Opinions on Chevy Volt Battery part time BMS ?

[Ampster]

To be clear, i mean the main contactor terminals have to be connected so DC current being disconnected under load is flowing in only one direction.

(ie the main contact terminals have a + and - )

No worries, it doesn't matter with Kilovac. I hung out on the diyelectricar.com forum for a few years and never heard that one before. But I never heard of Gigavac either. FWIW, I also thought DC flowed in only one direction. Never seen otherwise.

[tom rickard]

When you get to higher current contactors they often have arc suppression circuitry, this only works for one polarity. In an EV, the contactor primarily is used for a low voltage disconnect. In a stand alone power system, you are as likely to get a full charge current failure as a low voltage failure.
(i've seen more systems fail due to overcharge than low voltage)

In any case, plenty of people are using the Gigavac as a disconnect for both HV and LV - that doesn't make it right.

[Salts]

I'm still not sure what you have against a linking wire (eg 10a) between each parallel pair? I can't see a problem with this approach.

Nothing against it specifically, but I do have a few concerns that I haven't worked out yet. What is (eg 10a) ?
1) What ga wire is needed?
2) I would have to fuse each wire, that's 96 fuses and 96 fuse holders. (Technically, I think it I could get away with 84 fuses) That's still a lot of possible bad connection points, points of possible failure, and points of more resistance. The Chevy Volt has a BMS wiring connector built into the cell modules, all I have to do is use the original connector, but as was stated, the wires aren't big enough for paralleling cells and dealing with the eddy currents.
3) I'm not sure if I understand the various failure modes that can happen with paralleling 8 cells with the BMS that way. I am hesitant to do things that I don't fully understand.
4) If a single cell in one string starts to have problems, the BMS won't know it until it starts to affect the other cells. I'm not entirely sure if this is even a problem or not because I think that eventually it would start to drag down the other cells and the BMS would at least see that, maybe not as early, but it should show up eventually.


I did consider using 4 BMS's and paralleling two strings to each BMS. That would cut the number of fuses down by half to just 48. Still a lot of fuses though.

This battery bank is going to be in my basement. It will all be mounted inside a large industrial Hoffman steel box that is 4ft tall, 3ft wide and 20 inches deep with a hinged door. I think its made of 14ga steel. Obviously, I need to balance safety and cost so I'm not for or against anything yet.

You guys are a boat load of help.. Can't thank you enough.

[Ampster]

It sounds like you have done a lot to reduce the risks associated with Lithium batteries. I disagree with the comment about absolutely needing a BMS to prevent a fire. They can give you an early warning that a cell has become weak but they can't necessarily prevent a thermal runaway. There is even a lively debate about whether a BMS creates more risks than benefits. I am not of that belief. I think a BMS is a very important management information system. You can be a BMS with a multimeter. As I have said earlier, it is most important to observe the cell differences as the charge or discharge cycles approach the knees of the charge/discharge curves. Sacrificing some capacity by conservatively setting your high and low voltage set points can extend the life of your pack and reduce the risk of thermal runaway.

The idea of connecting all the cells to be able to have one BMS is more about convenience. In the case of the Volt batteries it is not very convenient. You are correct in your comment #4 that paralleling the cells might mask the problem that only one cell is going bad.

[Salts]

It sounds like you have done a lot to reduce the risks associated with Lithium batteries. I disagree with the comment about absolutely needing a BMS to prevent a fire. They can give you an early warning that a cell has become weak but they can't necessarily prevent a thermal runaway. There is even a lively debate about whether a BMS creates more risks than benefits. I am not of that belief. I think a BMS is a very important management information system. You can be a BMS with a multimeter. As I have said earlier, it is most important to observe the cell differences as the charge or discharge cycles approach the knees of the charge/discharge curves. Sacrificing some capacity by conservatively setting your high and low voltage set points can extend the life of your pack and reduce the risk of thermal runaway.

The idea of connecting all the cells to be able to have one BMS is more about convenience. In the case of the Volt batteries it is not very convenient. You are correct in your comment #4 that paralleling the cells might mask the problem that only one cell is going bad.

Thank you for your help, you are a wealth of knowledge.

