Tesla to unveil home storage batteries

Exiled_Tyke

Martyn1981 wrote: »

That's really important, and was the point I was trying to address / ponder with this comment:
Mart.

Apologies. I missed that in the midst of all the other interesting stuff in the discussion. Glad it prompted yet more interesting insights from you though!

Cardew

Martyn1981 wrote: »

I think I need a battery expert now to chime in, but have read about the C ratings for batteries, such as life expectancies at C100, C20 etc. As I understand it these relate to the number of hours that the battery will be discharged over.

The slower a battery is discharged the more efficient it is, and the more cycles it will probably last.

Looking at the Nissan Leaf it has a 24kWh (or 30kWh) battery but an 80kW motor. So foot to the floor acceleration is placing a massive draw on the battery. Theoretically C1/3(?)

I'm assuming, and I admit 100% that it's an assumption, that a battery that can cope with those rates of draw/discharge will not suffer significant life expectancy reductions from a 1kW or 2kW domestic draw.

But I could be completely wrong, and monies potentially saved on leccy bill will be lost against battery replacement costs.

Hope this makes sense.

Mart.

A crucial factor in the life expectancy of a lithium-Ion battery is the charging rate, see:


https://www.microchip.com/stellent/groups/designcenter_sg/documents/market_communication/en028061.pdf

Far too often, the charging system is given low priority, especially in cost-sensitive applications. The quality of the charging system, however, plays a key role in the life and reliability of the battery. In this article, the fundamentals of charging Lithium-Ion (Li-Ion) batteries are explored.

I had wondered how Tesla managed this with a volatile input from solar panels.

zeupater

Martyn1981 wrote: »

Hi Z, interesting discussion, but I'm not following the dead weight issue. There's no need to have larger batteries than needed as a car, and no need to have discharged batteries in the morning, as there's (always?) going to be 8hrs or so of lower demand, lower priced leccy at night ....

Hi

The issue as I see it is that users will attempt to utilise the battery capacity to their best advantage, after all isn't that why the concept would be considered. If the idea was to plug the vehicle in at 5pm and a timer only allowed power to flow from the batteries until (say) 9pm, or the power was available on demand via grid smart control in order to supplement the grid, then I'd be more in agreement .... however, I'd guess that anyone spending money on batteries would expect a little more.

As you know, the evening peak demand is due to evening domestic use, that's watching TV, cooking, lights, kettles etc so that's the time to make best domestic use of the energy in the batteries - that's for both consumer & grid, however to achieve this the driver would need to plug the vehicle in as soon as it was pulled onto the drive in the evening, even if it was going to need to be unplugged then plugged in again if used again later ... there must also be enough charge to meet the 'feed in' criteria. Then, of course, most would require the batteries to power lights, fridge, freezer, kettle, toaster etc in the morning ... all thieves which would likely steal around 5% to 10% of an EV's range before it's even moved, that's the equivalent of throwing a half-hundredweight sack of potatoes in the back of the vehicle if fully charged .... you're effectively unnecessarily transporting partially charged batteries, ie the 'dead weight' is the uncharged portion ....

In addition there's not only the battery charge rate and quality which Cardew raised above, but also the effect of the depth of discharge on the LiFePO4 (?) cell degradation .... there's a considerable difference in expected lifespan, moving from ~70% average discharge (~5000cycles) to ~80% will probably almost halve your useful-life battery expectancy (~3000cycles) - that's a pretty expensive decision to make for a car owner and explains why I believe that EV lease companies would frown at providing domestic power within their existing contracts .... I'd expect a ~£50/month battery lease on a 7000mile/year vehicle lease to double to cover the effect of additional usage on their asset life, which makes the prospect particularly unattractive ....

HTH
Z

Martyn1981

But Z, this deadweight argument is again based on cars that will be used again that night, or cars that are already heavily discharged - being discharged further.

Both of these seem easily avoided, discharge limits, and simply not opting to utilise a car if you plan to go out.

For the numbers we need, you don't need all cars helping, just some, and I'm pretty sure that a large percentage, possibly a majority will not be going out again, on average.


Moving away from EV's to domestic batts, and regarding charging, I understand the Powerwall comes with a charger, so (and I admit I'm assuming again) I'd expect Tesla to have installed an appropriate unit.

