Almost seems like its better NOT to have inverter communication?
- Thread starter Alkaline
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Maximum current draw is calculated independently from storage capacity.
If i have a potential maximum current draw of 10kw (approx 200amps @ 48V), for a maximum of 0.5C discharge i need a 400ah bank. In most installs i do, the maximum instantaneous current draw dictates the battery size, the storage capacity is then generally enough. In many cases the storage is over what is required, then supercaps can reduce battery size if there are inductive loads forming most of the peak loads.
As discussed, using integrated systems allows the battery to be easily protected from over current if multiple batteries are in parallel and one goes offline.
(ie two 200ah batteries in parallel for a 200 amp load - if one battery goes offline the system needs to limit current to 100amps)
For me the main advantages of integrated systems are:
- taper current while cell balancing
- limit discharge/charge depending on temperature
- switch additional loads based on SOC.
- ensure multiple charge sources are working together
- single diagnostic / data logging point
- less system components
- by far the cheapest system to put together (may be situation dependent- this is for Australia)
To build a system of non-communicating components that will achieve the same things is far more complex.
If your system doesn’t accurately control charge current relative to temperature and SOC, and you are relying on charge controller setpoints be prepared for short cell life.
Keep in mind that while this is fine in e.g. Australia where 'winter' isn't a big deal, in other places in the world, battery size in off-grid situations has to be related to power generation at worst times. In those cases, battery size is driven by the amount of days of autonomy you need and thus daily consumption patterns, not max draw.
As for communication between all parts, I have been doing that with my equipment where I have a central hub (a Pi) which not only logs the data of all the attached hardware (and graphs it with Grafana), it can also be used to control loads, power on water heating when the battery is full, etc. While this is nice to have, it's not exactly critical. The only thing I make sure of is that if the BMS cuts of the battery, the inverter and charge controllers never come back online unless I do so manually. Because of the design, the BMS shutting off the pack indicates a catastrophic failure somewhere, so nothing should move unless I say so.
timselectric
If I can do it, you can do it.
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If it works for you, that's great. Every system is designed around its particular environment and needs.
In cases where the battery may be 300mv out of balance as it approaches full (think: large grade-B DIY battery, such as mine) - it makes a lot of sense. Even though my charge voltage is 56v (3.5vpc), I can very easily cross over 3.65v on a cell which the BMS would shut down and then I have no battery and my system shuts down. In my case, my BMS comms is saving me by ramping down the allowed charge current before it's too late. In my case this extra "thing to fail" is actually preventing failure.
Like most things, depends on the system.
Like most things, depends on the system.
Sanwizard
Solar Wizard
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That is exactly what I worry about. Going away for a couple weeks, and coming back to find the power has been out due to a runaway cell shutting things down, or a rainy week where the BMS shuts things down due to low voltage.
I imagine the second scenario would be managed by the inverters pulling from the grid to charge the batteries though, and the BMS active balancers should prevent the first one. I also see why having independent parallel batteries makes sense, since a failure of one will not bring down the entire system.
I guess there are always trade-offs.
By the way, has anyone besides me been informed that new NEC code calls for zero exposed DC cables inside and outside?
This makes my Aims DC switch illegal, since it has external MC4 connectors. I currently live on Long Island( at least until Thursday, when I move to South Carolina!) I told the buyer he needs to remedy that issue with a new permit.
What are you guys using in the US as an emergency cutoff switch?
Attachments
Revive an oldie.... been thinking about this myself...
I cant find much from inverter manufacturers on the benefits & why...
BUT there *could* be many, many benefits - but they come with a risk, its all software, so is the programmer going to make a good decision for you?
Safety/Misconfiguration: If there's some differences in config, the inverter could chose the safer option, i.e. you configure 200A discharge, battery says max 100A, so it sets 100A max. Same for voltages etc. So a safety net feature. Useless if you're doing things right, but maybe insurance for a business installer putting these in houses with kids that wanna fiddle.
Invalid config/Hysteresis: I think this is the name... config is a bit funky... turn off at 51v & on at 51.5v. You're discharging & it hits 51v & turns off. Then the cells go back to 51.7 & it turns on. Then it discharges & turns off... on/off... on/off... on/off...