This goes back to using just one BMS and swapping it from string to string at regular intervals. Turn BMS Off (battery contactor opens), remove BMS Cell Tap Plug, swap in a different plug, turn BMS on)
Also, I don't plan to push my batteries to their limits. I have no problems with adding more battery capacity in order to stay away from those charge-discharge knees. I plan to charge to 4.1 and will discharge to 3.5. I'm having a hard time figuring out exactly how much energy this will harvest from my 2014 Chevy Volt pack, but my best guess is that it will give me about 10kw-11kw from a 16.5kw pack. The good news is that what I give up in capacity, I get back in life cycle longevity of the pack.

[jflorey2]

The problem is that I have 8 individual 12s strings (Chevy Volt) and each of them would require a BMS which would be very very expensive.

You can get a 12S li-ion BMS from Alibaba for $10.00 each. (I would post a link but that doesn't work well here - search for Battery PCB BMS for 12S 44.4V Li-ion/Li-Polymer Battery Packs)

I assume $10.00 isn't very very expensive to you.

Then there's the Sunny Island. I think it is preferable to have a BMS communicate with it so that it benefits from the best charging protocols. Problem is that I know nothing about how CAN BUS works.

CAN BUS is just the physical layer. It's basically a serial interface. The hard part is what protocol rides on top of the CAN bus. MODBUS is a popular one.

So what's better: 1) A cheaper BMS like the Zeva (Master + 8 Slaves) but no Sunny Island CAN BUS communication, or 2) A higher quality Rec-BMS on a single string but with full Sunny Island communication through the CAN BUS ?

I'd do one good BMS so the Sunny Island gets good info and the rest cheapo BMSes. You can use the onboard power switch for the BMS, or use a contactor (or remotely triggered breaker) to cut power when a cell goes out of bounds (safer.)

I started to think that I should start with the Rec-BMS. Install the battery safety contactor and have the CAN BUS control the charging. Then later, I could get a Zeva BMS (Master + 7 slaves) for the rest of the pack. I could wire the Zeva BMS so that it also controls the battery safety contactor along with the Rec-BMS through a relay. If both BMS's don't agree then the contactor stays open until they do.

That sounds good. Again, consider a remotely operated breaker rather than a contactor; safer and less power required.

[jflorey2]

Exactly what is going to cause a fire with switched sensing only wires that is not going to happen with 3 individual BMS's?

To do that you will have to switch at least 13 wires, which means a lot of wires and a lot of relays. When you switch the BMS will likely open the safety contactor because the sensed cell voltage will go to zero momentarily. So you put in a filter or some caps or something to "hold up" either the battery sense leads or the output to the contactor. Now you've slowed down the response of the system, and it will not work as designed. Will it still protect the pack? Maybe, maybe not. Plus you are now dealing with 39 wires - and those are raw cell voltages, and if any of them gets misconnected or shorted, a fire is inevitable.

[Salts]

To do that you will have to switch at least 13 wires, which means a lot of wires and a lot of relays. When you switch the BMS will likely open the safety contactor because the sensed cell voltage will go to zero momentarily. So you put in a filter or some caps or something to "hold up" either the battery sense leads or the output to the contactor. Now you've slowed down the response of the system, and it will not work as designed. Will it still protect the pack? Maybe, maybe not. Plus you are now dealing with 39 wires - and those are raw cell voltages, and if any of them gets misconnected or shorted, a fire is inevitable.

Hi jflorey2,
Thank you for your input. The REC-BMS I am considering using has this long green plug that all the BMS cell tap wires connect to. After they are all connected to the green plug, the plug is then inserted into the BMS. My idea is to order 7 extra plugs so all that I have to do is turn the system off, unplug a single connector, and plug the next one into the BMS. Restart the system, and I'm done. In other words, I'm not physically switching 13 wires going into screw terminals and having to connect and disconnect each individual wire. Its just a single multi-pin plug that each 12s string will have pre-wired.

Yes, the main battery contactor will open and shut down the system, I will shut down the system manually in the proper order so as not to put any stress on the BMS or the Sunny Island.

