Tesla Powerwall: Engineering Analysis

[dschlink]

Russ - my cabin is on time-of-use payment with PGE; this is particularly valuable for me as it has a heatpump and in the winter when I'm not there, I have the thermostat programmed to raise the temperature at night when I'm paying about 1/3 of the regular rate.

I could see charging the battery at night and using it to reduce daytime costs; but for power backup, I'd need 4KW or two units.

 

[nitsuj]

The problem for now is that there is no residential market that I'm aware of for this concept. In order to make it useful, your electric rate would have to vary from day to night (as it already does commercially). So this is a product without a market (except for the secondary purpose of being a whole house backup).

Ontario, Canada has "smart meters" for time of day costing, there is a price differential though I'd be surprised if the unit could pay for itself before the battery capacity becomes nill.

Also I haven't seen the very important value for it's wattage/amperage capabilities. Powering an oven/dryer maybe out of the question for a single unit, even a microwave takes a lot of wattage.

 

[rollingstein]

Also I haven't seen the very important value for it's wattage/amperage capabilities. Powering an oven/dryer maybe out of the question for a single unit, even a microwave takes a lot of wattage.

5.8 amp nominal, 8.6 amp peak output

Remember also that the electrons flow though every possible path in the entire interconnected grid, which is owned by many parties. CA belongs to the interconnection that goes all the way to Colorado. You can't use that grid without following the rules of the grid operator, even if you own the source, the destination, and the most direct path between them. That's logical because what you do affects everyone.

Have you seen situations where someone had to make an unscheduled shutdown of a baseload power station e.g. Coal just because demand dropped to a level so low that even after shedding all the peaking plants, generation exceeded the ability of the grid to absorb power?

 

[OmCheeto]

Ontario, Canada has "smart meters" for time of day costing, there is a price differential though I'd be surprised if the unit could pay for itself before the battery capacity becomes nill.

Also I haven't seen the very important value for it's wattage/amperage capabilities. Powering an oven/dryer maybe out of the question for a single unit, even a microwave takes a lot of wattage.

According to the U.S. Energy Information Administration:

In 2013, U.S. electric utilities had 51,924,502 advanced (smart) metering infrastructure (AMI) installations. About 89% were residential customer installations.

Residential  Commercial  Industrial   Transportation             Total
46,083,727   5,597,147     242,783              845        51,924,502

ref

Though this doesn't guarantee that "Time of Use" is available to all customers, but only that it could be.

It is available through my utility company:

https://www.portlandgeneral.com/residential/your_account/billing_payment/time_of_use/pricing.aspx

Peak   $/kWh
On     $0.12581
Mid    $0.07222
Off    $0.04195

Just eyeballing it, it looks as though it would be cheaper to be a mushroom.
hmmm...

My usage:

1000 kwh/month average = $126/month on-peak & $42/month off-peak = $84/month savings
$84/mo * 12mo/yr = $1000
Cost of Tesla battery = $3500
ROI = 3.5 years.​

hmmmm...

 

[rollingstein]

My usage:
1000 kwh/month average = $126/month on-peak & $42/month off-peak = $84/month savings
$84/mo * 12mo/yr = $1000
Cost of Tesla battery = $3500
ROI = 3.5 years.

1000 kWh/mo is approx. 40 kWh / day assuming a pretty invariant usage pattern.

That sounds like you'd need four Powerwalls? $14,000?

 

[nitsuj]

1000 kWh/mo is approx. 40 kWh / day assuming a pretty invariant usage pattern.

That sounds like you'd need four Powerwalls? $14,000?

What's more Om's ROI takes 3.5yrs to reach cost...what's the battery life expectancy after 3.5 years of daily use?...will it catch fire?

 

[OmCheeto]

1000 kWh/mo is approx. 40 kWh / day assuming a pretty invariant usage pattern.

That sounds like you'd need four Powerwalls? $14,000?

Ah! You are correct. So that would be 14 years for ROI, which is longer than the batteries last, so this would not be a good investment for me.
Unless of course, I switch to another form of heating.
My summer usage is only 14 kwh/day, so I could probably get by with 1 unit.

Wait a second. No, I don't need 4 units. I only need enough units to cover "On-peak" rates.
Good grief. This is worse than figuring out taxes.

This may take me a while to figure out...