Cell inbalance during charge/discharge - if 1 cell is about to go high/low voltage, the inverter could throttle the charge/discharge so that it doesnt trip the BMS. So adjust the max power given the conditions.
BMS Mosfets: if the config is a little out between the Inverter/BMS (similar to safety above), the BMS could trip a protection. You're potentially turning the mosfets on/off under load which should be fine, but it could be better to gently adjust power instead of a hard shut.
Now - the last two are great features, however you could do those pretty easily with some external software controls & adjusting the config of the inverter. It also only happens when you've got less than ideal setups.
I cant find much from inverter manufacturers on the benefits & why...
BUT there *could* be many, many benefits - but they come with a risk, its all software, so is the programmer going to make a good decision for you?
Safety/Misconfiguration: If there's some differences in config, the inverter could chose the safer option, i.e. you configure 200A discharge, battery says max 100A, so it sets 100A max. Same for voltages etc. So a safety net feature. Useless if you're doing things right, but maybe insurance for a business installer putting these in houses with kids that wanna fiddle.
Invalid config/Hysteresis: I think this is the name... config is a bit funky... turn off at 51v & on at 51.5v. You're discharging & it hits 51v & turns off. Then the cells go back to 51.7 & it turns on. Then it discharges & turns off... on/off... on/off... on/off...
Cell inbalance during charge/discharge - if 1 cell is about to go high/low voltage, the inverter could throttle the charge/discharge so that it doesnt trip the BMS. So adjust the max power given the conditions.
BMS Mosfets: if the config is a little out between the Inverter/BMS (similar to safety above), the BMS could trip a protection. You're potentially turning the mosfets on/off under load which should be fine, but it could be better to gently adjust power instead of a hard shut.
Now - the last two are great features, however you could do those pretty easily with some external software controls & adjusting the config of the inverter. It also only happens when you've got less than ideal setups.
Nobodybusiness
Solar Sponge.
I have the option to do it either way.
Have done it both ways but when doing closed loop the inverter only sees my SOKs instead of both SOK and DIY.
In closed loop the inverter caters its charge to the SOKs.
I just run open loop now and set the max voltage in the inverter.
Does it matter?
Dunno.
The batteries get charged daily.
curiouscarbon
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Properly integrated communication
No communication
Improperly integrated communication
descending order list of my preference.
using JBD BMS and victron inverter, i am still figuring out an optimal solution.
using a raspberry pi is the easiest "glue" for now maybe (hub)
one day it could all be RJ45 ethernet. although CANbus seems more popular now.
the devices seem to have various "dialects" and prebuilt batteries seem to be supporting more dialects over time.
choosing graceful failure modes ahead of time is a big goal for me. thanks for all the insights in this thread.
No communication
Improperly integrated communication
descending order list of my preference.
using JBD BMS and victron inverter, i am still figuring out an optimal solution.
using a raspberry pi is the easiest "glue" for now maybe (hub)
one day it could all be RJ45 ethernet. although CANbus seems more popular now.
the devices seem to have various "dialects" and prebuilt batteries seem to be supporting more dialects over time.
choosing graceful failure modes ahead of time is a big goal for me. thanks for all the insights in this thread.
Nobodybusiness
Solar Sponge.
I just use the set points in the inverter.
charge voltage
Float voltage
Low voltage
Shutdown voltage.
Restart voltage .
As long as the inverter can read the voltage and act accordingly I don’t see any real advantage other than maybe tapering current.
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I’m quite satisfied with my open loop system. I don’t believe I’d be able to do this without the active balancers that I’ve always had in one configuration or another. The time at bulk, absorb, full or whatever you want to call it is not configurable in my AIO, it has its own algorithm which seems to do some time adaptation, however it is undoubtedly too short for a passive balancer. But I look at it as not a bad thing because of the active balancers the batteries spend only a fraction of the time at a high state of charge, hence much less stress. The inverter doesn’t have to wait for the last slow to passive balance battery to say “hey I’m done with my thing up here, let’s all float”.
timselectric
If I can do it, you can do it.
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Current automatically tappers, as the batteries reach the set voltage. Closed loop, offers me no benefits.
If you have low quality batteries/cells. Or don't know how to setup your charging/discharging profile. Closed loop, can do it for you.