This is an off-grid system for my home. We do not run a surgical operating room where having power all the time is a life or death issue. Its just an off-grid emergency system to keep the freezers cold, the sump pump running, and a desktop computer or two going.

I have a lot of flexibility to work with.

[Ampster]

When you get to higher current contactors they often have arc suppression circuitry, this only works for one polarity. In an EV, the contactor primarily is used for a low voltage disconnect. In a stand alone power system, you are as likely to get a full charge current failure as a low voltage failure.
(i've seen more systems fail due to overcharge than low voltage)

In any case, plenty of people are using the Gigavac as a disconnect for both HV and LV - that doesn't make it right.

I think this conversation is becoming a distraction to the needs of the OP. In his situation the voltage is 48 volts and the maximum amperage of two 5k inverters at full load is 200 Amps.
I do agree that high voltage and low voltage disconnect logic is needed in a stationary pack. Damage to lithium batteries can happen at either end of the charge/discharge curve and that is why that is necessary.

[Salts]

I think this conversation is becoming a distraction to the needs of the OP. In his situation the voltage is 48 volts and the maximum amperage of two 5k inverters at full load is 200 Amps.
I do agree that high voltage and low voltage disconnect logic is needed in a stationary pack. Damage to lithium batteries can happen at either end of the charge/discharge curve and that is why that is necessary.

Rec-BMS offers a 48 volt contactor that I plan to order with their BMS. I have two 175amp DC circuit breakers I plan to use with 2/0 battery cable. I purchased a TEMco 11 ton hydraulic crimper and will probably also add solder to all the large battery cable lugs in order to reduce any and all possible points of adding resistance to the connections. Total battery cable length should be less than 10 feet to each Sunny Island.

The Sunny Island also has its own DC Breaker built in. I'm wondering if I should add a fuse or if the DC breakers will be sufficient.

[Ampster]

You can get a 12S li-ion BMS from Alibaba for $10.00 each. (I would post a link but that doesn't work well

That is a good suggestion. I would put emphasis on the reporting ability of a cheap BMS. If you can't set voltage parameters then all it would be useful for is a remote cell voltage reporting device. That make diagnosis simpler.

I'd do one good BMS so the Sunny Island gets good info and the rest cheapo BMSes. You can use the onboard power switch for the BMS, or use a contactor (or remotely triggered breaker) to cut power when a cell goes out of bounds (safer.)

That sounds good. Again, consider a remotely operated breaker rather than a contactor; safer and less power required.

Another good suggestion. The only issue is it would not provide fail safe protection in the event that the power to the BMS contactor circuitry failed. If CAN BUS communication is successful the Inverter might detect the BMS going offline and shut down. As suggested earlier that might be the situation and the remote breaker would work well. It is a much better solution for a stationary pack that will be in the on position all the time compared to an EV that is on and off several times a day.

[jflorey2]

Thank you for your input. The REC-BMS I am considering using has this long green plug that all the BMS cell tap wires connect to. After they are all connected to the green plug, the plug is then inserted into the BMS. My idea is to order 7 extra plugs so all that I have to do is turn the system off, unplug a single connector, and plug the next one into the BMS. Restart the system, and I'm done. In other words, I'm not physically switching 13 wires going into screw terminals and having to connect and disconnect each individual wire. Its just a single multi-pin plug that each 12s string will have pre-wired.

So far I have killed two battery packs based on the "I'll just check them periodically" approach. I would not rely on anything that requires plugging and unplugging things to maintain safety.

Yes, the main battery contactor will open and shut down the system, I will shut down the system manually in the proper order so as not to put any stress on the BMS or the Sunny Island.

That's sorta hard on the system. Charging all that capacitance at once that often may lead to some early failures. EV's use precharge systems and minimize unnecessary system capacitance to deal with this; home inverters do not.

This is an off-grid system for my home. We do not run a surgical operating room where having power all the time is a life or death issue. Its just an off-grid emergency system to keep the freezers cold, the sump pump running, and a desktop computer or two going.

I'm actually not worried about reliability at all. If it breaks or shuts down on you, you can fix it, and the most you will lose is a freezer or two full of food. I am more worried about an unseen high impedance cell getting 8 volts across it and starting a fire. You generally can't "fix" a lithium ion fire that starts in your house. You can't extinguish such fires and you can't move the battery out of your house once the fire starts.

