will be much smaller so nothing to worry about.

Hi diyfan

Some have gone down that road successfully, there are some good YoTube videos showing how to tear down the BEV packs. I would recommend you try and stick with LIFEPO4 (LFP) chemistry batteries as they are safer to work with compared with the higher density NMC type cells.

If I was building a pack again from new, I would go for new A grade LIFEPO4 cells from China like many on here have done. Or grade B if your on a tight budget. There are a few threads on here, some are quite long and take a bit of reading but the nuggets of info are in there,

A couple of threads worth starting on are,

Joeboy's epic, Idiot's guide to a self build battery
https://camelot-forum.co.uk/phpBB3/view ... =14&t=1194

And my, Lithium Cell Balancing at Nowty Towers - Resurrections
https://camelot-forum.co.uk/phpBB3/view ... =14&t=1013

How about ten of us club together and order one of these each?

https://www.alibaba.com/product-detail/ ... 227773ZN4c

I don't think there are regulatory pitfalls as the paperwork you need to do concerns the g98/99 /100 inverter rather than the battery.

I was considering the same plan but as has been pointed out, getting them at a price that makes it worthwhile is the issue.

I did buy some ex-hybrid modules (vw iirc 44v 25ah) and converted them into a 24v 150ah battery which i am currently using, but tbh, although the price/kwh came out ahead of Joeboys big diy china battery, if i factor in my time cost it was not so good a deal.

If i do go down the ev battery route I'd probably look at sourcing a battery inverter that could run off the ev battery standard voltage rather than converting to 48v, but then i'm fairly comfortable working with higher voltages d.c.

Thank you so much for your kind feedback, this is really a strong encouragement for me to charge ahead. I have some experience in smaller scale solar, battery and electronics projects but the scale I am looking at now will come with new challenges. So let me share some more detail and I would appreciate the scrutiny and pointing out gaps in my plan would be welcome.

The inverter - Sunsynk ECCO 8.8KW - it can output up to 8000W from battery to grid and the same into the battery (up to 190A). So over 3 hour window it should be able to pump out 24KWh. The same goes for filling up with cheap power if forecast estimates a murky day ahead.

The cells - KIA NIRO EV 64KWh, there is an ebay advert for £3800 but I can get it for £3300 delivered. I only had bad luck with battery cells from China, I found 95% of time they lie about specs and quality. 200Ah cells I bought previously turned out to be 140-150Ah with quite high internal resistance. I suspect they are just old cells slipped into a new case and sold as new. I just don't feel lucky enough to try the same again. With second hand EV cells I know they were made to last and must have high C rating. Cycling 32KWh battery made from these cells at my anticipated 8KW rate is going to be a walk in the park.

BMS for batteries - Undecided and looking for advice. I am looking at some on Fogstar website. I imagine any programmable, common port, 16S, 100A, RS485/CAN with Pylon emulation. But actually just as a layer of protection, thinking to set Inverter treat battery as dumb and set manual voltage limits. BMS still can do its own thing just not talk to inverter. Hook them up in parallel like in my crude schematic.

Welcome to the forum. There are a number of bms's and balancers sourced and operational within this thread.

https://camelot-forum.co.uk/phpBB3/view ... =14&t=1194

A number (many) of us sourced batteries from China through a company called Deriy.. They are excellent as was their point of contact Colin Deng. Colin has left Deriy but is still an active excellent member and source of batteries. 100% of us found Deriy & Colin to be excellent supplying A grade batteries. Worth noting.

Are you needing to charge at 8kW? Wondering if that is actually needed even though the inverter can do it. I ask as it will impact your build costs re cable & component sizing.

Many of us are on Intelligent Octopus tarriff which gives a regular 6 hour charge window overnight. I have a 28kWh battery stack, depending on grid voltage I can see up to 3.85kW in the charging window from my hybrid inverter. I am very rarely at bottom soc and that charge rate and time window gets the home stack to 100% every time.

Hi @joeboy

Thank you, I will check out the link, and will also consider the cells from China. But what would be an advantage over BEV cells?

As for 8KW, this is purely from economical standpoint. The faster I can pump power between the batteries and grid, the bigger is earning potential. I am on Octopus (non-intelligent) Flux tariff and it only gives 3 hours of good export price and 3 hours of low import price. At 8kw I could earn £1.97 per day but at 3.85kw that would drop to £0.97 You see, the only way I can justify getting all these toys is that they will pay for themselves (or even make some money) during their life span. I have thrown your charging number into a little spready to work out the figures. In my view having beefier inverter definitively pays off.

Switching to intelligent Flux makes things much worse as peak export price is only good for 1 or 1.5 hours a day instead of 3 hours. I coded a simple simulation and plugged in half-hourly historical prices for last 12 months and potential earnings were only about half in like for like comparison with "dumb" Flux. But you did not mention which Octopus Intelligent tariff you have so that could change economics. What are you on?

diyfan wrote: ↑Fri Dec 01, 2023 10:48 pm Hi @joeboy

Thank you, I will check out the link, and will also consider the cells from China. But what would be an advantage over BEV cells?

As for 8KW, this is purely from economical standpoint. The faster I can pump power between the batteries and grid, the bigger is earning potential. I am on Octopus (non-intelligent) Flux tariff and it only gives 3 hours of good export price and 3 hours of low import price. At 8kw I could earn £1.97 per day but at 3.85kw that would drop to £0.97 You see, the only way I can justify getting all these toys is that they will pay for themselves (or even make some money) during their life span. I have thrown your charging number into a little spready to work out the figures. In my view having beefier inverter definitively pays off.

Switching to intelligent Flux makes things much worse as peak export price is only good for 1 or 1.5 hours a day instead of 3 hours. I coded a simple simulation and plugged in half-hourly historical prices for last 12 months and potential earnings were only about half in like for like comparison with "dumb" Flux. But you did not mention which Octopus Intelligent tariff you have so that could change economics. What are you on?

The sooner Octopus accept Ebikes as legit connection devices for IO the better. IO is a 6 hr window overnight with further slots daily given depending on excess power sloshing about on grid.

I didn't do a deep dive on EV v's home batteries when heading for a diy pack. If I had I'd have looked at battery chemistry, whole pack design for ripping down, works to ensure home pack is well built with no risk of arcing. Individual battery unit charge/discharge capacity. Warranty.

I'm not saying you can't quite happily repurpose an EV pack to a 48V home pack and kudos if you do. Buying in grade A cells with posts fitted, certification and correct for job busbars supplied makes for an easy build.

Out of everything I've said I'd say a deep check that the EV pack units can charge/discharge at 8kW would be my priority one, then safe battery chemistry for home use. Beyond that, its really just time v's money and we'll be happily watching.