I don't see the link between PV and EV's since they'll be mains charged, so whether some, all or none comes from the PV, the rate will be stable and as chosen by the charger/settings.

Mart.

Martyn1981

Just to be clear, I'm not criticising either domestic storage nor EV's, just looking at multiple sources of storage.

I can envisage a possible sharing of costs for domestic storage, but only properties with PV would have this. Otherwise the leccy suppliers or DNO's would do better to install large scale units at a lower cost.

The idea surrounding EV's and domestic back feeding, is that these could apply to non PV households, and non PV generating times such as the winter.

Looking at the Tesla cars (sorry for any Tesla Powerwall confusion) these are up to 85kWh, so ten (actually 12) times bigger than the daily use Tesla home battery, and ~20 times larger than any storage system I'd be able to make good use of.

Tapping into one of those for 1 or 2kWh in the evening seems highly sensible, but again, only IF there is some sort of adequate financial system to justify it.


In a funny way, I suppose the PV + storage systems would shift generation forward to the evening peak, whilst EV's would shift it backwards, borrowing some from the EV, before replacing it a few hours later.

Mart.

BTW anyone seen the Tesla car and home hydro storage video - very specific circumstances, but who wouldn't want one?

almillar

zeupater -

Then, of course, most would require the batteries to power lights, fridge, freezer, kettle, toaster etc in the morning

EVs already have enough smartness and connectivity to manage their own charge and to allow users to manage it to. The idea that the usage pattern will be so rigid, as you say, is a simple one to fix, and I imagine by the time this technology arrives, there will be plenty of ways to disable/enable using the car's charge, from your smartphone. You can already do this with the car's heater right now, why assume this control won't be available for using the battery?

-Deadweight
It's not deadweight. It's there to maximise range. Just because you don't use that range every day, doesn't mean it's deadweight. Indeed, if you never use that range, you could have bought a car with a lighter battery. But, you know, one size fits all, they don't custom make batteries suited to your specific range. Using that extra capacity you haven't driven on, to power your home, actually makes use of that 'deadweight' you've just carried around and not used!

Both of these seem easily avoided, discharge limits, and simply not opting to utilise a car if you plan to go out.

Bingo.

zeupater

Martyn1981 wrote: »

But Z, this deadweight argument is again based on cars that will be used again that night, or cars that are already heavily discharged - being discharged further.

Both of these seem easily avoided, discharge limits, and simply not opting to utilise a car if you plan to go out.

For the numbers we need, you don't need all cars helping, just some, and I'm pretty sure that a large percentage, possibly a majority will not be going out again, on average.


Moving away from EV's to domestic batts, and regarding charging, I understand the Powerwall comes with a charger, so (and I admit I'm assuming again) I'd expect Tesla to have installed an appropriate unit.

I don't see the link between PV and EV's since they'll be mains charged, so whether some, all or none comes from the PV, the rate will be stable and as chosen by the charger/settings.

Mart.

Hi Mart

Regarding the EV, I agree in theory ... but practice tends to be a little different. Remember, we have had both EV and PV for a number of years so you would have thought that the car would be plugged in whenever it's on the drive and the electron farming is supplying a bumper crop ... well, in the real world, that rarely happens - "It's sunny - is the car on charge ?" ... "No, I'm going out again in a minute and can't be bothered to pfaff about with pulling it up the drive & plugging it in just for a few minutes" .... the problem is that the term 'in a minute' is rarely less than a couple of hours & I'd reckon that the same conversation applies to a number of households and could easily be applied to plugging the car in at peak-energy demand times. MrsZ now only uses the plug when it's at work where she knows that it'll not be needed all day so it's worth the effort fiddling around with the cable .... added to this there's the 'planning to go out' - this is the very situation where the 'feed in' control mechanism needs to be strongest because not all journeys are planned .... think about a situation where a car has already supplied (say) half of it's charge to domestic appliances over a Sunday morning/afternoon, a day where it wasn't planned to be used and lunch is particularly energy intensive, then it's suddenly needed, range anxiety raised a few notches with future domestic usage likely quickly revisited .....

