charging a second battery bank , with dump load from a pmw

hi all , electronics are  new to me so if I make myself sound foolish please bare with me , I just need help and advise .

this will be a small  setup , about 500 ah at 24 volts with solar with a  mppt  and wind with a mpw  charging the main  battery bank , if the sun and wind was working  to their  max about 2 kw would be produced  so I want to use the dump load to the max , so can I do is and will it work safely  ????????????????

when the main battery bank is full but power is still in abundance , feed the dump load cable through a relay driver ( which I think turns it back into DC power? ) then straight into a second battery bank and inverter , this inverter would power say a 240 volt  air con units or fan heaters ,  when the battery bank  got to a set discharge level , they would just turn off safely , there will be resistors , in case for what ever reason they is no load .

hope that all made sense to you

many thanks



  • EstragonEstragon Registered Users Posts: 4,496 ✭✭✭✭✭
    I'm no expert on wind systems, but my understanding is that you need a "dump" load on a wind controller to avoid overspeed on the turbine in high wind and low charging load. This load would have to be available whenever condtions require. Water heater would be a typical load for this dump load.

    AC or heating would be more of an "opportunity" load which you might optionally use with good output and full batteries. You may not want heat/AC as an automatic, non-optional dump load though.
    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
  • cockneybob2cockneybob2 Registered Users Posts: 18 ✭✭
    hi estragon , thanks for comments ,  it would only never be seen and used as an opportunity load for sure , as it is based on the weather it may ,   in fact  never be used at all   , resistors  would be in place to automatically burn off the extra  power that is not used up
  • JohannJohann Solar Expert Posts: 245 ✭✭✭
    edited February 2017 #4
    Consider yourself lucky if you live in a place where you have strong usable wind to actually make power. Many places have wind, but not enough wind to make  power.
    Basically, wind power stays connected to battery #1 at all times. If your battery #1 is full then the controller #1 is connecting a load/resistors #1 to the battery #1 while your wind power stays connected. This will draw power from battery #1.
    Your load/resistors #1 could be a battery or let's call it battery # 2.  If battery # 2 gets full you need a controller #2 to dump power into your resistors to avoid over charging battery # 2. 
    If using a inverter at any battery then a low voltage shut off should be incorporated, since most inverters will shut down when the voltage  is way to low and when the battery is discharged or dead which kills batteries in no time.  

    Depending, you may need to control the amps into battery #2. You may even need a charger or generator to fully charge battery #2.

    I would make battery #1 bigger and not using battery #2 , dump power into a water heater with controller #1.
    When water heater is turning off, controller #2 can dump power into the resistors or a heater in winter.
  • mike95490mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Remember, the dump load is critical, and prevents either/both the batteries being overcharged or the turbine spinning unloaded and disintegrating.  A 100 gal water would be a minimum load, and remember, after 3 hours and the tank is hot, the thermostat goes off and you loose your dump load.
    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

    gen: ,

  • JohannJohann Solar Expert Posts: 245 ✭✭✭
    edited February 2017 #6
    mike95490 said:
    Remember, the dump load is critical, and prevents either/both the batteries being overcharged or the turbine spinning unloaded and disintegrating.  A 100 gal water would be a minimum load, and remember, after 3 hours and the tank is hot, the thermostat goes off and you loose your dump load.
    Mike, the turbine stays connected to the battery at all times, so it always stays ''loaded'' even while the controller is dumping power. The controller is dumping power from the battery to a resister which is heating up when dissipating power in a normal ciurcuit. The turbine never gets disconnected from the battery while that is happening.

    When the thermostat goes off a second controller can dump that extra power into a resistor.

    If the battery need to be disconnected at a point for maintenance for example, there should be a switch installed that will short circuit the 3 phase ac side at the rectifier, this will put a load on the turbine and act as a break.
  • cockneybob2cockneybob2 Registered Users Posts: 18 ✭✭

    hi guys , thanks for your comments , the reason I would like to channel the diversion mode / dump load power from the PWM  charge controller into another battery is to use the excess power to run , lets call them self indulgent mains AC appliances , instead of just  burnt off by water heaters or resistors .

    I want to  put as little strain on the main battery bank as possible , to get the max life span out  of it , I would never try and run those appliances from the main bank as it would mean monitoring all the time  so the % of it didn't drop to low , I do not  want to be  switching things on and off all the time .

    the impression I get is that I can put the dump load into another battery  ( is that correct ) ? if so , I believe the power from the dump load is pulse not pure  DC  , which is ok for resisters and DC water heaters  but is it ok to put straight into a second  battery or does in need to be  cleaned  / smooth through a relay driver ? .

    you said the second battery bank could  have its own PWM charge controller so it can dump load into a resister , great idea thank you , this    inverter would be  set to  a high % stop voltage , say 95% so never really runs down to far , the idea being ,  the appliances would only run when there was excess power .

    please let me know if this can work , its driving me mad .

    many thanks Robert

  • BB.BB. Super Moderators, Administrators Posts: 33,143 admin
    Yes, you are correct. It would work.

