Can anyone recommend something similar to a Cotek SR1000T suitable for LFP batteries?

[Chewie]

Hi everyone!
I'm new here, and I have a question:
I bought this: https://www.cotek.com.tw/sr1000t-1000w from ebay, and it's very close to what I wanted.
It has 240v AC and 48V DC inputs.
You can configure it so that DC is primary, and AC is secondary, and you can configure under voltage and over voltage limits, which is nice, but the highest voltage you can set it to swap from using DC back to using AC is 48V, which isn't good for my LiFePO4 batteries.
I would prefer it to swap over at 50 or 51V so I don't drain them too far, but I cannot set it this high.
This means I have to manually swap over with the physical switch on the front, or try and get a serial cable that works so I can control it from a computer.
Neither is ideal because I don't want my computer to be polling the inverter all the time, with the risk that it won't swap over at the right time (dodgy coding on my part, system crashes, or having to have the computer connected/on all the time etc).

Can anyone recommend a similar device where you can set a more sensible voltage or a LFP system?

I contacted the manufacturer, who at least replied, but said there is nothing I can do to change the limits.

I know there are ATS that are similar, but most seem to be 2 x AC inputs, so I still need a separate inverter, which would be annoying.

Thanks!

 

[sunshine_eggo]

You can configure it so that DC is primary, and AC is secondary, and you can configure under voltage and over voltage limits, which is nice, but the highest voltage you can set it to swap from using DC back to using AC is 48V, which isn't good for my LiFePO4 batteries.

Based on? 48V is a very common cut-off voltage and represents about 10% SoC AT REST for LFP. Given the battery will be under load, there will be an additional voltage drop, and it's almost a certainty that the battery will be > 10% when it cuts off.

 

[Chewie]

Well, you don't know how much load is on my system.
The BMS turns the battery off at 44V. I do not want it to get anywhere near that low because the battery cost a fortune and it needs to last....

 

[sunshine_eggo]

Well, you don't know how much load is on my system.

Well, it doesn't matter one bit. You may have missed the point that ANY load guarantees cut off at > 10%. Run a heavy load, you'll cut off at higher, i.e., if you want a higher cutoff at 48V, a higher load is better due to the voltage drop.

The BMS turns the battery off at 44V.

That's already conservative. Dead is 40V (2.5V/cell).

I do not want it to get anywhere near that low because the battery cost a fortune and it needs to last....

LFP is not a fragile little flower that requires babying. It is a robust chemistry that performs well and lasts long when operated within specifications.

You're looking at spending more money for no benefit. Cut off at 50-51V won't measurably extend the life of the battery. If cut-off is a regular thing, you need to invest your money in more capacity or lower utilization.

If you want to maximize life, there's more you can do on the charging side of things than limiting the discharge cut off. Charging is harder on them than discharging.

 

[Chewie]

LFP is not a fragile little flower that requires babying. It is a robust chemistry that performs well and lasts long when operated within specifications.

Yeah, "within specifications" - they have a limit on the number of cycles, for example from the actual battery manual:

• 3000 cycles at 100% DoD @ 1C (see chart tab)
• 4500 cycles at 80% DoD @ 1C (see chart tab)

You're looking at spending more money for no benefit. Cut off at 50-51V won't measurably extend the life of the battery. If cut-off is a regular thing, you need to invest your money in more capacity or lower utilization.

I'd love to have more capacity, but I don't have enough space for more panels
Yeah, lower utilization = turn things off and back to mains power.

If you want to maximize life, there's more you can do on the charging side of things than limiting the discharge cut off. Charging is harder on them than discharging.

ok, like what?

 

[sunshine_eggo]

Yeah, "within specifications" - they have a limit on the number of cycles, for example from the actual battery manual:

• 3000 cycles at 100% DoD @ 1C (see chart tab)
• 4500 cycles at 80% DoD @ 1C (see chart tab)

Are you discharging at 1C or a smaller fraction of that?

If < 1C, cycle life will be even higher.

A few hundred 100% discharges isn't going to cut the cycle life to 3000 from 4500. Maybe you'll only get 4250 out of it.

I'd love to have more capacity, but I don't have enough space for more panels
Yeah, lower utilization = turn things off and back to mains power.

You have to operate within all limits, so maybe that's what it takes.

ok, like what?

Limit charge voltage to 3.45V/cell with a 1-2 hour absorption. You might only attain 98% charge, but doing it over a longer period at a lower rate to a lower voltage is less stressful to the cells.

 

[Chewie]

Are you discharging at 1C or a smaller fraction of that?

I am pulling < 1KW from a 105A 48V battery
If my maths is correct, that is ~0.2C ?

If < 1C, cycle life will be even higher.

True

A few hundred 100% discharges isn't going to cut the cycle life to 3000 from 4500. Maybe you'll only get 4250 out of it.

OK
I never want to go to 100% DoD because my charge controller can't power itself from the panels diretly and I have to slightly charge the battery from the mains so that the controller will "boot" and charge from the panels again.

You have to operate within all limits, so maybe that's what it takes.



Limit charge voltage to 3.45V/cell with a 1-2 hour absorption. You might only attain 98% charge, but doing it over a longer period at a lower rate to a lower voltage is less stressful to the cells.

OK. I don't think I can do that with my controller. Plus obviously I want to collect as much sun as possible.
My charging current is hardly ever more than 30A

 

[sunshine_eggo]

OK. I don't think I can do that with my controller. Plus obviously I want to collect as much sun as possible.
My charging current is hardly ever more than 30A

What controller?