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Mackay's Marvelous DIY Powerwall
#11
http://windandsunpower.com/Download/Lead...ciency.pdf


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#12
Let's say you want to use 1kw to charge. Just by having the pip connected, you waste 50w. So the actual consumption is 1050w.

Let's assume the charger is 90% efficient. Your 1000w is now 900w.
Your batteries store 50% of that 900w. It's now 450w.
When you run from the batteries, you again pay a 50w penalty. You now have 400w.
The inverter is about 90% efficient. It's now 360w.

1050w in, 360w out. 34% efficiency.

That's assuming no large loads which will give a much worse result. Also ignored is the life used up in the batteries and inverter, which typically costs between 10c and 15c per kwh.
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#13
DIY TESLA POWER WALL Wrote:Let's say you want to use 1kw to charge. Just by having the pip connected, you waste 50w. So the actual consumption is 1050w.

Let's assume the charger is 90% efficient. Your 1000w is now 900w.
Your batteries store 50% of that 900w. It's now 450w.
When you run from the batteries, you again pay a 50w penalty. You now have 400w.
The inverter is about 90% efficient. It's now 360w.

1050w in, 360w out. 34% efficiency.

That's assuming no large loads which will give a much worse result. Also ignored is the life used up in the batteries and inverter, which typically costs between 10c and 15c per kwh.

I think you will get your point across better if you used units of measure that are relative to efficiency. By just using 'w" (watts) you are just representing energy at a single point of time.

You must use energy over time to get any real meaning. That's either in joules, coulombs, Ah or Kwh. Kwh is a probably the most accepted term (in this application) as it represents total energy over time. Ah doesn't take into account variations in voltage so should not be used in this application.

Efficiency (in this application) should be measured by calculating total energy in divided by total energy out, over a specific period of time. For example, total energy input into the system (from solar panels) over say, 16 hours divided by total energy taken from the system (output of inverter) over say, 8 hours.

To be honest, your example above is not a real calculation of efficiency and is very misleading.
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#14
If it makes you more comfortable, add the letter h after w for each number. Ie, 1hr charge and 1hr discharge. Semantics. None of those figures are time dependant so they stand regardless of whether the letter h follows them or not.

"very misleading"

That's a statement you can only make when you have equally qualified evidence to the contrary. But you don't. And here's why. All of these steps are less than 100% efficient. Do you disagree?
The input number for each gets progressively lower. Agree?
The overall efficiency is the product of each individual inefficiency (disregarding inverter self consumption) multiplied. Agree?

Overall efficiency = charger efficiency x battery efficiency x inverter efficiency
= 90% * 50% * 90%
= 0.9 * 0.5 * 0.9
= 0.405

But we have not allowed for inverter self consumption, so that will account for the slight difference in the examples. So please provide evidence that it is "very misleading"
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#15
DIY TESLA POWER WALL Wrote:If it makes you more comfortable, add the letter h after w for each number. Ie, 1hr charge and 1hr discharge. Semantics. None of those figures are time dependant so they stand regardless of whether the letter h follows them or not.

"very misleading"

That's a statement you can only make when you have equally qualified evidence to the contrary. But you don't. And here's why. All of these steps are less than 100% efficient. Do you disagree?
The input number for each gets progressively lower. Agree?
The overall efficiency is the product of each individual inefficiency (disregarding inverter self consumption) multiplied. Agree?

Overall efficiency = charger efficiency x battery efficiency x inverter efficiency
= 90% * 50% * 90%
= 0.9 * 0.5 * 0.9
= 0.405

But we have not allowed for inverter self consumption, so that will account for the slight difference in the examples. So please provide evidence that it is "very misleading"

OK, first up. Chill out a bit. I was only offering advice, not having a go at you.

Secondly, I didn't disagree with anything you posted - I just said meant that the way you presented it was "very misleading". Perhaps a better way to state your point would be to provide real evidence of efficiency rating of some devices currently on the market. To simply state that a battery is 50% efficient gives the impression is that it's a fact of life. That figure is true for a battery in bad shape, however I have test data that proves good quality Li-ion cells can be around 85%-95% efficient.
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#16
We are not discussing Li-ion, he is using lead acid (temporarily). So that's where the miscommunication is. All good. Even so, the math still would look terrible using Lithium.

Anyway, hopefully some reading this will attempt to do some math before committing vast amounts of time and money into a flawed power system.
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#17
DIY TESLA POWER WALL Wrote:We are not discussing Li-ion, he is using lead acid (temporarily). So that's where the miscommunication is. All good. Even so, the math still would look terrible using Lithium.

Anyway, hopefully some reading this will attempt to do some math before committing vast amounts of time and money into a flawed power system.

OK, DIY TPW you got me there. I missed that he was using AGM until he gets his Li-ion bank up and running. Also have to agree, AGM is not the best type of battery for a solar setup. They are more suited to emergency back-up installations. Battery companies like them because they are cheaper to make, don't last as long as other types of batteries (have to buy more batteries) and they're easier to transport than FLA.

However, you can make them perform better and last longer if you treat them nice. The biggest issue these batteries have is not sulfation, it's heat. Heat from either charging or discharging them too quickly. Anytime you are heating them you are going to be off-gassing and evaporating electrolyte that cannot be replaced. Once they dry out, they're useless!

So looking more at Mackay's proposed setup, I agree with @DIY TESLA POWER WALL here. It's not going to be practical to charge these up with off-peak grid power and then drain them with a pool pump. If you don't want to kill those batteries and you want to improve your efficiency, discharge them at around the C12 rate. So if they are 125Ah, that's 125/12 which is around 10A. Since your running at 48V (50V nominal), that's a 500W load - which is probably a lot less than what your pool pump will draw but probably OK to run a PC for 5-6 hours a night. (Insert aforementioned inefficiencies here Smile )

Same goes for charging them, if you charge them too fast you will kill them quickly. Again if you limit the charging current to around C12 (~10A) you'll be topping them up gently which will increase the battery life and improve the efficiency. (C12 charge/discharge works well for a 24 hour cycle as well)

As for your pool pump, I would just run it from off-peak grid power. 8kWh seems to be very high, you must have a massive pool or you're running it longer than needed. Just for comparison, I have a 750W pool pump that only runs for an hour/day over winter (0.75kWh/day) and 4 hours/day (3kWh/day) during the summer swimming season. I use a timer to run it when my small 1.6kWh grid-tie solar system is putting out the most in the middle of the day.

Anyway, hope this helps
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