Low voltage Disconnect

wellbuiltwellbuilt Solar Expert Posts: 683 ✭✭✭✭
 Hi all , I was wondering what a good low-voltage disconnect for a 48 volt system with a 430 ah battery . 
 My battery’s are reading 49.9 / 50 volts +- 
 With no in coming power 
 I powered up my microwave for 2 minutes
 and the inverter turned off just shouts of the 2 min ? 
 I’ve never had this happen with my power tools kinda weird  
   My low voltage disconnect is set for 48.8v 
 should it be lower ? Battery’s all have good sg and I went into float yesterday ? 
  Thanks for the reply in advance 

Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .

Comments

  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Usually suggest around 11.5 (12 volt bank) or 46.0 volts as a LVCO level for "lightly loaded" set-point. And 10.5/42.0 volts as a cutoff for heavier loads (such as during starting a well pump.

    You seeing ~50 volts (12.5 volts) as a resting voltage on a battery bank (not under charge) is usually indicative or a "good bank" at something like ~70% or so state of charge.

    If this was the resting voltage of a bank that just came off charging and was supposed to be ~100% SoC--I would guess that this is not great but a sign of possible sulfation of a lead acid bank. And the next step would be to try an do some EQ charging (charging at ~60-62 Volts--Try for 2.5% to 5% rate of EQ charging current) and charge until the specific gravity of each cell stops rising ("new" full charge SG value)--And stop EQ if bank is overheating (and continue EQ the next day). Industrial batteries such as Rolls seem like they can take a few cycles of "hard" EQ over a couple days/several weeks of normal cycling and a (for example) weekly "hard" EQ to bring back up the "full charge" SG voltage.

    If you cannot get the SG to rise and/or your battery bank cannot support the microwave loads today--Then the bank may be on its (for you) last legs of service life (if a bank cannot do what you need anymore--Then it has lost it usefulness for you).

    Of course, there are the usual checks for bad/loose wiring... Look for any warm/hot connections/cables under heavy current (be careful not to burn your hand on a hot connection). What is the Bank voltage and voltage at the AC inverter DC input terminals under load/around when the shutdown occurs. Is the inverter shutting down at ~42.0 volts (normal LVCO voltage from factory)? Is there a large drop from Battery Terminals to inverter DC terminals? Are any electrical connections hot (dirty/loose/corroded connections/wiring)? Check for excessive voltage drop with meter set to (for example) 2.000 V max scale and when under heavier load or charging, see if you can find a terminal (or cable) with excessive voltage drop compared to rest of connections/cables.

    Also check the terminal of each battery (cell) under charge/discharge. Look for batteries that are "different". A high voltage during charging may indicate a failing battery. A low voltage during discharging may indicated a failing battery. At resting, a low battery voltage may indicated a shorted cell, etc...

    You have two parallel strings? Each string taking/supplying its share of total current (i.e, not one string with 30 amps and the other with 3 amps)?

    You have a hydrometer and DC current clamp DMM?

    If you do not have a current clamp meter--I have take voltage readings on cable voltage drop (under load/charging) on two identical length cables in two strings and use the cable like a shunt resistor. If one cable reads 0.020 volt drop and the other 0.010 volt drop--Then the cable with the smaller drop is probably carrying ~1/10th the current (the days when there were no "cheap" DC current clamp meters and I was "too cheap" to by the expensive meter).

    Age of bank? Have you gotten your 3-5 years or upwards of 7 years (OK to pretty good) service life for low cost Golf Cart batteries from your bank?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Combining with your other thread (keep to one discussion/thread to avoid confusion):
    wellbuilt said:
     Hi all , I was wondering what a good low-voltage disconnect for a 48 volt system with a 430 ah battery . 
     My battery’s are reading 49.9 / 50 volts +- 
     With no in coming power 
     I powered up my microwave for 2 minutes
     and the inverter turned off just shouts of the 2 min ? 
     I’ve never had this happen with my power tools kinda weird  
       My low voltage disconnect is set for 48.8v 
     should it be lower ? Battery’s all have good sg and I went into float yesterday ? 
      Thanks for the reply in advance 

    Yes 48.8 volts (aka 12.2 volts) is probably too high of LVCO (low voltage cutoff) for your bank under load.

    The typical starting LVCO number would be 11.5/46.0 VDC for a loaded battery bank at something like 50% state of charge (hot/cold/new/old/charged/discharged--Lead Acid batteries -- Using a "single voltage number" does not really work for all conditions).

    I have seen that some higher end inverters--You can set a short term and long term LVCO/Alarm value... Such as under 46.0 volts for >5 minutes. and under 42.0 volts for under 10 seconds.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • wellbuiltwellbuilt Solar Expert Posts: 683 ✭✭✭✭
     Hi bill , thank for the reply , every thing is great with the system , I just may of never set the low voltage disconnect 
     when I set up my system , it must be factory default setting .
       I just never use the microwave before day light so this is the first time 
     the 50 volts was at day break after being on all night and under some load , lights,  radio , ceiling fan and other stuff 
       SG  is all good every day this time of year 1.275 or better with good water in them . 
    The battery’s are 3 years old and going strong so far . 
     I’m going to drop the LVD to 12v and see how that works , thank bill 
    Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .
  • BB.BB. Super Moderators, Administrators Posts: 32,008 admin
    Wellbuilt,

    Remember that LVCO is really there to protect the AC inverter from high DC input currents (power=voltage*current)... If you have low input voltage, you will get high output current for the same wattage (aka inverter = constant power device -- Aka power output stays the same as input voltage/current vary).

    Also, from what I understand/have seen, when the DC input voltage falls too low--You run the risk of "browning out" the AC voltage (falls below the 105-110 VAC or so minimum you would like to see). For for many motor loads (such as refrigeration compressors), they draw much more current and can/will overheat with low input voltage, or if the compressor is stalled from low AC voltage...

    So--LVCO is not really there to "protect" your battery bank. And the 10.5/42.0 minimum VDC voltage is really just the point where a FLA battery bank has pretty near zero energy to supply--So no reason to design/run inverter at or below these lower voltages to generate your AC power.

    If you do a good job of keeping track of battery bank/state of health--There is not much reason to worry about LVCO going very "low" (say under 11.5 VDC)--Other than to avoid unexpected Inverter "blackouts" starting pumps, running microwave (running the microwave at 11.0/44.0 VDC for 2 minutes is not going to "kill" anything).

    If, for example, you where going on a trip and leaving the system unattended... You might set 12.0/11.5 volts as your cutoff to "protect" the battery bank if "something goes wrong" (stretch of bad weather, solar panel charge controller stops working, etc.)... At the possible expense of spoiled food, mess in fridge/freezer, and such.

    What the inverter does after an LVCO event may be an issue too... Some inverters shut down (on LVCO or even High voltage cutoff) and need a "cold reboot" to restart (shut down DV input voltage to "restart"). Others may shut down at LVCO voltage and restart once the batteries are getting a good charge again (i.e., >12.8 or >13.8 volts).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • wellbuiltwellbuilt Solar Expert Posts: 683 ✭✭✭✭
     Thanks bill every thing is working very good 
      I just drop the lbs a little lower it’s not really a problem 👍
    Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .
  • mike95490mike95490 Solar Expert Posts: 9,361 ✭✭✭✭✭
    My XW has a 46V low battery disconnect, never been tripped
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • wellbuiltwellbuilt Solar Expert Posts: 683 ✭✭✭✭
    Thanks Mike that is what I was thinking .
    Out back  flex power one  with out back 3648 inverter fm80 charge controler  flex net  mate 16 gc215 battery’s 4425 Watts solar .
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