Need a parallel switching solution for battery bank

norwich2norwich2 Registered Users, Users Awaiting Email Confirmation Posts: 4
We are installing a PLX Legion solar system that uses batteries while on grid to store and release power during darker hours. Those same batteries, however, could also be available as part of a Legion off grid system when the grid goes down. Since Legion does not recommend that the batteries connect to both these systems at the same time, I need a switching solution for multiple 24v batteries. Basically, I need to quickly disconnect batteries from one connection to the other when the power goes out, then back again later.  IS there an economical parallel switching solution I can use? I'm ok if it is a single manual switch and maybe even a dozen individual switches (one per 24V battery), but I don't want to physically have to unscrew the battery connections and screw in another set when the power goes out. Too much effort and risk with a system fully charged. Ideas? 


  • littleharbor2littleharbor2 Solar Expert Posts: 1,572 ✭✭✭✭
    Marine type A-B switch? This one, pictured, looks to be able to switch from Battery bank one to battery two without going to both at once. Many switches I've looked at give you no choice than to go  A - A/B - B, then off.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • BB.BB. Super Moderators, Administrators Posts: 31,170 admin
    You probably are going to need circuit breakers anyway (for safety). Just use A circuit breaker to Bank A, and a second circuit breaker to Bank B, and so on... You do need do make sure you get DC rated breakers for your system. The behaviour of DC current is quite different than AC current, and using AC breakers on a DC system can spell big trouble.

    There are interlocks (that bolt onto standard breaker panels) you can use to make sure only A or B is on (transfer switch), and not both (if that is an issue). And/or you can use lockouts (keyed locks to breaker(s)) so that only you or others trained to transfer power can throw the switches when needed.

    Is this the "system" you are planning on using?

    You mentioned you want to time shift your energy usage (charge during the day, use battery power/assistance during the night?). Have you checked out the price of doing this? So far, I have not seen any utility power that is more expensive than using solar+inverter+battery systems... Typically, a battery based solar power system (from my back of the envelope calculations) costs something like $0.50 to $1.00 per kWH minimum, and even in California, at this point, we are pushing around $0.40 per kWH in summer afternoons and $0.20 per kWH at night--So, for those of us on Time of Use power plans, it makes even less sense to run battery power at night (although "afternoon runs until something like 8-9pm these days--So running batteries in the evening makes a little more sense).

    Figure out the cost of hardware/installation, new batteries every 5 years, and new electronics every ~10 years, then divide by the number of kWH over that 10-20 year time period that the system will produce, and see how much it costs/saves you.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • 706jim706jim Solar Expert Posts: 380 ✭✭✭✭
    I use one of those marine switches to change between two banks of 4 L16's. Works great.
    Island cottage solar system with 2400 watts of panels, 1kw facing southeast 1kw facing southwest 400watt ancient Arco's facing south.Trace DR1524 MSW inverter, Outback Flexmax 80 MPPT charge controller 8 Trojan L16's. Insignia 11.5 cubic foot electric fridge. My 27th year.
  • norwich2norwich2 Registered Users, Users Awaiting Email Confirmation Posts: 4
    edited October 2019 #6
    Yes, that is the PLX Off Grid Controller that goes between batteries and the Off Grid Inverter.  The other thing connecting to batteries is the Legion Battery Commander. That device works in tandem on the back of the panel with the microinverter. The microinverters chain to dedicated circuit breakers, BTW.  The microinverter and Battery commander are both wireless controlled and I can set battery max/min charge levels/etc.  The only time I need to disconnect battery commander connections to the batteries is during a grid outage, when I would "flip" to the Off Grid commanders and Inverters to power select circuits (e.g. freezer).  I think I will need to switch about 12 24V batteries (three sets of 4, 4 to each of 3 off grid Commanders). On the other side of the switch are 12 battery commanders.  It looks like the Marine switch would work but it seems I'd need 12 of them.  Is there a parallel switch solution that could switch 6 or 12 at a time?
  • EstragonEstragon Registered Users Posts: 4,478 ✭✭✭✭✭
    Why so many batteries?  In general, it's better to use larger, lower voltage blocks (or individual 2v cells) in one or two series string(s).  What does the battery commander actually do?

    For example, 4 x 6v L16 size in series would give you ~350ah at 24v.  This would be 12 cells to check/water (if flooded lead acid) vs 144 with 12 x 24v batteries, and fewer interconnect wires to troubleshoot.
    Main daytime system ~4kw panels into 2xMNClassic150 370ah 48v bank 2xOutback 3548 inverter 120v + 240v autotransformer
    Night system ~1kw panels into 1xMNClassic150 700ah 12v bank morningstar 300w inverter
  • norwich2norwich2 Registered Users, Users Awaiting Email Confirmation Posts: 4
    Actually the 24vs are two 12v in series. The commander charges manages electric flow into and out of batteries over a course of the day. When it comes out it routes to microinverter.  It comes in through the microinverter on teh panel as well. There is an app that lets you monitor and manage all the BCs and MIs.

