Inverter with adjustable low voltage disconnect (LVD) for LiFePO4 cells?

[ckuttruff]

Really appreciate people taking the time to respond. Looking forward to getting an inverter that will work for me, misc fuses, etc that I need, and getting things wired up. I'll document as I go and maybe post an update to this thread when I get things sorted.

I also made a video of my bottom balancing process and little gotchas I ran into:

https://www.youtube.com/watch?v=Hb27Rnc5QNg

Gladly if I did anything terribly wrong, I can't be terrified if you let me know, cause I survived and my cells seem fine; but I will certainly appreciate the education going forward

[Sunking]

wow, okay, thanks so much for the info. so charge cutoff voltage is listed at 3.6, but looking at the discharge curve from CALB there's nothing more in 'em about 3.4, so no point going above that anyway, but (0.9 * 3.6 => 3.24). That would be my target? Or is 3.3 plenty safe?

Great you understand. Start at 3.4 or 27.2 volts and see where the weak cell ends up at. You want the weak cell to reach 3.4 to 3.5 volts. All other cells will be slightly lower.

Herre is my experience and hundreds of other EV builders use, and EV goes up to 45S. 3.4 VPc is the sweat spot +/- .02 volt. So with 8S you are going to end up as low as 27.0 up to 27.4 volts. Caution, make darn sure you did a good job of Bottom Balance. You want all those 8 cells wired in Parallel and discharge to 2.4 to 2.5 volts rested over night. So when they have rested over night they are no higher than 2.5 volts. Then you can wire them in series and charge them up.

When you charge them the first time, watch them closely as they go above 3.35 volts. One cell will start to shoot up. When you see that one weak cell start go higher than the others. STOP and record pack voltage. That is your starting point. Then for the next couple of days, zero in on your operating voltage. You will likely need to turn it down 0.1 to 0.2 volts like from 27.1 to 27.0..

Motor Verks is one of the top custom EV builders and largest supplier of CALB Cells in the USA ran by Jack Rickard a retired Electrical Engineer. He only uses Bottom Balance. What Jack does and seems to work great is to monitor cell voltages on the initial charge, and waits until the first cell reaches 3.5 volts, record pack voltage and Stop. He is done.

[Sunking]

FWIW an excellent tool is a Revoletric PL8. It is an RC Hobby charger made to charge any battery type of today and tomorrow up to 8S Lithium up to 40 Amps. You can Bottom Balance or Top Balance with them. You can measure capacity, internal resistance, charge, discharge, or anything you want to do. Once catch is they require a heavy duty 12 to 24 volt DC Power Supply

You can buy them together.

PL8
Power Supply 24 volt 50 amps.

It will charge any battery from a single cell AA NiCd 1.5 volt 1 AH battery up to 8S Lithium 500 AH and everything in between. There is no battery it cannot charge or do diagnostics.

If you use it for only one thing, keep track of the Cell Internal Resistance. When you see Ri go up, you know the end is near.

[ckuttruff]

FWIW an excellent tool is a Revoletric PL8. It is an RC Hobby charger made to charge any battery type of today and tomorrow up to 8S Lithium up to 40 Amps. You can Bottom Balance or Top Balance with them. You can measure capacity, internal resistance, charge, discharge, or anything you want to do. Once catch is they require a heavy duty 12 to 24 volt DC Power Supply

Thanks, yeah, I have the PL6 and it works well with a 12V 30A supply I have. I did hit some interesting snags and not sure if PL8 behaves the same way. But once the batteries were already below 3V, it would refuse to discharge further, and even with a 12V battery I hooked up, the regenerative discharge was extremely slow (like 20mA slow). Not sure if that indicates defect, but there's another problem also with the A123 setting; when below 2.75 or so it would slip into this LOW CHARGE MODE and give warnings about bad cell, etc and then charge at low current and turn off pretty quickly. Took an eternity when I was charging 'em back up from bottom balancing and had to end up hooking them up in series again to even charge them with the PL6. Frustrating cause I gotta bottom balance 'em again. Not sure if you have any tips or ran into any of that before. Thanks again

[Sunking]

Cannot comment on the PL6. On the PL8 you have to go into Settings and change defaults. Another good one is an iCharger 3010B. It is only 1000 watts, but a lot more flexible than the PL8. The 3010B goes up to 10S

Personally I prefer the iCharger over Revoletric.

