Greetings from London, UK

hi all!

I've jumped on this forum after seeing the viral "build your own powerwall for $300" video, I have access to hundreds of 18650's as i work in IT so I figured I'd do a bit of research into this and find out how feasible it is.

So far what I've read about the battery packs seems simple, i already have equipment to test/charge/discharge the recycled cells I'd be taking from work.

I've looked around at prices of solar panels etc and that seems reasonable too.

Wood i can get for free as well and the smaller parts like fuses and wiring etc are pretty cheap.

The big partthat seems unclear to me is around the inverterand battery management, I can see theres a forum section for BMS so I'm guessing these manage the charging and discharging of the batteries so I guess plenty of options to read through there.

The information on inverters (especially in the UK) seems a bit scarce though? can anyone recommend any UK inverters that will allow me to hook up some solar panels and powerwall plus connect it my house mains?

Also how is it defined when the powerwall is actually used? I'm guessing this is something programmed into the inverter?

Appreciate any help, if this ends up being something I go ahead with I'd create aUK "how to" video / walk through post.

The 300USD then you dont get the fancy stuff below i will write about

BMS: You can go from monitor by eye and just take it carefully to the china versions of bms for 20USD that doesnt do much to balancers for 20USD per cell to full blown BMS systems like Batrium for or cheaper ones from 200-2000USD.
I prefer Batrium if you go bigger system.

Inverters depends on how you want to hook it up. Offgrid is easiest where you only have a switchover panel and send some of the outlets or parts of the house to the inverter. A cheap version is the PIP4048 that is common.

Im not sure what your "define when powerwall is used"?


If you check my setup out below you will se a combination of chargers alone with the offgrid pip inverter with the hybrid version inverter as well.

I offer this for peer review!

Particularly in an urban UK environment the big worry with DIYis battery safety and not voiding your house insurance... This also applies to DIY mains electric wiring. At minimum, I think we'd need anything properly 'signed off' by a formally qualified electrician.

Second-use cells have no warranty and no certainty that they wouldn't misbehave. Hence you need to design to detect early signs of misbehaviour and handle the situation safely.
Restricting yourself only to cells that are not overdischarged (initially showing less than 2.5v - and certainlyhave not 'blown' their mechanical pressure protection to show 0v), don't overheat on charging, do still have around 2000 mAh capacity (on a 1 amp discharge) and don't self discharge from full (4.2v) by more than about 0.1v over a couple of weeks SHOULD weed out those that are an obvious liability. And you will need many hundreds of such good cells for a reasonable powerwall.
But the weakest cell in the pack is the one put under the most strain and likely to deteriorate fastest - weakest link in the chain!
Connecting every individual cell with a link that will fuse should 5 amps go through thatindividual cell is a prudent precaution against the drama of a cell going short circuit (and having its parallel fellows rapidly discharge through it).

There are various ways of minimising the damage/deterioration of the myriad cells that comprise your battery. But you can't completely avoid it, the assembly will (at best) slowly lose capacity over the years. That is what you hope for, not any accelerated local failure.
General wear is greatly reduced by limiting the cell voltage swing to between 4.0 and 3.0v - likely doubling the cells cycle life compared to 4.2/2.7v. However this reduces the effective capacity per cell, so for the same pack capacity you need more cells in parallel.
More cells in parallel (keeping everything else the same) has a second anti-ageing benefit - less current per individual cell.
Keeping serially connected cells at a very similar voltage makes sure that they are all exposed to the same voltage limits. The whole-string total voltage only indicates the average.
This brings up the need to keep those serially connected units "balanced" or equalised in voltage.
Auto-balancing seems to be the sole function of many "BMS" products, and it seems that such things would typically only 'top balance' - equalising at a full 4.2v ... which isn't particularly helpful if you are working on a reduced voltage range!
Some may turn on a warning led if they detect a serial cell going below 2.7v. Helpful for an e-bike, but woefully inadequate for a 'consumer' powerwall.
Batrium (plus a Win10 pc for stup and review) can do much much more than that. It uses Canbus (which I had naively thought to be automotive) to control inverters and chargers (but only those Canbus-equipped ones that it knows how to talk to). This enables Batrium to safely shut down your battery, etc,if it gets worried about what it is seeing. It is also programmable, providing one way of imposing those more cautious voltage swing limits, for example.
Batrium ain't cheap, almost 500 quid for a starter kit (before vat, duty, etc), but I can't see your insurer letting you off without that sort of protection.
Batrium-controllable grid-tied inverters aren't cheap either. 3kW output would allow you to power just one standard UK 13amp wall socket.
You need a BIG system to run circuits with significant power-draw potential and a big lithium battery bank has the potential for more than a fireworks display. Given the total cost of doing things 'properly', the savings through using salvaged cells becomes a smaller proportion of the total than one might initially consider,

All this is leading me towards the idea of a distinctly limited off-grid informal and auxilliarysystem. But that would still be quite a project!

