I am installing a 48v Quattro system with Chinese 3.2 v 280ah batteries. Will the Overkill BMS be sufficient to work with the Victron? I have looked at the REC-BMS. The REC-BMS communicates with the Victron through Ve.Can. But I can't tell through a reading of the Victron literature if it needs the information to operate. The REC-BMS costs more than the batteries. Any help appreciated.
Victron Quattro and Overkill BMS
- Thread starter Solar_Man
- Start date
Welcome to the forum.
Overkill and most BMS works independently from the source/loads. It has its own safeties, e.g., if the LV disconnect trips, the unit will still permit charging and vice-versa.
Overkill only offers up to 8S, so I assume you'll have 2X 8S batteries in series?
You will program your Victron hardware to operate INSIDE the limits of the BMS, so it's never an issue. You don't want the BMS being the traffic cop that helps you avoid an accident. It's the safety belt that saves you when things go sideways.
In some cases, a BMS can open or close a circuit that can signal Victron hardware to stop. With a Vic MPPT solar charge controller, give two pins an open circuit, and it disables charging. Same thing on a group of pins on the Quattro. These are nice to have to avoid having the battery go open circuit, but it's just another way of doing things.
Overkill and most BMS works independently from the source/loads. It has its own safeties, e.g., if the LV disconnect trips, the unit will still permit charging and vice-versa.
Overkill only offers up to 8S, so I assume you'll have 2X 8S batteries in series?
You will program your Victron hardware to operate INSIDE the limits of the BMS, so it's never an issue. You don't want the BMS being the traffic cop that helps you avoid an accident. It's the safety belt that saves you when things go sideways.
In some cases, a BMS can open or close a circuit that can signal Victron hardware to stop. With a Vic MPPT solar charge controller, give two pins an open circuit, and it disables charging. Same thing on a group of pins on the Quattro. These are nice to have to avoid having the battery go open circuit, but it's just another way of doing things.
7kw panels, same Quattro as you have there, SmartSolar MPPT RS 450|100, 32a Victron Autotransformer this year. Next year another 7kw panels with Fronius on AC side. 56 kwh of chinese batteries. Residential. Getting ready for the apocalypse.lol.
Nice. So you need the split phase for your home AC panel and 240VAC appliances? Make sure the autotransformer works as a step up + split phase. It looks like it, but the documentation is a little hazy.
You can always just run a second Quattro in parallel to provide split phase. That's what I plan to do.
You can always just run a second Quattro in parallel to provide split phase. That's what I plan to do.
I will add a second Quattro next year with the second set of panels. This year I'm working mainly on a subpanel for refrigerators and freezers and hot water. We had two hurricanes this year with almost two weeks on gas generators between the two. After I build this out I'm going to stack 30 spare panels in a building in case of storm damage. I got them for $40 each for 250w panels used.
the_colorist
"Move over... let me fix it" Installer/Engineer
You don't need the BMS to operate however the advantage to using the REC-BMS is not so much safety but the longevity of the cells. This is an over simplification but the REC-BMS monitors the cells and when the first cell reaches the target voltage (no more than 3.5VPC would be recommended), it instructs the Victron system (CC and inverter) to back down the charging current to a place where that the first cell doesn't exceed the full charge voltage but the rest of the cells can balance out.
There is some debate on how necessary this is. IMHO and from field installations with these cells, I've seen it's very possible to regularly have a few cells that tend to get out of balance from the others by a few points and it semi-linearly gets worst the farther you go past 3.4VPC because these are A- cells that are not from consecutive serial runs. They come from various batches.
The farther those cells go past 3.5VPC for X period of time at the end of the charging cycle, the more they will degrade over time. There has been more about this coming out as more lab and field testing is being done. There are some white papers on the subject. The issue here is that the "blind" BMS units have no way to communicate the needed realtime charging current once the first cell hits the target voltage so you always have some cells popping way up there for awhile unnecessarily. Not high enough to trip the 3.65V but possibly high enough to degrade some over time.
Again there is some debate on the subject but one thing I have found through the years is I monitor the updated specs and target values of the largest commercial providers to verify my own findings and lab/field test reports I read. I have seen them change over time to where even the top end and new Chinese companies (ZURI for example) have their target max charge voltage now set to 3.45V max charger with a trip cut off of 3.55V.
