ᐅ Assistance with Solar Panel Decision-Making

Bidirectional charging has become quite smooth by now. There are plenty of wallboxes that support it with all common electric cars.

The bigger issue is usually the warranty. Who covers the cost if the battery is worn out after 4 years, having reached its cycle life but the warranty period has not yet expired? This needs to be negotiated with the car manufacturers. Otherwise, you have great solutions, but nobody wants to use them before the warranty expires, and generally not after either, since that would accelerate the battery’s—and thus the car’s—depreciation.

Personally, I would rather rely on “seasonal storage.” Some companies already offer this, but it is still too expensive. Essentially, these are used car batteries that you can attach to the house wall and fully charge until autumn, then draw on them throughout the winter. This requires a highly efficient battery management system. It must detect when good weather is expected in the coming days, allowing the heat pump to consume power in the evenings. Otherwise, the heat pump stays connected to the grid, and the battery is preserved for standby operation, so it lasts as long as possible through the winter and is only completely discharged near the end.

Hi,

does anyone seriously plan or believe in something like this?

Evolith schrieb:

Basically, these are used car batteries that you can stick to the side of a house, charge them up nicely until autumn, and then draw from them over the winter.

To actually implement something like "charging fully until autumn and then drawing from it over the winter," we are certainly talking about 1000–2000 kWh (1,000–2,000 kWh) of storage capacity. That roughly means 20–40 car batteries weighing 300–500 kg (660–1,100 lbs) each. Including the necessary control and energy management systems, this would likely fill several 20-foot containers. You would definitely need a large section of wall to "stick them on."

And all this hassle just for one charge-discharge cycle per winter? ;-) Sorry, but that would be completely insane!

Second-life car batteries as large decentralized grid storage? Yes! But for private users?!

You can’t fool physics; complete self-sufficiency just isn’t possible in these latitudes, no matter what some visionary puts on their PowerPoint slides.

Seriously:

The idea of using old car batteries as stationary storage has been around forever. But are there actually such systems available for private users? I honestly can’t see a practical use case.

The storage would have to come from a used car—I definitely wouldn’t want a lithium-ion battery from an accident vehicle. So you end up with cells that are 6–10 years old and whose chemistry is only designed to last 10–15 years. This means you’ll be dealing with failures almost from the start. And if you have to "refurbish" the cells somehow (however that might work), you might as well just buy new ones.

Fun fact: Tesla’s latest generation Powerwall uses lithium iron phosphate (LiFePO4) battery cells instead of the previously used cells found in their cars. Even though I wouldn’t touch that company’s products with a ten-foot pole anymore—they are technically competent on this topic and know what they’re doing—there must be good reasons why they themselves are moving away from this concept.

Best regards,

Andreas

andimann schrieb:

Hi,

is anyone seriously planning or even believing in such a thing?



To actually implement this—“fully charged through autumn and then lasting over the winter”—we’re definitely talking about 1000-2000 kWh that such a battery would need to store. Roughly speaking, that’s about 20-40 car batteries weighing 300-500 kg (660-1100 lbs) each. Together with the necessary control and energy management systems, that would probably fill several 20-foot containers. You would need a huge wall of the house to “stick them onto.”
And all that effort for just one charge cycle per winter? ;-) Sorry, but that is completely crazy!

Combined car batteries as decentralized large-scale storage on the grid, yes! But for private users?!?

You can’t fool physics; fully self-sufficient operation in these latitudes is impossible, no matter what some dreamer pins on their PowerPoint slides.

Seriously:

The idea of using old car batteries as stationary storage has been around forever. Are there already such systems available for private users? Somehow, I can’t imagine a practical use case.
Such a battery would have to come from an old car—I definitely don’t want a lithium-ion battery from a crash-damaged vehicle. So you are talking about 6-10-year-old cells designed chemically for only about 10-15 years of life. That means you will almost certainly have to deal with failures from the start. And if you “recondition” the cells somehow (however that may work), you might as well just buy new ones.

Fun fact—Tesla’s new generation of the Powerwall uses lithium iron phosphate (LiFePO4) battery cells instead of the cells previously used in their cars. Even though I wouldn’t touch products from this company with a ten-foot pole anymore, they are technically competent in this field and know what they are doing—so there must be very good reasons why they themselves are moving away from this idea.

Best regards,

Andreas

Yes, as a matter of fact, there are already some companies doing this. In Norway, this approach seems to be working quite well. Friends of ours have a holiday home there and report that they increasingly see large batteries mounted on walls. These are roughly around 100 kWh (350,000 BTU) per wall, a rough estimate. Good cells from different batteries are pieced together to build these systems, which probably explains the high price. There’s a lot of manual labor involved.

You won’t get through the entire winter on a single charge either. The goal is more to cover the basic household demand (excluding heavy consumers like heat pumps, electric cars, or electric stoves) as much as possible during winter. When the weather forecast predicts plenty of sunshine in the upcoming days, the systems calculate the expected solar power generation and then supply power to the heavy consumers accordingly. How reliable this works, I cannot say. I only know the concept and can easily imagine how it could work.

The question is whether these are actually old car batteries or if they are off-grid systems from remote houses that get through the winter with a large solar array, a big battery, and a wood stove.

Alternatively, they might have flexible pricing models like Tibber and charge up when wind power floods the European market and drives prices down.

Evolith schrieb:

That's roughly about 100 kWh at the wall.

You can also buy large storage systems from us. For small businesses (also usable privately), 100 kWh systems cost around ±10,000€.
The models specifically designed for private users, ranging from 40 to 50 kWh, start at about 7,000€.

The main concerns are space, weight, and cooling.

Evolith schrieb:

I only know the vision and can easily imagine it.

You are confusing buffer tanks and seasonal storage. Batteries are unsuitable for the latter. The marketing claim "You get your own electricity in winter" is a different matter altogether. This very simplistic idea is just too exaggerated.