 

[CWatters]

Here in the UK... Electricity consumption per head is lower than in the USA. Many people with PV generate more than they use at certain times of the day (or even when averaged over the year). The subsidy/financial incentive they give you to install PV assumes you will export a fixed percentage of the electricity you generate even if you use it all yourself. So it's attractive to store and use all that you generate rather than export any.

It doesn't normally make sense to use electricity to heat water (gas and oil are cheaper) but some people install an extra heating element in their hot water tanks and use this to store the excess electricity from their PV system. There are devices you can get that detect you are exporting and turn on and modulate the heating element.

I think some people will also use Tesla batteries to store the excess electricity they generate in the day to charge their electric car at night.

One question I have... Tesla are offering a 10 year warranty which seems generous given that many cells are only rated for around 1000-2000 cycles?

 

[nitsuj]

5.8 amp nominal, 8.6 amp peak output

less than half of a standard outlet /cry presuming normal voltage, this thing can can't even produce just one horsepower (ideal use)

I think they maybe really sacrificing available wattage for battery life / safety. The battery for my remote control car toy (1.5kg) has an 11.1 lithium polymer that can do 1400 watts all day and stay only "warm". though would be lucky to get 500 uses and still have >80% capacity.

One question I have... Tesla are offering a 10 year warranty which seems generous given that many cells are only rated for around 1000-2000 cycles?

It would be interesting to have more info on the batteries. I don't get why this is suddenly a viable tech, which makes me think marketing rather reduced cost / improved tech. Has Tesla come up with new battery formulations, or reach new "economies of scale" with their batteries?

When will Tesla sell batteries for my remote control car toys? lol I want 10 year warranty with 3000 cycles."It's[/PLAIN] like crazy off-the-hook," Musk said during an earnings presentation on Wednesday. "The sheer volume of demand here is just staggering."

Anyone can go online and place a reservation, years in advance, with no money down and no commitment to buy."

Oh Musk, you're the king of optimizing investors with your optimism to get them investing or to stop asking about some sort return...again & again. lol That said GO MUSK!

If I had "venture dollars" hearing "It's like crazy off-the-hook," would be a bit too much of a ...warning sign..."cause like trendy isn't for things that cost thousands, so like I so couldn't"

 

[rollingstein]

It would be interesting to have more info on the batteries. I don't get why this is suddenly a viable tech, which makes me think marketing rather reduced cost / improved tech. Has Tesla come up with new battery formulations, or reach new "economies of scale" with their batteries?

If you look at their Powerwall website, other than whatever details are on that one page there's no other Literature. No spec. sheets, no test results, no whitepapers, no pdf's, no best practices, installation instructions. Nothing. Zilch.

To me that's a big red flag. It has the classic signs of a "magic" technological leap.

 

[nitsuj]

If you look at their Powerwall website, other than whatever details are on that one page there's no other Literature. No spec. sheets, no test results, no whitepapers, no pdf's, no best practices, installation instructions. Nothing. Zilch.

To me that's a big red flag. It has the classic signs of a "magic" technological leap.

Ah..so I'll use it to charge my Segway.

The magical technological leap is upon purchase it makes you feel good being a solution to an energy "problem".

What's more Musk backers see shimmers of hope when Musk adds a Musk multiplier to that sale preventing a balance sheet disaster. So there is some "magic" there too. take a sale of $3,000...multiply it by the number from an internet poll...report to media "The sheer demand here is just staggering!" & "...off the hook!".

What's the carbon foot print of one of these units?

 

[OmCheeto]

If you look at their Powerwall website, other than whatever details are on that one page there's no other Literature. No spec. sheets, no test results, no whitepapers, no pdf's, no best practices, installation instructions. Nothing. Zilch.

To me that's a big red flag. It has the classic signs of a "magic" technological leap.

They listed specs near the bottom of the page. I'll list the clues as to why I believe a 10 year lifespan is doable:

Specs

Technology

Wall mounted, rechargeable lithium ion battery with liquid thermal control.​

Models

10 kWh​

Power

2.0 kW continuous
​

I've never done testing on lithium ion batteries, so I can only take other people's words for how they function:

Understanding the life of lithium ion batteries ...

Note to journalists: Please report that this research was presented at a meeting of the American Chemical Society. (Yay! I think I'll add "journalist" to my resume.)

