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Building a Tesla inspired Powerwall for my reef.


1mp3r1al

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After attending the last WAMAS Annual 2018 Winter meeting, Julian Sprung talked about Disaster Planning. He mentioned he was planning to purchase a Tesla power wall for his home. After finding out from a WAMAS member there is a 5 year backorder, he seemed to have changed his mind.

 

I have built a Tesla inspired power wall solely dedicated for my reef using the same style batteries found in the Tesla power wall, but for a lot less. There is quite a bit of work involved in my project, so if you are a tinkerer as I am (we all own reef aquariums, so we are all kind of tinkerers right?), and looking for a battery backup system this may interest you.

 

You can view each video individually on my personal website, or watch on Youtube directly : https://www.youtube.com/channel/UCuYNu3VxihUFVfLRA4Ai-RA 

 

Episode 1 - http://iankreer.com/2017/building-a-tesla-inspired-powerwall-for-my-reef-episode-1/

 
 
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WOW!! Ian this is very impressive to say the least!! I may have missed it but what run time length will you get out of this battery pack from a full charge? Thanks for sharing and let me know when you go into sales production

 

Tom

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Thanks Tom!

 

I am still finishing the battery bank, so I haven't tested the total run length. My Apex tells me I use a total of 2.8 amp hours with the T5's, LEDS, Pump, both MP10's, and both 150 watt heaters on. 

The battery bank has a total of just under 1 Kilowatt hour (995aH). With these calculations and figuring 10% for loss, I would say I can power my reef for around 355ish hours with everything on including both heaters. Both heaters are rarely on at the same time, and just the 1 comes on around 6 times a day (winter).

 

995aH (total) % 2.8aH (tank) = 355 total tank run time. 355 % 24 hours in a day = 14.8 days total running time. This is when my bank is fully charged @ 29.4.

 

I hope this helps.

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This is some really high quality DIY.  Amazing that you're finding all these parts and getting it all together correctly by doing your research.

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Ian, what a great series! I mean, I'm impressed by the polish and clarity. 

 

However, something concerns me here in this quote:

 

 

I am still finishing the battery bank, so I haven't tested the total run length. My Apex tells me I use a total of 2.8 amp hours with the T5's, LEDS, Pump, both MP10's, and both 150 watt heaters on. 

The battery bank has a total of just under 1 Kilowatt hour (995aH). With these calculations and figuring 10% for loss, I would say I can power my reef for around 355ish hours with everything on including both heaters. Both heaters are rarely on at the same time, and just the 1 comes on around 6 times a day (winter).

 

995aH (total) % 2.8aH (tank) = 355 total tank run time. 355 % 24 hours in a day = 14.8 days total running time. This is when my bank is fully charged @ 29.4.

 

 

It seems that you're equating amp-hours to Watt-hours, and that's not quite right.  If you're drawing 2.8 amps to drive your tank, that's at 117 volts (RMS AC), consuming 2.8 A * 117 V RMS = 327.6 Watts. Over the course of 24 hours, that's 24 hrs * 327.6 W = 7,862.4 Whr, or 7.9 KWhr. 

 

Your batteries, on the other hand, give you approximately 1000 aHr at 4 volts, is that right? If so, that's 4KWhr of capacity to the lower discharge limit. 

 

Neglecting the DC-AC converson loss, you may be looking at roughly 2 hours of operating time for all those batteries.

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Ian, just adding to the math here. I wanted to dig deeper into 18650 cells that you're using. These appear to be 2600 mAHr cells. Nominal voltage is 3.7 VDC. And you're using 280 cells in your battery bank. Right?

 

So, 2.6 AHr * 3.7 VDC = 9.62 WHr per cell. For 280 cells, that's 280 * 9.62 WHr = 2693.6 WHr of capacity. So the cells can deliver 2.7 KW for 1 hour before they're exhausted (reaching the 3.3 VDC cutoff).

 

Calculating run time for this capacity for your tank's measured load: 2700 WHr / 327.6 W = 8.2 hours, neglecting conversion inefficiencies.

 

How did you get to the 995 AHr number that you used in your comment above?

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(edited)

I should have clarified, the 7S40P battery bank I made in the videos has 280 18650's with an mah rating of 2600 - 2240. Some batteries have 2600 highest, and some have 2240 lowest, so i am just going with all batteries have 2240 highest to even them all. I also have 7 other smaller battery packs I am including in series to the 7 packs you have seen in the video, and my total is 995aH after adding them in series. With the 7 packs of 40 each, my aH is 616ish.

