Backup solar battery system considerations.

[hsync33]

I’m currently looking to add some limited battery storage capacity to my solar system and looking for some thoughts / sugestions.

I have about 7KW of panels on an East / West configuration, producing roughly 5KW on the AC side. I’m exporting up to 16A (3.6-4KW) according to G98 limits and my battery charger is 1KW variable.

The battery backup is planned to stay in standby, mostly serving as a UPS for citical loads, though I will provision for the capacity to inject up to 500W into the grid to offset some consumption in the summer, after sun hours. I don't need a massivelly high capacity - But given they are a fire hazard I'm playing with the tradebacks of smaller vs larger size.

Large size
- Oversized enough that I'm always pushing power in / out at a low C rate for safety and longevity purposes.
- Peak power is able to instantly trip any fuses in case of a failure
- Internal failure means there is a large chance something will get severely damaged in the event of a fire.

Smaller size
- Lower fire risk in case of internal fault
- Lower longevity as cells with be cycled at higher C rates
- Higher fire risk as cells will tend to warm up quicker due to higher C rates

Next on, I started by looking into the batteries themselves, which run roughly into £1000/5KW as a final product with enclusure and BMS. At the same time I'm finding the bare batteries going for about the same price, but at 3x the capacity (320Ah). A BMS and an enclosure are not expensive enough to make up the difference, so what's the catch?

I ordered some from a brand called PWOD. Not so glowing reviews here, but good reviews on AliExpress. I'm having cold feet on that as the concensus is that Ali doesn't deal well with expensive returns and probably I should have dealt with manufacturer directly. Let's see and learn from that... I've went with the manufacturer direct back in 2014 ago on another project and also had to pay upfront, but at least the cells live to this day, altough lightly cycled.

Any advices on BMS? 16S, I want to have programmable cut off values. I was thinking Dali Smart BMS, but open to others. Charge / Discharge up to 1KW in a 48V system, so roughly 20A, but needs to be able to deal with fault transients without shorting our the mosfets. During winter the panels may be covered by snow for extended periods, so I would like the BMS to go dormant and self restart when charge is detected.

I'm also wondering what are your views on open vs enclosed batteries. Ideally I would like to have mines open, as hotspots always form on the inner cells of a pack, plus with an open enclosure I could regularly go to check the cell voltages and any swelling / leeks during rotine inspection.

These batteries will be installed on a basement, so year round temperatures are 3-15C. Pretty confortable for LiFePO4.

 

[wattmatters]

but good reviews on AliExpress

Buy nothing critical / expensive from AliExpress. It's the wild east and chances are you will get screwed. Alibaba yes (if the seller is well known/trusted to you).

Your use case is confusing.

If a battery is for back up and not going to be cycled, then I'm not sure lithium is the right chemistry. It's a lot of money to do no work and you do not want to be keeping lithium at a high state of charge for long periods.

If the battery is going to be cycled, frankly I think you are better to buy a ready made battery from a known reputable manufacturer. It just ends all the second guessing and will be safer.

 

[hsync33]

Buy nothing critical / expensive from AliExpress. It's the wild east and chances are you will get screwed. Alibaba yes (if the seller is well known/trusted to you).

Your use case is confusing.

If a battery is for back up and not going to be cycled, then I'm not sure lithium is the right chemistry. It's a lot of money to do no work and you do not want to be keeping lithium at a high state of charge for long periods.

If the battery is going to be cycled, frankly I think you are better to buy a ready made battery from a known reputable manufacturer. It just ends all the second guessing and will be safer.

I guess I messed up with that ALiExpress order, but let's see.

They will be slowly cycled depending on what available spare capacity I have, just not to a high discharge and charge rates as typically home storage batteries are used, where they are intended to supply the full household load as their primary function.

I have a smaller pack with 10Ah cells I use. That's been going strong for 10 years now, altough it is used to supply low voltage networking equipment / DVR / Alarm, that draws about 30W.

I figured out if I want to have the system running from the panels in case of grid failure, which does happen at times, particularly in winter with heavy snowfall, I might as well have a few KW available to run the gas boiler, circulation pumps, lights and anything else critical. Having the battery backup is the main aim and the grid injection is just a way to offset the cost of the batteries if and when I have spare capacity from the panels during the summer.

I'm generally not to found of sealed batteries with proprietary BMS controls. I'd rather use a high quality BMS or design my own and routinelly inspect and balance the cells and I guess most comercial options are really not designed for that. Anything with a warranty that it's worth the paper it's written on, will generally be prohibitivelly expensive and require a very restricted usage type, such as being coupled with their specific inverter. Once the warranty is gone, it's generally game over for any repairs that need doing.

Do you have any feedback on my questions above, in particular why ready made chinese packs are so much more expensive bought assembled? Any experience with them? Anedoctally the few fires I've seen, none was inside one of those packs, so there might be merit in keeping the battery inside a metal container

Also any feedback on good quality BMS's I should be looking into, if I want to go the comertial route?

Thanks!

 

[wattmatters]

Do you have any feedback on my questions above, in particular why ready made chinese packs are so much more expensive bought assembled?

Not really, prices are not really all that different here.

I looked at DIY but frankly the server rack units I have I could not build DIY for much less, let alone the quality of construction and superior form factor. The small cost difference just wasn't worth it and I could get a battery delivered within a few days, not wait months hoping my shipment actually arrived in good order only to then have to build it.

The BMS in my server rack units is the standard PACE BMS. Nothing fancy but it has been deployed in hundreds of thousands of said batteries. If building DIY I guess a JK BMS would be on my list. This is the sort of battery I have:

JK48V100PRO I Server Rack 51.2V 100Ah LiFePO4 Battery – JAKIPER BATTERY

eujakiperbattery.com

There is no inverter lock-in with 48 V architecture.

By the time I imported cells, all the hardware, a decent case, the fusing, breaker, BMS, display, terminals, comms and tools required to make a decent and safe battery, well the pre-made units are no more expensive.

I think the self assemble kits are pretty neat. But in the end they were saving maybe 8% of the total cost.

Cost differentials may well be location specific.

 

[wattmatters]

When I built a system purely for grid outage backup I went with pre-loved SLA data centre backup batteries (Enersys SBS 190F). Four of those for a 48 V system to begin with, then added another four in parallel later. That gave us about 12 hours of comfortable backup capacity, longer if we were frugal.

Data/telco centres cycle them out on a schedule and the batteries are designed exactly for this purpose. Can get them for not a lot of $, add a 4-5 kW AIO inverter and some relevant safety bits (fuse/breaker) to an AC distribution panel/outlet with suitable protection and you'll have a very nice power backup if using a generator is not your gig (not always suitable). Add a transfer switch and power inlet to the home and you'll be able to plug it in and cover the home's general power outlet and lights.

Leaving LiFePO4 fully charged all the time and rarely using them is not good for their health.

 

[DIYrich]

Large vs small: not the usual way to think about it. Most people size by how many hours or days of power when the grid goes down.

Pre made vs diy: pre made usually costs more. Certifications, and now may have internal fire suppression.

BMS is either on or off. Usually it is the inverter you turn on or off depending upon circumstances. If you do it all manually, the jk bms has an on/off switch. I'm guessing most do.

Open vs closed: closed, but vented. Reduces likelihood of dropping something onto all the battery connections and causing sparks.

 

[wattmatters]

I'm not convinced large vs small battery is much different from a fire risk POV. A devastating fire can be started with a match, it doesn't need a flame thrower. Most battery fires are caused by small battery failures, usually a charging fault because everything is made cheaply.

And yes, the primary issue is the lack of a clear scope. Define what you need a system to provide, then design it accordingly.

 

[SeaGal]

I’m currently looking to add some limited battery storage capacity to my solar system and looking for some thoughts / sugestions.

As @wattmatters said, bit confused about your intended usage. You mention battery and charger, but no mention of either and independent inverter for the batteries or a hybrid inverter? Can you explain a bit more?

DIY or pre-built will largely depend on your skills and interests.

Take a look at Fogstar as you are in the UK - they have both 5.1kWh and 15kWh prebuild battery packs, sell raw cells and also sell Seplos Mason kits which are ideal half-way house for those that can't decide between pre-make or DIY

Solar Battery Storage | Fogstar UK

When it comes to solar battery storage, we understand that choosing the right storage solution for your solar energy is crucial, which is why we offer a range of options to suit your installation.

www.fogstar.co.uk

I have JBD (Overkill BMS). JK and Pace have had good reports from others on here, but I have yet to find anyone recommending Daly after more than 18 months on this forum.

Depending on your intended usage, I'd suggest a regular G98 or G99 type tested grid-tied hybrid (such as a Solis or Sunsync) and battery pack to suit.

As for fire risk, I'd be careful to design the system properly in the first place, whatever size. A small 100Ah sized system can still provide enough power to create too much heat.

 

[DIYrich]

Large size

- Internal failure means there is a large chance something will get severely damaged in the event of a fire.

Smaller size
- Lower fire risk in case of internal fault

While there is more mass to eventually catch fire, the really bad fire starts in one cell. Same risk with small or large battery. If a small battery catches fire, you have a problem.

- Higher fire risk as cells will tend to warm up quicker due to higher C rates

Higher %c during discharge or charging. With a small battery, you have risk both ways. With a large battery, you are not likely to get to high discharge rates.

With 7kW of panels, if you have 35 kWh of batteries, you have about 0.2c charge rate, which will keep the temps low.
If you limit charging to 1kW, then a 5kWh battery would be the smallest I would go.

 

[hsync33]

As @wattmatters said, bit confused about your intended usage. You mention battery and charger, but no mention of either and independent inverter for the batteries or a hybrid inverter? Can you explain a bit more?

DIY or pre-built will largely depend on your skills and interests.

Take a look at Fogstar as you are in the UK - they have both 5.1kWh and 15kWh prebuild battery packs, sell raw cells and also sell Seplos Mason kits which are ideal half-way house for those that can't decide between pre-make or DIY

Solar Battery Storage | Fogstar UK

When it comes to solar battery storage, we understand that choosing the right storage solution for your solar energy is crucial, which is why we offer a range of options to suit your installation.

www.fogstar.co.uk

I have JBD (Overkill BMS). JK and Pace have had good reports from others on here, but I have yet to find anyone recommending Daly after more than 18 months on this forum.

Depending on your intended usage, I'd suggest a regular G98 or G99 type tested grid-tied hybrid (such as a Solis or Sunsync) and battery pack to suit.

As for fire risk, I'd be careful to design the system properly in the first place, whatever size. A small 100Ah sized system can still provide enough power to create too much heat.

I have a grid tie system with a G98 Stecca Coolcept grid tie inverter.
In between the solar panels and the inverter goes a growatt SP1000, which acts as an MPPT controller, diverting some of the power from the panels to charge a 48V battery. The actual ammount of power will depend on the setting I program on it and the available export. It prioritizes the grid tie inverter, unless there is excess export.

At night, the SP1000, can use the battery power to supply the inverter in order to offset some of the consumption.

You can think of it as an hybrid inverter. The Growatt can be set with voltage limits or it can communicate with a smart BMS. Currently I have a small battery pack of 48V @ 20A which is mainly used to supply DC networking equipment and the growatt works as a charger for this equipment.

I also want to to use this battery pack to supply power for a UPS or sine inverter for backup purposes. Electricity here is not expensive enough to justify using the setup as a hybrid inverter, so I don't have a great deal of interest in having a large battery pack to soak up all my solar. But having said that, in the summer, when the panels can provide ample excess power there is no reason not to use some of the excess energy overnight to offset the cost of the system a little bit. After all - batteries are consumables, so to an extend they will degrade, no matter what.

 

[hsync33]

When I built a system purely for grid outage backup I went with pre-loved SLA data centre backup batteries (Enersys SBS 190F). Four of those for a 48 V system to begin with, then added another four in parallel later. That gave us about 12 hours of comfortable backup capacity, longer if we were frugal.

Data/telco centres cycle them out on a schedule and the batteries are designed exactly for this purpose. Can get them for not a lot of $, add a 4-5 kW AIO inverter and some relevant safety bits (fuse/breaker) to an AC distribution panel/outlet with suitable protection and you'll have a very nice power backup if using a generator is not your gig (not always suitable). Add a transfer switch and power inlet to the home and you'll be able to plug it in and cover the home's general power outlet and lights.

Leaving LiFePO4 fully charged all the time and rarely using them is not good for their health.

My curent set of batteries was adquired 12 years ago for an electric car conversion. Link

First 5 years they were used on said car. After that things changed and I had to retire the car. With the 8KW battery it could go on 40 miles, but as life changed, 40 miles wasn't enough and didn't justify specialist insurance, when I needed another car to cover long distances.
After that they were used as storage batteries, discharged 25% at night and conservativelly charged to 3.4vpc. They haven't degraded much, but really with solar, they don't really sit for long at float voltage. In the same period I had several BMS's failing on other packs for a multitude of reasons. For me that's the devil of lithium batteries. Even poor batteries, if managed properly will live for many, many years!

That's also why I don't cycle them hard or recomend people to do so. LiFePO4 provides a nice steady constant voltage at 3.2-3.3V mark for a good portion of their discharge curve and even if one cell fails, if the equipment is set to shut down at a conservative voltage, it's unlikelly they will get low enough to be damaged. Consumer products, due to marketing must extract the last bit of energy to offer the best value possible and will rarelly last as long as claimed. So good to see others here thinking about with DIY on these things!

I'm not a great appologist of lead acid batteries, considering their performance, but then I was never really able to find a good 'bargain' on them. They were expensive as far back as I can remember. Back in they early 2000's I did use second hand 6V Telecom batteries on a offgrid setup. I also used brand new 12V AGM's. The AGM's did a great job as starter batteries and performed relativelly well for UPS Backup, but that was it. Once one starts to cycle them, they degrade quicky, it's virtually impossible to charge them quickly and the voltage is nowhere as stable as lithium.

Longevity wise I'm supper happy with my Lithium set which would require at least a 4x60Ah AGM to replace and would hardly ever come close to 12+ years this set has been in service. So for me that is excelent value for money.

So for now I am just thinking and playing with the idea to upgrade this system for the fun of it. But no matter how carefull one may be, I'm, concerned about the fire reports shown on this forum and trying my best to avoid human error being a factor there.

PS: I apreciate the feedback on the BMS's. Will need to look further into it.

 

[SeaGal]

Thanks for the additional info - seems you have a lot of experience with your EV conversion

I'm generally not to found of sealed batteries with proprietary BMS controls. I'd rather use a high quality BMS or design my own and routinelly inspect and balance the cells and I guess most comercial options are really not designed for that. Anything with a warranty that it's worth the paper it's written on, will generally be prohibitivelly expensive and require a very restricted usage type, such as being coupled with their specific inverter. Once the warranty is gone, it's generally game over for any repairs that need doing.

These echo my thoughts ^^^^, which is why I built my own battery pack. I like the control I have over the my system which I can tweak to suit my needs at any time I like.

As for cost comparisons... I'm sure I've posted this before, but if you're looking at (say) a 5kWh battery system you won't save much money wise. If you're looking at 15kWh+ then, sure, there are savings to be made if you have the skills and don't factor in your time. For me, it's as much as, if not more of, a hobby than a money saving eco thing.

 

[hsync33]

Similar!
Why do you think the 15KW battery pack will be more cost effective? I've seen that in china you can get a 300Ah pack for twice the cost of a 100Ah... Is that why?

I'm still apreensive on having such large batteries indoors, in the very unlikely scenario something goes wrong, on the other hand, I can probably forget them once they are installed. What kind of prices are people here in the forum getting for 100A+ cells? I'm assuming import from china, right?

I looked into car batteries, as they normally come with a nice enclosure, but there's not much to be had for any reasonable price and they all seem to be Li-Ion, several years old.

 

[SeaGal]

Why do you think the 15KW battery pack will be more cost effective?

If you are building your own, 300Ah cells are not 3x the price of 100Ah cells. And the fixed costs (DC wiring, fuses, isolators, BMS, case, bits for compression and inter-cell insulation) are almost identical if you are building a 15kWh pack, vs. a 5kWh pack. So the bigger one is more cost-effective.

I'm still apreensive on having such large batteries indoor

I wouldn't do that - not worth the risk, IMHO. I'm probably over-cautious, but I'm old and still here Mine are in a garage 30m from my house.

What kind of prices are people here in the forum getting for 100A+ cells? I'm assuming import from china, right?

I bought mine from Fogstar - EVE 304Ah cells are now only £81 each. You then don't have to worry about shipping, import duties and the worry they may not arrive. Again YMMV, but for me the cost-saving of direct import didn't exceed the hassle factor. Fogstar no longer do 100Ah batteries, but their 230Ah ones are only £40 each. So, you could buy those and only half fill them to get your 100Ah quota

I looked into car batteries, as they normally come with a nice enclosure,

... if you have space on your drive then a little Prius or Leaf makes a nice enclosure for the cells!

 

[robbob2112]

For you reading pleasure - the jury is still out on the cause other than we think the spark was an unsuitable type of fuse for the purpose. But the fuse that the vendor recommended.

House burned down

So, after 4 to 5 years of working flawless, the 100 kwh lifepo4 battery caught fire at night and burned our house down. Luckily our family just made in out on time. Inspection of the fire expert revealed that a melt fuse melted, and created an arc between the 2 points wich ultimatly started the...

diysolarforum.com

I know that I am going to line the walls and ceiling of any room containing batteries with firerock and/or durock. Firerock is gypsum board treated and thicker than regular sheetrock so it has a 4 hour burn time per thickness. Durock is cement board used under tile when you install it. One person has stated they are going to retrofit a sandwich of firerock-durock-firerock on walls and ceiling of their battery/electrical room.

In my case we are looking to buy property and build, so my utilities will be in a separate small building that is divided into a battery room and an electrical room.

Extreme ? yes. But if there is even a slim possibility of a fire that burns the house down I will not take chances.

 

[hsync33]

If you are building your own, 300Ah cells are not 3x the price of 100Ah cells. And the fixed costs (DC wiring, fuses, isolators, BMS, case, bits for compression and inter-cell insulation) are almost identical if you are building a 15kWh pack, vs. a 5kWh pack. So the bigger one is more cost-effective.


I wouldn't do that - not worth the risk, IMHO. I'm probably over-cautious, but I'm old and still here Mine are in a garage 30m from my house.


I bought mine from Fogstar - EVE 304Ah cells are now only £81 each. You then don't have to worry about shipping, import duties and the worry they may not arrive. Again YMMV, but for me the cost-saving of direct import didn't exceed the hassle factor. Fogstar no longer do 100Ah batteries, but their 230Ah ones are only £40 each. So, you could buy those and only half fill them to get your 100Ah quota


... if you have space on your drive then a little Prius or Leaf makes a nice enclosure for the cells!

I can see why people are not using AliExpress. Still remember paying $15 + duties for my 10Ah's, so good to see the prices today are a fraction!
A close check revealed they are now out of stock. How often do they have them?

It's funny these days no one seems to separate the batteries. My former supplier provided optional plastic moulds to keep them away from each other. So did the headways, also common then.
A) They heat and the inner cells get hotter than the outer ones
B) They swell when they fail. I can't immagine anything good from keeping them tight

 

[hsync33]

For you reading pleasure - the jury is still out on the cause other than we think the spark was an unsuitable type of fuse for the purpose. But the fuse that the vendor recommended.

House burned down

So, after 4 to 5 years of working flawless, the 100 kwh lifepo4 battery caught fire at night and burned our house down. Luckily our family just made in out on time. Inspection of the fire expert revealed that a melt fuse melted, and created an arc between the 2 points wich ultimatly started the...

diysolarforum.com

I know that I am going to line the walls and ceiling of any room containing batteries with firerock and/or durock. Firerock is gypsum board treated and thicker than regular sheetrock so it has a 4 hour burn time per thickness. Durock is cement board used under tile when you install it. One person has stated they are going to retrofit a sandwich of firerock-durock-firerock on walls and ceiling of their battery/electrical room.

In my case we are looking to buy property and build, so my utilities will be in a separate small building that is divided into a battery room and an electrical room.

Extreme ? yes. But if there is even a slim possibility of a fire that burns the house down I will not take chances.

I can't blame you on that!
Actually reminds me of British power plugs. I can remember more 13A fuses that melted or damaged the plug than those were the fuse actually blown when connecting 20A+ to the damm thing! I also had a dryer were this happened (rated 13A, but for 2 hours)

Here's a link showing what happens (so as to not clutter this with pictures)
That's also why I like to have reserve and have fuses of significantly lower current than the source is capable
If there is a short, the fuse will get vaporized instead of being kept there simmering. And sand fuses for DC, to estinguish the arc!

It seems in the last 10 years there's been a massive leap in construction techniques. Maybe outdoor buildings for electrical gear will be more common moving forward!

Though, mind those temperatures on the summer! Maybe this is the solution that will keep everyone happy

 

[wattmatters]

I'm not a great appologist of lead acid batteries, considering their performance, but then I was never really able to find a good 'bargain' on them. They were expensive as far back as I can remember. Back in they early 2000's I did use second hand 6V Telecom batteries on a offgrid setup. I also used brand new 12V AGM's. The AGM's did a great job as starter batteries and performed relativelly well for UPS Backup, but that was it. Once one starts to cycle them, they degrade quicky, it's virtually impossible to charge them quickly and the voltage is nowhere as stable as lithium.

This is what's confusing about your posts. You talk about wanting "UPS" or outage back up:

The battery backup is planned to stay in standby, mostly serving as a UPS for citical loads,

That's a use case where there is bugger all cycling. Hence why suggesting batteries designed specifically for that use case. The batteries sit there at float ready to supply energy when you need it. There is no daily cycling.

Then you say you don't like them because you can't cycle them!

So what exactly is the scope of what you are looking to do?

Get that sorted first. Then design a system to deliver on your scope.

 

[SeaGal]

It's funny these days no one seems to separate the batteries. My former supplier provided optional plastic moulds to keep them away from each other.

I (along with many others) have insulation sheets between prismatic cells to avoid shorts if the thin blue plastic outer layer gets damaged.

I can't immagine anything good from keeping them tight

Manufacturer recommendation (at least for my EVE cells) is support to limit expansion at full charge - there are many threads on that on here. IIRC it protects the internal bits from separating to improve longevity.

 

[hsync33]

I guess I messed up with that ALiExpress order, but let's see.

They will be slowly cycled depending on what available spare capacity I have, just not to a high discharge and charge rates as typically home storage batteries are used, where they are intended to supply the full household load as their primary function.

I have a smaller pack with 10Ah cells I use. That's been going strong for 10 years now, altough it is used to supply low voltage networking equipment / DVR / Alarm, that draws about 30W.

I figured out if I want to have the system running from the panels in case of grid failure, which does happen at times, particularly in winter with heavy snowfall, I might as well have a few KW available to run the gas boiler, circulation pumps, lights and anything else critical. Having the battery backup is the main aim and the grid injection is just a way to offset the cost of the batteries if and when I have spare capacity from the panels during the summer.

I have about 7KW of panels on an East / West configuration, producing roughly 5KW on the AC side. I’m exporting up to 16A (3.6-4KW) according to G98 limits and my battery charger is 1KW variable.

The battery backup is planned to stay in standby, mostly serving as a UPS for citical loads, though I will provision for the capacity to inject up to 500W into the grid to offset some consumption in the summer, after sun hours.

This is what's confusing about your posts. You talk about wanting "UPS" or outage back up:

That's a use case where there is bugger all cycling. Hence why suggesting batteries designed specifically for that use case. The batteries sit there at float ready to supply energy when you need it. There is no daily cycling.

Then you say you don't like them because you can't cycle them!

So what exactly is the scope of what you are looking to do?

Get that sorted first. Then design a system to deliver on your scope.

I apreciate the advice and it's sound, but I already defined the scope of the system on the first posts.
UPS as main aim, summer grid injection (small ammount for self consumption) as secondary, whenever there is spare capacity available.