SimpliPhi is another example. Thier integration guides now are using a max target of 3.5V for 3.5K cycles and 3.4V for 10K cycles.
The REC-BMS is expensive but after having spent nearly a year attempting to source Victron/SMA/Studer compatible BMS's, I would say they are not as expensive as many solutions. The only other solution that I know of that could be coming out soon and possibly cheaper is from EMUS and it's their new generation MINI BMS. The new version is supposed to come with Victron CAN integration but I haven't checked to see if it's out yet.
There are other options though:
Orion BMS
EMUS
Autarctech
Batrium
Simp BMS
There are more that I cannot think of right now. We use REC or ZEVA for SMA systems. Zeva is a super price but not compatible with Victron.
How much did you pay for your cells? And where are you sourcing the REC-BMS? The BMS itself is only around $315 Euros (~$381 USD). You will need other accessories such as a 48V contactor and a 50MV shunt. A 50MV shunt is about $30 or less. I'm sourcing 400A 48V heavy duty low-wattage contactors factory direct from China for less than $25 before shipping.
You'll probably have $500+ in the BMS before you are done depending on a few factors (do you want the REC BMS LCD? etc) but it should be considerably less than what you paid for the cells from the sounds of it.
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the_colorist
"Move over... let me fix it" Installer/Engineer
I suppose a clearer point would be that on the higher end or commercial systems, the BMS becomes the heartbeat of the system. It's the central controller. The inverters/charge controllers simply follow the BMS instructions sent out in every communication cycle. In the case of REC, that is every 100MS, with the possibility of a new instruction set every 100MS, particularly during the end of the charging phase.
The REC was $1000 with accessories, some of which I'll have to purchase for the Overkill such as contactor, software, cable. Two of them were $2000. The Overkill will $140 x 2 = $280.
With freight I paid $89 each for 3.2v 280ah batteries. I was exaggerating, the 48v battery sets cost $1400 for a 28kwh battery. $1000 seemed like a high fraction for the BMS.
Please help me understand. The Overkill will just shut the circuit off to the battery when charging is complete, while the REC will modulate the output of the MPPT? Are you familiar with Overkill?
Thank you for your thorough and helpful reply.
With freight I paid $89 each for 3.2v 280ah batteries. I was exaggerating, the 48v battery sets cost $1400 for a 28kwh battery. $1000 seemed like a high fraction for the BMS.
Please help me understand. The Overkill will just shut the circuit off to the battery when charging is complete, while the REC will modulate the output of the MPPT? Are you familiar with Overkill?
Thank you for your thorough and helpful reply.
the_colorist
"Move over... let me fix it" Installer/Engineer
Seems very expensive for the REC. What does that include? We don't pay that much but we also don't source all of the accessories from them directly. Under $500 for the REC BMS plus 3x temp sensor, CAN cable, and precharge. Directly from REC. All a person generally needs beyond that is the USB cable and the software. The LCD can be useful but I don't really find it that helpful. If something is amiss that you can't see on the GX, grab a laptop and plug in the cable. You can't change any settings with the LCD anyway.
I think I missed the part about 2 BMS units. Why did you need 2?
I looked over the Overkill. Generic Chinese BMS. IMHO too easy for the FET's to fail closed under load on those if you are even close to the load max which sounds like it could happen. 48v*100A=4800W. Twin 8AWG wires are a code violation (there is likely a debate somewhere on this). No comms for cell level charge calculations.
As the cells charge, they diverge some at the top/end of the charge even if they are "balanced". And it won't always be the same. I wouldn't have thought so but I have seen it. The issue is that as the cells diverge you're going to see a cell high 3.5VPC while other cells are 3.4VPC etc. The balancer is not fast enough to compensate for that.
Here is a screenshot from a video from David Poz with some NMC cells. I'm not on site today or I would grab a photo from one of my own installations. You can see here the cells diverging. This happens with those LFP's at the top of the cycle.
With the Overkill, it will let any cells go to the preset cutoff voltage (3.65V probably default) and if they exceed that for X period it will trip off the BMS and stop all charging/discharging. Very annoying. No charge current control at all. Very rudimentary, wears out the cells over time if this happens in every cycle.
The REC will begin modulation just as you said. As the first cell touches the value you set (3.5VPC for example) it will instruct the inverter and MPPT to back down the charge current to give the balancer time to work without overheating and also to keep the highest cell from going over 3.5VPC. Very smooth charging that keeps the cells from ever oscillating at a high voltage at the end of every cycle.
