Grounding 2 separate battery banks

BigTrouble
BigTrouble Registered Users Posts: 4
This is my first post. I've been following this forum since I built my small off grid system about 16 months ago. Thanks for all the great discussion and info. You guys have saved me a ton of aggravation and money.

My existing system is installed in a small beach house in Belize and consists of 750w array, into a TS 45a mppt controller, into 2 Dekka 12v 8g8d 225ah gel batteries in parallel for a 450ah bank. (Bank #1) My power consumption is a modest 1450wh daily, split 55% day, 45% night. I have a couple of small 12v loads directly from battery bank to a 12v distribution panel (hence the 12v system) along with 120v circuits powered by a GoPower 3000w inverter to a 120v distribution panel with the neutral bonding screw removed. The battery negative and 120v neutral are bonded inside the inverter and my single point of system grounding (per inverter instructions) is from the inverter chassis ground terminal to a ground rod driven 10 ft into the sand under the roof drip line of the house. All components are isolated by appropriately sized breakers.

The system has operated flawlessly for over a year, until I recently replaced the propane fridge that finally gave up the ghost with a modest electric one. This added approx 1400w to my daily power needs split 50/50 between day and night. I decided to add 1000w more panels into a TS 60a mppt controller into 2 additional 225ah batteries parallel for a separate 450ah bank. (Bank #2) 

I did not want to add the new batter's to the old ones, or have 4 batteries in parallel, so I am keeping the banks separate. I will leave the 12v loads on Bank #1 and run the inverter from Bank #2. I would like to install a battery selector switch to be able to utilize Bank #1 to the inverter when needed. 

Can I buss the negatives of the two banks together (they are charged by separate controllers) and only switch the positive leads to the inverter, or do I need a switch for the negatives to the inverter as well? (positive switch off,  switch negative to other bank, switch positive to other bank). If I switch both it will leave the bank serving the 12v loads floating with no ground, which I think is ok (?)




Comments

  • Estragon
    Estragon Registered Users Posts: 4,496 ✭✭✭✭✭
    If I'm understanding your setup correctly, you would normally just switch the positive.  This would be essentially the same as separate house and starting banks on a boat.  You could use a switch with bank1/bank2/both/none made for a boat to do this, but make sure it is rated to break load sizes you have.  A 3000w inverter at 12v potentially serious current (>250a).

    Both positives should have properly sized breakers or fuses.
    Off-grid.  
    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
  • BigTrouble
    BigTrouble Registered Users Posts: 4
    Thanks Estragon,
    I'll be using a Blue Sea Systems M-series battery switch rated for 775a @1 min. 500a @ 5 min. 300a continuous.... 4/0 interconnect cable at batteries and to inverter with 400a fuses within a foot of the battery on both positive leads to the switch.
    I wanted to make sure I could buss the negatives on the 2 separate banks together since they are charged by separate controllers.
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Based on my fridge experience you may need up to 500W of PV for that fridge, depending on ''Good Sun Hours", you will need in excess of 5 hrs full sun on the panels to power a standard top freezer fridge...

    I would also suggest you consider going to a 24 volt setup for your fridge, if your fridge has a surge of 1400W at 12v that is ~ 167A or more..  Look into the wire/cable size you will need for 12V vs 24V.. 

    There are significant costs as well as safety to consider....hth
     
    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
  • BigTrouble
    BigTrouble Registered Users Posts: 4
    Thanks westbranch,
    I know 24v makes more sense... when my inverter kicks the bucket I'll make the change. Meanwhile I already have the 4/0 cabling and appropriate fuses/breakers in place for the loads. 
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Well at least you will not need new cables when you do upgrade, just new smaller CB's... I should have added that ~1500W of PV will poweer the fridge for 24 Hrs, ie power the fridge in the day as well as charge the batteries for  overnight, but on startup it will draw from the batteries to cover the Starting Surge...
     
    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