Overkill bms and kisae dmt1250

[Pyke13]

Hi there folks, apologies if I’ve posted this in the wrong place or it’s been covered before. I’m new to diy battery building so please be gentle with me.

I’ve got the 12v 4s overkill bms and wondering if there’s any way I can use it to turn off my solar controller?
From what I’ve read and think I’ve understood, I don’t want the lfp cells sitting fully charged as it shortens their lifespan. My idea is to have the bms cut out at 90%soc and not reconnect until around 30%soc in summer and to turn back on at 80%soc in winter. I only get around 6-7 hours daylight in winter.
I have the kisae dmt1250 combined solar controller b2b charger, this unit has a master switch so can be fully turned off.

The reason for asking is I’m unsure how it handles dumping power if the bms disconnects from the charger.

Hope that makes sense and thanks in advance for any help and advice received

 

[DThames]

I recall hearing/reading that if you pay the $6 for the Bluetooth app and if you have the Bluetooth dongle, you can change most parameters of the BMS. So you could set the max cell voltage to whatever you desire.

 

[Pyke13]

I’ve got the Bluetooth dongle for it, setting up the app should be easy enough. What I’m struggling to get my head round is what does the b2b solar charger do with the power when the bms is shut down. Will it burn it out as it has no place to send it, like connecting the solar to a controller but not the battery. Or maybe I’m just over thinking the issue and finding a problem that doesn’t exist

 

[Dzl]

Hmmm that may be possible but sounds like somewhat advanced logic for a BMS not designed for charge control. A BMS that is designed for charge control like the SBMS or other higher end options is designed to operate in a way similar to what you are envisioning.

You would need to wire wire the BMS to control the controllers on/off switch (unless it has a dedicated remote input port of some sort). And you would need a BMS where you could manually set overvoltage and reconnect voltage for charging only.

While there are benefits to doing it at the BMS (cell) level, if you just want to accomplish the rough logic you outlined above, and have top balanced your pack, you can possibly accomplish this with the charger on its own. This is the more usual way to handle charge logic.

Looking at the Kisea Manual:

Lithium Charging Process
The DCto DC Battery Charger has a specific charging profile for Lithium batteries. It has its own charging voltage and current settings that needs to be set by the user. It also requires the user to set the charger termination current (“L” setting) to pass from Absorption to Float stage. Once the user defined charge voltage is reached, the charger will start reducing the current output and the charging process will terminate when the charging current drops to the set termination (“L”) current. This can take anywhere between 3 to 25 minutes, depending on the capacity (size) of the battery bank and its initial state of charge.The unit will then act as a power supply to support additional loads connected to the battery. A bulk re-start will occur when the House Battery Bank voltage drops to below13.3Vdc on DMT1230/DMT1250or 26.6Vdcon DMT2430.

It appears that this charger has a configurable lithium profile as well as a configurable custom profile (there is more info in the manual, beyond what I quoted above).

 

[Pyke13]

If I set the charger and the bms at the same top voltage it negates the need to switch the charger off.
Will it do the cells any harm sitting around 90% soc for 7 months? For two months it doesn’t get properly dark, more just twilight and even at midnight I can still get power out of the panels in mid summer.

Think I’m possibly overthinking this

 

[Dzl]

If I set the charger and the bms at the same top voltage it negates the need to switch the charger off.

Generally you set your BMS disconnects slightly higher than the charger. There are exceptions to this depending on your use case, but its the most common arrangement. Most people buying BMSes like the overkill solar BMS consider the BMS a second layer of protection that should sit there silently 99% of the time. There are some good intro resources made by @FilterGuy in the resources section.

These two are particularly relevant:
Lifepo4 voltage settings guide for BMS, Chargers, and Loads
Beginners summary of BMS functions types and features

For two months it doesn’t get properly dark, more just twilight and even at midnight I can still get power out of the panels in mid summer.

Your scenario is definitely unique!

Will it do the cells any harm sitting around 90% soc for 7 months?

It is not ideal. Generally the highest advisable storage voltage/SOC is at most 70%, with 30-60% being the recommended more often. I don't know a ton about the specifics of this, but from the little research I've done. What cells or batteries will you be using?