[ckuttruff]

Great you understand. Start at 3.4 or 27.2 volts and see where the weak cell ends up at. You want the weak cell to reach 3.4 to 3.5 volts. All other cells will be slightly lower.

Herre is my experience and hundreds of other EV builders use, and EV goes up to 45S. 3.4 VPc is the sweat spot +/- .02 volt. So with 8S you are going to end up as low as 27.0 up to 27.4 volts. Caution, make darn sure you did a good job of Bottom Balance. You want all those 8 cells wired in Parallel and discharge to 2.4 to 2.5 volts rested over night. So when they have rested over night they are no higher than 2.5 volts. Then you can wire them in series and charge them up.

Motor Verks is one of the top custom EV builders and largest supplier of CALB Cells in the USA ran by Jack Rickard a retired Electrical Engineer. He only uses Bottom Balance. What Jack does and seems to work great is to monitor cell voltages on the initial charge, and waits until the first cell reaches 3.5 volts, record pack voltage and Stop. He is done.

Really good to know. I was only bringing them down to 2.75 based on resources I was going off, but perhaps I'll dip them further; just concerned about the problem I ran into before when it basically thought I had a lesser number of cells and wouldn't let me continue to charge them in series. Was super frustrating and why I had to split 'em up to charge again - wrecking the balancing. This was after they sunk below 2.5 cause of the inverter. Not sure what threshold the Powerlab sets, but I honestly wish there was a more custom preset where you don't run into some of those walls. Based on the discharge curve, is the difference between 2.5 and 2.75 for bottom balancing significant?

Really appreciate the steps regarding monitoring and completing that process. Feeling great that a good bottom balance in conjunction with conservative cutoff thresholds and periodic monitoring I'll be in great shape with it. Again, thanks for your time and insights. Have already come across resources from EVTV and sounds like Jack is doing great work.

[Sunking]

Based on the discharge curve, is the difference between 2.5 and 2.75 for bottom balancing significant?.

IMO yes. You want all cells to rech 2.5 volts at the same time. It is a 0 Reference point, and the only point where you know what the exact capacity of every cell is and the voltage.

With Top Balance doe snot tel you anything other than every battery is at 100% SOC. It does not tell you the capacity, only voltage eis equal at the Top. When you hit the Bottom, cell voltages and capacity i sall over the place and unknown. One cell will crash, and the other 7 cells with energy left in them will eat the weak cell alive and destroy it by driving it into reverse polarity. Game over.

With Bottom Balance when you discharge all cells arrive at 2.5 volts at the same time with no energy left and the lights go out. Recharge and you are back in biz. Catch is if you set your LVD for 3.0 vpc, (24 volt LVD) they will never get that low or 20 volts on the pack. You got a 4 volt cushion.

Look with 4S, 8S, and even 16S your operating voltage is very narrow. Example On 8S you charge at 27 volts, and disconnect at 24 volts, a 3-volt range. Trust me you are going to notice if you have a bad cell when you discover 3.2 volts missing.

Although LVD is set for 24 volts, you start getting nervous when you see 25 volts because you are just about discharged at that point and will get your attention. Danger zone is 20 volts and less. With Bottom Balance you are protected with 4 layers of passive defense.

1. LVD is set to 24 volts, a full 4 volt cushion.
2. With Bottom Balanced, it is impossible to over discharge a pack and driving a cell into reverse polarity. No adjacent cells have any energy left in them.
3. No vampire boards aka Balance Boards to fault and destroy a cell.
4. Minimum Risk of over charge.

