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Seplos BMS

  • 05-10-2021 12:45pm
    Registered Users Posts: 1,856 ✭✭✭

    Since a number of us have bought the Seplos BMS I thought I would start a new thread.

    Documentation link here:

    I read the manual last night. The BMS seems to have decent build quality. The BMS wires are attached to two terminal connectors (black and white) and the wriring seems straight forward. The box I got - thanks @championc - has the BMS board, LCD, USB adapter and wire assemblies.

    I have three questions right now:

    1) Where do I get a battery NTC for battery temps? Or is that something that has to come inbuilt in the cells? Do the AliExpress CALB 200Ah cells have them?

    2) Is there any way to monitor it remotely? I have the RS485 to USB adapter so do I plug that into a USB wifi dongle?

    3) General question for DIY setups: I presume I connect the BMS to the battery, make sure everything is fine, make all the settings and then connect it to the solar inverter?



  • Moderators, Sports Moderators Posts: 17,885 Mod ✭✭✭✭slave1

    Good idea, I missed the championc collective order but sourced one and chap is setting it for 15%-95% available to use, which I will start off with and see how it goes

  • Registered Users Posts: 791 ✭✭✭reklamos

    For point 2, get elfin11 or a wifi version and this will save you from using multiple devices and conversions. I use it and it works really well. The only concern is, does the Battery software that comes with BMS supports modbus over TCP. If not then it is not that hard to write your own one and push it to the cloud or just self host it. This is what I have done done with Pylontech and as these look like Pylontech clones it should not be that hard. One thing I noticed though is the lack of comms ports on the BMS. Correct me if I'm wrong but there are 3: 1 for CAN/RS485/RS232 to communicate with PC or Inverter and the other 2 are RS485 but for daisy chaining batteries. So how do you connect monitor and inverter at the same time? Or are you supposed to pull battery data through inverter but then you'd need to set IDs somewhere.

    For point 3 you would I would think you will need to to some configuration on the inverter side also. First to let it know that you have battery and let it know what battery it is, set min/max settings etc.

    Anyway I am following this and happy to help where I can

  • Registered Users Posts: 1,856 ✭✭✭garo

    The BMS already comes with an RS485 to USB adapter to connect to a PC. I hook up a length of CAT5 to one of the RS485 ports and wire it up to the adapter as here:

    They don't mention what comm protocol they use so I can;'t answer your question on modbus over TCP. I suspect I could connect the RJ45 from the RS485 directly to my router if that were the case?

    There is a separate dedicated port on the board to connect to the LCD monitor in addition to the CAN and two RS485 ports. See the diagram below.

    I don't know why there are two RS485 ports and how the whole parallel packs thing conflicts with getting the battery data out. I suppose daisy chaining means each packs needs its own BMS so the master BMS will have one RS485 for communicating with the next pack and one for getting data out.

    Agree about point 3. I skipped the bit about inverter configuration. That comes before the final step. I currently have a Pylontech connected to the inverter.

  • Registered Users Posts: 791 ✭✭✭reklamos

    So the USB adapter just create a serial port on PC for it to communicate with the battery. I have checked the uploaded software and it only can communicate over serial port and there is no option for TCP(but I also cannot not read Chinese). Modbus over TCP would allow to connect battery to router via Ethernet or WiFi and manage it from anywhere. Now you will need laptop/PC close to battery or run cable to where you PC is. But as I wrote this should not be that hard to reverse engineer.

    I have watched Seplos video:

    So according to it CAN port can optionally be RS485/RS232. The 2 RS485 ports are used for interconnecting battery packs and for management software. So the first and the last packs would have one free RS485 port. The first pack would be your master and the rest slaves. You would connect management software to the available port on the master pack. This is similar way that Pylontech has, except Pylontech has dedicated ports for management.

  • Registered Users Posts: 1,856 ✭✭✭garo

    Thanks. Yeah I was coming to the same conclusion as you do here:

    The 2 RS485 ports are used for interconnecting battery packs and for management software. So the first and the last packs would have one free RS485 port. The first pack would be your master and the rest slaves. You would connect management software to the available port on the master pack. This is similar way that Pylontech has, except Pylontech has dedicated ports for management.

    The Pylontech uses the CAN ports for daisy chaining and talking to the inverter.

