diy solar

diy solar

Grid-tied, Battery backup system upgrade ready


New Member
Dec 26, 2021
We added solar well over a decade ago, and installed the first grid-tied, battery-backed-up (is the term Hybrid?) system in our city. About a year ago all of our Trojan 6v batteries all finally died. So I did the battery internet deep dive and ended up with two SOK 48V 100Ah Server Rack LiFePO4 Batteries, with plans to add more later. Then life got crazy and money got tight and the batteries have been sitting in my garage for over a year. But I'm now ready to move forward.

The original solar installers are long gone, but they installed a Xantrex (Schneider) 6048 Inverter and an Outback Charge controller. These two devices don't actually talk to each other (other than through voltages), which I think is less than ideal, but it's what they installed. The installation was pretty complex, including an outside shutoff that was required by the city at the time. I don't pretend to fully understand how it's wired. But I do understand the fundamentals.

My system has been down for over a year while I find the time and money to finish getting the new batteries installed. I was surprised to discover that my solar panel system can't work without batteries. I figured they were an add-on for when the grid goes down, but apparently, they are wired in such a way that without them nothing works. My bills have been higher. :(

My hope is that I can pretty much just swap out the Trojans for the two SOK Batteries (in parallel of course), reprogram the Outback to be a little more in line with the charging needs of LiFPO chemistry (it doesn't actually have any lithium battery modes, but I understand you can fake the settings pretty well). And turn it all back on. I've seen reports of similar setups that seemed to transition to LifPO rack batteries pretty smoothly. But those systems all seem to be pure off-grid. Where I get fuzzy and a bit nervous is how grid-tie plays into this. It is obviously already all wired for this, and I don't plan to touch anything on the AC side.

I do plan to put in an inverter pre-charge circuit (or probably just a 48v light bulb) button. Something like this: (

Some questions:
1) How drop-in replacement is this likely to be?
2) Do I need battery fuse(s)? I see these in all the rack system setup videos, but my lead-acid previous system didn't have any. I'm inclined to just for safety. But I'm reticent to mess with any more wiring than needed. I also seem to see a lot of bus bars in use in these videos, but since I'm drop-replacing, I don't see the need unless I'm missing something.
3) Does grid-tying change anything about how these rack-batteries behave?
4) Does anybody know the settings I should change on the Outback Flexmax (circa 2012-ish) to make it more lithium friendly?
5) What are the odds, SOK (or Current Unlimited) can get the Xantrex, Outback and batteries all talking on some kind of common language XAN/CAN/bus?
6) Not knowing the charge state of the SOKs, should I invest in a 48v stand-alone charger just to get them charged back up before the install? I haven't even tried turning them on in a year, so I don't know how dead they are.
7) I don't really have the budget, but should I just sell the Outback and get a charge controller that's more lithium aware? The Outback has been a workhorse and I'd rather not replace it. But, then again if I get a Schneider charge controller (MPPT 60 PV?), then I would assume they'd be able to finally have proper comms.
8) What also don't I know, that I probably should?
9) I can probably scrape a budget for an in-person consultation if anyone knows an installer in Utah who would be comfortable with a system like this. Or I'd be open to a virtual consultation if anyone is offering.
10) Thank you for reading this far!
Even though nobody replied, I figured I'd report back. TLDR; It works great (mostly)!

My setup just needs some fine tuning. Basically, I just pulled out my lead acid batteries, installed the SOKs and turned everything on, and it worked.

Answering my own questions:
1. Very
2. Nope, turned out there was a super beefy DC rated 250AMP DC breaker installed inside the Xantrex fuse panel.
3. Not really though I could use some advice. See below.
4. Still working on this, see below.
5. My understanding is that Schneider/Xantrex keeps Xanbus pretty close to the chest. And Current Connected have not yet set up a configuration for Outback FM80 series. So no, but doing it the old Voltage way is working okay, but I need advice. See below.
6. Turns out my SOKs sat in my garage for nearly 16 months and they were still 60% charged and fired up instantly and easily. Nice hardware!
7. Outback is doing fine!
8. How to adjust voltages to optimize grid selling and keeping the battery 90% or higher. Still trying to figure it out.