[karrak]

Ah, the great bottom balance versus top balance debate again. There are lots of reasons that I think top balance is better than bottom balance for off-grid systems charged by solar power. Here are the main ones

• The idea that there will be no energy left in the other cells to drive a weak cell to destruction relies on there being no drift in the bottom balance. The battery will go out of balance at the bottom end if there is any difference in the battery leakage of the individual cells. If we have a battery with 7 cells that have a 1% leakage over a year and one that has a 2% leakage and bottom balance it then charge it with 100Ah and store it for a year we end up with 7 cells with 99Ah of charge and one with 98Ah of charge. If we try to draw 99Ah out of the battery the weak cell will be at -1Ah.
• You will have no way of knowing if the battery has gone out of balance at the bottom end without doing a bottom balance. If you top balance you know every time your solar panels fully charge the battery if it is still in balance. Any imbalance will show up with the individual cell voltages deviating when the battery is fully charged. This can be easily corrected by adding or subtracting charge from the cells that are out of balance.
• With an off-grid system charged with solar energy you never really want your battery to be empty, quite the opposite you want it to be full as much of the time as possible so that you can get by during cloudy periods. Since I have been logging my power usage for nearly 3 years now my battery has only been down to a minimum of 9%SOC. On the other hand it reaches the high 90s% SOC for the majority of days of the year. The average SOC of my battery over nearly three years is ~77%SOC.
• With bottom balancing you maybe charging the weak cell to ~3.5V and the others to ~3.4V. That extra 0.1V every time you fully charge the battery will put the weak battery under more stress that the other stronger cells and will make it loose capacity at a faster rate than the others making the imbalance even worse.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[ckuttruff]

Ah, the great bottom balance versus top balance debate again. There are lots of reasons that I think top balance is better than bottom balance for off-grid systems charged by solar power. Here are the main ones

• You will have no way of knowing if the battery has gone out of balance at the bottom end without doing a bottom balance. If you top balance you know every time your solar panels fully charge the battery if it is still in balance. Any imbalance will show up with the individual cell voltages deviating when the battery is fully charged. This can be easily corrected by adding or subtracting charge from the cells that are out of balance.
• With bottom balancing you maybe charging the weak cell to ~3.5V and the others to ~3.4V. That extra 0.1V every time you fully charge the battery will put the weak battery under more stress that the other stronger cells and will make it loose capacity at a faster rate than the others making the imbalance even worse.

Karrak, thanks for your reasoning here. FWIW, when I bottom balanced and brought things up to 3.5, I only saw about a hundredth of a volt diff between the max and min cell. Not sure if I would expect to see much more drift over time, but seemed to be a good sign. Given that things are so close to 3.2 at the top, doesn bottom balancing make that point a bit more pronounced and thus easier to get them to the same SOC?

it's cool your custom monotoring system offers ao much insight into details over time. Will def check out that GH page. Thanks!

[Mike90250]

Ah, the great bottom balance versus top balance debate again. There are lots of reasons that I think top balance is better than bottom balance for off-grid systems charged by solar power. Here are the main ones.........[*]With bottom balancing you maybe charging the weak cell to ~3.5V and the others to ~3.4V. That extra 0.1V every time you fully charge the battery will put the weak battery under more stress that the other stronger cells and will make it loose capacity at a faster rate than the others making the imbalance even worse.[/LIST]

Simon

And you totally missed the cautions and bewares Sunking presented . You NEVER fully charge the cells. Full charge = shortened lifetime.
Bottom balance you monitor the weak cell and only charge IT to your personal top voltage

Top balance needs mucho monitoring & BMS and can easily drive the weakest cell into reverse charge.

< argghhh the new firefox wont' let me copy & paste because of clipboard issues and a pop-up sandbox >

There is much more you "gloss over" and it appears you are either itching for another vacation, Don't make me push that button again.

