To BMS or not to BMS?

[learning all the time]

Ok long explanation. I have a 25 ft motorhome. I just upgraded to 560AH LifePo4 batteries from Shenzen. I have an Overkill BMS hooked up to the system. I had shore power with a generator via transfer switch. I had a 2000w renogy inverter with two outlets plugged into it and 500w of solar. Everything was working fine and dandy. I upgraded to a Samlex Evo 3012 so all equipment would be able to power up either battery, shore or generator power. I am not beating my system down by running the AC or any high draw system.

Here is the problem: While hooked up to shore power the Overkill BMS keeps shutting down one of the cells for overcharge reasons. That cell gets to 3.65 while the others are at 3.43. It starts balancing then charging then cut off and repeat. While it does this it is causing my coach power to turn on and off. I have the fuse from the converter battery charger pulled as it is a stock converter and does not charge LifePo4 correctly. It is my understanding the converter should take over 12v duties and the battery should just charge. But when the BMS turns on and off my power turns on and off. Without the BMS I do not get the fluctuations with power.

I am tempted to remove the BMS but do not want to destroy the batteries. I read an article here that said it would be OK but then a lot of contradictory replies and responses. Has anyone had this issue or an issue like this? Is there a way to bypass this? I am truly at a loss why the battery charge is creating a fluction in power consumption. Please help.

 

[snoobler]

I am tempted to remove the BMS but do not want to destroy the batteries.

The BMS is activating to PROTECT the cell. Balancing is SLOW. double check that balancing is properly configured.

You need to adjust the absorption voltage down until it stops tripping off:

https://samlexamerica.com/documents/manuals/11025-EVO-2212-3012-2224-4024-0320_Lrez.pdf

 

[Craig]

Quick answer If your BMS is reacting to a cell overvoltage situation. You need the BMS as if you don't that cell will keep getting higher in volts if charging us not stopped. Not sure if you have other problems or not. But definitely it sounds as if a cell is out of whack.

 

[learning all the time]

The BMS is activating to PROTECT the cell. Balancing is SLOW. double check that balancing is properly configured.

You need to adjust the absorption voltage down until it stops tripping off:

https://samlexamerica.com/documents/manuals/11025-EVO-2212-3012-2224-4024-0320_Lrez.pdf

Thanks snoobler. I just printed the overkill manual at work so I will read up on balance configueration. I looked at it earlier but it saids very clearly do not do this if you do not know what you are doing. I will also look at the absorption rate of the samlex.

 

[learning all the time]

Quick answer If your BMS is reacting to a cell overvoltage situation. You need the BMS as if you don't that cell will keep getting higher in volts if charging us not stopped. Not sure if you have other problems or not. But definitely it sounds as if a cell is out of whack.

Thanks Craig. That cell definately charges faster than the others. I may need to pull out my actuall balancer and try it.

 

[HRTKD]

Something isn't right. You are correct that if the BMS shuts off charge and discharge, the converter in your Samlex should supply 12v power to the rest of the coach. But it depends on how you have it wired into the system.

Let's start with the BMS. The charge and discharge cutoffs are independent. It will decide that the batteries are at their 100% state of charge, but you should still be able to draw power from the battery.

With regard to the old converter, there should be an AC circuit breaker you can throw that will prevent the converter from getting any power from the shore/generator. I still have the old converter in my trailer, but I put it on a circuit breaker that is separate from everything else and I also disconnected the DC leads coming out of the converter from the main 12v distribution panel. It is effectively dead from both directions. I doubt this is your problem, but it might be good to disable it completely.

 

[Craig]

Something isn't right. You are correct that if the BMS shuts off charge and discharge, the converter in your Samlex should supply 12v power to the rest of the coach. But it depends on how you have it wired into the system.

Let's start with the BMS. The charge and discharge cutoffs are independent. It will decide that the batteries are at their 100% state of charge, but you should still be able to draw power from the battery.

With regard to the old converter, there should be an AC circuit breaker you can throw that will prevent the converter from getting any power from the shore/generator. I still have the old converter in my trailer, but I put it on a circuit breaker that is separate from everything else and I also disconnected the DC leads coming out of the converter from the main 12v distribution panel. It is effectively dead from both directions. I doubt this is your problem, but it might be good to disable it completely.

The Samlex at least my Samlex EVO will not work if the batteries are cut off. It will charge them if they are low but if I disconnect battery Loose all power AC and DC there could be a setting though that I do not know about as it has never been a problem for me. Even with a Chargery BMS if the discharge were to cut off the Samlex I think would shutdown if you had a relay between battery and Samlex. This is why I chose to kill the AC out of the Samlex with a relay so if battery drops short for some reason (say if there were a grid outage and batteries drained low due to no solar for a few days.)the Samlex would still run. I have Samlex set to only charge the batterys to 25% of capacity so I do not have o\vervoltage set up bt if I did I would do the same and cut AC power.

In all my experiences with this stuff I personally think its best to keep the SCC and the Inverter running as long as possible. So I have opted to Shut down the Panels before the SCC and the AC afer the inverter/charger.

 

[Cal]

I am tempted to remove the BMS ...

Would you remove the light bulb if your low oil level indicator turns on in you motorhome? Is that your fix?

 

[HRTKD]

The Samlex at least my Samlex EVO will not work if the batteries are cut off. It will charge them if they are low but if I disconnect battery Loose all power AC and DC there could be a setting though that I do not know about as it has never been a problem for me. Even with a Chargery BMS if the discharge were to cut off the Samlex I think would shutdown if you had a relay between battery and Samlex. This is why I chose to kill the AC out of the Samlex with a relay so if battery drops short for some reason (say if there were a grid outage and batteries drained low due to no solar for a few days.)the Samlex would still run. I have Samlex set to only charge the batterys to 25% of capacity so I do not have o\vervoltage set up bt if I did I would do the same and cut AC power.