Welcome, fellow UK builder here in a similar position. I wouldn't worry about the inverter quite yet, start building your packs first

Hi daromer, thanks very much for your quick response.

So my understanding is that a basic "off grid" inverter would be used if i wanted to just manually switch over the power supply to the powerwall, and a "hybrid" inverter (from what wikipedia tells me) would send all solar power to the house, store any excess power in the powerwall, then when the solar power is insufficient, say at night time, it will start to drain the powerwall. I'm also guessing if neither the powerall or solar have enough juice then the inverter will draw straight from the grid?

And hi zag2me, your thread was one of the first i read I need to understand how the whole setup and topology is going to work to understand if its even feasible before starting on anything. Building the battery packs is going to be the easy part since I already have all the resources, but its pointless if i dont have a way to wire everything up afterwards. If i work out the costs are too high then i wont even start the project so no point in building batteries now.

Also, I can't even find where to buy any of these inverters? there doesn't seem to be any suppliers in the UK for the models that keep getting mentioned?

It all depends on your goal
A simple inverter only runs from the battery and supplies AC. Nothing smart in it at all that can do witching
Off-grid inverter generally have solar charger, ac charge, can relay AC to loud and can alsy supply to load from battery bank
Hybrid is like the offgrid but it can also deliver excess energy to the grid. ie sell the energy.

Off grid and hybrid both can switch their output between battery/solar/grid depending on how and when you set it all.

If you just get yourself an inverter you need ATS switch, you need charger and so forth.

Have you seen this guy's youtube playlist on his (uk) powerwall?
https://www.google.co.uk/url?sa=t&s...CCUwAA&usg=AFQjCNF1sHlk2z9QIx28jtvfnGP_UlkifA
(If the link fails,Google UK powerwall, select videos, first on the list.)
His smaller green box is his Nedap PowerRouter hybrid inverter. Though lithium-capable it has 24v lead acid origins. He styled his battery box to match!

Your BMS confusion may be due to the different things that get called BMS...
Some are just providing some protection circuitry. Some are auto-balancers to even out the cells in a serial string. Some indicate problems by lighting a led.
Batrium seems to be in a different league offering programmable control of compatible connected inverters, chargers, solar charge controllers, etc. Which allows graceful automated handling of error conditions
It is nearly 500 quid for a starter kit, (plus shipping) then import duty, vat on the total and likely an agent's handling fee on top. Needing a Win10 pc for setup and monitoring shouldn't be a problem for you,but another extra for this Mac user.

Note that UK legislation now essentially requires formal qualification for work on new mains circuits. My strong suspicion is that house insurers would take a very dim view of an amateur home-made grid connected powerwall made from scrap cells and lacking a full fail-safe unattended auto-shutdown capability.
Misused Lithium cells do start fires. Big ones, quickly.To play safely, there is a lot to learn. A worrying number of youtube tutors wear rings and metal watches when working on their systems...
Overall, there is much more potential for drama with lithiumthan with lead acid batteries!

I think an off-gridbattery-backedsolar shed looks like a relatively uncontroversial learning-by-doing play experience.
But a grid tiedsystem opens up the financial rewardsof feed-in tarrif income, quite apart from buying less electricity - and system design (where in the system meters can be placed) has (or so I am led to believe) a significant impact on the financial benefit from a UK micro-generation system. Its an area I'd like to know more about.

A single UKstandard 13amp mains socket is good for about 3kW - a kettle. Bear that in mind when sizing and pricing inverters and calculating how many cells need to be paralleled to provide what the inverter can draw while keeping each cells current safely below 1 amp (based on 2000mA cells discharged at 0.5C). Systems can get big quickly. Conventional energy efficiency dramatically reduces the physical size and capital cost of a useful system.

If you ever find you have access to more cells than you need, or can handle, do let me know!

Thank you both for your posts, it helped clear a few things up.

A simple intervert requires manual use which isnt what i was after, I wasn't interesting in feeding to the grid either so I think the off-grid option would have been the best, though i think the cost of the inverters has made this project not feasible for me. My main objective was to see if i could reduce my electricity costs through a minimal expenditure and while the battery packs themselves would have cost me almost nothing to make, the BMS and PV panels would also be reasonable, I think the prices of the inverters has killed it for me. From what I can see they seem to be ranging around the 2000-9000 range.