NEW ORLEANS, April 10, 2013 — Scientists today answered a question that worries millions of owners and potential owners of electric and hybrid vehicles using lithium-ion batteries: How long before the battery pack dies, leaving a sticker-shock bill for a fresh pack or a car ready for the junk heap? Their answer, presented here at the 245th National Meeting & Exposition of the American Chemical Society (ACS), being held here this week, may surprise skeptics.

“The battery pack could be used during a quite reasonable period of time ranging from 5 to 20 years depending on many factors,” said Mikael G. Cugnet, Ph.D., who spoke on the topic. “That’s good news when you consider that some estimates put the average life expectancy of a new car at about eight years.”

Cugnet explained that the lifespan depends mainly on the battery’s temperature, state of charge and charge protocol. Battery performance begins to suffer as soon as the temperature climbs above 86 degrees Fahrenheit. “The higher the temperature, the lower the battery service life,” he said. “A temperature above 86 degrees F affects the battery pack performance instantly and even permanently if it lasts many months like in Middle East countries.”
​

This may take me a while to figure out...

This is worse than taxes and orbital mechanics, combined.

Winter and Summer consumption are best (June-Oct 14 kwh/day) and worst (Jan & Feb 50 kwh/day) cases.

Graphically, over my last 12 recorded billing cycles:

x = date y = kwh/day​

About the only things I've determined so far, is that:

1. a single Tesla Powerwall

a. will keep my refrigerator running during "On" and "Mid" peak hours during the summer (9.3 kwh)
b. will not cover my winter needs (33.3 kwh)​

2. I should do my laundry and dishwashing on Sundays (my water heater is set to 100°F, so I use the "Water Heat" mode on my dishwasher)​

So based on 1.b., I will have to determine a method of storing 23.3 kwh of energy via an alternative method, to make this "Wall" a viable option, for me.

Interesting that Tesla got nearly a billion dollars worth of orders for these type of things in the first week.

Tesla's Battery Grabbed $800 Million in Its First Week (BloombergBusiness)

 

[rollingstein]

a. will keep my refrigerator running during "On" and "Mid" peak hours during the summer (9.3 kwh)

How many Amps does your refrigerator pull? How many Amps will your Laundry / Dishwasher pull?

 

[OmCheeto]

How many Amps does your refrigerator pull? How many Amps will your Laundry / Dishwasher pull?

simple maths: Amps = Watts/Volts

You figure it out!

clues: I've never heard of a 220 volt home fridge, dishwasher, nor clothes washing machine. I seem to recall that there are 120 volt laundry dryers for apartment dwellers.
Ha! I just realized, that I've never been an "apartment" dweller.

hmmm...

 

[rollingstein]

simple maths: Amps = Watts/Volts

You figure it out!

Did you post the Watts somewhere? Maybe I missed that info.

I just wanted to point out that the Powerwall is rated for 6 Amps. I was wondering if you refrigerator + dishwasher / dryer combined load would be able to stay within those Amps. I didn't think it would work.

 

[OmCheeto]

Did you post the Watts somewhere? Maybe I missed that info.

I just wanted to point out that the Powerwall is rated for 6 Amps. I was wondering if you refrigerator + dishwasher / dryer combined load would be able to stay within those Amps. I didn't think it would work.

I don't know how much you eat, or roll in the mud, but I only have only one appliance that runs 24/7 in the summer.
The others can wait for Sunday.

 

[rollingstein]

I don't know how much you eat, or roll in the mud, but I only have only one appliance that runs 24/7 in the summer.
The others can wait for Sunday.

At least on Sunday you need something rated for the combined Amps?

 

[OmCheeto]

At least on Sunday you need something rated for the combined Amps?

I don't understand what you are asking/saying.

Or perhaps you don't understand my pictograph?

Summer rates start on May 1 ____ Winter rates start on Nov 1

What it means is that I can get off-peak rates all day Sunday. So I can run everything in the house, which is about 20,000 watts.

One funny thing is, that the way I cook, is already optimal for "Time of Use" metering. Ha!
I cook about 5 lbs of meat overnight at fairly low temperatures, and then take it out in the morning.
Then I store this in the fridge for the week, and heat it up in the microwave.

5 minutes of microwave cooking/day @ 500 watts = 42 watt hours
16 hours of refrigeration/day @ 500 watts = 8,000 watt hours
total = 8,042 watt hours

I'm thinking I will only need one of these things.
But I'm still working on the maths on how to heat my house in the winter.
It's been 25 years since I've done some of these calculations.