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  • Registered Users Posts: 1,856 ✭✭✭garo

    Just watched the video too and I have another question:

    The cell balancing diagram has resistors connected to each cell to dissipate the higher voltage. Is that built in to the BMS? I don't see how you can have only a single cell discharging without additional connections. The wires to each individual cell from the BMS are too thin and not meant to carry any load.

  • Registered Users Posts: 1,856 ✭✭✭garo

    This is an excellent video overview on how to connect the battery cells to the BMS. This puts everything into a Seplos case but the basic principles remain the same.

    I wonder if I should try to crimp their 2:1 connectors for the positive terminals. I just realised that I will need different size crimp connectors for the BMS.

  • Registered Users Posts: 1,856 ✭✭✭garo

    And this video shows the daisy-chaining. At 7:15.

  • Moderators, Home & Garden Moderators Posts: 4,435 Mod ✭✭✭✭graememk

    Yep, that's all bms's.

    That's how they balance, they bleed power from the high cells.

    It's fine for keeping them in check but to actually pull them into balance takes ages.

    I don't know the specs but it's less than 1 amp, at 3.2 volts

  • Registered Users Posts: 791 ✭✭✭reklamos

    Yes, balancing is extremely slow process I can see that on my Pylons. The whole night is not enoough to balance the cells when they are discharged but they do get there pretty quickly when being charged and once they are above 50%.

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  • Moderators, Home & Garden Moderators Posts: 4,435 Mod ✭✭✭✭graememk

    Also something to bear in mind, Pylon are LFP batteries too, so when its not full or empty, the voltage wont show any imbalance.

    you'll only see imbalance when nearly full where a cell shoots high or when nearly empty when a cell dips low.

    When I put in my ZEVA bms, I noticed mine was a bit out of balance, Started on a saturday, Finished on monday, Had my inverter trickle charging the pack (at somthing like 50-100w)


    after :

  • Registered Users Posts: 791 ✭✭✭reklamos

    Ok, I see what you mean. I do not let the batteries go very low as from what I read it is one of the things that they do not like is to be stored empty. Actually during winter I raise SOC as there is not enough sunlight to charge them to 100%. During summer I try to use as much as possible before it get to 100% and starts diverting to water. I have checked my logs and the cell are within 0.02 voltage tolerance when high or low in the pack.

  • Registered Users Posts: 1,856 ✭✭✭garo

    I assume the 24AWG communication wires - which are the only wires that connect directly to each cell - are NOT the path through which this 1A current flows? Or are they?

  • Moderators, Home & Garden Moderators Posts: 4,435 Mod ✭✭✭✭graememk

    They are. It's only something like 3w max.

    There's more power going down a USB cable.

  • Registered Users Posts: 1,856 ✭✭✭garo

    TIL that the Seplos BMS comes with an NTC temperature sensor - actually four of them - which you just screw onto four different battery terminals. Now this presents a small problem as the NTC ring terminals have holes than are smaller than the screws. Oh well! Probably need to use some SPU thermal paste from my PC builds to stick them on.

  • Moderators, Home & Garden Moderators Posts: 4,435 Mod ✭✭✭✭graememk

    You could just tape them to the cell or push them down the gap between the cells.

    Would give a better reading too instead of the terminal

  • Registered Users Posts: 1,856 ✭✭✭garo

    Thanks. Yeah I thought the terminal can give dodgy readings. Would tape work better than thermal paste? I have thermal paste from when I used to build my own computers. The gap between the cells sounds like a reasonable idea too.

  • Moderators, Home & Garden Moderators Posts: 4,435 Mod ✭✭✭✭graememk

    Dab of paste, and tape it on

  • Registered Users Posts: 1,856 ✭✭✭garo

    Yes that worked. In fact I had to use very little paste as too much would render the tape useless. Seems to have worked and I get valis temp readings from all four sensors within 1 degree of each other.

  • Registered Users Posts: 1,856 ✭✭✭garo

    Progress report. After almost two months busy with other projects, finally got a chance to finish this up. I had difficulty communicating with the inverter but contacted Seplos support and they sent me a new firmware specific to my Growatt inverter and it worked like a charm. Switched on the oven and quickly discovered that the SOC number was rubbish and the cells weren't as balanced as I had believed.

    Cells 15 and 16 are in fact the highest and lowest voltages. This is under load and BMS reporting 10% SOC.

