Finally found a LiFePO4 BMS with Low-temp Charging Protection

[BrianS]

First, thanks a ton for the quick reply. I've typed my question's in red under each of your responses below. You'll have to click on the expand button to read them and I think nomenclature is the sticky point here. I say that because in your first paragraph you allude to the fact that float and absorption voltage are settings in the charger but the following sentence it seems say that the BMS controls the charger, I'm confused...

OK, so float and absorbtion voltage are settings on your charger. The BMS will disconnect the charger when the voltage rises to the Trigger value, either on one cell or the whole pack. The release value is when charging will be re-enabled.

example: using an unregulated charger, cell 1 rises to 3.650v (the high voltage cutoff, at this point the cell is over 100% charged) the BMS will disconnect/ block any more charging current. It will stay like this until the cell drops down to 3.500v, (from discharging)

You should be using a regulated charger that has a voltage limit (absorption voltage limit) set at like 14.2 to 14.4

I'm using the Renology DCC50S to charge my cells. I was unaware that there is a way to set absorption voltage in the DCC50S.

With the lithium pack that I've been experimenting with, it tends to cut off for a single cell high voltage before the whole pack gets to 14.4v, but I am using the cheapest cells I could find. (possibly mismatched capacities)I have some good cells on the way.

Inverter cutoff is a setting on your inverter which is probably not adjustable, none of my inverters have that adjustment. The low voltage cutoff (pack or single cell) will cutoff the discharge current at whatever value you set it. Recommend 2.5v per cell / 10v pack.

This terms mentioned above are terms Will uses in his "Recommended Charge Profile for DIY Batteries" I assumed these were for a BMS not the charger. My bad...

Below 3.000v per cell there is like 1% of the capacity left so if you want to preserve cell life set the low voltage at 2.9v per cell / 11.6v pack

The Overkill BMS has settings called "Cell under volt" and "Batt under volt" in which should I enter the above value? I have a 12.8v pack does that make a difference?

likewise, they are almost fully charged at 3.4v per cell.

Again the Overkill BMS has both "Cell over voltage" and "Batt over voltage" in which should I enter the above value?

A warning tho- having the BMS cutoff current can cause voltage spikes up the line that might damage your charger or load. Thats why they should be regulated so as to not reach the BMS cutoffs if possible.

The Overkill BMS does not have a Cutoff current setting but does have both "Charge over curr." and "Charge under curr." setting. Is that the same and should I change those?

The Github and reddit pages are a work in progress, I will probably polish this post a bit and put it in both places.

If I missed part of your question please let me know.

One more item, most say you should not charge a LiFePo4 battery below freezing. In the "Protection" parameters section of the settings Overkill BMS has a field called "Charge under temp" and it is set at -1C. Is this a correct value as that figure is below freezing?

Perhaps if you could please take a look at the pic below and edit it to reflect best value for a LiFePo4 battery?

 

[Sgt Raven]

First, thanks a ton for the quick reply. I've typed my question's in red under each of your responses below. You'll have to click on the expand button to read them and I think nomenclature is the sticky point here.



Perhaps if you could please take a look at the pic below and edit it to reflect best value for a LiFePo4 battery?

View attachment 9716

The Renogy has different profiles depending on Battery type selected, but I think it is what it is and is not customizable.
IIRC what color is flashing depends on what battery type is picked.... for that...
You want to set both cell limits and total battery limits. Not going to say exactly what they should be. I'll let someone smarter list those.

 

[BrianS]

Thanks guys your help is very much appreciated. BTW please reread above post as I edited it and added some additional questions.

 

[Sgt Raven]

Thanks guys your help is very much appreciated. BTW please reread above post as I edited it and added some additional questions.

Just to be safe, it wouldn't hurt to set the cold charge cutoff to 3-4℃. Charge profile could be about the DC-DC/MPPT or about the BMS, they both play a part in it. The BMS is the last form of protection and if it doesn't have to do anything in protecting the battery, it's better that way. When it kicks in, it's a hard cut off and may not be kind to the rest of the system.