A more detailed report:
I started off by watching videos about pre-charging the capacitors without bothering to read the documentation on the SOK, turns out if you do things in the right order it does this by itself with no problem. It's right there in the F-ing instructions. :) I even installed one, only to not use it. So that was 1/2 day wasted. In the end I just plugged the batteries in using exactly the same cables I had been using for my lead acid batteries, even the jumper/bone cables between the batteries were exactly the correct length. It was about as drop-in replacement easy as I could have dreamed.

By the time I got around to firing it up, it was after dark so my Solar Outback charge controller was in full sleep mode. With the batteries at about 60% I wanted to get them charged up, so upon plugging everything in, I watched the battery slowly drain. It turned out I had the Grid > Battery charger in the Xantrex 6048 disabled, so it was pulling from the batteries to run the house. (Cool!) But I didn't want it draining too much, in fact I wanted to charge the batteries up since they'd been sitting for so long. So enabled the battery charger in the Inverter and it immediately started charging the batteries at full power, which for this inverter was 80AMPs. That seemed like a lot, and more than the manual recommended, so I turned down the charging power to 60%. And pretty quickly the batteries rose to nearly 85% by the time I was headed to bed.

So in the interest of seeing if the Outback could top off the batteries in the morning, I disabled the Inverter Grid > Battery Charger. When I checked the system in late morning it had in fact charged the batteries to 99.8%. Nice and the inverter was reporting it was feeding back to the grid. Nice 2X!

However, when I checked back in late afternoon, I was surprised to see the batteries were drained down to 54% SOC even though it was very sunny still and quite cool (ideal for solar). Yes we use a lot of power in our home. So I re-enabled the Grid > Battery charger in the Xantrex and let it do its thing. Pretty quickly the batteries started rising again, since both the Solar/Outback and Inverter were feeding energy to them.

I ended up diving into voltage settings on both the inverter/charger and solar/charger, since they were still set to my old Lead Acid settings. (It’s comforting to know the BMS’s in the SOKs won’t let me make to big of a mistake here.) But there’s so many different voltage settings, I’m getting lost in the terminology.

Ideally I want to optimize selling as much power to the grid as possible (when power usage is low enough), while keeping the batteries at 90%+.

Per reading through some threads I tweaked the settings a bit. I disable float on the Inverter/Charger, but I enabled it on the Solar Charger at just below optimal voltage for the batteries. And then I think I set the voltage “trigger” for the inverter charger to about 1 volt lower than that. With the logic that if the solar charger can’t keep up, which it can’t (I only have 18 200W panels that are 12 years old now), the grid/inverter/charger will kick in.

But there are so many settings on both the outback and the xantrex, and I’m getting lost. The xantrex has Battery Type (I set it to AGM since there’s no LiFPO4 setting), Batt Capacity (I set to 200Ah since I have two SOKs), Max Charge rate 60% (though the batteries didn’t seem to mind full 100%), 2-stage (no float), ReCharge Volts (??), Absorb Time (??), Chg Block Start (??).

And the OutBack FM80 has so many settings I don’t even want to list them all here, but needless to say I’m not clear how to set them to play nice with the SOK and 6048 and grid and of course the solar panels. I think the general idea is to have the Solar/Outback/Charge controller set to optimal voltage for the SOKs, and then the 6048/inverter/charger to a number below that to pick up the slack. But there's so many settings, even reading the manual I can't make sense of them all in the context of the SOK batteries.

So, overall great news, I highly recommend the SOK batteries. But some advice would still be appreciated!
Last edited:
You really cannot use voltage and expect to keep the batteries at any known charge level. You need to know Amp draw and state of charge. If you use voltage, your loads will change causing your sell back to be all over the place. I don't know what options you have with the equipment you have, but you will need some form of shunt to measure SoC, and then trigger sell-back operation off of that.

The FM80 has settings available that should be in line with what the SOK needs. You can probably ask SOK directly if you cannot find them in the manual for the batteries. All of the absorb, float, bulk, etc settings should be there. You can program the solar charger to be .05V higher bulk, absorb and float than the 6048. This will then prefer solar if you have no other way to do so.

80A is 40A per battery. Don't fear charging at 100% unless you are worried about the charger itself.