Opinions on Chevy Volt Battery part time BMS ?

[Salts]

The problem is when you go to switch the sensing wires the REC will see low voltage and shut down the system. I'm not sure what it's sensing interval is - if you are quick it might be ok!

I'm interested to see what you come up with.

I don't think you're even allowed to play with those wires when the system is active.. Judging by their instructions on what wire to plug in and in what order, I'm pretty sure (90% anyhow) that all I'd have to do is turn the BMS off and flop out the connector.

[tom rickard]

That's fine if you can live with power outages. Would be no problem to do

[tom rickard]

You should email REC directly with your situation, they were very helpful with my queries- their SI module used to cater for 15 cells, now it is 16

[Ampster]

The problem is when you go to switch the sensing wires the REC will see low voltage and shut down the system. I'm not sure what it's sensing interval is - if you are quick it might be ok!

If changing cables creates a problem for the BMS, a Y cable to keep one string powered while the other is disconnected might be a workaround.

BTW the OP mentioned a recharge unit and I have a suggestion. Forget it. They are only needed for EV motor controllers that have big capacitors that draw a huge sure for a second causing wear on the contactor. EVs use the contactor every time the car is turned on but in your case the inverter will always be on. Invest in a contactor that has a power saving mode. Those take very little power to keep them closed once the contacts are closed. The contactor is a fail safe because if the low voltage power to the BMS fails the contactor will open.

[tom rickard]

A Y cable (or double Y for 3 strings) is a good idea.

The precharge unit is to protect the Contactor when you start up the inverter. Even just one time the inrush current from the Inverter charging the caps can weld the contactor together.

I reckon there would be more than a few systems where the contactor will never open again (i personally have seen one)

I wouldn't use a contactor without a precharge unit, and i doubt you'll get a different answer from your contactor manufacturer.

The other common problem i see with off grid power supplies is that a single contactor is used. Most contactors are directional.

[Ampster]

A Y cable (or double Y for 3 strings) is a good idea.

The precharge unit is to protect the Contactor when you start up the inverter. Even just one time the inrush current from the Inverter charging the caps can weld the contactor together.

I reckon there would be more than a few systems where the contactor will never open again (i personally have seen one)

I wouldn't use a contactor without a precharge unit, and i doubt you'll get a different answer from your contactor manufacturer.

Yes I agree it is there to protect the contactor and I have experienced a contactor with welded contacts on an EV. I guess it depends on the inverter. Mine seems to have little inrush current but I have never measured it and I often turn on the contactor before I throw the DC breaker. I guess it is good insurance against welded contacts which creates a failure in the closed scenerio which could defeat the purpose of having a contactor in the first place.

The other common problem i see with off grid power supplies is that a single contactor is used. Most contactors are directional.

I have used KiloVac units but I have never seen the terminals as having polarity. Is that what you mean by directional? My KiloVacs seem to be agnostic about whether they are on the negative leg or the positive leg and there is no indication whether one terminal should be on the load side vs the source side. I am always open to new knowledge.

[Salts]

That's fine if you can live with power outages. Would be no problem to do

A power outage to flop out the BMS connector wouldn't be any problem at all. Besides, even if it was an issue, I could just put a bypass switch in to keep the contactor engaged while I swap the BMS cell tap connector. I don't think I would even bother though. Its my home, not a hospital with a life support system.

[jflorey2]

Oh man, thank you for all that, you're a huge help. Now I have to digest it for a bit while I do more reading. My current thoughts are that I'm going to go with the Rec BMS and just swap the BMS leads from string to string every couple of cycles.

Geez, don't do that. If you need balancing/monitoring on all of them, get a BMS for all of them. Don't skimp on protection; the downside is a fire.

[tom rickard]

I have used KiloVac units but I have never seen the terminals as having polarity. Is that what you mean by directional? My KiloVacs seem to be agnostic about whether they are on the negative leg or the positive leg and there is no indication whether one terminal should be on the load side vs the source side. I am always open to new knowledge.

Not sure on the Kilovac units, the ones i've seen incorrectly installed are Gigavac GX14.

They aren't bi-directional.

[Ampster]

Not sure on the Kilovac units, the ones i've seen incorrectly installed are Gigavac GX14.

They aren't bi-directional.

I am still trying to understand what you mean when you say bidirectional. Are you talking about current flow like AC and DC?

[tom rickard]

Geez, don't do that. If you need balancing/monitoring on all of them, get a BMS for all of them. Don't skimp on protection; the downside is a fire.

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

All well and good to be an alarmist, but all 3 packs will still have pack level over and undervoltage protection.

It would be helpful if you had knowledge of how the batteries worked, and what state you could see occurring that would cause a fire.

I can see issues with 3 BMS having communication conflicts on the CANBUS, so i wouldn't go that way at all.

[tom rickard]

I am still trying to understand what you mean when you say bidirectional. Are you talking about current flow like AC and DC?

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 - )

You need to check with your manufacturer, i know this is true for the Gigavac GX14, which some people were selling as suitable for off-grid power systems.

[Salts]

Geez, don't do that. If you need balancing/monitoring on all of them, get a BMS for all of them. Don't skimp on protection; the downside is a fire.

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.. 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. My network skills are basic at best and my programming skills are limited to some Visual Basic and database stuff. Not exactly hacker extraordinaire. The Rec-BMS comes pre-configured for controlling the Sunny Island.

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 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.
The Zeva BMS is just under half the price of the Rec-BMS, and if I add more Chevy Volt packs later, I can just add more of the cheap Zeva slaves.

Opinions?

[tom rickard]

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.

[Ampster]

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.

Yes that would eliminate multiple BMSs. The way the tabs are configured it might be tricky to drill holes. I will have to see what Dave Poz did.