I was looking real hard at the Renogy, but went with a Kisae instead. Neither is Right/Wrong, just a little different in some areas.
Time will tell if I made a good or bad choice.....

 

[OverkillSolar]

First, thanks a ton for the quick reply. I've typed my question's in red under each of your responses below. You'll have to click on the expand button to read them and I think nomenclature is the sticky point here. I say that because in your first paragraph you allude to the fact that float and absorption voltage are settings in the charger but the following sentence it seems say that the BMS controls the charger, I'm confused...



One more item, most say you should not charge a LiFePo4 battery below freezing. In the "Protection" parameters section of the settings Overkill BMS has a field called "Charge under temp" and it is set at -1C. Is this a correct value as that figure is below freezing?

Perhaps if you could please take a look at the pic below and edit it to reflect best value for a LiFePo4 battery?

View attachment 9716

First, thanks a ton for the quick reply. I've typed my question's in red under each of your responses below. You'll have to click on the expand button to read them and I think nomenclature is the sticky point here. I say that because in your first paragraph you allude to the fact that float and absorption voltage are settings in the charger but the following sentence it seems say that the BMS controls the charger, I'm confused...



One more item, most say you should not charge a LiFePo4 battery below freezing. In the "Protection" parameters section of the settings Overkill BMS has a field called "Charge under temp" and it is set at -1C. Is this a correct value as that figure is below freezing?

Perhaps if you could please take a look at the pic below and edit it to reflect best value for a LiFePo4 battery?

View attachment 9716

sorry for confusing the terms.

first, understand the limitations of a BMS- it is just an intelligent on-off switch. It doesn’t have the ability to regulate the power going in or out, it can only shut it off when something goes beyond the limits.

I am taking suggestions for the perfect set of parameters to ship with the BMS. I believe will has stated that +1C is good for the low temp cutoff. -1C is what the manufacturer shipped it with. I can have it changed in production or do it myself when I’m inspecting and shipping these. There is a lot of different opinions out there about the correct parameters including between battery manufacturers.

I need to get on my computer and reply to your notes, I can’t see them on my phone while typing

 

[OverkillSolar]

That's like, I'm curious what the difference beside the extra 2 wires, between the 100A and 120A versions. Not so curious I'm going to pull the heat shields off of them. Because I don't know if I could tell the differences if I did.... But it does make me go hmm. But if someone else did and made a video of it, I would watch it.

How hot do you think it was to melt the solder? My Chinese Ksger Digital Soldering Station, that uses T12 type tips, can go to 480℃, but I've read it is real hard on the tips and maybe the whole thing.

here is another option- the holes are big enough for a 10-32 bolt

 

[Sgt Raven]

here is another option- the holes are big enough for a 10-32 bolt

Thanks Steve, you're a help to the forums. and those who are using these BMSs...

 

[dashdrum]

If I had to say how hot I would say very, lol. I think it’s more a question of thermal mass.

I have not had a 100a unit in my hands. If someone wants to I would be glad to compare and contrast. I’ll put it on my list to take some naked pictures. Of a BMS.

DIY Solar After Dark - Naked BMS Pics!!!

 

[BrianS]

Perv’s!!!

 

[OverkillSolar]

I talked to my supplier overnight, they said they can ship some units without wires, but the production manager thinks its too hard to solder 8ga wires.

If you really want 8ga wires, Email me and we can make a deal. I think I can do the rework myself, and then you wouldn't have to worry about damaging your BMS after you buy it. There will be an extra charge for this, but maybe we can do the transaction outside of amazon to offset the cost.

I am going to do some testing today and measure exactly how much power is lost in the wires.

 

[BrianS]

Ask them to just leave the wires out? Are you putting two wires on per hole, is that necessary?

plz check your mailbox later I’ll send a PM to you Soon.

Edit: oh dear! My reading comprehension could use a buff.