This is the problem I'm working on:

Outside temperature is 43°F
Specific heat capacity of my house is 5000 Btu/°F
R-value is 13
Conductive surface area is 1900 ft²
If the house is heated to 75°F at 6 am, what will be the temperature in the house, 16 hours later?
And if it's less than 60°F after 16 hours, how much water, heated to 150°F would be required to maintain the minimum temperature at 60*F?​

I should have the solution by noon tomorrow.
But right now, my brain hurts, and I feel like watching some Hulu TV before bed.
Ciao!

 

[nitsuj]

They listed specs near the bottom of the page. I'll list the clues as to why I believe a 10 year lifespan is doable:

Specs
Technology
Wall mounted, rechargeable lithium ion battery with liquid thermal control.Models
10 kWhPower
2.0 kW continuous

How many Amps does your refrigerator pull? How many Amps will your Laundry / Dishwasher pull?

apparently 2,000 watts (household = 120 * 15 = 1800)

10,000 / 2,000 = 5 hours at 2000 watts

I'm more confused now...

 

[CWatters]

If you need more than 2KW...

The extra can be drawn from the grid.
or
If you are off grid multiple Powerwalls can be connected together.

 

[nitsuj]

If you need more than 2KW...

The extra can be drawn from the grid.
or
If you are off grid multiple Powerwalls can be connected together.

If it's seamless ( powerwall <-> grid) that's perfect, if not I think that's more math than most want to do in determining what to power with it to ensure all energy is used before night charging. I think this is a step towards higher production volume of batteries as opposed to a step towards convenience / energy cost savings / meeting market demands.

 

[OmCheeto]

If it's seamless ( powerwall <-> grid) that's perfect, if not I think that's more math than most want to do in determining what to power with it to ensure all energy is used before night charging. I think this is a step towards higher production volume of batteries as opposed to a step towards convenience / energy cost savings / meeting market demands.

I think you are correct, in that this is "more math than most want to do".
It's taken me an hour just to convert from imperial to SI units.
It may be 2 pm, before I have my answer.

...the problem I'm still working on:

Outside temperature is 43°F ( 6.11 °C )
Specific heat capacity of my house is 5000 Btu/°F ( 2.6 kwh / °C )
R-value is 13 ( (2.3 m² °C) / watt )
Conductive surface area is 1900 ft² ( 176 m² )
If the house is heated to 75°F ( 23.9°C ) at 6 am, what will be the temperature in the house, 16 hours later?
And if it's less than 60°F ( 15.6 °C ) after 16 hours, how much water, heated to 150°F ( 65.6°C )would be required to maintain the minimum temperature at 60*F?​

Thank god those silly Europeans haven't yet devised their own SI time unit...

 

[nitsuj]

apparently 2,000 watts (household = 120 * 15 = 1800)

10,000 / 2,000 = 5 hours at 2000 watts

I'm more confused now...

Ah I see now, it's 350-400 volts...presuming for efficiency & reliability reasons (not bad if a few cells are poor).

This unit is getting a bit more impressive...wish there was more info on the batts though.

 

[James Alton]

Ah I see now, it's 350-400 volts...presuming for efficiency & reliability reasons (not bad if a few cells are poor).

This unit is getting a bit more impressive...wish there was more info on the batts though.

7KW of energy storage could be very interesting to people that live in Northern Climates. That is enough power to run an oil furnace or perhaps a wood pellet stove for an extended period of time in case a power loss and make the difference between freezing and not! In the event of an extreme winter with an extended power loss could even save some lives. Generators are another option but the beauty of a charged battery is that you don't have to get it started for it to start producing power and gasoline units especially can be pretty unreliable. I could imagine powering my home for about 3 days using one of these batteries after removing loads that were not critical. Personally if I had to choose between relying on a generator or a charged lithium battery for backup, I would take the battery. And no, a bank of wet cell lead acids of the same capacity would not be a suitable replacement. Besides the corrosive/explosive gases, those batteries self discharge at an unacceptable rate IMO. I think that there will be a market for Elon's batteries, I might even help a little! James

 

[OmCheeto]

Ah I see now, it's 350-400 volts...presuming for efficiency & reliability reasons (not bad if a few cells are poor).