  • BB.BB. Super Moderators, Administrators Posts: 31,170 admin
    Is the commander a Battery Charger? Only gets charging power from the 120 VAC plug? I don't see a DC input.
    It is possible that you could connect the micro inverter outputs to the 3,000 Watt AC inverter?.

    Anyway, here is a different unit... The Battery Converter plugs between the ~399 Watt solar panel and the micro inverter. And the battery supported is only 40-60 AH (2x 12 volt @ 40-60 AH).

    And they power the micro inverter at night (at least until the batteries run down and/or the AC mains fail).

    I am not quite sure I understand the "need" for this type of system (power micro inverters after sundown)--Unless you are on a utility that "outlaws" backfeeding power the grid. Which is sound like is the design:
    Contains the solar energy that you produce behind your utility meter by regulating energy production of each Micro-Inverter so that energy production is less than or equal to energy consumption.

    Solar Regulator is connected in line with the Micro-Inverters to monitor home energy consumption, solar production and regulate energy production of each Micro-inverter to contain the energy that you produce behind your utility meter where the utility company does not own.

    Plug and play, installs in minutes. Setup and monitoring with smartphone app via Bluetooth connection available for Apple and Android.
    If you assume 60 AH @ 24 Volts and 50% cycling on the battery bank:
    • 24 volts * 60 AH * 0.50 max batt discharge * 81% system eff (battery and inverter--pure guess) = 583 Watt*Hours per day available for dark usage
    In California, that would be at $0.474 (wow--it has been a year since I last looked my Time of Use / Summer peak E-6 rate plan):
    • 0.583 kWH per day 'sifted' * $0.474 / kWH = $0.276 per day shifted power
    • $0.276 per day * 365 days per year * 5 year battery life = $503.70 shifted power used (best case) to avoid grid interactive/backfeeding
    So, what does [email protected] batteries + battery converter + wiring + etc.... Over 5 years (one per ~300 Watt of solar panel).

    And if you want backup power (which is what Norwich) is looking to do (take all of these 60 AH 12 volt x 2 strings and reconnecting to a single 3,000 Watt 24 VDC input AC inverter for backup power.

    It seems like it would work for a smaller system... For a 3 kW AC inverter, I would be suggesting a minimum of 600 AH @ 24 volt AC battery bank (maybe get by with 300 AH), but you are still looking at connecting a bunch of 24 volt @ 60 AH strings together in parallel (5-10 strings).

    And if the your power goes out at 10pm (making things up here to understand the system), I think the battery banks could already be discharged to 50% of capacity, and you would not have much energy for a power outage (using the battery bank for power shifting leaves no emergency reserve)--And would have to wait for the next sunny day to recharge for continued emergency power.

    This is an "interesting" system... All to get around the fact that, I guess, Norwich cannot backfeed power to his utility.

    I would certainly prefer a true hybrid power system (Grid Tied/Off Grid capable) with a standard battery bank (sized to power needs) of larger AH batteries (1-3 parallel strings). However, I do not know if the Schneider/Outback/etc. of the world can do this "neatly" yet.

    Last I read here, more or less, the XW Schneider hybrid inverter would draw ~2 amps @ 240 VAC even if there is enough solar+battery charge to run the home.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • norwich2norwich2 Registered Users, Users Awaiting Email Confirmation Posts: 4
    BB (and others)- Thank you for this well-thought out response. Much to think about. To answer your one question, the Battery Commander (BC) has a  + and a - wire that run down to the 2 12v series SLA battery pair.  The description link you sent is accurate...the BC is in between the microinverter and panel (I may have mispoke earlier).  The design of the system is so you have the option to not run power back to the grid and have that paperwork/etc, but you can choose to do that later on if desired. It also includes the app to manage the system. Not huge advantages, but useful flexibility.  My concern is that I ordered the off grid commander (OGC) from PLX that will each add up to 4 24v pairs to drive a single Off Grid inverter ( In addition to aggregating power, it says the unit will also automatically kick in when grid power goes down. Unfortunately I just found out that PLX (customer support) hadn't considered having both the BCs and OGCs connected at the same time. Their solution is to disconnect the BC wire ends and connect the OGC end to the battery bank if there is an outage.  To me this eliminates the automated capability of the OGC unless I have some sort of switch to speed it up.  I'm tempted to cancel the OGCs and put in another OffGrid solution, though I don't know which would work best in my case.  I do want to use the same batteries that the BCs are feeding under normal conditions. PLX did say the BCs shut down when the Grid goes out. What could I do? The Grid going down is infrequent, but I'm hoping to have an automated or nearly-automated failover to my batteries for the three circuits I want to power.  Can the XW Schneider hybrid inverter work in this role?
  • Dave AngeliniDave Angelini Solar Expert Posts: 5,563 ✭✭✭✭✭
    As Bill said there will be a loading on most any Hybrid inverter including XW if you can't sell to the grid. That load is the failsafe to prevent someone being shocked or electrocuted out in the utility.