 

[OverkillSolar]

First, thanks a ton for the quick reply. I've typed my question's in red under each of your responses below. You'll have to click on the expand button to read them and I think nomenclature is the sticky point here. I say that because in your first paragraph you allude to the fact that float and absorption voltage are settings in the charger but the following sentence it seems say that the BMS controls the charger, I'm confused...



One more item, most say you should not charge a LiFePo4 battery below freezing. In the "Protection" parameters section of the settings Overkill BMS has a field called "Charge under temp" and it is set at -1C. Is this a correct value as that figure is below freezing?

Perhaps if you could please take a look at the pic below and edit it to reflect best value for a LiFePo4 battery?

View attachment 9716

I read the manual for your Renogy DCC50S. it has 4 presets for battery types, no custom settings. Use the lithium setting.

If you set the over/under volt settings very conservatively to prolong the life of your cells you can expect the BMS to disconnect before the charger finishes a charge cycle. Other chargers with a custom profile could be set to stop charging at a lower value like 14v for a more elegant solution.

Always keep the lead acid starting battery connected to the alternator, that way if the lithium battery finishes charging and cuts off from overvoltage, the lead acid battery will moderate the power from the alternator and prevent any damage to it.

I do not know what will happen to your charger in MPPT mode if the BMS disconnects. Most of these things warn you not to disconnect the battery before the solar panels- it can result in damaging voltage spikes. For this reason it might be wise to keep the BMS over voltage limits high enough to not disconnect your charger. Another option is to make sure there is a lead acid battery in the circuit because it will always absorb those voltage spikes. A capacitor might work too, like the ones they sell for stereo systems.

yes, you can mix lithium and lead acid in a parallel battery bank.

The Overkill BMS has settings called "Cell under volt" and "Batt under volt" in which should I enter the above value? I have a 12.8v pack does that make a difference?

Again the Overkill BMS has both "Cell over voltage" and "Batt over voltage" in which should I enter the above value?

set "cell under volt" and "cell over volt" to the high and low limits for a single cell. (stock parameters are 2.200v to 3.650 volts) actual settings are expressed in millivolts.
set "batt under volt" and "batt over volt" to the 4x cell voltage minus a little bit. you can set them to exactly 4x the corresponding cell cell voltage but then it will always get to a single cell over/under condition first, because they are never perfectly balanced.

The Overkill BMS does not have a Cutoff current setting but does have both "Charge over curr." and "Charge under curr." setting. Is that the same and should I change those?

I was still referring to under/over volt settings. the over current settings will disconnect either charge or discharge current if it gets too high. for example if you have a 2000w inverter on a battery with 1 bms, it will be limited to about 1500w because the current draw from the battery is ~120a

if you start drawing 2000w like from a hair drier, the current will go up to ~160a and the BMS will disconnect the discharge current, shutting off the inverter. you would need 2 batteries (4 cells each) each with their own BMS to supply that much current to a large inverter.

 

[Sgt Raven]

I read the manual for your Renogy DCC50S. it has 4 presets for battery types, no custom settings. Use the lithium setting.

If you set the over/under volt settings very conservatively to prolong the life of your cells you can expect the BMS to disconnect before the charger finishes a charge cycle. Other chargers with a custom profile could be set to stop charging at a lower value like 14v for a more elegant solution.

Always keep the lead acid starting battery connected to the alternator, that way if the lithium battery finishes charging and cuts off from overvoltage, the lead acid battery will moderate the power from the alternator and prevent any damage to it.

I do not know what will happen to your charger in MPPT mode if the BMS disconnects. Most of these things warn you not to disconnect the battery before the solar panels- it can result in damaging voltage spikes. For this reason it might be wise to keep the BMS over voltage limits high enough to not disconnect your charger. Another option is to make sure there is a lead acid battery in the circuit because it will always absorb those voltage spikes. A capacitor might work too, like the ones they sell for stereo systems.

yes, you can mix lithium and lead acid in a parallel battery bank.

The Overkill BMS has settings called "Cell under volt" and "Batt under volt" in which should I enter the above value? I have a 12.8v pack does that make a difference?

