[quarterwave]

I have home back-up power based on an inverter charging a lead-acid battery (located in a sheltered area outside the home), which costs about $100/kWh. Usage is about 1-2 hours discharge per day. No matter how well serviced, I've found these batteries don't last beyond four years. Hence I'd pay even $400/kWh for a well-engineered deep-cycle battery that is safe, maintenance-free, and will last at least 10 years. Excluding balance of system, even.

[jacquesm]

> No matter how well serviced, I've found these batteries don't last beyond four years.

Wow, that's really short, especially with such a shallow discharge pattern.

[msandford]

Not necessarily! We don't know how deep the battery is being discharged. But we can make a guess. 4 * 365 = 1400 cycles.

http://www.mpoweruk.com/images/dod.gif

According to that chart (which is an approximation of course) 1400 cycles corresponds to about a 40% depth of discharge. Which isn't terribly shallow.

The other variable is the discharge rate, and the higher it is relative to battery capacity the worse the efficiency and also the propensity to fail early. A lot of times doubling the pack size can extend the pack life by more than two because the increased efficiency (the internal resistance is lower) reduces the depth of discharge by more than half.

http://batteryuniversity.com/_img/content/crate1.jpg

Of course it feels totally ridiculous to only use 20% of the nameplate capacity of the system, and much worse than using 40% which you can sort-of rationalize as "half" but if it decreases your dollars per joule, it might be worth it.

EDIT:

I should also mention that if you're constantly charging and discharging and you don't mind a little energy loss you should look at nickel-iron batteries. They're not terribly efficient nor are they cheap in absolute terms but they're basically bulletproof.

http://en.wikipedia.org/wiki/Nickel%E2%80%93iron_battery

http://ironedison.com/

[glabifrons]

I've wondered for years why people always gravitate towards Li-Ion cells when talking about a home storage battery. Li-Ion's advantages are far, far less important for a dwelling. Weight doesn't matter... you won't be moving them. Size doesn't matter nearly as much as it does in a vehicle... losing a few inches off an entire wall in a garage won't really be an issue for most people. Edison cells are incredibly durable and much, much more environmentally friendly than any other battery tech I know of. The lifespan is nothing short of incredible too... you won't need to change them out.

Thank you very much for the 2nd link. I was unaware any company was still manufacturing them. The last time I looked, the last company I could find that made them stopped a few years prior. I'm glad someone is making them still/again and marketing for an appropriate use.

The one odd thing is the price... for something as low-tech (relatively speaking) as an Edison cell, I'd expect them to be much cheaper. Must be the lack of competition.

[jacquesm]

They may seem to be low tech but the electrodes are works of art and manufacturing them is a lot more expensive than a lead-acid battery of comparable capacity. They charge slower too, but they'll stand up to abuse better than every other rechargeable battery tech. I looked at them for a long time before settling on regular lead-acid, cost and finding an inverter that would charge these properly were the major factors.

[it_luddite]

check out these _types_ of batteries.

http://www.sbsbattery.com/products-services/by-product/batte...

no recommendation on the supplier, but funny thing, the power companies and telcos have already figured this out for reliable DC power :)

[quarterwave]

Thanks, good point about telcos.

[sliverstorm]

Do we have any reason to believe lithium can achieve that? How long are the Tesla batteries supposed to last...?