But I prefer that my system does what I want, when I want.
If you have low quality batteries/cells. Or don't know how to setup your charging/discharging profile. Closed loop, can do it for you.
But I prefer that my system does what I want, when I want.
This is exactly where closed loop shines and is helpful in my system. As I approach 56v (3.5vpc), I usually have 150+ mv of difference and the closed-loop comms helps back off the current so that I don't trip the cell high-volt and disconnect my battery.
IMO, the charger should be one unit and not require "closed loop" to complete it. Either the batteries should be able to be charged by just applying a reasonable voltage/current, like pretty much every battery operated devices works these days, or the charger in the inverter should have all the charging smarts, like the chargers used in most (older?) RC drones and such. Plug in the battery and balance leads, tell the inverter/charger what type of battery it is and how many cells in parallel, and it does the rest. Those have been around for close to 20 years...
The former is obviously preferable, and will likely happen soon as BMS technology morphs into a complete, intelligent charger, balancer, conditioner.
IMO, arguing that you don't need/want closed loop is similar to arguing that you want to go back to the old carbureted engines of the '70s, which were great when they were tuned properly, but I much prefer just letting the car figure out what it needs to do to run optimally.
The former is obviously preferable, and will likely happen soon as BMS technology morphs into a complete, intelligent charger, balancer, conditioner.
IMO, arguing that you don't need/want closed loop is similar to arguing that you want to go back to the old carbureted engines of the '70s, which were great when they were tuned properly, but I much prefer just letting the car figure out what it needs to do to run optimally.
Too many people gloss over the main advantage of closed loop which is the ability to taper current based on temperature and SOC.
They are generally the same people that are satisfied to get ten years lifespan out of a battery that would last fifteen years treated properly.
They are generally the same people that are satisfied to get ten years lifespan out of a battery that would last fifteen years treated properly.
timselectric
If I can do it, you can do it.
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If your battery temperature is out of specs.
You should address that, directly. It's a design failure.
You should address that, directly. It's a design failure.
That's not how engineering works. Specs are limits, but there are many subtleties within those limits. Just staying within the limits may prevent immediate failure, but that's not the only factor.
Nobodybusiness
Solar Sponge.
What does your Solar system look like?
Jack Rabbit Off Grid
Solar Enthusiast
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I’m using old junk Magnum’s off Lifepower4’s with no coms with no problems. I have not even updated the BMS software in the batteries yet.
I do admit after a few months a battery or two will get a little out of balance.
I do admit after a few months a battery or two will get a little out of balance.
To me the best way if the system is compatible with solar assistant is no comms with the inverter to the battery and have SA read the soc. It will switch the inverter to not use the battery and go grid (if available) thus protecting the batteries. Also with the beta SA you can schedule battery usage.
Since SA and home assistant work nicely it gives you even more options to control it all and monitor it all.
Since SA and home assistant work nicely it gives you even more options to control it all and monitor it all.
How many engineering degrees do you have? Both questions are irrelevant.
Hedges
I See Electromagnetic Fields!
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I designed my system so I can drain the batteries in 20 minutes.
24kW of inverters, 20 kWh of AGM batteries for 14 kWh usable, and Peukert makes it about 8 kWh. 44kw surge.
(If I was reasonable and had only two Sunny Island, it would be good for about an hour.)
Purpose of batteries is to light up the island grid so GT PV can pump out 10kW during the day, to provide starting surge to A/C, and to provide 1kW or so throughout the night.
"Grid Backup", that's my special circumstances.
What BMS (outside of automotive) is even that smart?
I thought communications was mostly to prevent a runner, reducing current so balancing can occur.
We've seen reduced charge current specs approaching freezing, but I wasn't away of BMS implementing that. So I suggested a rectangular limit, pick a current around 0.15C and the corresponding temperature as charge cutoff.
Nobodybusiness
Solar Sponge.
I see. You have none but proclaim to know all about it because you read a book. Gotcha..
Nobodybusiness
Solar Sponge.
You should design one with all your degrees.
I don't proclaim to know all about anything. I have my opinions and you're welcome to state your own opinions, and point out where I'm wrong, but I don't welcome the personal attacks.
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