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

[ckuttruff]

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.

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I'm being polite and am indeed curious about automated monitoring, but honestly I have every intention of moving forward with a bottom balanced setup with as few components as are necessary. My cells are sitting at 2.6 in parallel right now.

I understand people's concerns on here (screwing up can mean serious damage or death with these tools). Definitely not making any decisions from a single comment or without understanding the implications and nuances adequately

[karrak]

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.

I am running some tests on the 200A SSR that I mentioned in my previous post. I have calculated the On resistances of the switch to be 0.002 Ohms. With no heatsink at 88A it runs warm to touch. 115A is pretty well the limit without a heatsink. There is a 400A version of this switch that should run easily at 100A without a heatsink.

At least the large relays will be de-energised when the LVD operates.

Latching relays are a good choice but need a special interface to operate.

My preferred option is still a remote on/off function in the inverter.

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

[karrak]

I'm being polite and am indeed curious about automated monitoring, but honestly I have every intention of moving forward with a bottom balanced setup with as few components as are necessary. My cells are sitting at 2.6 in parallel right now.

I understand people's concerns on here (screwing up can mean serious damage or death with these tools). Definitely not making any decisions from a single comment or without understanding the implications and nuances adequately

Both bottom balancing and top balancing have their risks if you are not doing individual cell monitoring. With bottom balancing there is more risk of cells being damaged or degraded by being overcharged, with top balancing there is more danger of cells being damaged or degraded by being over discharged. If the battery goes out of balance without you being aware of it there is danger of cells being damaged by overcharge or over discharge.

You can top balance a LFP battery with your charge controller and a singe 5 Watt 1 Ohm resistor. Just set the charge controller to do a long absorb time at your standard charge voltage, say 27.6V(3.45V/cell). When the charge current has dropped to less than C/50 measure the individual cell voltages, put the 1 Ohm resistor across any cells that are above 3.45 until they drop below 3.45V. I think a better approach is to log the voltages at the end of the charge cycle, work out how much each cell is out of balance using the charge curve I posted earlier and put the resistor across any high cells for a fixed amount of time to remove charge from the high cells.

If your CALB cells are as well matched as the documentation implies, your battery will be well balanced to start with at both the bottom and top end. Whether this will be the case after several years of operation is another matter.

I agree with you about not just looking at one persons point of view, I would add regardless how qualified they say they are or what they say their background is. Always have a look at the evidence that they provide to back up their argument.


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

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[karrak]

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.

More unsubstantiated rubbish,

For the record for the umpteenth time, I am a retired Engineer in Australia, I do not sell anything to anyone.

LFP batteries in the Space Station and satellites, you have to be joking. LFP batteries have one of the worst energy densities (i.e. they are one of the heaviest) of any of the Li-Ion batteries. No they do not use LFP batteries in satellites and the space station.

As for no BMS on the space station, I don't think so, look at page 16 of this NASA document

So what else is made up in your post?

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

[sensij]

I'm being polite and am indeed curious about automated monitoring, but honestly I have every intention of moving forward with a bottom balanced setup with as few components as are necessary. My cells are sitting at 2.6 in parallel right now.

I understand people's concerns on here (screwing up can mean serious damage or death with these tools). Definitely not making any decisions from a single comment or without understanding the implications and nuances adequately

Please start a new thread with follow on questions or updates to your system.