This unit is getting a bit more impressive...wish there was more info on the batts though.

I was assuming Tesla was going to use the same technology as their vehicles. I don't know about the battery, but the cells used are currently made by Panasonic. And they have 19 different versions!

http://na.industrial.panasonic.com/search/18650


ps. interrupting me will only delay the solution to my problem.
pps. and then there's lunch... I'm shooting for 4 pm at the moment.

 

[nitsuj]

7KW of energy storage could be very interesting to people that live in Northern Climates. That is enough power to run an oil furnace or perhaps a wood pellet stove for an extended period of time in case a power loss and make the difference between freezing and not! In the event of an extreme winter with an extended power loss could even save some lives. Generators are another option but the beauty of a charged battery is that you don't have to get it started for it to start producing power and gasoline units especially can be pretty unreliable. I could imagine powering my home for about 3 days using one of these batteries after removing loads that were not critical. Personally if I had to choose between relying on a generator or a charged lithium battery for backup, I would take the battery. And no, a bank of wet cell lead acids of the same capacity would not be a suitable replacement. Besides the corrosive/explosive gases, those batteries self discharge at an unacceptable rate IMO. I think that there will be a market for Elon's batteries, I might even help a little! James

Yea...it's good for down to -20c, I presume damage after that.

I am unsure of Li-ion, but know for certain that li-poly cannot be "stored" charged for any significant period of time (days). Hopefully Li-Ion doesn't have the same constraint.

Calling this unit a possible life-saver is a bit of a stretch no?

 

[James Alton]

Yea...it's good for down to -20c, I presume damage after that.

I am unsure of Li-ion, but know for certain that li-poly cannot be "stored" charged for any significant period of time (days). Hopefully Li-Ion doesn't have the same constraint.

Calling this unit a possible life-saver is a bit of a stretch no?

Li-Poly can be stored charged long term but the batteries I am familiar with will suffer some loss of capacity. With Li-Poly, the rule of thumb I have seen is to store at about 50% of charge to maximize the life expectancy. Interestingly, Li-poly is damaged by freezing temperatures mores when the battery is fully charged, at 50% charge the safe temperature is a lot lower. I am just guessing but am pretty sure that the Tesla battery is not Li-poly due to the cells being less stable. Li-Ion that I am familiar with is of a lower power density but likes being kept fully charged. I think that the self-discharge of Li-Ion is higher unfortunately but much less than wet cell lead acid.

Definitely not a stretch to say that having power available to keep your heating system going in event of a power loss could be a life saving system IMO. Not only do you tend to lose power in an extreme storm, you can also lose access to transportation. Combine this will someone that is elderly, and yes, backup power could be pretty important I think. James

 

[nitsuj]

Li-Poly can be stored charged long term but the batteries I am familiar with will suffer some loss of capacity. With Li-Poly, the rule of thumb I have seen is to store at about 50% of charge to maximize the life expectancy. Interestingly, Li-poly is damaged by freezing temperatures mores when the battery is fully charged, at 50% charge the safe temperature is a lot lower. I am just guessing but am pretty sure that the Tesla battery is not Li-poly due to the cells being less stable. Li-Ion that I am familiar with is of a lower power density but likes being kept fully charged. I think that the self-discharge of Li-Ion is higher unfortunately but much less than wet cell lead acid.

Definitely not a stretch to say that having power available to keep your heating system going in event of a power loss could be a life saving system IMO. Not only do you tend to lose power in an extreme storm, you can also lose access to transportation. Combine this will someone that is elderly, and yes, backup power could be pretty important I think. James

I'm trying to find data on storage voltage being nominal cell voltage and that degradation is inevitable when "stored" at those "max" cell voltages. I am of course presuming the pack charges the cell to near full.

Battery university (website) indicates the chemistry maybe nickle/cobalt!

Lithium Nickel Cobalt Aluminum Oxide2 (NCA) LiNiCoAlO2
(9% Co) N/A Electric powertrain (Tesla Model S), grid storageI agree having heat when it's cold could be a life saver, whatever the means.

I've only found this (assertion) which I experienced with my use of Li-Poly batt. Though I had not ever noticed the batteries at an "elevated" temperature.

Formation of the electrolyte oxidation at the cathode that may lead to a sudden capacity loss. Keeping the cells at a high voltage and at an elevated temperature promotes this phenomenon.