Let me know if you need any further explanation.
Have you bought the RS yet? Just curious how much you paid for it or what price you are considering. I ask because if you're interested in saving some $$, I'd recommend going the AC coupling route. It's generally cheaper with higher voltage strings which will save you on wiring. Other advantages also.
I think I missed the part about 2 BMS units. Why did you need 2?
I looked over the Overkill. Generic Chinese BMS. IMHO too easy for the FET's to fail closed under load on those if you are even close to the load max which sounds like it could happen. 48v*100A=4800W. Twin 8AWG wires are a code violation (there is likely a debate somewhere on this). No comms for cell level charge calculations.
As the cells charge, they diverge some at the top/end of the charge even if they are "balanced". And it won't always be the same. I wouldn't have thought so but I have seen it. The issue is that as the cells diverge you're going to see a cell high 3.5VPC while other cells are 3.4VPC etc. The balancer is not fast enough to compensate for that.
Here is a screenshot from a video from David Poz with some NMC cells. I'm not on site today or I would grab a photo from one of my own installations. You can see here the cells diverging. This happens with those LFP's at the top of the cycle.
With the Overkill, it will let any cells go to the preset cutoff voltage (3.65V probably default) and if they exceed that for X period it will trip off the BMS and stop all charging/discharging. Very annoying. No charge current control at all. Very rudimentary, wears out the cells over time if this happens in every cycle.
The REC will begin modulation just as you said. As the first cell touches the value you set (3.5VPC for example) it will instruct the inverter and MPPT to back down the charge current to give the balancer time to work without overheating and also to keep the highest cell from going over 3.5VPC. Very smooth charging that keeps the cells from ever oscillating at a high voltage at the end of every cycle.
Let me know if you need any further explanation.
Have you bought the RS yet? Just curious how much you paid for it or what price you are considering. I ask because if you're interested in saving some $$, I'd recommend going the AC coupling route. It's generally cheaper with higher voltage strings which will save you on wiring. Other advantages also.
Thanks for the explanation. The RS is on order. $1,180.00.
The REC price includes:
• REC Q-BMS x 2 - $490
• PC Software with PC to BMS cable – required for BMS configuration $110
• Current Shunt
• Pre-Charge Delay
• Tyco EV200ADANA Contactor (24/48V) $220
• CANbus Adapter – DB9 to WS500 Pinout RJ45
• Temp. Sense Cable/Current Shunt Cable
Optional items include:
• Bluetooth Module* for use with Free REC Smartphone App $175
• LCD Touchscreen* with connection/power cable $189
Total was $1000 for all.
I am building 2 - 16 cell - 48v batteries 280ah cells so I'll need to BMS devices.
I plan to run the panels up above 400v to the RS to help on the wiring. When I add more panels it is on a separate building that already has to split phase wiring 240v running back to the location of the RS and batteries.
My initial install is 28 kwh of batteries powered by the RS and Quattro. 28 kwh might be enough but my plans are to double that to 56 kwh and install 30 more panels on the 2nd bldg. I was going to put a Fronius out there to feed the AC back to the main building and the batteries.
My thinking is that 2/3 of the demand is during the day so the Fronius would directly power that demand. The batteries would handle the period that I'm not charging and would handle that. I might put a second Fronius on the first install to AC couple some of the 1st set of 30 panels.
In the Victron literature I recall some advantage to having the RS on the DC side of the Quattro.
Your thoughts?
The REC price includes:
• REC Q-BMS x 2 - $490
• PC Software with PC to BMS cable – required for BMS configuration $110
• Current Shunt
• Pre-Charge Delay
• Tyco EV200ADANA Contactor (24/48V) $220
• CANbus Adapter – DB9 to WS500 Pinout RJ45
• Temp. Sense Cable/Current Shunt Cable
Optional items include:
• Bluetooth Module* for use with Free REC Smartphone App $175
• LCD Touchscreen* with connection/power cable $189
Total was $1000 for all.
I am building 2 - 16 cell - 48v batteries 280ah cells so I'll need to BMS devices.
I plan to run the panels up above 400v to the RS to help on the wiring. When I add more panels it is on a separate building that already has to split phase wiring 240v running back to the location of the RS and batteries.
