Building a UPS for a house with an "off grid solar inverter"

[Fflint]

I've been a happy user of on-grid solar for quite a few years now, but every time we had a power cut (and they always happen at the most inconvenient time - once we were with no power for a week in winter) I would assemble an ad-hoc system of a cheap Chinese 4kW 12V inverter. 400ah of lead acid batteries, a modified 1000W computer psu as a 80A charger and a gas/petrol generator.

My house is on 3-phase power and all loads are spread out evenly. So I'd cut off utility power, short all phases together and supply power through my "ad-hoc" system. The biggest benefit was I only had to run the generator to top up the battery,or when I'd need to switch on any loads over 1kW(due to crappy inverter), not 24/7. The biggest disadvantages are, the time to switch everything to the off grid system and back as well as not using my 2.2kw of solar.

So having recently helped to build a small off grid solar system for a friend I decided to use a so called "hybrid inverter" as a sort of ups.

I've ordered a 7kW (14kW surge) 48v battery, 55v-450v solar "hybrid" inverter. What's hybrid about it you may ask? I think it's the fact it can take utility power and simply bypass itself supplying utility power to the loads. Charging batteries from solar or utility (or a generator). Then when there is a power cut it starts supplying power in 10ms as a ups would from the battery. The solar would be manually switched to the off grid inverter when there is a power cut.

I decided to get the isolar smh ii 7kw that looks like this internally:



I'm posting it to ask for your opinions. Is it the worst Chinese inverter you've ever seen? Or are there worse ones? My on-grid solar inverter is also Chinese by a company called deye and I had no issues in last 3 years, but this one does look a bit cheaper.

So in addition to the inverter I'm getting 16x100ah 3.2v lifepo4 cells and a 16s 200a (350a surge) 2a balance BMS. All these parts cost me $1150 delivered... (once they are hopefully delivered). This price is insanely low.

I initially planned to have to use an additional electric wire to feed the backup power into my house, but as the inverter simply passes through utility power I may get away with just using one of the phases instead and move all essential loads to it. When utility power is on, the inverter is bypassed so the phase delays all match fine. When the utility goes dark, the inverter supplies power to this one phase.

I'm planning to check how exactly it behaves with an oscilloscope, but I can't think of many disadvantages to this solution. The only 3 phase loads in the house are resistive (water heaters). If anyone can think of any reason why that may be a bad idea let me know please.

I'm not rushing into it. I already have an extra wire coming out of the house for backup power, but it was sized for 4kW. I'd much rather use the main wiring that is sized for 40kW.

I'm also interested in hearing what people use for alternatives. I briefly considered an AC coupled system with a battery powered grid forming inverter like the Victron Multiplus, but I decided against it due to price and greater simplicity of the system. Of course my system will not be fully automatic unless I can figure out a way to automate transferring solar power to the battery inverter, but i can't argue with the price...

[Stevietech]

How did you get on with this?

I'm looking at something similar with a hybrid but feeding it the output of my 4kw solar inverter as its "mains" power and using a pile of recycled 18650s for a battery (looking to cover the background load of freezer, kids tv, etc)

Genius idea using a computer psu as your 12v charging source!


(wife moved out 2 months ago, nobody around to tell me no when i get a dumb project idea.

[Fflint]

Thanks. I'm still in the building process. Somehow the original scope grew 3x. For example I decided to add more PV panels.

I can't say much about reliability of these Chinese inverters, but I regret not doing more research first and choosing EASUN's isolar or igrid models(or other clones of the same thing). What is the difference? Ability to parallel these devices. With my inverter it's 7kw and that is it. With the others you get (usually with parallel models) only 5kW but you can put them in parallel for up to 11 units. You can also do 3 phase with 3 units.

They are almost 2x the price and lower power. So it is not an easy decision, but as I'm expanding this system I think it would've been a better choice.

[tom66]

How did you get on with this?

I'm looking at something similar with a hybrid but feeding it the output of my 4kw solar inverter as its "mains" power and using a pile of recycled 18650s for a battery (looking to cover the background load of freezer, kids tv, etc)

Genius idea using a computer psu as your 12v charging source!


(wife moved out 2 months ago, nobody around to tell me no when i get a dumb project idea.

If you use 18650s, don't put them inside your house.  Put them outside, or in a shed, or in a separate brick building that you don't care as much about... Or use LFP cells that won't become a firey nightmare if something goes wrong.

