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 Privatteer » Sat Apr 13, 2013 5:42 pm

I'm assuming you still have a load attached which is the cause for the spikes in net amps?

Thanks for posting this info particularly the float volts it gives me an idea of what to aim for initially.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Gordon-Loomberah » Sat Apr 13, 2013 5:48 pm

Yes, there were 2 or 3 laptop computers, weather station, data logger, wireless router etc running, so the load does vary continuously.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Cherokee Solar » Sun Apr 14, 2013 8:51 am

Hi Gordon,

Interesting stuff. It will be interesting to see what, if any, difference the cooler temperatures of winter have on the batteries.

I realise it is still early days, but does the imbalance between cells mean that they have to get to Float or they become unbalanced?
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Gordon-Loomberah » Sun Apr 14, 2013 9:05 am

They just have to spend a bit of time at a suitable Absorb voltage, Float wont do anything much to balance the cells, as it isn't charging them.
Previously I found that 18 mins was sufficient time, I'm just trying to find out how much I can lower Vabsorb and still maintain balance without needing to increase time at Vabsorb.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby offgridQLD » Sun Apr 14, 2013 9:37 am

I'm just thinking your trying to correct a cell voltage difference at the top end.

But when dose this cell voltage gap start to form? Putting the BMS aside for a moment and make out you didn't have one. If the cells were all balanced to equal (capacity & voltage)

Then when charging the following day from say 25% DOD will the cell voltage difference only start to creep in when the cells reach a particular voltage?

Just trying to relate it to my own experience. If I get all my cells at the same voltage and watch them charging all cells stay spot on in balance for the bulk of the charge but as you start to go up the steep end of the charge curve things start to drift a little. I just experimented to find the voltage point were things started to drift and set that as my max charge voltage.

I'm not getting 100% capacity perhaps only 80% but they just don't get the chance to drift apart.

It would be great to see a log of say the best and worst cell in your pack showing the point where things start to drift.

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

Postby Gordon-Loomberah » Sun Apr 14, 2013 9:51 am

Yes, it is a near-top balancing that I am doing, not at a particularly high voltage. The one cell pair that was low had previously been the pair that went high first, and it was still low at Vfloat. If the cirrus doesn't get too thick I'll see how it goes today at 3.63V/cell, I wasn't near the batteries at the end of Absorb yesterday, so missed my chance to see how it went.

I haven't yet made up the necessary wiring harness for the cell logger (with such short leads they are clearly designed for compact batteries), but will one of these days when I have some spare time... ;)
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby offgridQLD » Sun Apr 14, 2013 12:29 pm

Yes with the cell log you wont find a 3m long harness off the shelf for them due to there original design intent . It's not to hard to lengthen the now 20cm? wires to the required length.I would just cut all the extension wires the same length say 2m long so they all have the same resistance. Nothing a bit of heat shrink and 10 min with the soldering iron wouldn't sort out.

But if your anything like me I have a gazillion little 10 min jobs that just always get put aside.

My corrosion issue looks like its creeping back on some of the terminals that exide reconditioned 6 months ago. I don't think I will take them up on there offer to replace them at 1/3 RRP as I just don't want to invest any more into a dead technology. Performance hasn't deteriorated from what I can make out so I will just see how they go. Great if they last another 5 years or so until we move in permanently as they will be ten years old by then. If they come up short and by the look of it they will. Then its 400AH of lithium at 52v for sure.

Kurt
Last edited by offgridQLD on Sun Apr 14, 2013 2:18 pm, edited 2 times in total.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby Privatteer » Sun Apr 28, 2013 6:13 pm

offgridQLD wrote:I'm just thinking your trying to correct a cell voltage difference at the top end.


What I am finding is I have one "weak" cell that lags behind the others in charge and as such does not get a full charge at "float" (55V). It was about 150mV behind the others. So it was sitting at 3.3V while the others were 3.45V(+/-7mV) and it stayed that way even after several hours.
Might not sound like much but it hit 2.9V during discharge while the others were all above 3.2V still. By some rough calcs it should have only been at 50% discharge.

These had all been balanced+charged in parallel before use.

Like Gordon I'm now trying some time at 3.6v and it has closed the gap. Hopefully the cell is not a dud I will be watching it tonight.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby davidg » Sun Apr 28, 2013 9:10 pm

I'm not as good with Lithium as I am with lead acid.
So I thought I would ask the magic question/s and see what others might suggest.
If I wanted a battery pack in Lithium at a nominal voltage of 48V give or take and I needed say 8-9Kwh hours of discharge capacity per day for one day and also for two days what size packs might be suggested and the suggested DOD etc that might be expected along with estimated total number of cycles to that DOD.

Just to got to see how far out my estimates are. Purely hypothetical of course.
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Re: Off-Grid: AC-what usable % of DC energy, now with Lithium!

Postby offgridQLD » Sun Apr 28, 2013 9:36 pm

16 x lifepo4 cells nominal voltage of 3.3v each is typically what's used on a 48v system. With no input charge (all loads covered by the battery) 20kwh pack size (400ah cells) could handle 8-9kwh and once in a while do it twice but it would be near on depleted doing it twice.

Though I'm not sure in your question if your 8-9kwh load is covered just from the battery's (overnight load) or if that is the 24hr load. Like in a typical off grid application where a good portion of the 24hr load would be covered by the charger - PV.

Then you say cover the load for two days. Is this two days without any charge input?

The thing with lithium is it works well with a small capacity battery and high output charger (large PV system). You don't need a large cell to make up for low C rating in both charging and discharging like you do with led acid.

So your question is a little open as we don't know what the application is and what the chargers potential is?
Last edited by offgridQLD on Sun Apr 28, 2013 9:42 pm, edited 1 time in total.
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