Again the Overkill BMS has both "Cell over voltage" and "Batt over voltage" in which should I enter the above value?

set "cell under volt" and "cell over volt" to the high and low limits for a single cell. (stock parameters are 2.200v to 3.650 volts) actual settings are expressed in millivolts.
set "batt under volt" and "batt over volt" to the 4x cell voltage minus a little bit. you can set them to exactly 4x the corresponding cell cell voltage but then it will always get to a single cell over/under condition first, because they are never perfectly balanced.

The Overkill BMS does not have a Cutoff current setting but does have both "Charge over curr." and "Charge under curr." setting. Is that the same and should I change those?

I was still referring to under/over volt settings. the over current settings will disconnect either charge or discharge current if it gets too high. for example if you have a 2000w inverter on a battery with 1 bms, it will be limited to about 1500w because the current draw from the battery is ~120a

if you start drawing 2000w like from a hair drier, the current will go up to ~160a and the BMS will disconnect the discharge current, shutting off the inverter. you would need 2 batteries (4 cells each) each with their own BMS to supply that much current to a large inverter.

mndare has found with the upgrade to 3 8AWG wires the BMS can handle over the 120A of the BMS, FYI... IIRC hes is in the 150A range without any problems......

 

[drinskta]

Anyone know where to get this BMS quickly. I ordered one from aliexpress over a month ago and it is still not here and the tracking says it was cancelled.

 

[OverkillSolar]

Anyone know where to get this BMS quickly. I ordered one from aliexpress over a month ago and it is still not here and the tracking says it was cancelled.

I have them in my hand right now, I can ship today. I just disabled the Amazon-fulfilled inventory so if you order it now I will ship it.

remember the BT module is sold separately.

 

[OverkillSolar]

mndare has found with the upgrade to 3 8AWG wires the BMS can handle over the 120A of the BMS, FYI... IIRC hes is in the 150A range without any problems......

I am doing some testing right now, measured 30mv voltage drop on each set of wires, 40mv drop across the circuit board, at 118 amps current.

That's about 7 watts of power lost in the 10ga wires

 

[OverkillSolar]

mndare has found with the upgrade to 3 8AWG wires the BMS can handle over the 120A of the BMS, FYI... IIRC hes is in the 150A range without any problems......

My concern is for the transistors not the wires. More current- more heat. If you provided active cooling I bet you could push more than that.

I am planning a series of "Torture Tests", including pushing the current higher until something pops.

Send me your wish list if you want to test something else.

 

[BiduleOhm]

Yep, if you can remove the heatsink after your tests to see what mosfets they use it would be nice.

 

[OverkillSolar]

Yep, if you can remove the heatsink after your tests to see what mosfets they use it would be nice.

They are all the same, HY4903

https://datasheet.lcsc.com/szlcsc/HY4903B6_C133393.pdf

 

[BiduleOhm]

Wow, 30 mosfets, that's a lot for 120 A.

They are 30 V 314 A 1.7 mOhms mosfets. Pretty high spec for a 120 A BMS but they aren't from a top tier manufacturer though. A quick search told me you can get them for 0.75 USD each by 100 quantity but you can bet the manufacturer in china can get them for a fraction of that, especially in 1000s quantities.

My guess to why they used so many of them is to make the thermal management easier and/or account for pretty loose tolerances between each mosfets.

So yep, you can theorically pass a pretty high current (even double of the BMS rating if you want...) if you keep the fets cold enough but there's other things to account for: the PCB traces are decent but sooner or later they will act like a fuse, there's 2 mOhms current measuring shunt resistors and I have no idea about how much current they can handle (if you want to know you can measure them, deduce the package and use the specs of similar resistors), there's some protections (probably bidirectional TVS diodes, 10 of them in a row at the top) probably against the inductive voltage spike at turn-off and they can't handle an infinite amount of energy either (can't find anything with 1950 IEZ or 1EZ).

Edit: ah, the package limits them to 130 A continuous each, but that's still a lot higher than necessary so no pb.