 

[James Alton]

I'm trying to find data on storage voltage being nominal cell voltage and that degradation is inevitable when "stored" at those "max" cell voltages. I am of course presuming the pack charges the cell to near full.

Battery university (website) indicates the chemistry maybe nickle/cobalt!

Lithium Nickel Cobalt Aluminum Oxide2 (NCA) LiNiCoAlO2
(9% Co) N/A Electric powertrain (Tesla Model S), grid storageI agree having heat when it's cold could be a life saver, whatever the means.

I've only found this (assertion) which I experienced with my use of Li-Poly batt. Though I had not ever noticed the batteries at an "elevated" temperature.

Formation of the electrolyte oxidation at the cathode that may lead to a sudden capacity loss. Keeping the cells at a high voltage and at an elevated temperature promotes this phenomenon.

I don't know why Li-Polys suffer from being left fully charged but that position has been consistent over the years. I have probably used more than 500 Li- Poly batteries of various sizes, some with greater than 400 cycles and so far have never had one experience a sudden loss of capacity. The capacity loss seems to generally be age related and how many cycles the battery has on it. None of my experience may be applicable to the Tesla cells but I will say that I am a Lithium convert pretty much across the board. I can charge the Lithium batteries for instance for a tool and fully expect almost a full charge a year later when I use it. Or the fantastic lithium primary AA and AAA batteries with a 20 year shelf life and a much better power density than the alkalines…they also won't kill your electronic devices from leakage I am told..so far so good! They are also apparently environmentally friendly compared to most other chemistries I am told. James

 

[nitsuj]

I don't know why Li-Polys suffer from being left fully charged but that position has been consistent over the years. I have probably used more than 500 Li- Poly batteries of various sizes, some with greater than 400 cycles and so far have never had one experience a sudden loss of capacity. The capacity loss seems to generally be age related and how many cycles the battery has on it. None of my experience may be applicable to the Tesla cells but I will say that I am a Lithium convert pretty much across the board. I can charge the Lithium batteries for instance for a tool and fully expect almost a full charge a year later when I use it. Or the fantastic lithium primary AA and AAA batteries with a 20 year shelf life and a much better power density than the alkalines…they also won't kill your electronic devices from leakage I am told..so far so good! They are also apparently environmentally friendly compared to most other chemistries I am told. James

Hmm... I wonder why your experience with them is different. Mine is mostly with 5AH 11.1v (3 cell) simple "shrink wrap" type lipo. rated for 35-70c.

Worst treatment seemed to be from dropping the voltage to low during use. This continued into the most noticeable degradation of the batteries..."voltage sag". Powering "Off-road" type vehicles (bout 3.5kg) would draw over 1,000 watts, I think it's max is 12.6v * 35c * 5ah = 2,200w so less than 50% of an overstated "nominal" c value. That and would endure some heat (40c) and g-shock (crashes).

I've got one left of three, It has about 250 cycles I'd guess, and further guess its about 50-60% original capacity 'n holding voltage. That said it still charges to 4.18ish volts on all three cells which is surprising to me, the others had failures mostly attributable to dropping voltage too low during use (voltage sag) and in turn not "balancing" properly and having to "junk" it (I had luck removing the "bad" cell from one of the packs...the 2 cell pack was poor performing but lasted a while longer). With that dropping voltage low seems to heat the batteries more which I think indicates resistance increasing / cell damage??

The competitive advantage of a Powerwall maybe it's liquid cooling.

Oh and for the record I too have never had a "sudden lose of capacity". We'll outside of trying to "revive" a small 3 cell I had discharged to nill by accident. It's voltage never came back high enough for the charger to attempt a charge.

Oh and Power tools are generally iron type, it's safest (ask Apple about li-poly) and more important for our different experience is it's more reliable, though less capable of high amperage. I don't think Li-poly's are shipped air freight by most postal. I think currier's still do.

 

[James Alton]

Hmm... I wonder why your experience with them is different. Mine is mostly with 5AH 11.1v simple "shrink wrap" type lipo. rated for 35-70c.

Worst treatment seemed to be from dropping the voltage to low during use. This continued into the most noticeable degradation of the batteries..."voltage sag". Powering "Off-road" type vehicles (bout 3.5kg) would draw over 1,000 watts, I think it's max is 12.6v * 35c * 5ah = 2,200w so less than 50% of an overstated "nominal" c value. That and would endure some heat (40c) and g-shock (crashes).