Edited by 1mp3r1al
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Also I charge each of my battery packs of 40 to 4.18-4.19. I could charge all the way to 4.22, but I don't want to overcharge the lower mah batteries, so I keep them charged to 4.18-4.19. These LG batteries can be safely discharged to 3.00 - 3.10 without damaging. Also keep in mind the 7S40P bank is always topped up via solar.

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Be careful. If you start stacking in series, the maH ratings don't add. For example, two 3V cells with a rating of, say, 1000 mAH will give you 2000 mAH at 3 volts or 1000 mAH at 6 volts. It won't be 2000 mAH at 6 volts. Either way, it's going to be the same mWHr rating based upon the cell's energy capacity. You can't create additional energy capacity by connecting cells differently. The other concern for any cell is the power capacity. That is, the ability of the cell to dump energy very quickly without damage. There are often tradeoffs in the internal construction of a cell between power and energy capacity. I'm assuming that these cells can dump the amount of current that you're expecting from them rapidly without damage.

 

So, when doing a back-of-the-envelope estimate, you'll take a look at the total energy (e.g. mW-Hr or kWHr are convenient units) that one cell can provide multiply it by the number of cells that you have, and divide that by the energy requirement of the load.

 

You have 2.7 KWHr in 280 cells based on the published capacity of a single cell - your cells appear to be the ones with the higher capacity. Your load is 327.6 Watts. So, that load draws 0.3276 kWHr of energy in 1 hour.

 

Divide the two and you get a little over 8 hours of run time. Just my calculation. It's been a long time since I designed some sophisticated battery operated equipment but, when I did, energy and power density,and various exotic cell chemistries were in the trade space that I was researching.

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I should have clarified, the 7S40P battery bank I made in the videos has 280 18650's with an mah rating of 2600 - 2240. Some batteries have 2600 highest, and some have 2240 lowest, so i am just going with all batteries have 2240 highest to even them all. I also have 7 other smaller battery packs I am including in series to the 7 packs you have seen in the video, and my total is 995aH after adding them in series. With the 7 packs of 40 each, my aH is 616ish.

 

As expected, cells with 2240 mAHr of capacity will give you even less run time when compared with 2600 mAHr cells.

 

The 3.7 volt number is an average between the peak open cell voltage (of about 4 volts) and the knee in the discharge curve - typically around 3.3 volts before the cell voltage begins to drop quickly. You can't count on the open cell voltage of 4 volts to last very long. It'll begin falling off the moment you start pulling energy from them.

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(edited)

Be careful. If you start stacking in series, the maH ratings don't add. For example, two 3V cells with a rating of, say, 1000 mAH will give you 2000 mAH at 3 volts or 1000 mAH at 6 volts. It won't be 2000 mAH at 6 volts. Either way, it's going to be the same mWHr rating based upon the cell's energy capacity.

 

Not series, I meant I am adding them in parallel. I bought another 15 packs, which gave me an additional 300 batteries. I made another 7 packs of 20 and connected them in parallel, and plan on adding another 140 soon. 

 

I plan to test the bank this weekend when I have time. I will start the bank at 29.4, run the aquarium for 24 hours without charging and see how much the aquarium pulls from it.

Edited by 1mp3r1al
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Not series, I meant I am adding them in parallel. I bought another 15 packs, which gave me an additional 300 batteries. I made another 7 packs of 20 and connected them in parallel, and plan on adding another 140 soon. 

 

I plan to test the bank this weekend when I have time. I will start the bank at 29.4, run the aquarium for 24 hours without charging and see how much the aquarium pulls from it.

 

Series or parallel make no difference in terms of available energy. It does make a difference in terms of resistive losses through wiring. The point that I poorly made was that you have to be careful with how you consider applying the capacity numbers like mAH and open circuit voltages.

 

Doubling the numbers will get you more time, certainly. As you've noted, you'll have conversion losses. Your service time will depend a lot on the power profile and average power consumption. You'll likely see greater inefficiencies during high power usage because of the series resistance in each cell. If you can draw 24 hours, that would be great. If not, you may be able to cut out the stuff that you don't need to reduce power consumption. Ideas for this might include:

 

1) Don't back up power to the lights.

2) Cut the return pump if it makes sense and only run a few in-tank pumps for circulation. If you have heaters in your sump, this might be a problem.

3) Cut back on your heaters. Perhaps keep a backup heater hidden in the tank and set for a lower temperature - say 68 or 70 degrees - powered only by your backup system.

 

These sort of things can help anybody considering extending the service time of a battery-backup system.

 

Good luck with your test. If possible, you may want to monitor the temperature of the cells to make sure that the power draw doesn't lead to dangerously high temperatures and a possible fire risk. 

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Very interested in how the test goes. There seems to be a major difference in the way the OP and Oragimi are calculating the time that this will run the system. Interested to hear the results.

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