Without restrictions on the times where the battery could be available to supplement the grid or smart-control, owners are likely to become very disappointed, very quickly ... but this probably isn't the main issue. The government has already provided an EV subsidy, therefore it's not likely to provide a reasonable second one for grid-assistance, therefore the risk will be bourn by the battery owner, which in many cases is part of an already limited (by mileage) usage policy, something which isn't going to happen without some form of billing .... anyone leasing a vehicle and/or battery or owning one expecting a battery-pack warranty to not be affected in any detrimental way must really step back into reality and think about the inevitability of the motor trade finding ways to at least mitigate cost exposure, but more likely leverage profit from the secondary use ...

HTH
Z

michaels

Our ev lives in the garage and is set to charge up so it will be full at 15:00 when DW goes to pick up the kids so it tends to get plugged in whenever we get home in the day, the plug is about 3 feet from the car charge point and the cable stays plugged in their so it is not a lot of a pfaff.

zeupater

michaels wrote: »

Our ev lives in the garage and is set to charge up so it will be full at 15:00 when DW goes to pick up the kids so it tends to get plugged in whenever we get home in the day, the plug is about 3 feet from the car charge point and the cable stays plugged in their so it is not a lot of a pfaff.

Hi

The issue is that MrsZ uses hers, it's not a second car to be used as a shopping runaround .... for example, she stopped off to see elderly relatives on the way home from work, came through the door at 6:15 and has just left again (7:40) to sort something else out ... she'll probably return around 8:30, just in time to get changed and go out for the evening at around 9:00 ... since leaving for work this morning, the car has had no time to be in the garage charging and it's been pretty dark, drizzling with rain and cold on the drive, so not quite ideal conditions for hanging around to play with a cable and plug just for a few minutes of charge - this probably isn't far from typical for many people .... a whole evening where the batteries wouldn't be available to supplement the mains at peak demand hours. Fortunately, she has access to a dedicated charge point at work, else the car would normally be used as an extended capacity hybrid ...

Don't get me wrong, I'm all for EVs and as we have taken so many efficiency measures we're ultra-low electricity users so we could probably utilise the battery of a Leaf or Zoe to power the house overnight without too much affect on the vehicle range .... however, ultra low electricity users are, apart from place such as this forum, pretty rare ... well under 4kWh would see us through most nights, I believe that Mart and a couple of others are roughly the same, but then again, there are others reporting to be careful with usage, yet still consuming 15kWh-20kWh (or more) per day, which would be more of a problem ....

I simply have a worry that thousands of pounds of expenditure on batteries to achieve a reduction in grid peak demand could be achieved in a far cheaper and more sustainable way. Take for example a 2KWh Maslow battery unit which is capable of supplying ~400W .... if this 400W is being used to power a 300W plasma TV and a 100W bulb then wouldn't it be better to invest in a £300 LED TV and a £5LED bulb? - same effect on the grid for about 10% of the overall expense ... as such the same applies to the EV batteries, what's cheaper - the additional wear on the car batteries, or efficient appliances ? .... it might be perfectly obvious for some, but just consider that the idea is that EVs will become mainstream .....

HTH
Z

# In the meantime, I got ready to go out half way through composing this !

Martyn1981

Interesting stuff Z, I suspect reality will be somewhere between my optimistic view and your pessimistic view, though hopefully both opinions are within the main body of the bell curve.

I'd like to see some sort of subsidy support for domestic batts, shared across households, leccy suppliers, DNO's and govt, but larger systems are cheaper, so without a domestic PV contribution, I'd expect leccy suppliers and DNO's to do their own larger scale systems.

But ..... the thought of those huge batteries sitting in driveways .... it's very tempting!

By huge I should explain that when I read about the big batts the off-gridders have, they seem to be in the 25kWh to 60kWh range, but these are usually lead acid, so at 50% DoD are not that much bigger than a Leaf (24kWh or 30kWh) Li-ion that can be used happily to a safer depth say 80% DoD.


Myself and Wifey can't really justify a second car, but if we did get one, I'd love an EV. Perhaps park it up for a few days a week when generation is really high, but obviously expect most charge from the grid, and always have the other car for emergencies (range anxiety).

Mart.