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 » Sun Apr 28, 2013 9:41 pm

Hypothetically speaking...
For 1 day in reserve @ 8-9kWh/day I'd say 300AH
For 2 days @ 8-9kWh I'd suggest 600AH, nominal voltage will be ~53V for 16 cells.

I've assumed ~96% efficiency for charging and ~85% overall inverter efficiency, min SOC ~30%

Cycles to this depth... good question, but I would guess somewhere in the range of 3000-5000, if manufacturers numbers are to be beleived.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby davidg » Sun Apr 28, 2013 9:50 pm

Gordon-Loomberah wrote:nominal voltage will be ~53V for 16 cells.
I've assumed ~96% efficiency for charging and ~85% overall inverter efficiency, min SOC ~30%


Voltage looks to me about right that's about where I figured, is that down to 30% SOC left in the pack? 70% DOD?
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Gordon-Loomberah » Sun Apr 28, 2013 10:03 pm

Yes 30%SOC = 70%DOD... only slightly rounded to fit the standard 300AH cell size. 600AH could be with 2 X 300AH set up as cell pairs like I have done (with 400AHs), or by using a 600AH (not sure if available) or 700AH which is available, at least from Winston.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby offgridQLD » Mon Apr 29, 2013 2:25 pm

My understand was that Winston battery filed for bankruptcy.You can still get the cells though so I'm not 100% sure what happened . Some googling would dig up the answer.



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

Postby Privatteer » Mon May 06, 2013 1:55 am

Don't know how Gordon has gone with his testing but currently I have ended up with:

Equalise daily 3.68V 15min.
I initially had this lower but I have one problem cell as mentioned before. At lower levels it was drifting down and not getting a full charge. I started at 3.625V and worked my way up.
I even removed this cell again and externally charged it. Has the right capacity to rough tests but seems to lag behind others in series.

Absorb 10m 3.5v (counter starts once volts reached)
Float 3.4v +/-0.02A Cancel after 30min below float voltage and returns to absorb.

Reasoning behind the equalise is in cloudy weather a couple absorb cycles can occur and I did not think it necessary to "balance" quite that often. Equalise might even get moved out to 2-3 days apart once I am sure the cells are going to play nice together.

When going from below Float to Absorb the MPPT throws everything its got at the batteries. Once its hits 3.5v current quickly tappers off, was 0.10A at 10min.

I will also mention I found cell-log 8s cable 100cm
here
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby offgridQLD » Mon May 06, 2013 7:30 am

All interesting data. 3.4v is the true fully voltage(off the charger) for lifpo4. So your float voltage sounds good.

I would still prefer a balancing system started to balance at a lower voltage and with a little more grunt.

I would be keeping a check on that cell as you are . Bad week cells more often than not show up from the get go with lithium rather than becoming bad.

Nice find on the 1m long cell log8 harness. :D Now it will only be 50mm to short :lol:

Sorry if I missed it before what cells are you using and configuration AH and system voltage and your BMS - balancing system?

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

Postby Gordon-Loomberah » Mon May 06, 2013 8:23 am

I'm currently doing a 24min Absorb at 3.65V, and Float at 3.41V. I often check the cells during Absorb, and I too currently have one that is usually a bit lower- at worst up to 0.14V lower than the highest cell at the start of absorb, but usually not quite that low. If there is still over 0.05V range near the end of Absorb (if I happen to be checking) I'll give them another 6 mins Absorb time.
Previously I had one high cell, but after a while it came back, and a different one was high instead, but now it has settled back, and I've had the low one for a couple of weeks.
I have the FM80s set to re-Absorb if the voltage drops to 26.3 (3.29V/cell), which it almost never does if there is a reasonable amount of charging, even when I have some big loads.

Privatteer, what C% does your current drop to by the end of your absorb and EQ times? With my 800AH I usually get down to 3-5A, ie ~0.4-0.6% after 24 mins Absorb.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Privatteer » Mon May 06, 2013 9:17 am

Equalise was still pushing 2amp. However as mentioned after 10min at the absorb current was down to 0.1Amp.
Winston WB-LYP40AHA 3.2v nominal. 2.5v min 4v max. 48v config 16 cells. Its only a small system running dedicated load as testbed.
Using the 800mA cell protection from evpower. BMS is a custom picaxe board. I would also prefer if thr balance units kicked in earlier but I did not like the resistive ones.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Gordon-Loomberah » Mon May 06, 2013 11:53 am

A short while ago I checked my cells, once only a minute after Absorb started and there was a 0.06V range, and then 21min into Absorb it was down to 0.03V: 3.63 to 3.66V. That was after a 33% discharge since the sun went behind the hill about 3:30pm yesterday.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby offgridQLD » Mon May 06, 2013 12:18 pm

30mv Sounds good to me Gordon .

using a cell log 8 to measure the balance variation of a factory matched 5S lithium 5ah battery pack.This is After balancing on a dedicated balance charger I usually see around 6-12 mv variation.

Some of my old packs are 30 - 50mv out if you treat them bad . Apply a small load to zap the surface charge off them and they are come iinto line better.

I'm totally sick of seeing big KWh numbers being fed into my led acid to recover from a relatively small consumption totally inefficient.

I just hit float about 5 min ago (little cloudy) and pumped 5.2kw into them just to recover from the 18hrs of loads from 6pm yesterday until now (wasn't even there) just fridge and modem a few system running loads . That's around 2kwh used hmm Hungry buggers :lol:

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