What kind of inverters are you looking at that costs 9000GBP?!?!?1 Thats like half of my install that has 17kW of solar + 100kWh of battery...

A 4kW inverter you can get for fraction of that for instance the infamous PIP https://goo.gl/JgwhNu (Ebay Link)

ah thats a bit more reasonable, I'll have a read through the manual later and see what I can learn.

In the mean time I've started harvesting 18650's from work and I've ordered the "Opus BT - C3100 V2.2" charger/analyzer, it seems to be the cheapest and best option according to this thread:

http://secondlifestorage.com/t-Differences-between-discharging-testing-devices

It seemed to be the cheapest option to support charging and testing on all 4 slots.

It is probably the cheapest and good from far but far from good
I hate this thing. It is one of the reasons why I made this thread.

DarkRaven said:

It is probably the cheapest and good from far but far from good
I hate this thing. It is one of the reasons why I made this thread.

Click to expand...

why do you hate it? there are some inconsistencies in charging? how bad?

The build quality is not very good, that's why I didn't keep mine. The structural integrity and rigidity of the case is not very good and the fan is unacceptable. I've never properly tested them myself because of that and learned only recently that it is also lacking in other areas like accuracy and consistency. Probably not so much while charging, but discharging.

It gets the job done but frankly I think it is not a good idea to settle with such a low standard. I'm a bit concerned that this is such a popular device by now. Maybe it's exaggerated, I don't know. What I do know is that, as a customer, I'm not satisfied with the Opus.

I have two MC3000 now. They are certainly better in terms of build quality. If they produce better results is still to be tested so I won't judge for now. They were 75 EUR each, so more expensive than the Opus. But if they are better, and I'd say there is a decent chance, then I won't complain.

DarkRaven said:

The build quality is not very good, that's why I didn't keep mine. The structural integrity and rigidity of the case is not very good and the fan is unacceptable. I've never properly tested them myself because of that and learned only recently that it is also lacking in other areas like accuracy and consistency. Probably not so much while charging, but discharging.

It gets the job done but frankly I think it is not a good idea to settle with such a low standard. I'm a bit concerned that this is such a popular device by now. Maybe it's exaggerated, I don't know. What I do know is that, as a customer, I'm not satisfied with the Opus.

I have two MC3000 now. They are certainly better in terms of build quality. If they produce better results is still to be tested so I won't judge for now. They were 75 EUR each, so more expensive than the Opus. But if they are better, and I'd say there is a decent chance, then I won't complain.

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im not really bothered about build quality, as long as it does the job im happy.

how inconsistent are we talking? do you have any rough figures?

I don't personally. SimonW reported in another thread that he got values that were all too high by 5% to 15%, seemingly depending on the mood of the device. Inconsistent results even in the same slot. I might get back to the Opus to test it myself, that's what that thread is about.

DarkRaven said:

I don't personally. SimonW reported in another thread that he got values that were all too high by 5% to 15%, seemingly depending on the mood of the device. Inconsistent results even in the same slot. I might get back to the Opus to test it myself, that's what that thread is about.

Click to expand...

5% is ok butyeah 15% is pretty bad. either way im keeping costs to a minimum so its good enough for me, thanks for the heads up though I'll keep it in mind when i start seeing crazy results on my tests :/

10% in avg here. Its not really bad. Its a number! Use the number as a guide and sort by that. You know that the numbers are what they are so its easily taken into account in later stage.

The important part is that the numbers are rather consistent.

I've got an Opus and its fine in my opinion. No issues with build quality.

zag2me said:

I've got an Opus and its fine in my opinion. No issues with build quality.

Click to expand...

Thanks for the input.

Right I've managed to collect 297 cells, for the moment I'm not expecting to gain any more. My work contract ends tomorrow so if my next rolehas some in recyclingI'll keep collecting but for now these 297 are what I'm working from.

Since my build is going to be quite small (297 cells with about 600w of solar) I'm thinking of getting the MPP Solar2424MSE:

http://www.ebay.co.uk/itm/MSE-3kva-...inverter-40A-mppt-solar-charger-/131622539095

It's listed at 24v so I guess I should build my wall something like 7s42p?

Yes, 7S42P seems fitting if all your 297 cells are good and usable and you can build 7 packs with the same capacity, preferably without too much of a gap between the capacity of the cells.