You CANNOT push your scheme of charging to neophytes (with all the accessory gear & wires needed) without it being called attention to.

[karrak]

karrak thanks so much for that info! Going to carefully review those CC values (esp boost vs float charging since those terms are new to me). Hoping I didnt make a poor choice on my controller. Bought it from a local shop earlier before delving into more research.

On paper the controller looks good. Difficult to know how well built it is or reliable it will be in the long run as it is a new unknown brand (at least outside China).

I will definitely look at your monitoring README on github. Very open to such a system. My cells are all between 118.1 and 118.8Ah from the manufacturer slip that came with the cells. And internal resistance is 0.27 for all but one (0.28), so they are very well matched. Confirmed this when bringing cells up (in series) to about 3.5V.

I am a great fan of individual cell monitoring party for nerdy reasons and more seriously to pick up any faults that may occur. I know of one case where a CALB cell lost about 25% of its capacity over a period of time and a separate fault where a balancer board failed and dragged down one cell. Both these faults were picked up by cell monitoring and corrected before any damage was done. Most of the time you can get by without having individual cell monitoring, rather like most of the time you can get by without having any fuses, but when things go wrong ...

Also was planning to leave a good amount of room at top and bottom for cutoff to account for slight imbalances (and of course manually check regularly to ensure that too much drift isn't happening)

Leaving a good amount of room at the bottom is fairly easy although it does reduce the usable capacity of the battery. Leaving a good amount of room at the top end when charging from solar is much more difficult. The reason is evident in the charge/discharge curves for LFP cells.
ChargeDischargeCurves.jpg

The big problem with solar is that the charge current is not constant. The maximum charge rate from your panels is ~0.5C. If we extrapolate from the graph at that rate a cell could be at around ~85%SOC when it reaches 3.5V. If there is cloud cover the charge rate could be a tenth of the ~0.5C rate. If we follow the 0.05C charge curve we see that when the cell reaches 3.5V the cell would be at ~99%SOC.

Super open to consult with a pro, so if you know any folks near Portland, OR you'd recommend, let me know.

More chance of me knowing someone near Portland, Victoria, Australia. I have been to Portland OR. Very beautiful area.

and regarding remote off switch, I'm definitely down to find an inverter with that feature, but ideally I'd like the system to shut off automatically in case of low voltage since I won't always be the one in there to hear an alarm. Though if there are really no good options for that, I feel bad for returning the Go Power inverter since it workes well otherwise. Do you have a brand / model you'd recommend for my setup?

Tyco make some large relays but they aren't cheap and use quite allot of power. I am currently looking at a Chinese 200A Solid State Relay (SSR) which is looking promising. I add a warning that you have to be very careful when buying Chinese parts that they are not poor copies or that they will live up to their specifications. Something like this SSR could be controlled by your charge controller.

I don't have a LVD besides the 24V LVD on my inverter as I reason that I will be around when the power is being used at a large rate or it will take several days for the fridge and other equipment connected to the battery to run it down if I am away from home and I can and do check the state of my battery via the Internet.

I think it is well worth getting an SOC meter that uses the current going into and out of the battery to calculate the SOC. These meters can be very accurate when used with Lithium batteries because of lithium batteries coulomb/current efficiency of >99%.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[Sunking]

ckuttruff you ned to ignore Karrak, he is a fraud and only here to make trouble and has been banned twice for giving dangerous and bad advice. He is also a Chi-Com junk peddler dealer out of Australia with no engineering background. He is a pretender.

Karrakl does not know anything about Bottom Balance. He has never tried it or know anyone who has. Me I have done it both ways. 7 years ago I was as ignorant as Karrak is today. Being an Engineer with 40 years battery experience I pounded my chest Lithium Ion Batteries Shall Be Top Balanced, there is no other way to do it.

