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LiFePO4 (LFP) bank - a work in progress

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rossw:
Some of you will have been following my trials and tribulations of batteries, and may remember my first ever bank.
These were a batch of basically freebie AGM cells. I had no way to monitor them, no way to really charge them properly, they were originally 300AH cells but by the time I got them, who knows...


They died a premature death when the contractors building the place drained them absolutely dead flat and left them that way while I was away. NOT IMPRESSED.

Unable to find replacements, I limped along on some flooded 220AH cells for a while, but they really were not up to the job asked of them.


Then I got onto some 500AH AGM cells - again, used but at a price I could afford (and I needed something that could hold up a bit better than the trojans were)


Almost 4 years on, several of the 500AH cells were getting a bit soft, and while trying to find some replacements, I was offered 50 (again, used and removed from service) for a price that was quite attractive. So I grabbed 'em


And after some time to tickle the old cells back, figured I may as well put them back into service


So now lets move on 4 more years. My love of AGMs has waned. Two broken shoulders (at different times) and I really don't enjoy lifting them (at 34kg / 75 lbs each), and with the extra power demands of more equipment, regular use of lathe, welder, grinders and other power tools, combined with poor monitoring of the banks and insufficient charge to regularly float them, they've been getting to be a problem. I'm over splurging $500 here and there to pick up some replacement cells and replace the dying ones. Worse, the rate of death has been increasing dramatically. I stopped buying cells and cannibalized the oldest bank, taking the best cells to prop up the other banks. When I ran out of them, I had to ditch the second bank to keep the primary bank operational. Of course, this reduced my capacity back to only 500AH (if the cells were all new and in good condition, and neither of those applied!). With a typical overnight demand of 150AH, that was hammering the cells pretty hard. They could manage it if I'd had a really good day, but introduce cloud and they were really struggling. The generator was running at least morning and evening, and more often than not once in the middle of the night.

It was way past time to take the plunge and get something different. Over the years I've looked at NiFe, and discarded them (too many reasons to go into here). I was sorely tempted to get a decent pack from a forklift, but they've got other problems.

So in the end it's come down to LFP (Lithium Iron Phosphate) as a cell with decent life, suitable voltage characteristics, manageable charging regime, reasonably resistant to abuse and "safe enough" to put inside the house.

So I decided initially on 300AH cells. They claim their 2000+ cycle life at 85% depth of discharge. I wanted to be able to handle 200AH overnight,  which is only 66% (so should lengthen the life). 200AH is equivalent to only discharging my old lead-acid cells 20% (for the 1000AH bank) or 40% (for 500AH bank). First pleasant surprise was when they turned up. It took me 6 trips from the delivery van, each time carrying less weight than one of the old 500AH cells. These suckers are SMALL and LIGHT compared to the AGMs!


Each cell is light enough to carry with one hand, and small enough to be "cute"!


There was a miscommunication about the cell interconnects, and they sent the wrong ones, so I had to wait on replacements. They duly arrived (they are about 2mm thick x 45mm wide - 90 sq mm CSA, or roughly equivalent to 000 wire). I quickly modified each with a couple of M5 bolts to attach the battery monitoring and cell equalizing equipment in a manner that would not require taking the bank out of service to do so.


So the whole new bank takes up half the height and half the depth as the AGMs that it replaces, yet has twice the usable capacity in 1/4 the space! It also only weighs 172Kg (380 lbs) compared to 816Kg (1800 lbs) for EACH BANK in AGMs!
This picture looks pretty messy, because the battery monitors are only temporarily connected, and the cell equalizing is just dropped in place, nothing has been tied up neatly yet... but here it is!


Before you ask - yes, there is a lot of space behind and in front of each cell. The plan is that when I save up my pennies, I can buy another 16 cells and put them cell-by-cell beside the existing bank to double the capacity.

So far, these are performing absolutely brilliantly! Better than I'd hoped.

rossw:
Addendum: we had sun yesterday, but not enough to get the cells fully charged. Woke up to 100% overcast day today. The batteries were holding well, but we had a couple of loads of washing to do so I thought there was no point discharging the batteries unnecessarily and ran the genset. (Probably didn't need to as it turns out!) The generator explains the sudden jump in volts shortly after 8am until 10am.

Here's the individual cells:


And to put it in context, here's the whole string:


By comparison, the old AGM cells, taken in arguably their "peak" operating conditions - summer, within a week of the summer solstace (ie, the longest sun day), with minimal loads on, no lights etc, behaved like this!

bj:
   Very nice Ross.  I have often thought (dreamed) of them for various projects, but never gotten
serious about it.
    With your reduced depth of discharge, a predicted life of 6+ years?
    Much drool on my keyboard.
    Very nice indeed.
    Edit----just did a bit of dirty math on life curves,  probably more like 10 years?

rossw:

--- Quote from: bj on April 26, 2014, 06:52:43 am ---    Edit----just did a bit of dirty math on life curves,  probably more like 10 years?

--- End quote ---

That's around the numbers I came up with too. But then factor in the "real life" conditions. I don't always draw that much overnight - generally only winter with long nights and pumps etc running. In summer it can be as low as 60-70AH overnight.

And if I do save up and get an additional set in parallel, I could be looking at 25 years.

Remember also, that their "life" figure is where the cells retain only 80% of their original capacity. So by "end of life" they'll act like they're only 240AH cells, not 300...

MadScientist267:
Worked out well Ross, congrats. ;)

Didn't know about the Trojans, thought you had run AGM all the way through for the house... Sucks about the contractors and the original AGM set, what they do, burn them up thru the inverter with tools and such?

Steve

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