    That said, there are ways to write modbus code to XW to have it do most anything you want. You have to have the skill as it is not supported by Schneider or Outback. There are also ways to do what you want with solenoids and contactors.

    Are you sure you are not overthinking this? If you don't have outages that often most use a genset. I did not read all the posts here.
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • BB.BB. Super Moderators, Administrators Posts: 31,170 admin
    edited October 2019 #12
    What is the primary reason for your system? Is it to 1) save money on your utility bill, or 2) to have full solar backup for frequent power outages and save a bit of money on the utility bill when the grid is up and running?

    1) Just do a standard Grid Tied Inverter system (central or micro inverters). If your utility prevents GT Solar connections and back feeding to the grid--That is their (and the state public utility commissions') power system/their rules. There are utilities that have biling plans that make GT Solar less economic (typically high monthly connection fees, low $/kWH power costs, etc.). And some actually "outlaw" the connection of GT+Battery systems to the grid (they don't want people "buying" cheap power at night, and selling it back to the utility during summer afternoons at peak retail rate).

    2) Backup power systems are almost always uneconomic vs utility power. They only make sense if you need backup power (personal choice). A full blown solar+battery bank+hybrid inverter system--When you include initial install, new batteries every 5-7 years, and new electronics (inverter, charge controller, monthly minimum power connection fee, etc.) every 10+ years... Typically you are looking at $0.40 to $0.50 per kWH minimum "all in costs" for your "free solar power" (and can run $1.00-$2.00+ per kWH for full off grid systems--where you have to over design the system to provide winter power in poor sun, vs the lots of excess unused power in the summer). Whereas a typically GT Solar+Inverter only system, probably costs something like $0.10-$0.15 per kWH, has no maintenance costs (other than inverter failure every 10+ year, possible solar panel failures in 20+ years).

    A backup genset... Assume $4.00 per gallon for gas, ~4kWH per gallon, or ~$1.00 per kWH that only takes money from your wallet when the power is out for a couple hours to a couple days per year... + the $ for the initial generator cost and installation+transfer switch+protected power sub-panel, if needed.

    And if you have natural gas or propane on site--That is much better than storing gasoline/diesel on site.

    I store 20 gallons of gasoline (with preservative) and recycle once a year into the vehicles. And have an Honda eu2000i that will keep me happy on something like 2 gallons of gasoline per day--20 Gallons = 10 days of no power--Plus I have fuel for a gasoline cook stove (if I am out of 20 lb bottles of propane), and fuel for the car if needed. $8.00 per day (our gasoline price went from $3.00 per gallon to $4.00 per gallon in the last few months) if I don't have utility power... And I have had a genset and fuel since 2000 (y2k--wife wanted backup power) and never needed the genset yet. Probably about time to go out and buy a second Honda eu2000i (or whatever is current) for a second (actually third) spare genset.

    For folks in California, the picture is a bit muddled. Because of the last few years of "electric utility related" massive brush fires (there is lots of blame to pass around in my humble opinion), our local utility has been threatening to shut of power to large areas of the state (mostly rural) for hours to days at a time during the summer/fall. I am not sure they have actually done this yet (perhaps a couple times so far), but that does put the ball into the (especially) rural consumer's court and make having a full off grid solar power system + generator back (if needed) more useful. And simply disconnect permanently from the grid if the utility is "not reliable" any more, and the State of California keeps escalating power bills.

    The PLX system look way too complex (too many bits and pieces that can fail, and operator error) to be a good deal for the long term (you will probably responsible for maintenance, and what if a BC fails 5 years from now and you cannot get a replacement for the custom product)...

    Regarding the battery connections... I think you are going to need double pole breakers (like 240 VAC breakers) and open/transfer both B+ and B- lines... I don't think the system was designed with a common ground (negative) line between each BC unit. That leaves out most DC standard transfer A/B marine switches--They only switch the positive lead.

    I highly suggest you go through some paper designs first, and run a little math/spreadsheet on your loads and equipment costs, to calculate the actual cost per kWH for the various solutions. And all that goes back to measuring your loads (Watt*Hours per day) and basing your design on supporting those loads.

    And look at your location too... Solar panels only work in full sun. Any shading (trees, overhead wires, roof stacks and chimneys, neighboring buildings, hills, valleys, local weather conditions, marine layer, non-southerly facing panels, etc.) only serve to dramatically reduce your harvest.

    A Kill-a-Watt type meter is a great way to start, and not bad for figuring out conservation measures (what to turn off when not used, or replace if consuming more power than a modern energy star replacement would use... Using a laptop computer (at 30-60 Watts) vs a desktop at ~300 Watts, etc...

    It is almost always cheaper to conserve power than to generate it.

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