Seplos BMS v2 Charging Current Restricted to 100A

[Rob99]

I have one of my Seplos battery packs connected to my Sunsynk 8kW inverter but even though I set the inverter to charge at 150A (0.5C on a 304Ah pack) it seems like the BMS is restricting the actual charge to only 100A according to the BMS page on the inverter.

I've looked at the BMS settings but can't see anything which suggests this current limit is changeable so don't know why a 200A BMS is restricting the charge current. The only thing I did see was that there is a function switch in battery monitor for setting "Single BMS 1.0C Charging" but worried that it's the wrong thing to switch on.

Any thoughts?

 

[SeaGal]

I don't know anything about the Seplos, but is is normal that the BMS will tell the inverter at what current to charge. Typically there should be a setting in the BMS to set a maximum charge current.

Furthermore a BMS will likely reduce charge current when the SOC approaches 100%, cell voltage hits a pre-defined value or the temperature is low.

Suggest you look into Seplos BMS config in more detail - hopefully someone with a Seplos can chip in - lots of Seplos users on here.

 

[Steve_S]

I have one of my Seplos battery packs connected to my Sunsynk 8kW inverter but even though I set the inverter to charge at 150A (0.5C on a 304Ah pack) it seems like the BMS is restricting the actual charge to only 100A according to the BMS page on the inverter.

I've looked at the BMS settings but can't see anything which suggests this current limit is changeable so don't know why a 200A BMS is restricting the charge current. The only thing I did see was that there is a function switch in battery monitor for setting "Single BMS 1.0C Charging" but worried that it's the wrong thing to switch on.

Any thoughts?

Most BMS' including Seplos are this way. A 200A Discharge capable BMS will only do 1/2 that for charging and that is quite normal. With FET Based BMS' they tend to cap out at 250A Discharge capability. After that amperage level you have to step up to Relay/Contactor models to handle the higher amperages.

No inverter/charger will override a BMS as the BMS controls the actual battery pack.
NOTE the lurking Gotcha !
If you had 2x 304AH Battery Packs, each with a 200A Seplos BMS, they can collectively take that 150A and split it between them giving 75A to each pack.

Apparently Sunsynk & Seplos BMS' can use CANbus to interface which offers some interactive controls but from my understanding (a tad out of date) it has limitations AND depending on the source for the BMS, they may also be partially locked out (some settings cannot be changed without tech PW. PACE BMS' are notorious for that.

 

[Lt.Dan]

The Seplos BMS is absolutely rated for 200a charge and 200a discharge. I did it every day for a year.

That is of course, unless you only have a 100a BMS.

It is normal that whatever you program the SunSynk to, will not stay, because the BMS will force whatever it wants. If it is a 200a BMS, and the SunSynk is only showing 100a max, then there is a setting in the BMS that is limiting it.

 

[houseofancients]

Most BMS' including Seplos are this way. A 200A Discharge capable BMS will only do 1/2 that for charging and that is quite normal. With FET Based BMS' they tend to cap out at 250A Discharge capability. After that amperage level you have to step up to Relay/Contactor models to handle the higher amperages.

No inverter/charger will override a BMS as the BMS controls the actual battery pack.
NOTE the lurking Gotcha !
If you had 2x 304AH Battery Packs, each with a 200A Seplos BMS, they can collectively take that 150A and split it between them giving 75A to each pack.

Apparently Sunsynk & Seplos BMS' can use CANbus to interface which offers some interactive controls but from my understanding (a tad out of date) it has limitations AND depending on the source for the BMS, they may also be partially locked out (some settings cannot be changed without tech PW. PACE BMS' are notorious for that.

this is confirmed...

there is an option switch in the bms configuration to disable that, however , i would strong advise NOT to charge more that .5C
If you need to charge at more current, add a battery...
If your sunsynk is grid-connected, it will joyfully send the remainder to the grid

 

[houseofancients]

The Seplos BMS is absolutely rated for 200a charge and 200a discharge. I did it every day for a year.

That is of course, unless you only have a 100a BMS.

It is normal that whatever you program the SunSynk to, will not stay, because the BMS will force whatever it wants. If it is a 200a BMS, and the SunSynk is only showing 100a max, then there is a setting in the BMS that is limiting it.

had nothing to do with the sunsynk, it will happily charge at higher amps...
This is being limited by the BMS as it should

 

[houseofancients]

btw i am running your exact bms and inverters ( be it, i run 2* 12k 3phase version of the inverters )

 

[SeaGal]

there is an option switch in the bms configuration to disable that, however , i would strong advise NOT to charge more that .5C

Just trying to understand the logic here (for interest only as I don't have a Seplos or Sunsynk)...

