Grounding Battery Bank?

midijeepmidijeep Registered Users Posts: 96 ✭✭
edited October 2016 in Solar Beginners Corner #1
I am getting ready to install a 48v battery bank to a Grid-Tied SMA 6048 Sunny Island and SMA SunnyBoy 3000TL-US-22. 
With this setup, do I need to earth ground the battery bank from the negative of the battery terminal?

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,736 admin
    The short answer is, usually, yes you want to tie the DC negative bus with a minimum of 6 AWG cable to the house/building common ground point (ground rod next to outside of foundation, tied to cold water pipe (or both). Follow local code (at least).

    Now--Why. First, the system will (usually) work without the battery being grounded (nothing magic with grounding). However, there are some caveats--The first depends on the solar charge controller. The new code usually has a DC ground fault detection system (typically a fuse between DC bus negative and greenwire/earth ground. This was an attempt by code folks to reduce the chances of arc faults in the solar array (hot wire to metal frame short). If you ground the battery bus directly, it bypasses (pretty much) the fuse (or ~1 amp breaker used to detect "ground faults" and turn off the solar charge controller). Personally, I believe this is a very bad idea (putting a fuse/breaker between battery negative and safety ground). This is/was a long dicusssion and, with some controllers, DC Arc Fault detection was built into the solar charge controller (a better/safer solution than fuse/breaker in safety ground). Can discuss more, if you wish.

    Second reason to tie battery ground to common building safety ground (same common ground location use by your main AC panel when its green wire ground bus is tied to "earth ground"). "We" (standard practices) put a fuse/breaker in the Hot Wire (Positive, AC Hot, etc.) to protect against short circuits. If you "float" your DC negative bus, you should put fuses in the "return wire" too (2x the number of fuses/breaker, expensive, etc.).

    Grounding the DC negative bus prevents the return (negative) wiring from becoming "hot". Better safety (most people are used to negative ground) and if you have (for example) an RS 232 or RS 485 communications connector between a solar charge controller and a PC / laptop--The common ground cannot go hot (i.e., positive battery lead shorts to grounded metal)--Prevents the two grounds (solar ground and computer ground) from have 12-48 volts difference (and frying your RS 232 cable and Printed circuit board traces). I have seen a fair number of these "different" ground potentials happen in big computer rooms/buildings that would fry a terminal or printer comm port.

    Another is lightning and static charge protection. Solar arrays are (many times) on a roof and can build up a static charge--And if there is direct (or nearby) lightning strike, you want the current to go to ground on the DC side of the solar power system (same thing with AC neutral-earth bonding, again most of the lightning energy goes to ground instead into building/loads/people). Of course, if you have lightning in your area--We need to have more discussions.

    Note--All reasons pretty much point to DC battery bank negative grounding--Except for the first one (ground fault detection in the solar array). The code people "forced" a solution which, I personally believe, was very bad for overall safety. But because different brands/models of controllers implement the DC grounding differently--Need to make sure that your system is done correctly (or to code--whichever you want--I can only make suggestions).

    Note--There is an issue with AC grounding with Inverters... For most TSW/PSW inverters (true/pure sine wave)--They have isolated AC outputs and you can ground bond one of the outputs to create a ground bonded neutral (common north American wiring). However, if you have a MSW inverter (modified square wave inverter)--Most of them do not have their AC output isolated. if you ground the negative Bus and Ground one of the AC outputs (trying to create a ground bonded neutral)--It will smoke the supply.

    For almost all MSW inverters, you cannot ground bond one of the AC outputs to create a "neutral"--Both outputs must float (especially if your ground bond the negative battery bus).

    Complicated answer to a simple question--Sorry...

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • midijeepmidijeep Registered Users Posts: 96 ✭✭
    edited October 2016 #3
    Bill ,Thanks for the detailed information!
    I neglected to mention that I have a Midnight Solar E-Panel connected to the sunny island along with a Auto Former.  The electrician wired all 3 together, which has a tranformerless SunnyBoy 3000TL-US-22 tied into the grid.  Do I run a  6 AWG cable ground wire from the  (-) battery terminal  to the ground terminal located inside the Sunny Island [it is located beside the (+ ) & (-)  battery terminals] or take a 6 AWG copper cable and run it directly to a ground rod outside?




  • BB.BB. Super Moderators, Administrators Posts: 29,736 admin
    I am not sure I am clear on your wiring.

    In general, we want "single point grounding". On the DC side of an off grid system, your current is something like 3-10x higher than on the AC side. If you have a ground jumper that ties an AC ground and a DC ground together, it is possible for the High DC Current (a 100 amps more or to the AC inverter's DC input) to find an alternative/parallel path through both the DC return wire and the DC ground to AC ground back to DC Battery common/ground. And you can "toast" the AC ground wiring (14 AWG wiring is rated for 15 amps maximum).

    Generally, Midnite has good documentation and following it is a good start.

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