 

[Pyke13]

The batteries won’t be in storage they are in a mobile home. So they are in constant use. My agm’s are in float mode for almost 2 months in summer and I need a genny to keep a charge in them in the middle of winter. I’m in the north of Scotland around 58 degrees north or about the same distance north as south Alaska.
The cells are 8x 272ah lishen cells 4s2p 12v.
The reason for going for such a large bank is purely to get me thru winter, i will run a genny once a week to keep them topped up.

Thanks for your help, I will read both the links you posted

 

[Dzl]

I referenced storage voltage because you mentioned:

Will it do the cells any harm sitting around 90% soc for 7 months?

Which whether it is technically storage or not sounds a lot like the characteristics of storage or maybe a UPS. I think its not the end of the world to let them sit at that high state of charge for months on end, but it is not ideal and it will reduce cycle life.

I think your goal of setting a lower reconnect voltage and/or lower float and absorb voltages for the months where you have a surplus of PV makes sense if you can figure out a way to implement it. I am not exactly sure how configurable the Kisae is.

 

[Pyke13]

Worst case scenario I can switch it off when the battery is full and switch it on a few days later. I would obviously rather have a way of doing it that reduces the risk of user error. Mainly me forgetting to switch it back on. I will set the reconnect voltage as low as possible for summer and change it for winter. I was kinda hoping that I could set the limits thru the bms as it will be easier to program due to having Bluetooth

 

[HRTKD]

My Victron solar charge controllers let me set to the high voltage threshold to whatever I want. 13.9v? Sure, not a problem.

As stated by @Dzl, I'm using the best practice to set my charge controller's high voltage threshold just below the high voltage threshold for the BMS. I run battery warmers in the winter, so I don't use a lower state of charge target.

 

[Pyke13]

I’ve had a good look thru the kisae manual today and found the variable charging profiles for lithium and the programmable setting.
Lithium,
Bulk 13.9v - 14.6v
Float 13.5v - 14.2v
Bulk charge is up to 50a with a float of 5, 7 or 10a.
The restart voltage is 13.3v
I’m thinking of using this in winter set at
Bulk 14.2v &50a
Float 13.5v @ 5a

The programmable settings are
Bulk 13.8v - 14.8v
Float 13.0 - 13.8v
Bulk charge is up to 50a wth a float of 5,7 or 10a.
Restart voltage is 12.5v.
I’m thinking use this in summer at
Bulk 14.2v @50a
Float 13.0v @5a

Can anyone see any issue with these setting or suggest what I should be setting the profiles at?
It’s only a case of pressing a button to switch between the settings.

 

[Pyke13]

My Victron solar charge controllers let me set to the high voltage threshold to whatever I want. 13.9v? Sure, not a problem.

As stated by @Dzl, I'm using the best practice to set my charge controller's high voltage threshold just below the high voltage threshold for the BMS. I run battery warmers in the winter, so I don't use a lower state of charge target.

Are your batteries inside or in a locker? Mine live in a box under a kitchen cupboard. I’ve just put a thermometer in there to find out what temp it’s at. It’s around 20 degrees in the rest of the van. I will leave it half an hour and take a reading

 

[HRTKD]

My batteries are installed inside my RV trailer. But the trailer is at the storage lot, not connected to shore power. So it's on its own. Solar panels on the trailer keep the batteries charged, the warming pads keep the batteries warm enough (in the range of 35° F to 45° F) that they can safely accept a charge.

 

[Pyke13]

I live in a converted box truck so it’s never that cold in here. The temp in the battery box is 14.7oC.
Even in the height of summer it’s never more than 23-25 degrees C in here. With the wood burner on it’s never cold in here and keeps the temp in the low 20’s.
I’ve ran everything inside apart from waste water and I put salt down the drain to keep it from freezing.
The batteries should be kept within safe charging temperatures. The only time they will get cold is if the trucks parked up and not in use for a few days and I can turn the solar off in that case.

 

[Pyke13]

That’s my house at the end of the rainbow

 

[Pyke13]

I’ve had the system set up and running for a while now and all seems well. I have noticed the kisae isn’t cutting out at 14v and reconnecting at 12.5v as it should in programmable mode.

I’ve set the programmable mode to two settings bulk and absorption, does the absorption mode need time to complete before the unit turns off?

should the charger turn off or go to sleep until it hits the 12.5v restart voltage? It seems to keep constantly charging even when the display is saying full. As soon as the volts drop the charger is goin fb back into bulk mode.