My initial install is 28 kwh of batteries powered by the RS and Quattro. 28 kwh might be enough but my plans are to double that to 56 kwh and install 30 more panels on the 2nd bldg. I was going to put a Fronius out there to feed the AC back to the main building and the batteries.
My thinking is that 2/3 of the demand is during the day so the Fronius would directly power that demand. The batteries would handle the period that I'm not charging and would handle that. I might put a second Fronius on the first install to AC couple some of the 1st set of 30 panels.
In the Victron literature I recall some advantage to having the RS on the DC side of the Quattro.
Your thoughts?
the_colorist
"Move over... let me fix it" Installer/Engineer
Sure a few thoughts off the top:
1. Did you select the Q-BMS for a reason or was it recommended to you? The reason is because you need the REC SI BMS W/ Victron integration. That said as long as at REC they know you are integrating with Victron, they will make sure you get a BMS with the right software. Otherwise it's not going to help you.
www.rec-bms.com
2. How much was the shunt?
3. That's crazy for the Kilovac. See here:
4. That cable is not for the Victron GX system. It's for the Wakespeed WS500 regulator:
wakespeed.com
More to follow, accidentally sent so just doing a quick edit on the end.
1. Did you select the Q-BMS for a reason or was it recommended to you? The reason is because you need the REC SI BMS W/ Victron integration. That said as long as at REC they know you are integrating with Victron, they will make sure you get a BMS with the right software. Otherwise it's not going to help you.
BATTERY MANAGEMENT SYSTEM – REC BMS
www.rec-bms.com
2. How much was the shunt?
3. That's crazy for the Kilovac. See here:
4. That cable is not for the Victron GX system. It's for the Wakespeed WS500 regulator:
WS500 Advanced Alternator Regulator - Wakespeed
The WS500 is the only alternator regulator to utilize current, voltage and temperature to deliver the most precise and effective charging possible.
wakespeed.com
More to follow, accidentally sent so just doing a quick edit on the end.
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the_colorist
"Move over... let me fix it" Installer/Engineer
5. I'm sure they told you but you can only use the LCD or the BT module at any given time on your REC BMS. Unfortunately not possible to use both right now.
6. If you order directly from them, they have an option for 3 temp sensors. It's usually a good idea on such a large bank.
7. I should have made this #1 perhaps. Are you intending to connect both of the battery banks to the same Victron inverter system? If so, this configuration won't work. You have 2 options in that case:
#1. Buy a Master + Slave + Slave BMS configuration from REC.
#2. Install a 2P16S bank (32 cells, single bank)
This may not be the case for you but I have been seeing some confusion in the DIY world on how BMS's are supposed to work VS how the Chinese BMS units work. The correct way for a single inverter system with multiple battery banks is there are slave modules monitoring each bank and reporting their findings to a master BMS. The master BMS makes the charge parameter calculations in realtime from those and sends those up to the inverter system. Then the inverters and charge controllers in the system follow those instructions. REC has this system available for very large systems when it's needed. I've had extensive discussions with them about it.
In your case, if you are not building 2 entirely separate (AC & DC) systems but instead building one large system, then both they and I would recommend a 2 parallel and 16 series singular bank (32 cells). This makes the bank much more stable as well since you have 2 cells in parallel instead of 2 strings in parallel. We try to avoid parallel string unless we have to.
More thoughts to come later. Let me know if you have any questions.
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OverkillSolar
Solar Enthusiast
- Joined
- Mar 26, 2020
- Messages
- 188
The 16s 100a BMS (available starting next week) has 20 Pairs of FETs, each rated 120a per the datasheet: https://datasheet.lcsc.com/szlcsc/1912111437_CRMICRO-CRSS042N10N_C410926.pdf
I have not seen any of these fail closed under normal loads. (speaking about all the models) I tested a sample of this 16s unit by directly short circuiting it, and it was able to break the current twice before failing on the third attempt. It still broke the circuit, but the smoke came out.
I have tested the 4s BMS with a 300a load with no failures. Thermal management is the primary limitation.
And I don't think it's fair to call it generic.
I'm not sure about codes for DC systems, but a quick search found that parallel conductors can be used in AC systems per the NEC as long as they are properly installed:
366.20 Conductors Connected in Parallel.
In the 2017 NEC, parallel conductors inside an auxiliary gutter must be grouped together to prevent current imbalance in the paralleled conductors due to inductive reactance.
www.electricallicenserenewal.com
Regarding communications, The BMSs do communicate everything you want to know, but the protocol is not going to be compatible with victron. We do plan to make a module that will interface these systems, but the to-do list is getting pretty long.