Are you saying that a 200A Seplos limits charge to 100A unless overridden because it thinks 100A is 0.5C?

If so, that's totally illogical for Seplos's to do that because the C value is dependent on the Ah rating of the cells (304Ah in the OP's case) & nothing to do with the 200A max current value of the Seplos BMS.

 

[houseofancients]

Just trying to understand the logic here (for interest only as I don't have a Seplos of Sunsynk)...

Are you saying that a 200A Seplos limits charge to 100A unless overridden because it thinks 100A is 0.5C?

If so, that's totally illogical for Seplos's to do that because the C value is dependent on the Ah rating of the cells (304Ah in the OP's case) & nothing to do with the 200A max current value of the Seplos BMS.

actually, one should do the training/calibration of the BMS, which OP clearly hasn't done

that will tell the BMS what the total AH of the bayyery is , based on a flag that is set when there is only a single bms in the chain it will limit to .5C ( which is the max charge rate for 95% LifePO4 cells)

by default , seplos set the total AH of the battery to it's max : 200A

i would argue it makes a hell of a lot of sense

 

[SeaGal]

Thanks - that makes it much clearer and the OP's issues should be resolved once he configures the BMS for 304Ah capacity (which presumably has to be done anyway to get a meaningful SOC from it).

I agree that setting a max charge rate to equal 0.5C makes a lot of sense.

But the logic of Seplos defaulting to having a cell capacity equalling a 1C discharge rate of the BMS's max current seems odd. If you had a 100A Seplos BMS attached to a 280Ah pack, as must be very common, it would default to assuming 100Ah cells and 50A max charge. Or a 200A Seplos on a 50Ah pack would default to a 2C charge rate.

 

[houseofancients]

Thanks - that makes it much clearer and the OP's issues should be resolved once he configures the BMS for 304Ah capacity (which presumably has to be done anyway to get a meaningful SOC from it).

I agree that setting a max charge rate to equal 0.5C makes a lot of sense.

But the logic of Seplos defaulting to having a cell capacity equalling a 1C discharge rate of the BMS's max current seems odd. If you had a 100A Seplos BMS attached to a 280Ah pack, as must be very common, it would default to assuming 100Ah cells and 50A max charge. Or a 200A Seplos on a 50Ah pack would default to a 2C charge rate.

unless they want to cater to special case cells/circumstamces that actually allow 1c discharging ...

There for they added a switch to do just that, but it has to be a deliberate choice of the end user, with the understanding how the BMS works

also, the training of the BMS is in the manual, and was also printed BOLD and BIG on the box the bms came in when i bought it...
Maybe they left that out when OP bought his, but letting the bms learn is acually pretty important for this bms to function at it optimum




btw, i have uploaded the manual to the resource section here quite some time ago ;-)

 

[Rob99]

btw i am running your exact bms and inverters ( be it, i run 2* 12k 3phase version of the inverters )

Good to know
I guess you're quite happy with your Sunsynk's.
I chose Sunsynk as I wanted a flexible, controllable inverter which I could connect my Loxone automation system to, and nothing else came close. So far I'm pretty impressed, although occasionally frustrated by some very bizarre hardware issues, chief amongst them being the inability to keep time even when sync is ticked. I can get round this by just writing the correct time to the registers but I really shouldn't have to (come on guys, it is the 21st century, how hard can it be?)

actually, one should do the training/calibration of the BMS, which OP clearly hasn't done

Ah, should I have done something else with the battery pack?
I did a top balance as advised on here then a full charge from the inverter, at which point the BMS reset the SOC to 100% and then a discharge down to 20% (didn't think it wise to go lower than that).
My second battery is ready to hook up over the weekend so should I do a full charge and discharge and let it discharge until the BMS stops it?
I want to make sure I get this right before I start using the batteries properly.

 

[Steve_S]

The BMS does not care what cells you have, the BMS is rated for what IT CAN DO not what the cells can or cannot do.
Take a 16S Pack of 100AH cells. That battery pack is 100AH, Capable of 100A (1C) output and the cells can only take 50A (0.5C) charge.
Add a 200A BMS and it will work BUT the cells can still only output 1C and take 0.5C "safely" so the BMS must be adjusted according to the cells.
Using 300AH Cells is no different, the BMS is still capped to its design and at this time you will be terribly hard pressed to find a FET Based BMS that handles more than 250A discharge.