I've got one left of three, It has about 250 cycles I'd guess, and further guess its about 50-60% original capacity 'n holding voltage. That said it still charges to 4.18ish volts on all three cells which is surprising to me, the others had failures mostly attributable to dropping voltage too low during use (voltage sag). With that dropping voltage low seems to heat the batteries more which I think indicates resistance increasing??

The competitive advantage of a Powerwall maybe it's liquid cooling.

Hello, My larger batteries are 8AH 6S being used in series giving 12S (around 50 volts) . I am not sure of the exact power since my tester pegs at 150 amps. I am very careful to avoid getting the cell voltage too low, normally finishing with at least 3.7volts per cell. I have some 5 year old Li-polys that are nearly as good as my new batteries in capacity that balance almost perfectly. I have had one pack get hot and puff up but 0 failures power failures to date. I think that you are right that dropping the cell voltage too low is probably the most damaging thing that you can do to these batteries. Even shorting them is apparently less of a problem since whatever contacts you had vaporize and the battery is still fine..don't ask me how I found that one out! (grin) I don't use multi cell batteries when any of the cells show voltages that are much lower. If I get a new battery and one cell won't balance properly I won't use it as is and sometimes remove the bad cell. I have found that generally the lower voltage in an unbalanced battery is due to a cell outgassing due to some rupture in the packaging so any attempt to rebalance does not help. If you open the outer packaging you can normally see a whitish powder where the leak is. Discharging to a low voltage also creates a lot of heating and if one cell is bad especially in the middle of the pack it's not good, especially if the pack is loaded heavily. My batteries normally finish even in the summer just slightly warm. James

Hmm... I wonder why your experience with them is different. Mine is mostly with 5AH 11.1v (3 cell) simple "shrink wrap" type lipo. rated for 35-70c.
In order to revive an overly discharged Li-poly, you have to use an alternate charging source ( I keep an old Triton for this purpose) to gently lift the voltage enough to reach the minimum cell voltage that the Lipo charger will begin charging at. I would always use the Lipo charger to do almost all of the recovery charging, it usually takes only a couple minutes on the Triton to get back up to minimum voltage. Needless to say, all overly discharged batteries should not be recharged and you need to be present to keep an eye on things. I have recovered the same TX battery that was run flat about 3X and while I am sure the capacity is down it still balances fine, no puffiness at all and it has been used for several additional years.

The early Lipos were pretty volatile but things seem to have changed a lot. Li-polys are now being used inside the cockpit of some manned aircraft. Would make me a little nervous but I have not yet heard of any disasters. I guess anything that has a high energy density can be somewhat risky...

Yes, I knew that the tool batteries were of the iron phosphate (A123 if I am not mistaken) type chemistry. Still good batteries. I really like the stable voltage they hold during discharge. I am wondering how the Lithium primaries such as the energizer provide 1.5v when Lithium cell voltages are higher. Are they actually putting a voltage regulator into the battery itself?

I hope that the Tesla Powerall concept does well. I glad to see the discussion and hope to learn more about them.

James

Worst treatment seemed to be from dropping the voltage to low during use. This continued into the most noticeable degradation of the batteries..."voltage sag". Powering "Off-road" type vehicles (bout 3.5kg) would draw over 1,000 watts, I think it's max is 12.6v * 35c * 5ah = 2,200w so less than 50% of an overstated "nominal" c value. That and would endure some heat (40c) and g-shock (crashes).

I've got one left of three, It has about 250 cycles I'd guess, and further guess its about 50-60% original capacity 'n holding voltage. That said it still charges to 4.18ish volts on all three cells which is surprising to me, the others had failures mostly attributable to dropping voltage too low during use (voltage sag) and in turn not "balancing" properly and having to "junk" it (I had luck removing the "bad" cell from one of the packs...the 2 cell pack was poor performing but lasted a while longer). With that dropping voltage low seems to heat the batteries more which I think indicates resistance increasing / cell damage??

The competitive advantage of a Powerwall maybe it's liquid cooling.

Oh and for the record I too have never had a "sudden lose of capacity". We'll outside of trying to "revive" a small 3 cell I had discharged to nill by accident. It's voltage never came back high enough for the charger to attempt a charge.