I don't know why you think this would wear out the cells. If one cell goes overvoltage before the pack, this means they are not balanced. This is the entire reason why the cells need to be top balanced.
In a properly functioning system, the charging sources will limit themselves to stop charging at the correct voltage level (3.500v per cell, IMO), and the cells will all be below the overvoltage cutoff.
Let me know if I missed something else in this thread.
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OverkillSolar
Solar Enthusiast
- Joined
- Mar 26, 2020
- Messages
- 188
I have to point out that the cells can not do this on their own, this only makes sense if there is a large active balancer connected.
Anyway, if this is happening, you need to top balance the cells, or the cells are damaged and mismatched.
The Overkill Solar BMS that I'm using (two of them) tracks the number of times the BMS has had to enable the high voltage disconnect for both the cell and battery levels. So far, those numbers are big fat zeroes.
My cells were well balanced (top balancing) to start with and continue to stay balanced. The BMS is doing some balancing. My charging devices are doing a good job of not providing more charge than necessary. I've put in the time to make sure all these devices are synchronized in their setup. I'm not charging or discharging at more than .5C. I don't see any problems.
My cells were well balanced (top balancing) to start with and continue to stay balanced. The BMS is doing some balancing. My charging devices are doing a good job of not providing more charge than necessary. I've put in the time to make sure all these devices are synchronized in their setup. I'm not charging or discharging at more than .5C. I don't see any problems.
the_colorist
"Move over... let me fix it" Installer/Engineer
@OverkillSolar Good to hear from you. Glad to have your input. My assumption of generic was that it appears (externally) to be similar to so many Chinese BMS units I have reviewed. I was thinking Overkill was just a generic reseller of those so my comments were a bit harsh. No offense was intended to you directly whatsoever!
The Chinese units that I have reviewed in the past were very rudimentary, didn't trip until extremes, didn't have much settings. Just "last-ditch effort" scenarios. I tend to try to avoid those BUT on my side I tend to try to operate in tighter tolerances and do the best I can to study the chemistry, whitepapers, field tests, lab tests etc to optimize. IMHO what happens at the end of every charge cycle is compounded and has an effect over time.
With respect to what I wrote above, it was less generic and more about the specific cells he is working with here. They are grey market and tend to drift worse than others because of the IR and other issues IMHO.
Good to know. I worry about FET's failing closed, it has happened in the past but properly sized (by the user buying the right capacity BMS) it definitely sounds like it should be fine based on your comments above. Some BMS's use temp compensation with comms to the inverter to back down current or load shed as the temp rises.
I'm glad you found that. I would recommend a re-read of the opening statement:
"Parallel conductor installations are covered in NEC 310.10(H) and are permitted for each phase, polarity, neutral, or grounded conductor in sizes 1/0 AWG and larger."
Anything smaller and if you lose one conductor, you are likely to have the insulation in the other one melt before any breakers trip, if they do. The thermal limits of the larger cables allow them to handle higher surge currents and the likelihood of one failing of that size is reduced significantly over say, an 8 awg.
I may be misunderstanding your comment here but if I'm understanding correctly though, here are my personal thoughts:
I do agree that a large difference is definitely a result of either bad cells or improper balancing. However, there is always still a small amount of imbalance at the top of the charge cycle with these cells or so I have found. And they continue to drift which gets worse at higher C rates and the farther in the knee a person goes (Ohm's law & fundamentals of the chemistry).
At the top of every cycle, there will always be a "first cell" that reaches the target voltage by 0.002 or greater probably, regardless of top-balancing. Here is a comment from the REC SI manual:
"The communication between the REC BMS and the SMA Sunny Island charger is established through the CAN bus.
All the parameters that control the charging/discharging behavior are calculated by the BMS and transmitted to
the Sunny Island unit in every measurement cycle.
The charging current is controlled by the Maximum charging current parameter. It’s calculated as Charging Coefficient('C','H','A',’C') x Battery capacity.
The parameter has an upper limit which is defined as Maximum Charging Current per Device ('M','A','X',’C') x Number of Devices ('S','I','S',’N').
