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Mackay's Marvelous DIY Powerwall
#1
Greetings,

Thought I would start a thread for my own build progress.

As anyone who has done this before, or has started doing this knows, this is a long process. I started on 23rd September dismantling my first laptop battery. Now on 8th October I have 14 laptop batteries left to dismantle (plenty more to come) and have salvaged 204 cells from 40 laptop batteries. Out of those cells 39 have been capacity tested and recharged and are labelled ready for service (RFS). I have started using some of the lower capacity cells to put some charge in to the cells that read below 3v so the charger will charge them.

Still waiting on my TP4056's to arrive (25 of them), 25 x 4 cell battery holders, the plastic holding thingies hat I ordered on 17th September and was only told yesterday that there has been industrial action in the postal service and as such have been delayed significantly...awesome, 3d printing it is. The PIP4048MSE will arrive next week so I am very much looking forward to that!

In order to get this project off the ground relatively quickly, I acquired some 6 volt 130Ah AGM batteries from work. They are reclaimed but still tested to about 125Ah capacity. The idea is to hook them up to the PIP4048MSE so I can run my PC's and pool pump off the batteries during the day and recharge at night off peak. The pool pump uses about 8Kwh per day so the more I can use off peak for that the better, plus I can add solar panels when I get them to run it off grid as much as possible.

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#2
You are probably not saving any money charging from off peak. Agm batteries retain about 70% of the energy put into them. And if your load current is high you will only get 35-50% as high discharge current depletes the battery faster
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#3
looking good so far I'm going to try to talk on here some times will this site work on a phone?
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#4
It seems to work on a phone. I am currently using an iPhone to check all of this goodness out.
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#5
phillip20 Wrote:looking good so far I'm going to try to talk on here some times will this site work on a phone?

Seems to be in my tests.
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#6
DIY TESLA POWER WALL Wrote:You are probably not saving any money charging from off peak. Agm batteries retain about 70% of the energy put into them. And if your load current is high you will only get 35-50% as high discharge current depletes the battery faster

Not sure where you got your statistics from? If I put 100Ah in to a battery I can take 100Ah back out, assuming the batteries are still decent. A poor quality or near end of life battery may be different. It may also happen if you try and charge the batteries at more than 10% of their rated Ah. EG if you have a 100Ah battery and you charge it at any more than 10A.

In my case I have 6v 130Ah batteries x 8, but they are actually 2v 130Ah cells but each battery has 3 2v cells in them. So if I take my worse case scenario load of 4kw from my inverter, at 48v that will equal 83A draw from the batteries (assuming 100% efficiency). That means only 10.4A per 6v battery or 3.5A per 2v cell. That is only 2.7% of their rated capacity.

Obviously nothing is 100% efficient but at least that gives plenty of wiggle room, and I won't be using the inverter at 4kw anyway, will probably max out at about 2.5kw.

I may not save any money recharging in off peak over the long run but at least my PC will have clean power all the time, and won't rely on a UPS to switch over in the event of a power failure. And this setup will allow the connection of solar panels as I get them.
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#7
Simple logic. If charge voltage is ALWAYS HIGHER than resting voltage, and even if amps in = amps out (which it doesn't) then how would it be possible to get 100%?

Input power, consists of volts x amps. Let's do volts first.

Charge voltage = 14.4
Resting voltage = 12.85

12.85/14.4 = 89.2%

But it doesn't stay at 12.85 does it? Even at 100% full, put a large load on it and the voltage falls to say 11.2v. Now it's 11.2/14.4 = 77.7%. Or, go down in state of charge.

But remember, I am still assuming amps in = amps out. But that doesn't happen. Typically you get 80 out for every 100 in. Its just fact.

So let's adjust our results:

89.2 x 0.8 = 71.3%
77.7 x 0.8 = 62.1%

Here's a scientific paper showing 71%
http://www.battcon.com/PapersFinal2013/1...Energy.pdf
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#8
14.4 is not the charge voltage of AGM, 13.8v is. 14.4v is used if you are trying to clear it of sulphation. In some cases the charge voltage can go as high as 14v in certain temperature situations but 13.8v is the nominal charge voltage.

AGM's aren't designed for high current loads, unless they have been specifically designed for that. Like I said you shouldn't normally exceed 10% of the Ah rating for both charging and discharging. The battery voltage under normal load conditions will sit around the 12.5v mark and will stay that way until it starts to get low in capacity. To keep your batteries in good condition and to last a lot longer you shouldn't let them get below about 12.2v but at the most 12v. So let's say thats the 70% discharge mark (which is about accurate in my tests of the batteries I have). If the batteries reach 12.2v (or 48.8 in my system) then it will kick the charger in to recharge them.

A full AGM battery will rest at 12.8v with no load, it will drop to 12.5v under 'normal' loads. It will stay at 12.5v for the first ~70% of it's capacity.
A fully discharged AGM under 'normal' load will sit at 11.6v (will bounce back to 12v with no load in most cases, unless the capacity of the battery has been diminished)

Your logic doesn't take in to account the battery resting at 12.5v under load for ~70% of the time.

PS That link doesn't work for me, 404 error.
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#9
Lots of quoting voltages for each state of charge, but no actual rebuttal nor evidence to back up your statements.

ShaneE Wrote:If I put 100Ah in to a battery I can take 100Ah back out

Says who? Please provide evidence. Energy is not amps. Amps in is not amps out.
I know this first hand. I had a coulumb meter running on my agm bank. Every day, without fail, the soc (which is amps out vs amps in) would reach 100% long before the charge controllers reduced the current to a trickle. Therefore, when the amps out were replaced by an equal number of amps in, the batteries were still not charged and more amps were required. Thus proving that amps in does not equal amps out. I searched Google high and low, and was unable to support your claim that amps in equals amps out. So you are on your own there.

ShaneE Wrote:at 48v that will equal 83A draw from the batteries (assuming 100% efficiency). That means only 10.4A per 6v battery

What? If it's 83A, and you have 8 batteries in series, then each battery (and each cell) has a flow of 83A. Show the math giving you 10.4.
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#10
ShaneE Wrote:Your logic doesn't take in to account the battery resting at 12.5v under load for ~70% of the time

That's because logic doesn't factor in fairy tales. How about we factor in 90% efficiency on the inverter? Now your 83A is 92A. And I assure you, at 92A you do not have a voltage of 48v. More like 45. So now your current needs to be 98A
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