Reycled 18650s can have faults which are hard to detect but essentially lead to the cell forming a short circuit within itself after 10-100 cycles of normal usage.  This will often lead cells in parallel with it to dump enough current in it for that cell to go into thermal runaway. 

[Siwastaja]

LFP isn't a magically safer alternative. Of course if you compare random china crap LFP and random china crap LCO/NMC/NCA, the LFP is probably a lot safer. Then again, if you compare a genuine 18650 cell made by Samsung or Panasonic (or some of the well-regarded Chinese manufacturers) to some random Winston Chung LFP brick crap, the latter is definitely more dangerous, even if it uses a "safer" chemistry. The problem is that even LFP chemistry is not safe enough to be idiot proof. It just needs higher temperature to enter thermal runaway (i.e., maybe 350degC instead of 200, or so?), and energy released during the event is smaller (maybe a quarter or so?), but it still burns down your house if it fails.

DIY EV/energy scene has used a lot of these China LFP bricks and also seen many fires as a result. You can find promotion videos where a single Winston Chung style LFP brick is punctured and shorted and only vents smoke but does not ignite; in free outdoor air, in isolation. Things might get different when you have a dense pack of 16 cells.

On the other hand, I have never succeeded getting a brand 18650 to even smoke. This is not to say they are infallible, but they have protection mechanisms not present in the LFP bricks at all.

[Fflint]

LFP isn't a magically safer alternative. Of course if you compare random china crap LFP and random china crap LCO/NMC/NCA, the LFP is probably a lot safer.

There are a lot of scammers selling very old swollen, sometimes cracked 9 Yr + old forklift truck batteries (usually plastic encased, these have not been manufactured in China since 2018 - if anyone sells plastic encased lifepo as "new" he's a scamner). Then when people complain they bullshit their customers. "oh, the battery swelled in transport, you need a compression fixture and cycling to fix it", "oh, it measures less than half capacity, it needs few dozen cycles to wake up" and so on (these are bullshit if it's not obvious).

However, even with this old crap. I'd rather have a lifepo failure than any other lithium chemistry failure. Every other popular lithium chemistry (except lto) produces it's own oxygen in thermal runaway. It's essentially impossible to extinguish without pumping it full of water under pressure. Lifepo on the other hand does not. Lifepo cell material doesn't burn at all(electrolyte does, but more about it below) and there is no oxygen emitted. In fact driving a nail through a lifepo cell and not observing fire is one of the standard certification tests auto grade lifepo have to meet in China and every Chinese manufacturer has a test report in its datasheet.

However, a lifepo is an energy storage device and when punctured or has a manufacturing fault that results in internal short it gets very hot (~230C based on research). The electrolyte used is a flammable liquid that usually vents through a safety vent. If it finds a source of ignition(for example a typical melt fuse melting at the same time) .... It will burn. There is a guy on diy solar forum that lost his house that way.

Also if lifepo cell finds itself in a high temperature fire sustained by something else it might emit hydrogen. Which is explosive with air. This in addition to the electrolyte is the reason for various "I heard multiple explosions" claims when a house with a lifepo battery catches fire for unrelated reasons.

For example there was a case in Germany few months ago where a guy had a DIY install and his house got ignited on the roof where pv panels were located. The fire spread thrpugh and these "explosions" ensued. What were the headlines? "A DIY battery burns a house down" no matter it was the panels that started the fire. So mind for insurance reasons, if your house burns down for any reason and you happen to have a diy battery anywhere near. It will be the battery's fault. Period.

So yes, LFP is great, but it's not magical. As for other chemistries. I would not even think of using them anywhere near my home (except lto, but it's cost prohibitive.

[Psi]

Yep, with these sort of items you really want to choose an aliexpress seller based on recommendations from someone else who buys from a lot from them and has tested them and built a reputation with the seller.  Ideally someone which orders from them in an ongoing basis.
You tell the aliexpress seller that you are buying from them based on recommendation of XYZ .

At least this way you know the seller 'can' ship good items (the ones they sent to the other guy gets) and by telling them you were recommended by that person the seller is less likely to screw you over because they risk annoying their other customer who is recommending them and bring in more sales or ongoing sales.

There's a few solar channels on youtube that order batteries from aliexpress/alibaba for their own solar business/projects and have given links to the good ones they tested.

[jbb]

A kWh-sized battery pack is capable of setting almost anything on fire.