    Same without any load:

    As soon as load was removed BMS started reporting 100% SOC. I set the nominal capacity to 200Ah via the configuration software. Need to read up on cell balancing now.

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  • Moderators, Sports Moderators Posts: 17,885 Mod ✭✭✭✭slave1

    I have a bench power supply I can loan you once I setup my own batteries (to top balance), I got my Seplos last week but unfortunately cables and LCD screen were missing so they are being posted out

  • Registered Users Posts: 421 ✭✭Geeyfds53573

    thanks Garo I wondered about this - I assume the firmware update is straight forward through the software? - re balancing I might invest in a dc battery charger as I think I’ll be needing it from the sounds of that… appreciate the feedback

  • Moderators, Home & Garden Moderators Posts: 4,435 Mod ✭✭✭✭graememk

    Is there anyway of resetting the BMS to a lower SOC?

    eg can you knock it back down to 50% and then start a manual charge (ie if you were to charge overnight) and charge until all the cells are 3.5+ (total voltage of 56-58v) to get a proper "100%"

    Off grid garage has a good video on charging curves on of lifepo4

    Some things to remember

    Under heavy loads, ie oven! the pack voltage can easily drop 1-1.5 v (or even more at a low SOC)

    Voltage is not a good indication of balance or SOC in LFP batteries, as the 3.3x can be anything in that mid range of SOC.

    Voltages under load will be generally lower and voltages under charge is higher.

    I didnt have a Bench power supply in july, but I was able to trickle charge my pack to balance it again, took about 3 days.

    ZEVA not seplos but should be similar:

    shortly after i started the "balance" after charging to full. the BMS cut off the charging anycells above 3.4 were basically full but I had 4 cells running low, with cell 2 being the lowest.

    after a day :

    just waiting on Cell 2 mainly, cell 11 came up fairly quickly after that. After another day, At about 50w charging (allowing the bms to bleed off the high cells) I had the pack balanced.

  • Moderators, Home & Garden Moderators Posts: 4,435 Mod ✭✭✭✭graememk

    Also verify the voltages on the bms with actual measurements with a multimeter, incase there is a loose/bad connection somewhere.

    I had noticed on mine that because my pack is split on 2 rows of 8, the measurement on cell 9 was a little bit off (high on charge and low on discharge) due to the extra bit of cable (voltage drop) joining the batteries. I mitigated that a little bit by doubling up the cable joining the 2 halves of the pack.

  • Registered Users Posts: 791 ✭✭✭reklamos

    according to picture below that is in Seplos box, you need to do full cycle at 0.5C for it figure out SOC

    But as @graememk wrote make sure your connections are good. Watch this, the guy shows what happens when connections are bad. His channel is pretty good and he is using seplos.

  • Registered Users Posts: 1,856 ✭✭✭garo

    Thanks slave1. Sucks about the screen and wires. I'll try and figure something out locally first but appreciate your offer and will take it up if nothing else materialises by then.

  • Registered Users Posts: 1,856 ✭✭✭garo

    Yes firmware update was pretty straightforward. They have a very good illustrated guide which I followed to the letter and had no issues. I got a "File not found" error the first time I tried but restarting the battery monitor software fixed it.

  • Moderators, Home & Garden Moderators Posts: 4,435 Mod ✭✭✭✭graememk

    you might have to put the inverter into manual or default battery mode, (ie lead acid mode) to get it to charge manually.

    Settings something like max voltage of 57/58v min voltage 40v. (the bms will take over anyways and cut the power if it goes too low or too high)

  • Registered Users Posts: 1,856 ✭✭✭garo

    Thanks Graeme. Yes it is possible to lower the SOC using the software and then set charging again. Going to try that now. I had the cells connected in parallel for over two weeks to let the voltages equalize and when I initially connected the BMS - 2-3 weeks ago I verified that the voltages being reported by the BMS are the same as that by my multimeter and they were all within 10mV of each other IIRC.

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  • Registered Users Posts: 1,856 ✭✭✭garo

    @reklamos @graememk Yes that's correct. There isn't any way to force a charge while the BMS is controlling it. My inverter can be set to charging or discharging and that's what I did at the start to drain the battery. What's weird is that after I stopped discharging at 10% SOC it jumped straight back to 100% and now isn't charging or discharging. I think I need to connect the BMS software again to see what's going on. I'll report back in a bit.