 

[Sipma02]

I was (and still sort of am) having this same issue with my Kisae DMT1250. I had to set the termination current to 10 amps for my 280ah lifepo4 bank. I'm still learning how it affects the switch from bulk to float/absorb charge, but it usually disconnects when fully charged now. I think my issue is with the overall balance of the pack.

 

[onemanvan]

I've been using the Kisae DMT1250 for the past 4 years with my DIY lithium build. I've experienced problems with both the lithium & programmable charge profiles. No doubt some of that was due to my ignorance of how the charger works. Nevertheless even after gaining a better understanding of how it works I still feel as though it sometimes has a mind of it's own... As others have mentioned the BMS should sit in the background 99% of the time. It's primary purpose is to manage any catastrophic faults. It's my understanding that best practice with regard to charge profile is to incorporate cyclical charging. IE: charge the pack to a conservative threshold then disconnect the charger - when the pack voltage sags to a given threshold turn the charger back on. The overall goal is to set HVDR such that, under normal operating conditions where good solar input is available, the end of day state of charge of the battery should be somewhere around 90%. So - to answer your specific question - if your batteries are being used on a daily basis it won't hurt to maintain them at 90% SOC. Personally I'm not confident that the Kisae can reliably & accurately do this. As I've already said - it sometimes seems to have a mind of it's own... Which is why I made a couple of modifications to it.


1) The Kisae has an ON/OFF switch - by adding an SPST relay in parallel with the ON/OFF switch I can leverage the relay port of the Thornwave battery monitor to turn the Kisae ON/OFF remotely. In this way I have very good control over the charging profile. FWIW: I consulted with Ricardo at Kisae technical support about the relay mod and he said it was OK to do this.

Note: I repurposed the 'Ignition start' terminal on the Kisae to serve as a ground connection for the relay. The Thornwave relay port can take a small ( less than 1 amp ) load to ground when configured to do so.



2) The Kisae has a temperature sensor port. This can be repurposed to serve as a protection against charging when the temperature approaches freezing. I used a simple bimetallic snap switch attached to the battery pack to achieve this.





I believe this article best describes the preferred charge profile characteristics for lithium batteries:

https://2n1s7w3qw84d2ysnx3ia2bct-wp...Phosphate-LiFePO4-Generic-Charge-Settings.pdf

PS: As an added bonus the Thornwave Powermon can be connected to an external shunt to provide an accurate gauge of SOC. I know - the Overkill can provide SOC as well - but IMHO the Powermon is more accurate & reliable...

 

[Pyke13]

The overkill bms does measure state of charge but doesn’t measure current draw under 1 amp or at least not very accurately. The aili battery monitor and the Kisae both seem to agree on state of charge going by when they both say the battery is fully charged.
I’ve found a way round the kisae so I’m cycling the batteries, I’ve added a switch and simply switch it off when the battery is fully charged and switch it back on when I drop to around 12.7v. It’s not the most sophisticated method but it works. I’ve also got a Christmas tree voltage gauge to help me keep an eye on voltage.
As winter comes I will just leave it on all the time. It’s not an ideal solution but it’s a solution.

 

[jayswain]

I was (and still sort of am) having this same issue with my Kisae DMT1250. I had to set the termination current to 10 amps for my 280ah lifepo4 bank. I'm still learning how it affects the switch from bulk to float/absorb charge, but it usually disconnects when fully charged now. I think my issue is with the overall balance of the pack.

View attachment 49339

I'm also seeing odd behavior when using the DMT1250 and a 200ah battery w/ built in BMS. It seems that when nearing the end of the absorption phase the amps and voltage will bounce, ranging from 0a to +9a. Sometimes the charger will then enter the Float stage, but I've also seen a handful of times that the charger doesn't fully go into the float state, but also not displaying a charge state (no BUL, Abs or FLO reading, just the ch1 voltage). I talk about it in another post here: https://diysolarforum.com/threads/p...gramming-profile-for-newbie.11379/post-883599

Current Lifepo settings I have for the charger are inline with the battery spec:

Abs: 14.6
Flo: 13.6
Termination current: 5a (battery suggests 4a)

I'm curious if I should be concerned when I see the charger in this non-charging state, when it seems to be feeding power to the system, but it doesn't seem like the battery is taking it. (Battery monitor shows +30a, but charger doesn't display ABS/FLO). I'm also curious of what termination current you ended up with and if you experienced any of this or similar behavior, and if it improved it.