charging and 38s lifepo4

[OhYou_]

Figured I'd make a thread on this.
I have a 7.5KW 120/208v battery backup ups unit that I got for nothing. It is normally set up with 10s of SLA batteries for 120vdc.
looks like going off discharge rate charts, it should operate in line with 38s of lifepo4.

Having a hard time finding a charger for this though. closest one i could find is this https://www.aliexpress.us/item/2251801616611891.html
if it is to be believed, that is some sizeable bit of kit, but obviously its nolabel so.

Does anyone have some suggestions on charging?

Or should I scrap the whole idea as not worth it? I feel the cost of a similar capacity 48v inverter outweighs the extra cost of a big bms and charge controller but idk. Saves a ton on all the little things related to moving around the power since I'm using way less wire size.
also some hidden benefits of just having a 120vdc circuit means I can just plug in a surprising amount of 120vac items in and run them on dc no problem. I was running laptop chargers and other switchmode psu stuff straight off the batteries just to try it. surely my induction oven wouldn't care either.

 

[sunshine_eggo]

Does the UPS not have it's own charger?

IMHO, 40S is likely more practical:

10S*13.8V (typical UPS float charge voltage) = 138V

138V/3.45V (a reasonably safe "float" voltage at near 100% SoC) = 40S

 

[OhYou_]

Does the UPS not have it's own charger?

there is no grid, so not really.
Even if I were to use a backup generator and the onboard charger, it only accepts 208Vac. I do not foresee ever using the charger

the UPS should be perfectly fine running 34s-42s but I have chosen 38 because it aligns the best with 120vdc

 

[sunshine_eggo]

there is no grid, so not really.

It wasn't clear you were looking for a solar charger because I never go to aliexpress unless I want to get scammed.

Even if I were to use a backup generator and the onboard charger, it only accepts 208Vac. I do not foresee ever using the charger

Makes sense.

the UPS should be perfectly fine running 34s-42s but I have chosen 38 because it aligns the best with 120vdc

Consider that "12V" batteries use 4S even though their nominal voltage is 12.8V. 4S/40S is the same ratio.

When battery builders spec a battery, they are not necessarily trying to hit the same nominal voltage. If they did, all 48V LFP would be 15S. Instead, they are attempting to best mimic the entire envelope of operation. As a result 16S LFP is almost a perfect drop-in for typical 48V system operating ranges.

Higher voltage also means lower current of for the same power and additional energy storage.

In any case, high voltage MPPT are pretty rare. Sigineer used to have a line of them, but they seem to be discontinued.

I would punt at this point.

 

[OhYou_]

38 or 40 I'd still be looking for a charger that does 120vdc.
Or I could perhaps use multiple chargers in series if they did not share a common ground with the panels like most do.

 

[OhYou_]

I went with a slightly different version of the one I linked. I will see how well it works when it arrives.

What sold me eventually on the 120vdc bank idea is I realized I could potentially run a inverter A/C unit off of the dc side and not have to run the big inverter for 12 hours a day all summer.
This and I can use my existing solar charger to maintain the 12v system, by just simply connecting the pv inputs straight to the 120v battery bank. I'll have to see what the losses are like but it should be fairly efficient and I can always disable charging at night if needed.
I plan on having a large solar array now with this charger, it will support 21kw, and my plan is to just cover my shop roof in panels.

 

[zanydroid]

The rectifier on your 120VAC devices may not be rated to carry 100% duty on one side, whether passive or active rectifier

PV inputs aren’t necessarily designed to handle the available fault current of a battery, I think some even have a bypass relay that shorts the input in some cases. They also sometimes will check the impedance of the input, and battery is lower than it expects. There are a few threads here on tricking a PV input.

 

[OhYou_]

I received the somewhat questionable charge controller and it is certainly questionable but it will work for my application.
unfortunately the "user entered" charge settings are surprisingly problematic.
I wanted to test the system out with a 30s li-ion pack which has the charge/discharge limit of 100v-126v.
the lowest voltage the controller stops at is 130v kek.
I solved this by setting a voltage offset which is supposed to be used for calibration of the unit but I just set it to max, which is 4.9v, for a grand total of 124v measured at the pack when it hits cutoff. (which it overshoots cutoff by almost half a volt on the display)
all of that wouldn't be a problem if I wasn't considering a massive 30s li-ion based setup, so with that in mind I will be forced to 32s if I go that route still.

PV inputs aren’t necessarily designed to handle the available fault current of a battery, I think some even have a bypass relay that shorts the input in some cases.

Good to know, I will install an appropriate fuse. I'm aware of the rectifier issue and will monitor the situation, but I have done limited testing with quality adapters and did not see issues.

pic of bench setup to test out the charger, 140vdc source acts as the solar input since I am not home atm and still need to buy some small things to hook it up to the array.