[steffenfrost]

Lifetime cycles are an issue for Li-ion batteries.

For example, lets look at the Tesla Powerwall numbers. The Powerwall warranty covers 85% of capacity with 740 cycles in the first two years -- whichever comes first. Then it covers 66% capacity with 1,087 cycles in three years, and finally, it covers 54% of capacity for 2,368 cycles in five years. http://www.greentechmedia.com/articles/read/is-teslas-powerw...

Running the numbers for the Powerwall, 85%, 66%, and 54% of 7kWh is 5.4, 4.6, and 3.8kWh respectively.

(740cycles x 5.4kWh) + (1087cycles x 4.6kWh) + (2368cycles x 3.8kWh) = 17994.6kWh

Thus the cost per kWh stored is

$3000/17994.6kWh = 16.7 cents/kWh.

Not included is the shipping and installation cost, which could add another $300 - $500, which would run

$3300/17994.6kWh = 18.3 cent/kWh.

[fencepost]

From what I've read[1] about lithium-ion batteries, it seems to me that if you don't actually need to utilize the full capacity and don't need to have the full capacity always available, you can get some pretty drastic improvements in battery lifespan by limiting the charge to 70-80% of maximum capacity and trying to avoid discharging below 40-50%.

That doesn't mean you can't go outside those ranges in special cases, just that it's best for the battery to spend as much time as possible within that range. For a Powerwall, I'd take that to mean that charging it to 100% every day with solar then draining it to 10% every night recharging an electric car is going to reduce its lifespan pretty fast, but charging it to 80% daily and using it to reduce your grid draw without fully discharging may let the unit last for years with minimal decline in capacity.

It's like a lot of things - if you're constantly bouncing against hardware limitations, there's a good chance you're putting more stress on the hardware and are going to see shorter lifespans for it. It may not be readily visible, but effectively batteries are a moving part subject to wear so treat them as such and plan for it.

[1] http://batteryuniversity.com/learn/article/how_to_prolong_li...

[tedsanders]

That's true, but I wouldn't call the effect drastic (as long as you stay in normal voltage ranges).

For example, your link says that 100% depth of discharge lasts 300-500 cycles, 50% lasts 1,200-1,500 cyles, 25% lasts 2,000-2,500 cycles, and 10% lasts 3,750-4,700 cycles.

You might think wow, 3,750-4,700! That's way more cycles than the 300-500 of a 100% discharge cycle! This is true, but usually the important comparison is total kWh stored, not total cycles. Each cycle of a 100% loop is 10x more valuable than a discharge of a 10% loop. So the proper comparison is not 3,750-4,750 vs 300-500, it's 3,750 vs 3,000-5,000. So the shallow discharge cycle comes out on top, but not by a huge margin.

Compounding this is the fact that lithium ion batteries are always losing capacity while being stored at room temperature. So this is another advantage of deeper cycling of batteries. You get more value out before they age.

Anyway, you likely knew this, but I thought I would share in case it was educational for others.

[infogulch]

If this can extend the cycles to 100,000 (from the article), this brings it the cost down dramatically. Using a conservative half of 100k:

(50,000 x 5.4kWh) = 270,000kWh

$3300 / 270,000kWh = 1.2¢/kWh, more than a factor of 10!