On another forum, an ebiker charged his battery overnight in his garage. This was a little 500 watt-hour battery. The battery caught on fire and destroyed his garage. He was VERY fortunate they were able to save the house.

[Salts]

So let me see if I have this straight. You're suggesting I use the Rec-BMS on one string and hook up the rest with the cheap Chinese BMS's just to use as monitoring packs? Or should those cheap BMS's also perform balancing? I've heard people say that the BMS is one of the leading causes of pack failures.. I "assumed" they were talking about the cheap BMS's and not the units that cost several hundreds of dollars.. Am I wrong? I'm open to all input.

So far I have killed two battery packs based on the "I'll just check them periodically" approach. I would not rely on anything that requires plugging and unplugging things to maintain safety.
That's sorta hard on the system. Charging all that capacitance at once that often may lead to some early failures. EV's use precharge systems and minimize unnecessary system capacitance to deal with this; home inverters do not.

The Rec-BMS will have a precharger... They suggest I use it so that's what I think I should do.. Won't hurt anything right? Their operating manual even has the precharger drawn in with the Sunny Boy. Sales technique or legit component requirement?

I'm actually not worried about reliability at all. If it breaks or shuts down on you, you can fix it, and the most you will lose is a freezer or two full of food. I am more worried about an unseen high impedance cell getting 8 volts across it and starting a fire. You generally can't "fix" a lithium ion fire that starts in your house. You can't extinguish such fires and you can't move the battery out of your house once the fire starts.

On another forum, an ebiker charged his battery overnight in his garage. This was a little 500 watt-hour battery. The battery caught on fire and destroyed his garage. He was VERY fortunate they were able to save the house.

I'm a bit confused on how you are suggesting I use the cheap BMS's. Monitor only with ability to open the main battery contactor? Or have them fully functional and balancing the cells?

As for fire considerations, the entire pack will be inside a Hoffman hinged cover box that is constructed of 14ga steel. It is 4ft tall, 3ft wide and 20 inches deep. I know I'm going to have to vent it with a muffin fan. Not entirely sure how I'll set that up but I will take the possibility of a fire into consideration and maybe use some 4 inch stainless flex pipe and run it a few feet to cool any combustion gasses should they happen. I had actually considered installing a fire sprinkler in the cabinet.. If the temperature gets past a set point, the little wax plug melts and douses the entire inside of the cabinet. Same way the sprinklers work in any commercial building. I installed sprinklers in my garage because I do a lot of fabrication work in there and welding sparks go everywhere. Luckily I've never needed them, but it lets me sleep easier at night knowing that a stray spark in a garbage can isn't going to burn my home down.

Thank you again for your suggestions.. this is all good stuff.. I'm learning.

[Mike90250]

... I have two 175amp DC circuit breakers I plan to use with 2/0 battery cable........
......Total battery cable length should be less than 10 feet to each Sunny Island.......

2/O cable is rated for 145A, unless you are buying high temp insulation. And the distance you plan to run seems excessive with marginal wire.

If you have double checked your specs and are happy, proceed.

[Salts]

2/O cable is rated for 145A, unless you are buying high temp insulation. And the distance you plan to run seems excessive with marginal wire.

If you have double checked your specs and are happy, proceed.

Hi Mike, thank you for the prudence in double checking things like this.

I'm using Carolprene 2/0 90C Welding Cable (600V) Made in the USA

https://www.awcwire.com/mfg/general-cable/product/general-cable-carolprene-welding-cable

Specs say its good for 223 amps continuous duty. I don't have enough stuff assembled to check my distances yet, but I'm guessing from the 2" wide x 1/4" thick bus bar inside the battery box to the Sunny Island inverter(s) should be around 6 feet.. It might be as much as 8 feet, but I doubt it.

I have two DC circuit breakers Eaton Heinemann GJ1 175ADC w/2sec delay. Considering the addition of a fuse to back them up. If nothing else, they make for a good disconnect switch.

Thank you for watching out.