Grid tie with battery back up

A company I do a bit of work for installed a xantrex sw inverter/backup system. Turns out the system drains the batteries down to a certain level every night, then spends around 4 hours the next day recharging the batteries before any power is sent back to the grid. Concequently the customer is unhappy with the amount of savings on his bill, which is pretty much nothing. Do the batteries need to be cycled daily to maintain their integrity? What about every other day? Should 1/4 of the panels be used to re-charge the batteries and the others used to feed the grid, which would require another inverter of course. I noticed they are using the c-40 charge controllers as well, what type of difference would a MPPT controller like an outback make?

Pardon me for not having all the system details, I have only been there once and I was just along for the ride. I am being sent back to install a 3rd party data logging software and PC anywhere on a computer connected to the SW inverter so they can moniter and change parameters as necessary.


  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Grid tie with battery back up

    that much drainage from the batteries is either the culmination of the phatom load of the inverter being on or there are other loads on draining it down further as well. looking at the specs for idle and search mode show 16w and 1w respectively.
    i don't know if that would account for the losses seen or not, but i suspect somebody overlooked something even if that something is the inverter being left on during sunless periods.
  • Frank
    Frank Solar Expert Posts: 54 ✭✭✭
    Re: Grid tie with battery back up

    Something isn't right with the setup. We use a SW inverter and it cannot drain the batteries downl, assuming that power isn't lost every night. "Sell" is the same as "Float" which means that the grid will maintain setpoint voltage at any time there's no DC input and the panels will provide the energy when sunlight is available. Excess will be sold after satisfying local loads. Post some more details and lots of folks can help.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,404 admin
    Re: Grid tie with battery back up

    From the Xantrex Manual for a generic SW inverter/charger, there are a couple of modes buried under menus (assuming I have the right manual):
    Set Grid Usage

    • FLT - Float will try to maintain the batteries at the float voltage level. This can be used when the
    source of power is a utility grid or a generator. When AC power is available, the inverter will complete
    a full three stage charge cycle and then hold the battery at the float level until the source of utility
    power is no longer available. This is the default setting and is appropriate for use with stand-alone
    systems with back-up generators or utility back-up systems. FLT mode does not sell excess power
    into the utility grid. If a DC power source is available and the battery is full, its power will be used to
    directly power the AC loads connected to the inverter output even though the AC power is also
    connected to the loads. If more power is available from the DC source than is required to power the
    AC loads, the battery voltage will increase above the float level. An external charge control device –
    such as the Trace™ C40 Load/Charge controller - is therefore required to prevent over charging of the

    • SELL – SELL mode enables the inverter to “sell” the excess power to the grid (AC HOT IN 1
    only). This mode must have the approval of the local power utility prior to its use. In the US,
    utility companies are required by law to purchase any excess power generated by their customers;
    however, they decide what can be connected and what safety requirements must be met. Be advised:
    some utilities will be more receptive than others will. The most advantageous configuration is called
    “NET” metering where only one meter is installed and spins either direction. The purchase and sell
    prices are equal. “Dual” metering requires two meters and is less desirable since the power you sell is
    usually worth only a fraction of the price for the power you purchase. Power from any DC source, such
    as a solar array, and a battery can be sold. When power from a DC source is available, it will be used
    to power any AC loads connected to the AC output first. Any excess power available from the system
    will be sold “into” the utility gird through the AC HOT IN 1 terminals.

    • SLT - The silent mode does not maintain the battery at float voltage all the time. The battery
    charger only operates for part of each day. AC power from the utility grid is passed through the
    inverter to the loads 24 hours a day. Once a day, at the time prescribed by the BULK CHARGE
    TRIGGER TIMER, the batteries are given a bulk and absorption charge cycle. The inverter will
    perform a bulk charge once per day from the grid, charging the battery to near the SET BULK VOLTS
    DC setting until the battery charger has held the battery near the SET BULK VOLTS DC setting for
    the ABSORPTION TIME period setting. The inverter will then go totally silent and will wait for the utility
    power to fail, or until the next day when it performs another bulk charge. After each power outage, the
    inverter will perform another bulk charge cycle once the AC source has returned. This is typically used
    only in utility back-up applications.

