Low Voltage Disconnect Settings

tkc100tkc100 Solar Expert Posts: 67 ✭✭✭✭
What is the general consensus out there for the proper setting for a load controller?
I have a 12 battery bank I want to protect from being overly discharged.
I am operating under these assumption please correct me it I’m wrong.
12.7 volts ≈ 100 % SOC
12.2 volts ≈ 50 % SOC or 50 % discharged
11.98 volts ≈ 20 % SOC or 80% discharged
Ideally I want to keep the SOC from going below 50%

The problem I am having is where to set the LVD. My load controller’s settings are 10.5 to 12.5 volts in increments of 0.2 volts. There is a 6 minute delay built in between the time the disconnect voltage as been hit and the actual disconnect.

How do I account for the battery voltage under load? The actual SOC is not indicative of battery voltage when the battery is under a load. Is there any rule of thumb you all have come up with, a kind of average that seems to work under most circumstances or should I set it to 11.98 volts to keep it from ever discharging more than 80%. As I’m writing and thinking I don’t know if that would work either.

What does experience say?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    Re: Low Voltage Disconnect Settings

    Maybe nothing below 11.5 volts unless very heavy surges? The six minute timeout may be good for averaging through surges.

    Might even be more difficult to choose... MorningStar on some of their units says that they have a LVD offset based on current draw of the load (more current, lower LVD setpoint?).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • GreenPowerManiacGreenPowerManiac Solar Expert Posts: 453 ✭✭✭
    Re: Low Voltage Disconnect Settings

    Some of the charge controllers have dip-switches and others are fixed at a LVD. SunSaver is fixed and it's a pain to reset when the batteries are too low. (The LVD light stays on until a certain battery voltage is met). As you can tell, I've left my inverter on all night a few times and woke up to a drained backup battery bank and an error light on the charge controller.....lol
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  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Low Voltage Disconnect Settings

    Technical note: inverters should not be connected to a charge controller's "LOAD" terminals. These are designed for low current DC devices such as lighting and are limited to (usually) the max current of the controller, which an inverter can easily exceed.

    Further; if your batteries are going dead overnight because you left your inverter on, you haven't got enough batteries. The stand-by draw needs to be taken into account when determining the total load that needs to be supplied.

    Just thought I'd mention that in case anyone reading this thread was getting the wrong idea about how to hook things up. :roll:
  • tkc100tkc100 Solar Expert Posts: 67 ✭✭✭✭
    Re: Low Voltage Disconnect Settings

    I think you are right BB it’s a moving target. I suspect some EE boy somewhere has come up with an automated solution. A load controller with the ability to follow the bouncing ball but for me right now I have a Xantrex C-60 configured as a load controller.

    Cariboocoot this particular unit can handle 60 amps continuously and 85 amps surge. If that were not enough with a relay it could be made to handle any load but that’s not the point of this thread.

    I was hoping that those of you with more experience than me might have come up with a reasonable compromise for setting the LVD. The real unknown that might make such a rule of thumb improbable is each and every system will experience different loads. I think I will begin with erring no the side of being safe and set it a 12 volts even. If the thing is always tripping I will consider a lower figure if it never trips I will consider a higher number.

    For the uninitiated out there you might be wondering what all the fuss is about. Stated simply discharging your battery bank too deeply is very detrimental. Consider the attached graph. Ignore the little comment tags this was converted from a PDF file. The graph is also a depiction of a AGM battery, expect less from the standard golf cart battery.
    Battery life is directly related to how deep the battery is cycled each time. If a battery is discharged to 50% every day, it will last about twice as long as if it is cycled to 80% DOD. If cycled only 10% DOD, it will last about 5 times as long as one cycled to 50%
    The most practical number to use is 50% DOD on a regular basis. This does NOT mean you cannot go to 80% once in a while. It's just that when designing a system when you have some idea of the loads, you should figure on an average DOD of around 50% for the best storage vs cost factor
    Now I for one am not willing or perhaps not able to constantly monitor the battery bank voltage to ensure the best life for my batteries, hence the load controller.
  • petertearaipetertearai Solar Expert Posts: 406 ✭✭✭✭
    Re: Low Voltage Disconnect Settings

    The battery monitor I have can have an output programmed into it to disconnect at a predetermined SOC.
    Why not use the battery monitor to decide when to disconnect ?
    2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . victron battery monitor . 24 volts 450 ah surette batterys . off grid  holiday home 
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Low Voltage Disconnect Settings
    The battery monitor I have can have an output programmed into it to disconnect at a predetermined SOC.
    Why not use the battery monitor to decide when to disconnect ?

    What do you have for a battery monitor?
  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    Re: Low Voltage Disconnect Settings

    I have always thought that a Battery Monitor would make a much more accurate "switch" for charging (auto-genset, turn off loads for low battery, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tkc100tkc100 Solar Expert Posts: 67 ✭✭✭✭
    Re: Low Voltage Disconnect Settings

    I currently have the TRI-METRIC 2025 and when I say current I mean I really haven't become accustom to using it yet.
    I does provide a Low Battery warning.
    Flashing B.Lo No audible warning or cut off
    You all are right in that a meter such as this does provide a much better sense of SOC but here the rub for me.
    I'm not the only person present. I keep a pretty vigilant eye on things but my wife however is not inclined to.
    I could have spent twice as much and gone with something in the Xantrex line. They have a low battery disconnect built in but I didn't.
    I have got the C-60 controller just laying around and thought I would give it a useful purpose.
    I feel better with a fail safe
    Knowing that I won't forget, overlook, come home or wake up to a dead battery bank.
  • mike95490mike95490 Solar Expert Posts: 8,470 ✭✭✭✭✭
    Re: Low Voltage Disconnect Settings
    tkc100 wrote: »
    ...... They have a low battery disconnect built in but I didn't......