Well BS. Then I heard about Bottom Balance approach. I shrugged it off for a year as BS. But then I opened my mind and started reading, studying, and talking to engineers that actually use and design this stuff for aerospace and electric automobile manufacturing. My Aha moment came after talking to an Engineer, an old friend of mine from the Navy now working for Lockhead Martin making satellites explained how LM managed LFP batteries in satellites and the space station.

James explained Middle Balancing idea they stole from commercial EV manufactures. LM and other government defense contractors match their cells within 1% capacity. To prep the batteries all they do is put all the batteries in parallel, charge to 60% and let them set a week while wired in parallel. After that they go into service. I asked about battery management and balancing. He just laughed at me and said what BMS? They do not use anything, all they do is charge to 90% (3.4 vpc) and Float the battery. There is no LVD or Vampire boards because there are not needed and just add extra weight and space required to hold it. Weight and space are not in abundance on a satellite. For each pound of payload cost S10,000 of fuel to get it in orbit

That got me real curious and I found a couple of engineers that work for Nissan and Chevy. Like LM EV manufactures match their cells at 1%. and Middle Balance their cells initially. Life is a little easier on EV because none use LFP cells, they use LMC or LiCo. They use Slope Charge because LMC and LiCo charge/discharge is not as flat like LFP. With NMC voltage to SOC is somewhat accurate and predictable. Anyway here is the takeaway, EV manufactures do not allow their customers to ever fully charge or discharge a battery. They cannot because if they did, they would go bankrupt with battery warranty claims.

OK after doing some more research I put things together. Talked with a lot of folks who Bottom Balanced and then went back to my contacts at LM, Nissan, and Chevy and talked about Botom Balance techniques and how it mimics Middle Balance used by NASA and EV Manufactures. Guess what, it works and works quite well. You eliminate a lot of risk and expense. But the biggest benefit i slike EV and NASA you run in the Middle by never fully charging or discharging doubles to quadruples cycle life.

Now what we all agreed upon is Bottom Balance is not something most consumers can implement, nor does industry want to promote. You have to know something about Lithium Ion batteries so you can spot trouble if a cell fails or the pack becomes unbalanced. As a manufacture I would not want the public to know about it. As a manufacture I make a lot more money selling you new batteries ever few years and an expensive BMS that also shortens battery life.

So to run Bottom Balance, you need to know what you are doing. Karak does not know what he is doing, and must use Top Balance. Either that or vested interest in Top Balance and Chi-Com junk batteries he peddles.

[Sunking]

And you totally missed the cautions and bewares Sunking presented . You NEVER fully charge the cells. Full charge = shortened lifetime.
Bottom balance you monitor the weak cell and only charge IT to your personal top voltage

Top balance needs mucho monitoring & BMS and can easily drive the weakest cell into reverse charge.

Mike why don't you just ban the SOB for the thirrd time and make it permanent? You now darn well he only came here to get under my skin and make trouble. Heck he admitted it. Nuke the SOB.

[jflorey2]

Tyco make some large relays but they aren't cheap and use quite allot of power. I am currently looking at a Chinese 200A Solid State Relay (SSR) which is looking promising. I add a warning that you have to be very careful when buying Chinese parts that they are not poor copies or that they will live up to their specifications. Something like this SSR could be controlled by your charge controller.

I would also add a few warnings:

-SSR's often have a LOT of loss; heat sinking is critical

-Large relays often take a lot of current to keep them pulled in; this can cause the problem you are trying to prevent (overdischarge)

-Latching relays can be found that can switch relatively high currents - and latching relays take no current when they are not being switched (nice for battery systems.) Tyco has a 190A latching relay that is good to 42 volts - which means 24V but not 48V systems.

[Sunking]

I would also add a few warnings:

-SSR's often have a LOT of loss; heat sinking is critical

Jeff, Karrak sales that junk. What else can he say? He is vested, he is not going to bite the hand that feeds him. He will ride that pony until bankruptcy, then take the money and run.