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by 54mf 3975 days ago
"For storage a $65 lead acid automobile battery does the trick. It’s 12V so can be charged directly from the solar panel, and holds 420Wh, way more than I use in a day. That’s $0.15 / Wh so I don’t see why everyone is so excited about Tesla charging $0.43 / Wh for the Powerwall, sans inverter and installation."

I'm not smart enough to take this apart, but my gut tells me the best-in-class engineers and (tens/hundreds of?) millions of dollars in R&D at Tesla aren't going to be outwitted by Mr. I'm-Repulsed-By-The-Idea-Of-Kitchens. Anyone want to take a swing?
3 comments
soneil 3975 days ago
Car batteries simply aren't designed for deep-discharge. It actually damages (sulphates) them. They're designed to provide thousands of cycles at 2-5% discharge. Using them for deep discharge, you're likely to see them increasingly unusable (or at least, holding nowhere near the designed charge) after 30-150 cycles.

So that 420Wh battery is most likely designed to actually provide about 21Wh. Past that it's a good assumption it's lifespan is being degraded faster than design.

To match a 7kW powerwall with these figures, he'd need roughly 350 such batteries (nearly $23k) to stay within a 2-5% discharge cycle.

So his setup may be capable of delivering 420Wh cheap. Or it may be capable of lasting 1-2000 cycles. But not both.

(Deep-discharge SLA are available. They're usually marketed as marine batteries rather than car batteries. They'll survive such usage better - a couple of years, rather than a couple of months in this scenario. But still not tesla's claimed 5000 cycles.)

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rtkwe 3975 days ago
That battery has very poor deep cycle and recharging characteristics. Standard lead acid car batteries don't like being discharged below ~50% like others have pointed out. Because of this the battery is going to lose most of it's capacity quickly and need to be replaced long before practically any other choice, there are lead acid batteries called deep-cycle that are designed to be drained and recharged.

Like most things you can slap together something that works on the surface much cheaper in initial costs than the commercial equivalent.

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shawabawa3 3975 days ago
It's summed up nicely here:

> http://www.powertechsystems.eu/en/technics/lithium-ion-vs-le...

Basically, lead acid have much lower lifespans, and also degrade very quickly if discharged below 50%. So straight off you have to double the size of your battery to not massively shorten the lifespan, so his costs are already closer to $0.30/Wh.

Also, lead acid batteries are larger/heavier per Wh, and even if you never discharge below 50% last less than half the time of Lithium batteries

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