    However, I dislike complexity of design in systems that are supposed to work for years with little maintenance. You would need at least two charge controllers on bank "A". One to move current to battery bank "B", and a second controller to dump excess energy from "A" to a resister bank when "B" is full. And, you really should have a backup dump controller+load on "A" in the case if both dump controllers/loads fail.

    Second--If you do not have enough energy--Battery bank "B" will be the first to suffer--Needing you to use a backup genset and/or you will need to manually manage loads on bank "B" (and "A"?) to ensure that you do not kill either bank.

    Dump controllers are not as good at regulating battery bank state of charge--You either are dumping charging current into the bank or dumping load. With series (standard configuration) solar charge controllers, you are either charging or "off"--This is better.

    Now--Normally, in a system with a solar array, you would have a series charge controller for the solar array regulation, and a second (one or a pair) of dump controllers setup as dump controller(s) to regulate the wind turbine charging (the solar controller would be set, for example, to charge to 14.6 volts, and the dump controller set to charge to 14.8 volts. Wind turbines typically do not have predictable/daily charging unlike solar. Gives you the advantage of better charging controller with the series type solar charge controller and the dump controller to control over charging when needed during stormy weather/windy season). That is another charge controller (with PWM controllers, you can share one solar array with two PWM charge controllers to separate battery banks).

    I would suggest that you simply have one battery bank / charging setup (solar+wind)--And simply mange your secondary loads (turn them off when poor charging conditions are present--or when the battery bank is at a lower state of charge).

    Things fail--And they can fail in different ways. A failure of a series charge controller will stop charging your battery bank (not good for the battery). A failed dump controller (or dump load) will over charge the battery bank and can run the risk of fire. (note: series charge controllers have been known to fail and over charge batteries too, just that failing shorted seems to be less common).

    I believe that a single battery bank will be much easier to maintain, debug, and safer than a "master/slave" battery bank setup.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cockneybob2cockneybob2 Registered Users Posts: 18 ✭✭

    thanks bill , lot to take in , back to the drawing board I think

    many thanks for the   advise


  • BB.BB. Super Moderators, Administrators Posts: 33,143 admin
    There is one "master/slave" battery system that people have used successfully. The standard 24/48 volt master battery, and a smaller 12 volt battery for 12 VDC devices (such as HAM radio gear).

    Use an MPPT type charge controller to take the higher voltage battery and down convert to the 12 volt battery. However--As you can tell, this discharges the main battery bank in favor of the lower voltage battery bank--So you still have to monitor your loads.

    By the way, how well is the wind turbine working for you? In general, most installations do not seem to reliably generate much useful power. There are a few people that have done well, but most build their own turbines on put them on 60-90 foot towers.

    From a person that ran a Skystream turbine for a few years (grid tied--So this is the "best" harvest that you will get--Battery charging always "tosses" some energy away when the batteries are full and you do not have enough loads--Note that the Skystreams did seem to be the best producing "small" wind turbines--When they worked):

    Very important historical information (In my humble opinion):

    92 84
    97 164
    246 472 2,480
    265 348 244 371 18286 90 84 250 100194 131* 2,345
    101 130

    * December, 2007 - Skystream shut down from software problem with cold and would not restart.

    * January, 2008 - Skystream down until the 27th. Produce 98kWh in the 5 days remaining in the month.

    * December, 2009 - Skystream shut down several times and would not restart on it's own, production very low for wind speeds.

    * January, 2010 - Production very low for wind speeds.

    * February, 2010 - Production very low for wind speeds.

    Seems to have worked.

    Remember that this was from a customer that had heavy support from the vendor (Skystream, now out of business). Was unhappy, complained, got support, was then "happy".

    Good wind near Lake Erie or other Great Lake... (as I recall)?

    For most people, I would suggest solar panels is a better/lower maintenance/cost system... However, if you are in Peterborough, EN, United Kingdom area--You probably don't get much winter sun (fixed array, 45 degrees):

    MonthSolar Radiation
    ( kWh / m2 / day )

    3 hours of sun per day of sun is pretty reasonable. 1 hour of sun per day--Not great.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • cockneybob2cockneybob2 Registered Users Posts: 18 ✭✭

    hi bill , thanks again for your incredible detailed and knowledgeable input / comments .

    I am only at the planning stage of a boat fit out in the near future , I think I got a little carried away with myself and the expectations of what a solar and wind system can and will produce .

    the idea was to use the free / excess power but now looking at the reality of it all , it would never be free due to the high setup costs , for something  at best would only happen  once in a blue moon so it doesn't make sense .

    one battery bank and monitoring the input power , then using  appliances where applicable is the prudent and sensible way forward .

    like a lot of thinks and ideas in life , are start with good intentions but then find out that in fact has no merit .

    once again , thank you for your input , it has been invaluable .



  • jonrjonr Solar Expert Posts: 1,386 ✭✭✭✭
    > Use an MPPT type charge controller to take the higher voltage battery and down convert to the 12 volt battery.

    If you have a spare battery and are willing to manually shuffle batteries around, this same technique could be used to make sure that each battery in a string gets a full charge every two weeks.   Discharge it to match the batteries in the string before putting it back into the string.

    Of course using a generator is easier....

    I am available for custom hardware/firmware development

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