When any cell reaches the voltage interval between Balance Voltages Start and Balance Voltage End, the charging current starts to ramp down to 1 A x Number of Devices until the last cell rises to the End of Charge Voltage. At that point the Maximum charging current is reset to 0 A and the charger is disabled also via the BMS I/O interface. End of Charge, SOC hysteresis and End of charging cell voltage hysteresis prevent unwanted switching. SOC is reset to 100 %, Power LED turns ON 100 %. "
This is cell-level charging. In my humble opinion it's best to make sure you have every cell gently finish each charge cycle without oscillating over the target voltage so that is what I tend to advocate when a person can afford it.
Other BMS units actually modulate the charge voltage sent to the inverter/charge controller in every measurement cycle (~100ms) to more precisely control the end of charge current. For some applications, this provides even better current control than manipulating a charge current target value.
BYD battery systems simply shut down if you do not have communication. Tesvolt as well, along with other commercial and high-end residential systems. LG too as I recall.
The wear effect is linear (I'm sure you aware of this but I'll mention it for others in case they are curious). The farther up the knee you take a cell for any given length of time, the more it's lifespan is reduced. Even commercial providers are finding this out after having put cells in the field and operated them for a few years farther into the knees. So for longevity of banks with cells that tend to drift, IMHO one needs to be aggressive about tapering the current as the first cell reaches the target voltage in every cycle.
Can a person just use a tail current and absorption phase? Yes, I think so but for those interested I would recommend a test with this and extensive cell voltage logging (every 100MS). Then graph it and see if (using a top balanced pack) any given cell oscillated past the target during the absorption phase. I think the results will be interesting.
I posted a photo above of a bank of cells being pulled/pushed out of balance. To get a concept of those, that is an LG factory-built and balanced pack. It was installed and operated for at least 1000 cycles (a bit of a guess but should be accurate based on the capacity loss) with a commercial BMS with 1000s of other modules. Each module had a BMS slave with passive balancing. And yet, without any individual cells being touched from the commercial installation, David Poz saw an imbalance and he felt he needed to cycle the BMS at the top a few times to bring them back into balance (no active balancing). Following that he had more usable capacity. As he cycles those cells, if no balancing at the end of or during every cycle is applied, they will once again drift out of balance.
Good to know! How many cycles in are you?
So again, no offense intended with my comments. I tend to be harsh in steering people away from the low-end BMS's that they may see are low cost now but could cost them money 2K cycles later. One of those "every cycle counts" scenarios. But I can see that is not the case with yours!
The Chinese units that I have reviewed in the past were very rudimentary, didn't trip until extremes, didn't have much settings. Just "last-ditch effort" scenarios. I tend to try to avoid those BUT on my side I tend to try to operate in tighter tolerances and do the best I can to study the chemistry, whitepapers, field tests, lab tests etc to optimize. IMHO what happens at the end of every charge cycle is compounded and has an effect over time.
With respect to what I wrote above, it was less generic and more about the specific cells he is working with here. They are grey market and tend to drift worse than others because of the IR and other issues IMHO.
Good to know. I worry about FET's failing closed, it has happened in the past but properly sized (by the user buying the right capacity BMS) it definitely sounds like it should be fine based on your comments above. Some BMS's use temp compensation with comms to the inverter to back down current or load shed as the temp rises.
I'm glad you found that. I would recommend a re-read of the opening statement:
"Parallel conductor installations are covered in NEC 310.10(H) and are permitted for each phase, polarity, neutral, or grounded conductor in sizes 1/0 AWG and larger."
Anything smaller and if you lose one conductor, you are likely to have the insulation in the other one melt before any breakers trip, if they do. The thermal limits of the larger cables allow them to handle higher surge currents and the likelihood of one failing of that size is reduced significantly over say, an 8 awg.
I may be misunderstanding your comment here but if I'm understanding correctly though, here are my personal thoughts:
I do agree that a large difference is definitely a result of either bad cells or improper balancing. However, there is always still a small amount of imbalance at the top of the charge cycle with these cells or so I have found. And they continue to drift which gets worse at higher C rates and the farther in the knee a person goes (Ohm's law & fundamentals of the chemistry).
At the top of every cycle, there will always be a "first cell" that reaches the target voltage by 0.002 or greater probably, regardless of top-balancing. Here is a comment from the REC SI manual:
"The communication between the REC BMS and the SMA Sunny Island charger is established through the CAN bus.
