DC Transfer Switch

Options
Philip C
Philip C Solar Expert Posts: 45
Just wondering what others use for Manual DC Transfer switches? Here is my objective, we have two off grid PV systems, and in the event one inverter or battery bank goes down, I want to be able to transfer the PV from one array to a second battery bank. I need to be able to transfer the output of two Xantrex MPPT 60 charge controllers to a second battery bank, and looking for suggestions?

Thanks,
Philip

Comments

  • waynefromnscanada
    waynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Options
    Re: DC Transfer Switch

    What are you using right now as a disconnect between your controllers and your first battery bank? You could leave that in place and add a second identical disconnect between the same controllers and the second battery bank. Then you could connect either battery bank, or both if you so wished, to be charged.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
    Options
    Re: DC Transfer Switch

    Is the second battery bank much smaller than the primary bank?

    I would look at using a DC to DC converter/battery charger (RV's use these a lot) to charge from the primary bank to the secondary bank. If the secondary bank is a lower voltage, you can even use a MPPT type solar charge controller to charge the lower voltage bank from the higher voltage bank (the MorningStar 15 amp 12/24 volt MPPT controller was designed to support this function too--And most good quality MPPT charge controllers should work fine too).

    If it is not too large of battery charger and not done too often--You could use the Inverter and an AC battery charger to recharge the second bank.

    Not a big fan of putting more heavy current/low voltage switches/connections/wiring between charge controllers and battery banks. Although, if I had to do that, I would use something like this switch:

    wind-sun_2203_31289030Blue Sea Battery Switch 1-2-OFF 350 Amp

    Note there are "Make Before Break" (above) and "Break Before Make" switches... You can only use a Make Before Break switch if the two battery banks are the same voltage (otherwise you would be shorting a 48 volt to a 12 volt battery bank during the switching operation).

    Also, your actual connections/products will depend on if the two banks are next to each other with common ground bus, or if they are separated battery banks (long wire runs) and have isolated grounding.


    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Philip C
    Philip C Solar Expert Posts: 45
    Options
    Re: DC Transfer Switch

    Guess I should have been more clear, if this was my system, I would have no problems adding a second set of breakers. But being a customer of mine, I would like to have more of a transfer switch.
    Far as having the big system charge the smaller battery bank, my simple solution to this was placing an AC transfer switch on the load side of things. So if the smaller battery bank is low, one can transfer the AC loads off the small system to the larger one, allowing for the PV system to do its thing.

    But here is the concern, we had a battery failure on the small system, forcing all the AC loads to be transferred to the large system. But I did not have a way to transfer the PV modules to the large system in the mean time. It would be nice if there were a simple way to transfer the PV output to the large battery bank in a similar situation.

    Thanks
    Philip
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
    Options
    Re: DC Transfer Switch

    I don't know of a handy two pole transfer switch (spring loaded knife switch)--They are probably out there somewhere.

    I have seen people make transfer switches out of two pairs of circuit breakers and an interlock that prevents both breakers being on at the same time...

    Here is a site with an interlock design you can probably make your self out of a thin plate of aluminum that slides back and forth.

    And here is another lever type interlock (thread on Mike Holt's forum).

    And here is a search that returns various transfer switch/interlock options...

    Square D makes some breaker interlock systems... However, I am not sure if the $120 is for the interlock (way too expensive) or includes a box+bus bars too...
    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Options
    Re: DC Transfer Switch
    BB. wrote: »

    transfer switches out of two pairs of circuit breakers and an interlock that prevents both breakers being on at the same time...

    -Bill

    Siemens makes one with 3 breakers on each side
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada