@jamespa 

You are in a similar situation to me James, although I am a little further down the line having installed a small 5 kWh battery.

What I will say for sure is that having the battery does increase your self usage and has reduced my running costs with my ASHP. Im currently trying to make the case for increasing the size of my battery but at the moment dont have enough data.

The 5K price tag seems a bit high and having been speaking with @transparent you could certainly build a much cheaper one or buy a Fogstar 15.5 kW for £2500 if you are up for a bit of DIY.

I havent braved the Agile tariff yet but am making good use of Intelligent Go. Did you mean to say your saving was £1 per day not £10, that was the figure I was working on. The thread with my recent discussion is this one.

https://renewableheatinghub.co.uk/forums/energy-storage/battery-sizing-how-low-can-you-go

Posted by: @bontwoody

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@jamespa 

You are in a similar situation to me James, although I am a little further down the line having installed a small 5 kWh battery.

What I will say for sure is that having the battery does increase your self usage and has reduced my running costs with my ASHP. Im currently trying to make the case for increasing the size of my battery but at the moment dont have enough data.

The 5K price tag seems a bit high and having been speaking with @transparent you could certainly build a much cheaper one or buy a Fogstar 15.5 kW for £2500 if you are up for a bit of DIY.

I havent braved the Agile tariff yet but am making good use of Intelligent Go. Did you mean to say your saving was £1 per day not £10, that was the figure I was working on. The thread with my recent discussion is this one.

https://renewableheatinghub.co.uk/forums/energy-storage/battery-sizing-how-low-can-you-go

Yes sorry £1 per day is what I meant.

1K per kWh is a figure I have seen quoted more than once for a 'professional' install.  £160 per kWh changes the equation big time if its achievable.  I don't mind some DiY but get the impression we are moving towards batteries being external to limit fire risk?

@jamespa Mine is outside at the moment but I will be moving it indoors. The sea air is hard on it :-). I believe the LiFePO4 are much less likely to catch fire than the older chemistry. I did start to watch a Youtube video on the subject but i think it was mostly concerned about installations in lofts.

Wow! Please slow down @jamespa and @bontwoody - You're opening far too many issues at once!

I can provide the technical background, together with typical prices and suppliers.
You will then need to evaluate how that fits with your own ideas of viability.

1: Battery Types and regulations.

I'm going to assume that we're ignoring the older lead-acid batteries, and the newer cells based on sodium ions.

There are six main families of lithium chemistry which you're likely to encounter.

a: Lithium Manganese Nickel Cobalt (LiMNC) has high capacity (watt-hours) per unit mass, at around 250 Wh/Kg.
Its light weight makes it the dominant chemistry used in transport (EV's, electric bikes and hoverboards)
as well as portable tools (power-drills, hand-held vacuum cleaners, vapes).

LiMNC cells are usually cylindrical, and connected together within a plastic outer casing.
Each cell has a nominal 3.7v, and can deliver very high currents when required.

An LiMNC battery can be damaged by over-charging, or mechanically, by hitting it.
A poor quality charger or inaccurate temperature sensing is the usual reason for failure.
In either case an internal short-circuit can create rapid, violent combustion.
The reaction is exothermic, which makes it almost impossible to extinguish.

b: Lithium Ferrous Phosphate (LiFePO4 or LFP) has a lower capacity of about 150 WH/Kg and is most commonly used for energy storage.
It is very stable and offers lower reduction of capacity over thousands of charge-cycles.
Typically, it will still hold more than 90% of its specified capacity after 6000+ cycles.

Each cell offers 3.2v, and wouldn't normally be charged or discharged above its 'C' rating.
The C-rating is the current required to take the cell from 0% to 100% full within one hour.
Thus a 105Ah cell would be charged or discharged at no more than 105Amps.

If you short-circuit an LiFePO4 cell, it can deliver 10,000+ Amps and weld the ends of the wires together!
But the cell wouldn't normally be damaged from such an accident if you quickly disconnect it.

Drilling through an LiFePO4 cell whilst it's in use is discouraged,
but is unlikely to cause a fire.

c: Lithium Titanate (Li2TiO3 or LTO) is less common, but can also be used for energy storage.
It operates at 2.3v, and is usually a large cylinder with a screw thread at each end.

To make up a nominal 50v battery, you might use 20-24 LTO cells.
That's why many BMS units facilitate 24 cells, like this one from JK.
It's more expensive than LiFePO4, but can be charged and discharged extremely rapidly... in just a few minutes.

d: Lithium Polymer (LIPO) is used in thin pouches for laptop batteries, or rectangular packs for drones and model aircraft.
It's lightweight and flexible, offering the same 3.7v per cell as LiMNC.

LIPO cells have a low self-discharge of around 5% per month.
Their overall lifetime is unlikely to be greater than 1000 charge-cycles, and shouldn't be stored above 50% full.

e: Lithium Disulphide is the most common lithium chemistry which shouldn't be re-charged.
It's most often used in mission-critical devices which exhibit low-current drain such as domestic smoke-alarms and multimeters.
For that reason it gets packaged in the common sized AA (1.5A) and PP3 (9v) battery casings.

f: Lithium Nickel Cobalt Aluminium Oxide (LiNiCoAlO2 or NCA) has the highest energy density by far, and can deliver thousands of amps.
It's less stable and has a lower lifetime than LiMNC.
You're unlikely to meet it in raw cells, but is sometimes used in bespoke equipment such as professional camera flash units.

Of these lithium chemistries, it is most likely that we will continue buying LiFePO4 cells for domestic battery storage for at least the next three years.

If you'd asked me the same question two years ago (2022), I would have predicted that Sodium-ion cells would be taking over due to the scarcity and high price of lithium carbonate ore.
However, investors and speculators misjudged the speed with which we'd migrate to using EVs.
As a result, battery-quality lithium has fallen in price by 80% over the last 20 months.
That means the argument in favour of sodium has been undermined.

The most common domestic storage battery arrangement is sixteen LiFePO4 cells in series, giving a nominal 52v DC.

Cell capacities range from around 105Ah to 320Ah.
It is usually the case that 280Ah cells are the cheapest per watt-hour.
I can currently buy a set of 280Ah batteries from an agent in China for about £1500, including shipping and taxes.
That equates to a battery with a total capacity of about 14kWh.

The PAS63100 safety standard for domestic storage batteries was released in February 2024.
However it's been published by the IEC who write the electrical wiring standards for UK, rather than engineers familiar with battery technology.
As a result it's lumped all lithium battery technologies within the same group and assumed the worst possible safety scenarios.

PAS63100 requires batteries to be outside the main building element, ignoring the fact that LiFePO4 cells shouldn't be used below 0°C, for example.

Since PAS63100 isn't mandatory, it's unlikely to be well regarded or implemented in its current form.

@transparent Thanks transparent. I noticed the similarities in this thread to mine and thought I would save you the trouble of going over the same ground twice 😂

@jamespa, I've sent you the data I collect regularly which should help replace predictive models with actual data. My situation is not all that different since my PV array produces about 4.5MWh per year, I have a 6kWh battery and an EV charger, and I'm on Octopus Agile tariffs.

Obviously, time period the data covers includes several changes to the way I do things, so there are still a number of unknowables. Nonetheless, feel free to ask any questions that occur to you.

Battery enclosures:

The majority of storage batteries installed in UK are LiFePO4, operate at 52v nominal and have a capacity of 4-5kWh.
Each box has its own Battery Management System (BMS), protective fuse/breaker and high-current connectors.
The capacity being offered is constrained by the need to keep the weight below 25Kg, which is a 'one-man lift' within Health & Safety guidelines.

That makes for a relatively expensive unit... and hence high prices.

The cost is significantly reduced if the enclosure and LiFePO4 cells are shipped to site separately and then assembled.
Sixteen 280Ah LiFePO4 cells weigh 96Kg, and a steel enclosure adds about 30Kg more.
That's why Seplos developed the Mason box, which is discussed here on the Forum,
and Fogstar have released their own higher-quality equivalent here in the UK: discussed here

Fogstar also supply LiFePO4 cells and ready-to-build kits.

You don't have to buy a steel enclosure with integral BMS electronics.

Since you're unlikely to ever move the battery once it's assembled, it's also possible to place the cells on open shelving,
provided the strutts can hold the weight of course.

I have a preference for placing the cells with the terminals facing outwards.
If I accidentally drop a metal tool, then it continues to fall to floor instead of creating a short-circuit.

Location:

Storage batteries and inverters need to be in close proximity.
The thick cables are expensive, and you want to avoid voltage-drop along their length.

An unheated ante-room is a good location, but not if that's your only means of exiting the home.

I had a suitable room which already contained the consumer unit(s), which in turn, backed onto the Smart Meter in its external enclosure.

Storage battery capacity and size of inverter(s).

Apart from cost, the factor most likely to dictate the battery capacity required by a home is sufficient to tide over the most expensive time of day when on a Time-of-Use (ToU) tariff.

Readers here will know that Octopus leads the field in offering a wide range of different ToU tariffs.
This time line shows three of them at the height of the 2022 energy crisis, when there was significant price variation.

As Britain migrates towards Net Zero there will be ever more electricity generation from sources with fluctuating output.

Although we currently imagine a storage battery with capacity to run the house during the early-evening peak, it makes good sense to ensure that this can readily be expanded to last a full day or even longer.

It's highly unlikely that a home would recharge an EV from energy which has already been stored within the home.
It's just too inefficient to convert electricity from 240v AC to DC twice.

That means the highest domestic demand is likely to be a heat-pump.
How much power is required by a heat-pump depends on whether it runs continuously, or is cycling on/off.

When the heat-pump starts it will typically draw 30A until the compressor has brought both circuits up to operating pressures.
The minimum sized inverter for a heat-pump alone is therefore 7½kW

Typically the rest of the home might require a further 7½kW for cooking, lights and other appliances.

Although I don't (yet) have a heat-pump, I've currently got three 5kW inverters operating in parallel... which satisfies the thinking above.

And I've gradually increased storage capacity to 56kWh... sufficient to run for a couple of days, even if I'm using workshop machinery and there's no solar input.

The best approach is to plan flexibly from the start.
If demand is greater than first calculated, then add another inverter in parallel.
Or if more capacity is required, then add another battery (or two).

Posted by: @transparent

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It's highly unlikely that a home would recharge an EV from energy which has already been stored within the home.
It's just too inefficient to convert electricity from 240v AC to DC twice.

I would amend that slightly to say it's highly unlikely that a home would want to recharge an EV from....

From the inverter's point of view, an EV charger is just another load, so by default it will satisfy that load from battery first which is absolutely not preferred behaviour. That is why I have my automation system set such that whenever the EV charger is strutting its stuff the battery's minimum SOC is set to 90% and then reset back to 10% when the EV charger is no longer drawing a charge. Ideally future inverter and charger designs would allow for communication so as to separate out that load from any others and operate the battery accordingly, but for now it's necessary to work around the limitation.

BEWARE of IMPOSSIBLE OFFERS!

When evaluating the much lower costs of assembling your own DIY battery, we all tend to look around for the best price.
However, there are occasionally offers which really are too good to be true.

I often received MailShots from the Chinese online sales platform Alibaba.
That's not a surprise; I use Alibaba from time to time when making large purchases from Chinese suppliers.

In the past 10 days two of these Mailshots have featured an offer for LiFePO4 280Ah cells thus:

Those prices are impossibly low...

and yet so many other aspects of the 'advert' are attractive

• the seller has been Verified by Alibaba for over 2 years
• the stocks are already in Europe, thus available fast and with taxes/administration easier to sort out
• the cells are 'Grade A'
• the Mailshot is from Alibaba themselves, rather than an unknown vendor

If I click on the link I am taken to the main sales page for that Supplier:

and if I proceed to the product-selection frame, it confirms that these 280Ah cells really are $20 (USA) each:

Of course, the Shipping Costs still need to be added, but you'd expect that because the Seller doesn't yet know my delivery address.

It's very unusual for Alibaba to promote such an impossible offer as this.
The correct price should be around $70 each, plus shipping and VAT.

Alibaba risks its own reputation as a trustworthy market-place, but I can only use their complaints system if I've actually purchased the products in question.

I've therefore communicated my concerns to a colleague in mainland China, and he's agreed to file a complaint with Alibaba on my behalf.

@transparent You have previously commented that there is little or no correlation between retail price and carbon intensity.  Are you able to share the argument/reasoning/data for this in a way that is simple to understand