The inverter looks quite tidy, actually. But a couple of concerns:
- how well supported are those heatsinks? They could be a reliability problem if they’re putting lots of stress on the leads of all those power transistors
- the heatsinks may be live. Not necessarily a problem, but keep it in mind if you’re operating without the front panel
- cable management of the lead going to the front panel - could it get caught on a heatsink and make a short? Especially a live heatsink?
- the external connections through terminal blocks is… novel. I’d be concerned about the long long way that the red and black lead have to go before coming down to the PCB.

Note: that’s a hot take from one photo. There are a lot of details we can’t see. For example, the insulation details of any isolating transformer (s) can best be determined by dissection. After that you no longer have a functional inverter which is a problem…

Creepage and clearances on the PCB would also be one to look out for. Those could probably be investigated non-destructively.

[Fflint]

Yep, with these sort of items you really want to choose an aliexpress seller based on recommendations from someone else who buys from a lot from them and has tested them and built a reputation with the seller.  Ideally someone which orders from them in an ongoing basis.
You tell the aliexpress seller that you are buying from them based on recommendation of XYZ .

At least this way you know the seller 'can' ship good items (the ones they sent to the other guy gets) and by telling them you were recommended by that person the seller is less likely to screw you over because they risk annoying their other customer who is recommending them and bring in more sales or ongoing sales.

There's a few solar channels on youtube that order batteries from aliexpress/alibaba for their own solar business/projects and have given links to the good ones they tested.

Indeed. However it is worth mentioning there are certain sellers (perhaps all the same person/business) on Aliexpress that sell one type that is fine - for example 280ah higee cells (well, they are already a year old, but unused, they pass the capacity test and both AC and DC internal resistance - so good in my book) and at the same time he is scamming on plastic encased cells.

The fact a youtube got good cells of one type is not a guarantee their seller is not scamming.

At this stage I decided to never buy lifepo cells on Aliexpress again. It simply doesn't offer any protection unless you use a credit card and issue charge back.

Lifepo scanners found a "trick" to bypass normal buyer protection. Your standard protection (at least in the EU) on Aliexpress relies on the fact Aliexpress has local warehouses and you can send the item back for a full automatic refund.

Guess what, it only works for single parcels up to 30kg. It is not mentioned anywhere on Aliexpress terms and conditions and all these lifepo cells are sold as "free return" "guaranteed by Aliexpress buyer protection" etc.

Aliexpress buyers protection my arse. The company is full of imbeciles. If the automated system generates a shipping label you cannot use (because again it is a single parcel limited to 30kg while for example 8 200ah lifepo cells come in two parcels 26kg each) customer service is unable to do anything.

In fact customer service can't do anything anyway on Aliexpress. They can only push buttons to guide the automated system a little. If that system does contain a provision for a buyer to send back using his own shipping number and it generates labels up to 30kg good luck.

I've been engaged in a case for 3 months by now(and you can't review during the time BTW, because they insert a wrong shipping number so it never bocomes "delivered" unless you click "I received the item as described" in which case the seller gets the cash) . I've had a lot of "your money is safe" "don't worry you've documented your case well" etc "escalated to the higher team" "well provide a response in 2days" (stuck for over two weeks once - now I contact them on day 3). And multiple times after "escalating" I was told "why don't you just send it together on the label generated?" despite drawing them a literal fucking picture like for 3 year old. Seriously. Here is a photo of your label, it says up to 30kg and here is a photo of two boxes 26kg each. Nope. Not. Able. To. Return.

And if you ask for a full refund they stall forever. Aliexpress stated in March of 2024 when the EU regulator opened a case into them that they "abide by the customer laws in the EU". Bullshit. They are enablers for the scammers on their platform.

You may say (to me) "you knew very well this is a scammers market, why did you buy" and I would answer I only bought because of a promise of "buyer protection" and "ability to return". So yes there is a scammed in a form of a seller. But the platform itself is a far bigger scammer. They promise this "protection" but they don't fulfill randomly. And don't tell me "it works for me". It worked for me too, for 8 years until it didn't, at the moment I needed it most.

So, no, I do not buy lifepo on Aliexpress anymore and I don't recommend anyone to do it.

How do you thd buy lifepo without paying the ridiculous markup of local sellers? One word. Alibaba. Yes, I always thought Alibaba is even worse than Aliexpress in terms of scams. However on Alibaba there is a perceived trust for sellers that have been there a long time. So it is not easy for a seller to just give up his brand and start a new shop if bad reviews start coming in.

Also there are certain very long lived shops one can trust. I recommend buyers seek these that have been in business for at least 3 years and have recent good reviews.