In all my experiences with this stuff I personally think its best to keep the SCC and the Inverter running as long as possible. So I have opted to Shut down the Panels before the SCC and the AC afer the inverter/charger.

Kind of makes me want to avoid any of the inverter/charger combinations. My independent converter certainly doesn't work that way.

 

[Craig]

I can not argue with this. They definitely have their pros and cons

 

[snoobler]

Would you remove the light bulb if your low oil level indicator turns on in you motorhome? Is that your fix?

I was trying to get to something like this, but I had nothing so succinct...

 

[learning all the time]

Something isn't right. You are correct that if the BMS shuts off charge and discharge, the converter in your Samlex should supply 12v power to the rest of the coach. But it depends on how you have it wired into the system.

Let's start with the BMS. The charge and discharge cutoffs are independent. It will decide that the batteries are at their 100% state of charge, but you should still be able to draw power from the battery.

With regard to the old converter, there should be an AC circuit breaker you can throw that will prevent the converter from getting any power from the shore/generator. I still have the old converter in my trailer, but I put it on a circuit breaker that is separate from everything else and I also disconnected the DC leads coming out of the converter from the main 12v distribution panel. It is effectively dead from both directions. I doubt this is your problem, but it might be good to disable it completely.

HRTKD you are correct. I can disconnect the AC switch on the converter however: when plugged into shore power the unit still needs it to do converting. My wiring is very basic. Shore power to the grid input, generator to the generator input and of course output main line of the breaker. I have verified that it is specifically with the BMS. Just not sure how to get the BMS to communicate with shore power. After a few minutes it does stop and everything is happy again. I could just run the coach of battery and use shore power to charge but want a solid fix.

 

[learning all the time]

Would you remove the light bulb if your low oil level indicator turns on in you motorhome? Is that your fix?

Of course not. Don't be silly. I would definately remove the thermostate if it was stuck and my vehicle was not really overheating though. It is a temporary fix til a real solution can be found.

 

[Dzl]

To BMS or not to BMS?

To BMS

Here is the problem: While hooked up to shore power the Overkill BMS keeps shutting down one of the cells for overcharge reasons. That cell gets to 3.65 while the others are at 3.43. It starts balancing then charging then cut off and repeat. While it does this it is causing my coach power to turn on and off.

Doesn't the JBD (overkill) BMS control charging and discharging separately? If a cell reaches the cutoff point and the BMS cuts charging, loads should be be unaffected right? How/why are you losing coach power when the BMS disconnects charging?

I would definitely remove the thermostat if it was stuck and my vehicle was not really overheating though.

In this case it sounds like your vehicle is overheating though:

That cell gets to 3.65 while the others are at 3.43.

The BMS is doing what it is there to do. Cut charging and/or discharging to protect the battery in an HVD/LVD situation. Seems the problem is either mismatched or unbalanced cells. Removing the BMS won't fix either problem, but will hide the problem. <-- not a good thing

 

[Craig]

The BMS is doing what it is there to do. Cut charging and/or discharging to protect the battery in an HVD/LVD situation. Seems the problem is either mismatched or unbalanced cells. Removing the BMS won't fix either, but will hide the problem.

I disagree it will not only hide the problem it can lead to complete pack demolition. If that cell keeps charging to 4 or 5 volts he will have real issues. That cell will keep getting higher and higher until it kills itself. The other cells will get lower and lower and the SCC won't know any better

 

[Dzl]

I disagree it will not only hide the problem it can lead to complete pack demolition. If that cell keeps charging to 4 or 5 volts he will have real issues. That cell will keep getting higher and higher until it kills itself. The other cells will get lower and lower and the SCC won't know any better

We don't disagree, we are just making the same point in different ways (maybe not clear enough in my case)

"Removing the BMS won't fix either, but will hide the problem" was intended to sound like a non-solution and a bad choice. In my experience hiding a problem rather than fixing it, generally leads to trouble, particularly if in hiding it you remove safeguards which are in place to protect the system.

 

[Craig]

We don't disagree, we are just making the same point in different ways (maybe not clear enough in my case)

"Removing the BMS won't fix either, but will hide the problem" was intended to sound like a non-solution and a bad choice. In my experience hiding a problem rather than fixing it, generally leads to trouble, particularly if in hiding it you remove safeguards which are in place to protect the system.

Lol I was only trying to say it could be much worse than hiding the problem. The hidden problem could be disasterous. And joking with you at same time.

 

[Craig]

How about this Craigism "To BMS so as not to be a mess!"

 

[Gazoo]

I just upgraded to 560AH LifePo4 batteries from Shenzen.

I can't remember everything everyone has posted in other threads but did you top balance the cells? And if you did which method did you use?

Also check the obvious. Check the voltage of the cells right at the cells terminals. Don't rely solely on what the BMS is telling you.

 

[HRTKD]

Doesn't the JBD (overkill) BMS control charging and discharging separately? If a cell reaches the cutoff point and the BMS cuts charging, loads should be be unaffected right? How/why are you losing coach power when the BMS disconnects charging?

I know that my Overkill Solar 4s BMS works this way. I was on shore power a week ago and the batteries were topped up already. In fact, I looked at the BMS configuration app and it showed that charging was cut off, while discharge was still active.