Comment by ChuckMcM | Hacker News Reader

Comment by ChuckMcM | Hacker News Reader

Nice, that came in lower than I was expecting. I was thinking $5K for the 7kWh unit. Of course that is the "installers" price so who knows what those folks will consider the 'retail' price. My wife still has her solar installer certification so we could presumably get one that way.

It also makes for a really interesting opportunity for grid tied solar inverters. Now you want the inverter to power the house first, then push power to the batteries and only if they are full push it back to the grid. Software update for sure :-) of course it might make more sense to leave it DC for the push into the battery and only have the AC conversion happen post battery, so charge controller between the battery and the DC disconnect.

On a safety note I'd also really like to mount this outside, preferably against firebrick rather than my house. I realize the batteries are much safer than they have ever been, but still a cascading lithium battery failure inside my garage is not my idea of a party

On the grid tied solar inverter front - the spec sheet says the battery voltage is 350~450 V, so we're looking at 108 lithium ion cells (25 x 18650 cells?) at 400 VDC nominal. This is quite different from typical lead acid battery pack voltage of 12, 24, or 48VDC that's used for battery backup storage, so a lot of existing solar battery storage infrastructure may not even work... this means that people may need to buy a whole new set of supporting hardware to integrate this into the existing solar systems instead of being able to update the software on existing hardware.

Lithium Ion also has quite a different (and much less forgiving!) charging cycle that requires much more monitoring of things like temperature, though I'd imagine a lot of that would be built-in as a safety mechanism directly into the Powerwall.

What I've heard is that National Electrical Code becomes much more stringent on battery systems greater than 48V, with the line drawn at 48V due to it being used widely in the phone landline system. I'm not sure how true that story is, but I'd imagine extra care is probably warranted. 10kWh is about 9kg of TNT. :-)

48V is under the threshold at which electrical shocks become dangerous. 48V or less is "low voltage" class. And, 48V is a multiple of both 12V and 1.5V (most individual battery cells are 1.5V), making it much easier to hook up by using a combo of parallel and series connections.

Yes talk to telco Engineers from the power side of the biz and they will have war stories about accidents and close misses with the Central Offices (Exchanges) DC power.

High Voltage/Amperage DC is quiet different to AC power and i suspect that building/wiring codes are going to need to be updated if this local storage takes off.

dropped a wrench across the terminals on the deep cycle backup batteries inside a telco switching center. Had to disconnect the whole bank of batteries to fix it because the wrench welded itself to the terminals. it wasn't a tack weld either.

That makes for a very exciting day.

The one I remember is decorators placing a paint tin across the bus bars - which explosively painted the walls

Yes, it would be interesting to know if there's an advantage of connecting so many cells in series

Most systems limit themselves to something like 48v as you said (I think 48v is the threshold of "being dangerous")

Higher voltage means lower current and thus lower voltage loss, and lower percentage losses for the same voltage loss, so you don’t need lots of copper.

Yes, but you're throwing it into an inverter, and it's not like you're carrying it too far. High voltages also present problems in itself (isolation, etc)

Now, on a second thought, maybe it's more efficient to do 400VDC to 120VAC/240VAC (don't know the exact values)

DC-DC buck converters may be a little simpler and more efficient than boost converters, I don't really know.

Solar systems are generally HVDC off the panel. I think in practice you'd tie the three sources (grid, panels, battery) together in the AC domain with a computer monitoring consumption and generation then signalling the battery system to charge or discharge.

How about stuck to the side of your house?

From further down the comments, this is a rebranding of a battery already offered to SolarCity customers.

http://www.solarcity.com/residential/backup-power-supply

The image on SolarCity has been updated to show Tesla branding.

Apropos of nothing but that product page has a lovely clean design.

That was my thought took until I remembered people already steal copper wiring whether or not it's carrying load. That would be a tempting target for theft. I think I'd stick it in my garage instead, but that's just me being paranoid.

Good point, hopefully the controller is DRM'ed to keep it from being used by a thief.

Outdoors you're going to need to heat and cool it more - you want those cells kept within a fairly narrow range to keep the lifespan up.

But you might not want someone to vandalize or steal the battery either which can happen if it's outdoors. Maybe it would be best to put it in a separate shed with lock or something.

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