Off-Grid: AC-what usable % of DC energy, now with Lithium!

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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Gordon-Loomberah » Sat Mar 09, 2013 10:38 am

Gordon-Loomberah wrote:I dont know if the Lithium batteries will show any reduced capacity at colder temps, but haven't read anything to that effect.


The good news is that Li cells are only very slightly affected by cold temps, I read the other day that at about -18C, Lead-acid is down to 50% available capacity, but Li only drops by 9%. I'm not likely to see temps that low here, so even on a frosty -5C morning, which I haven't seen for many years here, I'll still have close to 100% rated capacity.

Another point is that there is a danger of freezing the electrolyte in Lead-acid batteries at very low temperatures when they are partially discharged and have a low SG, ie, it's mosty water, but no danger of that with Lithiums.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby offgridQLD » Sat Mar 09, 2013 5:22 pm

Great data Gordon. Its good to see they are living up to there expectations.

With lead acid battery's part of the need to have larger capacity is just to cover for there relatively pore C rating of led acid compared to lithium's.

Take a 800AH lithium bank and a 800ah led acid bank at 24v and try running the same 3000W load and watch the voltage sag on the led acid.You just cant get a way with small led battery's and large loads even if the cycle life was identical.

I would encourage anyone who wants to get a feel for lithium's to purchase a small bank to play with even a 20 - 60 Ah 12v pack for a camper van or car project. Its a real eye opener and you wont regret it.

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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Tracker » Sat Mar 09, 2013 6:29 pm

Gordon-Loomberah wrote:..... they specify: "Cycle life: 2000+ to 80% DOD, 3000+ to 70% DOD, measured, not just claimed", so I would estimate something in the 4000-5000 range of cycles to 50% DOD.....

So in summary, what you are declaring is that you really must consider designing for a 40 to (60???)% dod, in order to get the best service life from the Li. Batteries...
Correct me if I'm wrong.... a 20% dod, is bad for them, also..? so the ultimate life curve, sounds a bit like the old bell curve..

My existing "SOLAR ASSISTED" system uses 6units a day (during 10AM to 8PM), and so the ideal would be a 12unit system = 250AH @ 48V...
I suspect that I will try for a 400AH one and thus I could double it to 800AH when I went Off-Grid..
We really bashed the existing system yesterday with the RAC, and consumed 16Units from the solar..

It sounds stupid, but I can't wait for my Pb's to die.. but for study reasons I will just have to wait..
The existing.. Pb.. is 600AH...
..
.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Gordon-Loomberah » Sat Mar 09, 2013 8:36 pm

Tracker wrote:So in summary, what you are declaring is that you really must consider designing for a 40 to (60???)% dod, in order to get the best service life from the Li. Batteries...


I'm not declaring anything like that at all! LiFePO4 chemistry isn't really bothered by lower %DOD cycling- the cells wont suffer stratification and sulphation like Lead-acid batteries. I suspect the 70 and 80%DOD cycle numbers are given in the specifications because the batteries are primarily destined to be used in EVs, at least that is their primary market. 20% DODs are not a problem, and much of the past few months here has seen 15-20% DODs overnight.

I sized my batteries to cater for cloudy weather periods and also for a large range of DODs that are the norm here. Anything from 10-15% DOD overnight if I'm not using much power, to several cloudy days taking them down to 70 or 80%DOD, and then summer, when I might use more than 100% of their capacity for a few days in a row when processing fruit. Big power use, high current discharges, but supported by ~4kW of PV, they are the ideal battery for the job.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby offgridQLD » Sun Mar 10, 2013 8:45 am

When you go lithium you basically throw away the led acid rule book.

One of the biggest advantages of lithium is you can stop all the worry regarding anything thing less than 100% SOC as evil place to be. It's surprising how many idiosyncrasy there is with led its like a black art keeping them healthy.

Just as a example think of your consumer goods like mobile phones. How much care and love do you give the lithium battery in it. I only charge my phone when its almost flat.When you think about it. It gets charged at a very high C rate and at times discharged at a very high C rate considering its small capacity Yet mine is almost 5 years old and going strong despite the quite hard life.

Can you imagine trying to run your mobile phone from a small led acid battery Other than the size / weight issue. Having to charge your mobile every time it was less than 30% DOD and having to do it every day or risk insulation. Topping up the the electrolyte in your phone all all the other nonsense.

Regarding cycle life there are a few people on the electric bike forums I have been a member of for about 7 years. Some members living in the city have done away with there cars for some time now. There is a few examples where they ride to work each day and the the small lithium battery receives a 80% DOD twice a day they also use it over the weekend and after work. I can think of one guy who has been doing this for 5 years and the battery's are working fine with close to 4000 cycles on then. (ebike's abuses the battery more than a off grid home ever would 1c charging rate and 1-2c discharging rate)

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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Cherokee Solar » Sun Mar 10, 2013 2:09 pm

offgridQLD wrote:One of the biggest advantages of lithium is you can stop all the worry regarding anything thing less than 100% SOC as evil place to be. It's surprising how many idiosyncrasy there is with led its like a black art keeping them healthy.


Ha! Lead acid batteries are a great technology - well, as long as you don't need to use them! hehe! :D
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Gordon-Loomberah » Sun Mar 10, 2013 2:17 pm

Cherokee Solar wrote:Ha! Lead acid batteries are a great technology ... :D


Perhaps for the 20th century, but we've moved on from there now :lol:
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Gordon-Loomberah » Thu Mar 28, 2013 1:21 pm

After a bit over a week of having all my DC loads running through the load shunt, rather than only logging the inverter input, it appears that the charging efficiency of the LiFePO4s is very close to 96% :!: That's a little better than I was hoping for- these batteries are advertised at 95%.

It's clear that a bit extra Absorb time needs to be added (I'm using a bit under 20 mins generally) after a couple of days of not reaching full charge, as cell balance tends to drift out a bit and not get back to normal with those short Absorb times. The cells get to within 0.1V, but a second 20 minute Absorb brings them back to under 0.03V range.
Once they are nicely balanced the end amps comes down too, to about 2.5A, as seen in the below graph at the end of Absorb today. 2.5/800 = 0.3%

EndAmps20130328.gif
click to de-fuzz


The sawtooth pattern of the Net Amps curve seems to be related to the behaviour of the FM80 at higher step down ratios. It is much smoother at 60->28V cf 85or90->28V, but the 1kW test array in this test will be 2kW operating at more like 45V once mounted on the new alt-azimuth tracking mount
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Privatteer » Thu Mar 28, 2013 1:51 pm

Was looking at some of the li batteries and they claimed 5000 cycles to 70%. At one cycle a day even its over 10 years. Good enough for me.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby davidg » Sat Mar 30, 2013 1:41 pm

Gordon, here is a long question, primarily it is around lithium but I have mixed in other questions with it, to form a more complete understanding. I'm sure you have been asked something along these lines someplace, either I can't find it or I have not really looked properly. Some of it I know is here.

What size bank do you have and what would the typicla number of kWh per day used and yet another what is the numbe of no sun days you target or have acheived, yes I'm aware that here is no such thing a no sun day but there are days where ytou only get a small amount, which leads to the next question and that is what would you consider to be the worst amount of energy you have derived from a no sun day and how many days in a row would that likely to be. :?:
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