There are a few Chemistry Variants of LFP that will allow for 1C Charge & 2C Discharge, but you can't get them.... They all goto EV's.
Then we have LYFP (Uttrium Doped) that can do it and even be charged at -20C but you are paying big for that... Home ESS = ouch cost.
Now we have LMFP (Manganese Doped) that blows them all out of the water but this is the latest chemistry and will not be publicly available till at least 2026 and even then it will be terribly limited. People have NO CLUE about how many variants are out there... even within single manufacturers.

Sadly, people have this notion that you can "push in" more juice than the design allows for, kinda like "topping your gas tank till it overflows cause you keep pulling the pump trigger... While not realizing they are harming their investment after which they bitch, complain and point fingers at everyone else but themselves.

 

[houseofancients]

Good to know
I guess you're quite happy with your Sunsynk's.
I chose Sunsynk as I wanted a flexible, controllable inverter which I could connect my Loxone automation system to, and nothing else came close. So far I'm pretty impressed, although occasionally frustrated by some very bizarre hardware issues, chief amongst them being the inability to keep time even when sync is ticked. I can get round this by just writing the correct time to the registers but I really shouldn't have to (come on guys, it is the 21st century, how hard can it be?)

this has to do with the migration to the UK they are currently do for the Userdata ( and the time sync location) from china to the EU
i have temporarily diasabled time sync, inverters keep time pretty well without sync, will turn it back on when migrations are completed

Ah, should I have done something else with the battery pack?
I did a top balance as advised on here then a full charge from the inverter, at which point the BMS reset the SOC to 100% and then a discharge down to 20% (didn't think it wise to go lower than that).

top balancing is indeed what is needed..
in order to trigger the bms learning, charge until the BMS does a high voltage disconnect, then do a continues discharge until bms low voltage disconnect...
after that your bms will be trained, and you can charge it to it's specs ..
sunsynk specific , make sure you switch your inverter to voltage and make sure the low voltage setting is low enough so the bms can trigger the disconnect, and make very sure to change those back

My second battery is ready to hook up over the weekend so should I do a full charge and discharge and let it discharge until the BMS stops it?
I want to make sure I get this right before I start using the batteries properly.

yes sir, see above

 

[Rob99]

The BMS does not care what cells you have, the BMS is rated for what IT CAN DO not what the cells can or cannot do.
Take a 16S Pack of 100AH cells. That battery pack is 100AH, Capable of 100A (1C) output and the cells can only take 50A (0.5C) charge.
Add a 200A BMS and it will work BUT the cells can still only output 1C and take 0.5C "safely" so the BMS must be adjusted according to the cells.
Using 300AH Cells is no different, the BMS is still capped to its design and at this time you will be terribly hard pressed to find a FET Based BMS that handles more than 250A discharge.

Yeah I understand that and have no intention to charge more than 0.5C but does seem a strange inbuilt limitation if the BMS has been told I have 304Ah cells.

 

[Rob99]

in order to trigger the bms learning, charge until the BMS does a high voltage disconnect, then do a continues discharge until bms low voltage disconnect...

I assume leaving the "out of the box" BMS settings as they are

sunsynk specific , make sure you switch your inverter to voltage and make sure the low voltage setting is low enough so the bms can trigger the disconnect, and make very sure to change those back

So I switch battery type to AGM (V) and then set the shutdown voltage to lower than the BMS default. Do I leave it on AGM while doing the charge/discharge routine and then switch back to Lithium after?

 

[Rob99]

this has to do with the migration to the UK they are currently do for the Userdata ( and the time sync location) from china to the EU
i have temporarily diasabled time sync, inverters keep time pretty well without sync, will turn it back on when migrations are completed

I did wonder if this was connected to the Region1/Region2 thing. I think last thing I saw from Joel at Sunsynk was that the switch was being completed sometime next week.

 

[houseofancients]

I assume leaving the "out of the box" BMS settings as they are

So I switch battery type to AGM (V) and then set the shutdown voltage to lower than the BMS default. Do I leave it on AGM while doing the charge/discharge routine and then switch back to Lithium after?

only have to change that for the discharge...
if you do not, your inverter will disable charging before the bms does, therefor not ending the learning cycle properly

 

[houseofancients]

I did wonder if this was connected to the Region1/Region2 thing. I think last thing I saw from Joel at Sunsynk was that the switch was being completed sometime next week.

that is exactly it

seems they have been having some issues with that migration for a while no....

the inverter do check the NTP daily ( if reachable) , but really dont like the time shift from china to UK...

unfortunatly havent been able to set my own NTP servers in the inverter...

like i said, switching of time sync will be your easiest option, and so far ( i switched them 2 months ago) , they are still in sync upto the MS

 

[Rob99]

only have to change that for the discharge...
if you do not, your inverter will disable charging before the bms does, therefor not ending the learning cycle properly

Great, thanks