Oh and Power tools are generally iron type, it's safest (ask Apple about li-poly) and more important for our different experience is it's more reliable, though less capable of high amperage. I don't think Li-poly's are shipped air freight by most postal. I think currier's still do.

 

[nitsuj]

Hello, My larger batteries are 8AH 6S being used in series giving 12S (around 50 volts) . I am not sure of the exact power since my tester pegs at 150 amps. I am very careful to avoid getting the cell voltage too low, normally finishing with at least 3.7volts per cell. I have some 5 year old Li-polys that are nearly as good as my new batteries in capacity that balance almost perfectly. Discharging to a low voltage also creates a lot of heating and if one cell is bad especially in the middle of the pack it's not good, especially if the pack is loaded heavily. My batteries normally finish even in the summer just slightly warm. James

Oh wow those are some big cells! 50v 8ah in li-poly could produced mad power!

I've used the same cut off of 3.7-3.8v (rest) which apparently is rather kind to the batts. But during use have likely dipped bellow 3v at times. Sounds like you treat yours well, what are they powering?

I agree lipos were bad at first, and perhaps a bit had to do with the vastly different charging requirements to nicad and even nimh or whatever. i.e. most failures where likely over charging.

 

[James Alton]

Oh wow those are some big cells! 50v 8ah in li-poly could produced mad power!

I've used the same cut of of 3.7v (rest) which apparently is rather kind to the batts. But during use have likely dipped bellow 3v at times. Sounds like you treat yours well, what are they powering?

I hope that it is ok here to post a link? I did static tests at high power settings for pretty extended periods and my voltage held quite well on the cells I tested. It would probably be good to do some inflight monitoring to catch any packs with a weak cell early in the game.

Does anyone know if the Tesla powerall has any venting for cooling?

James

 

[nitsuj]

Does anyone know if the Tesla powerall has any venting for cooling?

James

Says "liquid thermal management" or something like that.

I cracked out laughing when you blipped the "throttle" at take off...WOW! Surprised the rest of the plane followed the motor lol Great Video

Have you measured the thrust of your set up? Yea if I had a rig like that you could bet there would be telemetry on pretty much everything.

 

[OmCheeto]

...
Outside temperature is 43°F ( 6.11 °C )
Specific heat capacity of my house is 5000 Btu/°F ( 2.6 kwh / °C )
R-value is 13 ( (2.3 m² °C) / watt )
Conductive surface area is 1900 ft² ( 176 m² )
If the house is heated to 75°F ( 23.9°C ) at 6 am, what will be the temperature in the house, 16 hours later?

And the answer is: 62.8 °F ( 17.1°C )
So one Powerwall unit will work for me.
Though, I'm retired, and don't get out much in the winter.
2 years ago, I would have laughed at heating my house to such a ridiculous temperature while I was at work all day.

I think I'll work on the heat capacity of an auxiliary water heater next.
I found one for $1,107.59 with a 105 gallon capacity. It has a 150°F maximum temperature.
Since it won't be hooked to the water mains, the associated plumbing shouldn't be too complicated, nor spendy.

I generally keep my house around 65°F in the winter.

And if it's less than 60°F ( 15.6 °C ) after 16 hours, how much water, heated to 150°F ( 65.6°C )would be required to maintain the minimum temperature at 60*F?

This shouldn't take too long to figure out.

105 gal * 8.34 lb / gal = 876 lb
1 btu = 1°F lb
ΔT water tank = 150°F - 65°F = 85°F
x btu = 876 lb * 85°F = 74,400 btu / 3400 btu/kwh = 21.8 kwh
ΔT interior exterior = 65°F - 43°F = 22°F
power loss at 22°F = 940 watts
21.8 kwh / .94 kw = 23.2 hours
required hours (off-peak): 16
7.8 hour surplus

hmmm...
That sounds ridiculously wrong.
Though, a 22°F ΔT isn't that much.

Reality check:
Ave temp for Dec 2014: 43°F
days measured: 34
actual kwh heating used: 1170 kwh
theoretical calculated: 770 kwh ( 0.94 kw * 24 hr/day * 34 days )

Not quite off by a factor of two. I'm guessing my R-value must be too high.

Anyways, this Powerwall thing should work for me.

Perhaps tomorrow I'll figure out the ROI on this new system.