Opinions on Chevy Volt Battery part time BMS ?

[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]

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.

[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.

[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.

[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.

[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]

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]

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

[tom rickard]

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

[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]

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!

The current between parallel cells will be low, i

[Salts]

I don't think they would be very large unless one cell loses a lot of capacity. Then you would only see it as the other cells attempt to equalize voltage. That would not occur at resting but only as they charge or discharge. I think 17 Amps is a good starting point.

That sounds like a plan. The important BMS functions of high voltage and low voltage disconnects would be in place no matter which string is connected to the BMS. I have a pack of mixed Nissan Leaf cells and I just use my Orion BMS for monitoring. I have turned off the balancing since. What I would look for when you are monitoring is the difference between the highest cell and the lowest. If that delta changes as you discharge the pack then that might be an early warning that one cell has slightly less capacity and is heading for the knee of the curve before the others. If there is any it will show up at 4.1 at the top and 3.5 at the bottom. Because I have some weak cells {60% of original capacity) in my Nissan pack I set my voltages at 4.05 and 3.80. When the lowest cell is at 3.82 I have a 120 millivolt difference to compared to the top cell at 3.94. At the top the difference is only 10 millivolts. I did not bin my modules before assembling my pack and it consists of modules from 3 different Nissan Leafs. I think you have the advantage of your cells all coming from the same pack and the Volt packs are more robust than the Nissan packs.
Regarding balancing, I don't think you will need to balance very often. When you do, you will only want it to happen when you are in the CV phase of charging because that is when the Amps begin to taper off and the balancing shunts will be more effective. Presumably you can set the balancing on or off or at least the voltage at which it should start.

I think that is an important issue. When I got my Outback Skybox I was hoping to have CAN BUS connectivity but found out that unless there is similarity between the CAN BUS statements (not sure that is the right term) you can only monitor the CAN BUS traffic but not create actions. That was why I was skeptical in my earlier comment.

Hopefully the REC gives you some remote capability with logging. That is what I really like about my Orion Jr. I can look at the graph of my cell voltages at different phases of my charge and discharge cycle.

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.

[SunEagle]

The chemistry is similar. The Volt was water cooled and the Leaf air cooled. The Leaf consists of modules containing 2P2S in a Sardine can like shape. The Leaf has had some problems with battery degradation especially in hot environments because they are air cooled. They can easily be reassembled into packs from 8 volts to 384 volts.

The Volt modules vary from 12 volts to 24 volts. The Volt modules are hard to separate into smaller groups.

Thanks. I have been getting a number of website advertisements that take me to a company selling the Leaf batteries in different configurations. Even though they are used batteries, most if not all are still expensive IMO.

[Ampster]

How much eddy current are we talking about? I'm going to have 8 packs paralleled and a 6000 watt inverter so max continuous current is going to be around 136 amps. Divided by 8 packs, that about 17 amps per 12s string. We're not talking time travel in a DeLorean with Flux capacitors here..

I don't think they would be very large unless one cell loses a lot of capacity. Then you would only see it as the other cells attempt to equalize voltage. That would not occur at resting but only as they charge or discharge. I think 17 Amps is a good starting point.

That David Poz on youtube ran his volt pack for a year without a BMS and the pack stayed balanced. So here's what I'm considering, please chime in with opinions. What if I just purchased the Rec BMS and hooked it to a single string. Every two or three charge cycles, I would unplug the BMS leads and plug them into the next string. All the strings would still be paralleled so most of the protections would still be in place.. the only thing that wouldn't be protected is the balancing of any of the packs that aren't connected.. and they would eventually get their "time" on the BMS to make sure the series is balanced, it just wouldn't be a continuous thing.

That sounds like a plan. The important BMS functions of high voltage and low voltage disconnects would be in place no matter which string is connected to the BMS. I have a pack of mixed Nissan Leaf cells and I just use my Orion BMS for monitoring. I have turned off the balancing since. What I would look for when you are monitoring is the difference between the highest cell and the lowest. If that delta changes as you discharge the pack then that might be an early warning that one cell has slightly less capacity and is heading for the knee of the curve before the others. If there is any it will show up at 4.1 at the top and 3.5 at the bottom. Because I have some weak cells {60% of original capacity) in my Nissan pack I set my voltages at 4.05 and 3.80. When the lowest cell is at 3.82 I have a 120 millivolt difference to compared to the top cell at 3.94. At the top the difference is only 10 millivolts. I did not bin my modules before assembling my pack and it consists of modules from 3 different Nissan Leafs. I think you have the advantage of your cells all coming from the same pack and the Volt packs are more robust than the Nissan packs.
Regarding balancing, I don't think you will need to balance very often. When you do, you will only want it to happen when you are in the CV phase of charging because that is when the Amps begin to taper off and the balancing shunts will be more effective. Presumably you can set the balancing on or off or at least the voltage at which it should start.

I was also looking into the Zeva BMS system. Its far cheaper than the Rec BMS but I don't think its CAN BUS is set up to talk to the Sunny Island.

I think that is an important issue. When I got my Outback Skybox I was hoping to have CAN BUS connectivity but found out that unless there is similarity between the CAN BUS statements (not sure that is the right term) you can only monitor the CAN BUS traffic but not create actions. That was why I was skeptical in my earlier comment.

Hopefully the REC gives you some remote capability with logging. That is what I really like about my Orion Jr. I can look at the graph of my cell voltages at different phases of my charge and discharge cycle.

[Salts]

That is a problem using a Chevy Volt pack. You really need 8 separate BMSs. The issue with trying to parallel Volt batteries is finding a way to connect the individual 3P buddy cells together. The connections need to be large enough to carry the eddy currents that will develope as the various packs charge and discharge. The existing BMS wires are too small for that.

With regard to CAN protocols do you mean the explicit messages to set voltage at the constant voltage phase and the current at which to stop charging during the CV phase?

How much eddy current are we talking about? I'm going to have 8 packs paralleled and a 6000 watt inverter so max continuous current is going to be around 136 amps. Divided by 8 packs, that about 17 amps per 12s string. We're not talking time travel in a DeLorean with Flux capacitors here..

That David Poz on youtube ran his volt pack for a year without a BMS and the pack stayed balanced. So here's what I'm considering, please chime in with opinions. What if I just purchased the Rec BMS and hooked it to a single string. Every two or three charge cycles, I would unplug the BMS leads and plug them into the next string. All the strings would still be paralleled so most of the protections would still be in place.. the only thing that wouldn't be protected is the balancing of any of the packs that aren't connected.. and they would eventually get their "time" on the BMS to make sure the series is balanced, it just wouldn't be a continuous thing.

I was also looking into the Zeva BMS system. Its far cheaper than the Rec BMS but I don't think its CAN BUS is set up to talk to the Sunny Island.