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  # 1296254 3-May-2015 19:52
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Many thanks Niel for your very comprehensive answer.  Having written a paper about it, there is no doubt that you are GZ's resident 'Go To Guy' for any questions on batteries.

So then, taking your answer of 2000 cycles max for the LiFePO4 batteries, it seems that we would be looking at a battery life of 5.5 years, if they were charged every day by PV Panels, and then discharged at night.  Perhaps a smarter strategy would be to charge them one day, and let them discharge for the next 2 days, thereby getting double the life (in theory).  I'm not sure if any inverters out there are smart enough to do this, or if in fact the life would be increased, given that the depth of discharge would also increase. 
It seems like we would be getting something for nothing, and experience has taught me that such theories are seldom borne out in practice.

Say for argument's sake that the Tesla battery does in fact last 5.5 years, with an intial cost of USD3500, at exchange rate 0.75 works out to NZ$850 per year approx, or about $71 per month.  This then begs the question:  Is it worth spending $71 per month to save a proportion of our electricity bill, which might not in fact add up to that much?  I can see that some further calculations need to be done.





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  # 1296260 3-May-2015 20:15
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i would imagine you could set that up, you could dump your power into the batteries one day then the next day dump it into your hot water.

 
 
 
 


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  # 1296270 3-May-2015 20:45
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My understanding is if you don't fully charge and fully discharge you get significantly more life out of your batteries.

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  # 1296289 3-May-2015 21:24
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Timmmay is correct, and Tesla has set it up that way.  It is one of the few bits of technical info I could find regarding this pack.  The other bit of info is the efficiency, however typical marketing hype they only give the DC efficiency which is useless information.  Virtually all the losses are in the inverter and the cooling system.

I seldom say what I've done with batteries as I don't want to be labelled an expert.  There are others that know lots more than me.  Just making the point that I don't just go by hearsay e.g. on hobby RC forums regarding batteries.  The paper I've written is anyway not that impressive IMO, just a collection of what I've learned over a few years, to share with other engineers.




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  # 1296293 3-May-2015 21:28
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Wonder if in a few years we will see the aluminium batteries for this application

http://arstechnica.com/science/2015/04/07/flexible-aluminum-battery-charges-fast-stable-for-over-7000-cycles/


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  # 1296309 3-May-2015 21:59
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The Tesla battery (7 and 10 kWh) would be able to output about 8A/230V if I have read the information correctly. That doesn't seem very impressive?




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  # 1296320 3-May-2015 22:32
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jarledb: The Tesla battery (7 and 10 kWh) would be able to output about 8A/230V if I have read the information correctly. That doesn't seem very impressive?

Doesn't sound too bad to me.  That's about 1.8kW, which is reasonable given the intended use of these batteries.  I wouldn't try to boil an electric jug or run a heater off it, but rather run lighting, computers, fridges and other low-demand loads.  This would mean cooking on top of the woodfire during a power outage, which we do anyway, and relying on the hot water cylinder being full of hot water, which is usually the case.

Most batteries don't put out anything like 230V, so I'm surprised if it's that high.  An inverter would be needed in any case, to power the AC wiring in a house, as there aren't many low-demand appliances out there that would cope with 230VDC.  Incandescent light bulbs would be one of the few I can think of, and maybe a toaster if it didn't draw too much power.





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  # 1296375 4-May-2015 00:57
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The piece of information that hasn't been disseminated very much is that the 10KWh pack is only rated for weekly discharge, so if you're wanting to use one daily, you need to base your calculations off the 7KWh pack. http://www.teslamotors.com/powerwall

The weekly discharge of the 10KWh unit makes the economics less favorable for it than I initially thought.

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  # 1296395 4-May-2015 07:54
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Also, the tesla page I posted has some other information: The batteries are 2.0 kW continuous 3.3 kW peak, (5.8 amp nominal, 8.6 amp peak). So if you need more than that to power your house (based on your use case) then you need to purchase more battery packs.  The package only covers batteries, no other components.  The output voltage of the battery packs is 350 – 450 volts. I believe they chose higher output voltage over lower output voltage in order to reduce wire size.

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  # 1297786 4-May-2015 17:24
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They chose a higher voltage because it reduces the stress on the batteries due to lower current flow.  Has nothing to do with wire size, there are established systems for getting high current out of batteries.  This started over 15 years ago, gets 0-100 in 1 second and pulls almost 3G: http://killacycleracing.com/

So with weekly cycles and a 10 year warranty, that is about 500 cycles.  They say they will consider offering an extension on the warranty after 10 years (I'd guess they would first look at the usage and the condition), so a 20 year warranty at weekly cycles would be 1000 cycles which is achievable with normal lithium batteries charged to 4.1V and discharged to no less than 20% (and used at a light current).




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  # 1297917 4-May-2015 20:40
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Niel: They chose a higher voltage because it reduces the stress on the batteries due to lower current flow.  Has nothing to do with wire size, there are established systems for getting high current out of batteries.  This started over 15 years ago, gets 0-100 in 1 second and pulls almost 3G: http://killacycleracing.com/
i found the source of my comment:

'Chief Technology Officer JB Straubel said that the Powerwall is a 400 volt battery that "doesn’t use heavy gauge wire, so that makes the installation easier." arstechnica I accept its framing the 'feature' in a consumer friendly manner.

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  # 1297997 5-May-2015 06:47
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It sounds better when they say it is designed to make installation easier rather than to make the battery last longer.  One focusses on the installer and the other on the product.  You know the size of house wiring is not bad, and that can easily carry 30A which is 12kW at 400V.




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  # 1298272 5-May-2015 11:49
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This may be useful to take the edge off peak demand. That is where all the issues are.

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  # 1298960 6-May-2015 12:12
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I would use something like this to heat water.   I have space for another element and another thermostat in my cylinder.  The cylinder is highly efficient [edit: ie well insulated].

So ...

1) Add a 1.5kW element powered to the powerwall (employ a spark wrangler to do this all safely and properly).
2) Add a thermostat to control the 1.5kW element, set to quite a high temperature (we have a tempering valve).
3) Set the thermo stat controlling the 2.4kW element to a lower value than the thermostat controlling the 1.5kW element.

The 2.4KW element now acts as a booster when cylinder temp drops too low.







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  # 1299125 6-May-2015 14:38
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Waht is the formfactor of those Li modules used inside the [Tesla] pack?

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