All the parameters that control the charging/discharging behavior are calculated by the BMS and transmitted to
the Sunny Island unit in every measurement cycle.
The charging current is controlled by the Maximum charging current parameter. It’s calculated as Charging Coefficient('C','H','A',’C') x Battery capacity.
The parameter has an upper limit which is defined as Maximum Charging Current per Device ('M','A','X',’C') x Number of Devices ('S','I','S',’N').
When any cell reaches the voltage interval between Balance Voltages Start and Balance Voltage End, the charging current starts to ramp down to 1 A x Number of Devices until the last cell rises to the End of Charge Voltage. At that point the Maximum charging current is reset to 0 A and the charger is disabled also via the BMS I/O interface. End of Charge, SOC hysteresis and End of charging cell voltage hysteresis prevent unwanted switching. SOC is reset to 100 %, Power LED turns ON 100 %. "
This is cell-level charging. In my humble opinion it's best to make sure you have every cell gently finish each charge cycle without oscillating over the target voltage so that is what I tend to advocate when a person can afford it.
Other BMS units actually modulate the charge voltage sent to the inverter/charge controller in every measurement cycle (~100ms) to more precisely control the end of charge current. For some applications, this provides even better current control than manipulating a charge current target value.
BYD battery systems simply shut down if you do not have communication. Tesvolt as well, along with other commercial and high-end residential systems. LG too as I recall.
The wear effect is linear (I'm sure you aware of this but I'll mention it for others in case they are curious). The farther up the knee you take a cell for any given length of time, the more it's lifespan is reduced. Even commercial providers are finding this out after having put cells in the field and operated them for a few years farther into the knees. So for longevity of banks with cells that tend to drift, IMHO one needs to be aggressive about tapering the current as the first cell reaches the target voltage in every cycle.
Can a person just use a tail current and absorption phase? Yes, I think so but for those interested I would recommend a test with this and extensive cell voltage logging (every 100MS). Then graph it and see if (using a top balanced pack) any given cell oscillated past the target during the absorption phase. I think the results will be interesting.
I posted a photo above of a bank of cells being pulled/pushed out of balance. To get a concept of those, that is an LG factory-built and balanced pack. It was installed and operated for at least 1000 cycles (a bit of a guess but should be accurate based on the capacity loss) with a commercial BMS with 1000s of other modules. Each module had a BMS slave with passive balancing. And yet, without any individual cells being touched from the commercial installation, David Poz saw an imbalance and he felt he needed to cycle the BMS at the top a few times to bring them back into balance (no active balancing). Following that he had more usable capacity. As he cycles those cells, if no balancing at the end of or during every cycle is applied, they will once again drift out of balance.
Good to know! How many cycles in are you?
So again, no offense intended with my comments. I tend to be harsh in steering people away from the low-end BMS's that they may see are low cost now but could cost them money 2K cycles later. One of those "every cycle counts" scenarios. But I can see that is not the case with yours!
Thank you to all for the comments. Very helpful. I find the entire idea of solar energy fascinating. Getting down in the weeds is a challenge for me.
I think I'll stay with the Overkill on this batch. My batteries are hopefully drifting this way on a boat. I have other questions that I'll search the boards for before asking here.
@the_colorist , I'd appreciate your comments on the Fronius system.
I am building an essential load system for food storage and hot water in the first phase that will operate off-grid. If I prove out that I can do it, I will double the capacity to make my entire home off-grid if necessary.
I copied this from the Victron training videos. Hopefully it is clear enough.
Does the 1 to 1 rule make the first arrangement preferred? I could use this for a short outage?
What if I made a wiring switch to move the Fronius to operate longer term.
The Victron literature indicates that a DC grid is desirable in the event the batteries are drained. They also show this scenario.
I think I'll stay with the Overkill on this batch. My batteries are hopefully drifting this way on a boat. I have other questions that I'll search the boards for before asking here.
@the_colorist , I'd appreciate your comments on the Fronius system.
I am building an essential load system for food storage and hot water in the first phase that will operate off-grid. If I prove out that I can do it, I will double the capacity to make my entire home off-grid if necessary.
I copied this from the Victron training videos. Hopefully it is clear enough.
Does the 1 to 1 rule make the first arrangement preferred? I could use this for a short outage?
What if I made a wiring switch to move the Fronius to operate longer term.
The Victron literature indicates that a DC grid is desirable in the event the batteries are drained. They also show this scenario.