    Charge controllers low battery disconnect can only handle a small load, but it could controll a relay to shut down the inverter. I thought the C-60 had a load terminal ?
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  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Low Voltage Disconnect Settings
    mike90045 wrote: »
    Charge controllers low battery disconnect can only handle a small load, but it could controll a relay to shut down the inverter. I thought the C-60 had a load terminal ?

    As per the original post, the issue is that Voltage alone is not a reliable indicator of SOC. Heavy loads may pull Voltage down below disconnect even though the batteries could sustain the load for some time. Hence the desire for some system that can shut things off based on real SOC readings.

    Sounds like a darn good idea to me: battery meter hits "60%" and you get a warning; "50%" and it's "lights out, everybody!" :D
  • tkc100tkc100 Solar Expert Posts: 67 ✭✭✭✭
    Re: Low Voltage Disconnect Settings

    The Xantrex C-60 can function in one of three modes, independently not concurrently.
    Charge controller, Load controller or Diversion controller
    As a load controller which is what I am playing with right now it is in series with the positive cable. Between the battery bank and load. It is capable of handling 60 amps continuously. It is only capable of sensing battery voltage as a means of determining SOC. That was the gist of this conversation. Since voltage is only half of the actual equation when it come to monitoring SOC for a system in use. The variation it voltage in response to a load makes it difficult to determine an actual SOC.
    I think you were referring to an earlier post in which there was some mention of not putting an inverter in series with a load controller.
    I am now beginning to think that at best it can function as a fail safe to avert a complete system discharge in the event other thinks get over looked. What I need to experiment with is the actual voltage setting.
  • tkc100tkc100 Solar Expert Posts: 67 ✭✭✭✭
    Re: Low Voltage Disconnect Settings

    Cariboocoot
    Right on!
    You must have posted while I was composing.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Low Voltage Disconnect Settings

    Maybe the good folks at Midnight Solar are listening?
    We want the next generation Classic controller to have a built-in battery monitor with fully programmable SOC AUX function. :D
  • tkc100tkc100 Solar Expert Posts: 67 ✭✭✭✭
    Re: Low Voltage Disconnect Settings

    Check out the attached file.
    I have had some misgiving about Xantrex product line over the past few years. They certainly have a good linage with their acquisition of Trace but the sell out to Schneider Electric I don't know, I guess time will tell.
    As always they're pricey $$$$
    On paper the monitor seems to do it all, including features I have not seen on other products like temperature compensation, LVD, generator start and a computer interface.
    For those that have the money to have it all, 480 some dollars this look like the monitor for them.
    I would love to have it but for now and for the foreseeable future my 2025 is going to have to do.
  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    Re: Low Voltage Disconnect Settings

    $480 sounds a bit high:confused:... For the base model, our host sells them for ~$240 each.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • tkc100tkc100 Solar Expert Posts: 67 ✭✭✭✭
    Re: Low Voltage Disconnect Settings

    BB
    If you get it loaded with the accessories necessary to do all that it is capable of doing.

    http://www.solar-electric.com/xabamoac.html

    $482.05 Pro meter temp sensor and wiring kit

    I think the $239 is the lite version with out the accessories which really aren't accessories if the monitor is to do all that it can.

    I would love to give one a try but there's a number of more basic things that will have to come first.
    I was just sharing a dream and if your going to dream why not big.
  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    Re: Low Voltage Disconnect Settings

    You don't need the wiring kit--Just get some CAT 5 or similar cabling and a couple of 1 amp fuse holders (to limit current in case of short).

    The remote temperature kits--probably very nice in temperature extremes. Most Battery Monitors (that I have seen) don't even have that option. But $65 ain't cheap.

    In any case, options are how these vendors really make their money on options (look at MorningStar's price list). They charge $26 for their remote temp sensors (probably still too much).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • petertearaipetertearai Solar Expert Posts: 406 ✭✭✭✭
    Re: Low Voltage Disconnect Settings

    Hi Wayne from Canada.
    I have Victron single batt monitor. Around $300.00 nz.
    2225 wattts pv . Outback 2kw  fxr pure sine inverter . fm80 charge controller . victron battery monitor . 24 volts 450 ah surette batterys . off grid  holiday home 
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Low Voltage Disconnect Settings
    Hi Wayne from Canada.
    I have Victron single batt monitor. Around $300.00 nz.

    Thanks a lot, I'll check it out :)
  • Lack_Of_SunshineLack_Of_Sunshine Registered Users Posts: 1
    Has anyone had experience of this ? http://www.pbase.com/mainecruising/smart_gauge
    Interesting read as It claims to accurately learn soc from voltage readings only and comparing it to pre set values.
    It has put me off getting an AH type of battery monitor as they calculate SOC based on the  100% capacity Ah rating as programmed by the user. In real life this rating is far from accurate especially as batteries age and even with new batteries.
    But can it really calculate soc just from voltage ?
    It also has a relay output to switch anything like an inverter for example which is what I would want it for.
  • BB.BB. Super Moderators, Administrators Posts: 29,717 admin
    Here is the mfg's website. Lots of technical discussions/white papers about their products and batteries in general:

    http://smartgauge.co.uk/

    I do not know anything about the product itself. But they seem to be a sharp set of folks designing the product.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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