    • LBX - The low battery transfer mode allows a system to automatically switch between utility
    connected and stand alone battery operation. In this mode, the inverter will power the loads from
    the battery and solar array (or other energy source) until the battery voltage drops to the LOW
    BATTERY TRANSFER VDC setting. It will then connect to the utility grid and charge the battery. The
    loads will be powered by the utility until the battery voltage reaches the LOW BATTERY CUT IN VDC
    setting. The inverter will then disconnect the utility and power the loads from the battery and any other
    source of DC power connected. This mode is often used instead of the SELL mode because approval
    from the utility is not required - no power will be sent into the utility distribution system when LBX is
    selected. To use the LBX mode, the AC source (utility power) must be connected to AC1 input only,
    transfer to the inverter will not occur if the AC source is connected to the AC2 input. If AC is present
    on the AC1 input in the LBX mode, the AUTO GENERATOR CONTROL MODE will be disabled.

    CAUTION: If the system is not properly sized, the LBX mode can result in frequent transfers
    from the battery to the utility and result in poor performance of the system and excessive
    energy consumption from the utility. The daily output of the alternative power source (solar,
    wind etc.) should be able to meet the daily power requirements of the loads being operated
    under typical conditions. See the low battery transfer mode section for more information.

    The manual talks about "Modes":
    • Utility Backup Mode - Phase synchronized fast AC transfer switching for utility backup power supply
    applications. Includes adjustable AC transfer voltage and line conditioning ability.

    • Utility Interactive Mode - Excess power from charging sources or stored power from the battery can
    be “sold” back into a utility grid. Also allows selling of the stored energy in the battery during a specific
    time period.

    • Energy Management Mode - Onboard clock to set inverter and charger operating time periods. This
    mode can be used with “time of day” metering to shift energy consumption to off-peak periods.

    • Peak Load Shaving Mode - Used to limit the draw of AC loads from a utility grid by powering it from
    the batteries. The batteries are recharged when the AC loads are reduced. This can “level” the load
    on a utility.

    • Low Battery Transfer Mode - Automatic transfer of the AC loads from the batteries to the utility when
    the system reaches an adjustable low battery voltage setting. Independent settings allow control of
    when the AC loads return to battery once it is recharged.

    Often, the inverter will be set-up to operate in several modes at the same or different times - such as
    operating as an inverter/charger in utility back-up mode with automatic generator control mode and
    generator support mode during extended utility outage periods.

    It would be best to download and print out the manual... There are a lot of settings that need to be correctly done to sell power to the utility (only after approval by the utility)... It is possible that the inverter/charger is set to simply use utility power only where there is insufficient battery power available (not sell mode is used when selling back to the utility is not allowed).

    Also, an interesting warning that you don't need a battery charge controller while selling power because any excess power is simply sent out to the utility. However there would be an issue if the system was disconnected from the grid for an extended period as it (apparently) has no ability to reduce the amount of power from the solar panels once the batteries are charged... You would either need to use a secondary relay to turn of the panels, or a solar controller to properly charge the batteries in a long term off-grid situation.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Brock
    Brock Solar Expert Posts: 639 ✭✭✭✭
    Re: Grid tie with battery back up

    Ya they shouldn’t be in that mode, they are loosing more power running the batteries down each night and recharging them. They would be much better off with the inverters in float "FLT" mode where it sells any access power back to the grid. I just with you could set it to silent "SLT" when the sun goes down. But anyway it sounds like it is disconnecting from the grid every night. I forget the setting that does that, I used to do that when I was “on peak” when our grid power was more expensive but I didn’t want to ruin my batteries in a year or two so I just went back to float mode.

    I leave mine in float for about 2 weeks then cycle the batteries a bit then letting them run down 10% to 20% then fully charge them and back to float mode. The batteries certainly don't need to be cycled daily and will wear them out a lot faster.
    3kw solar PV, 4 LiFePO4 100a, xw 6048, Honda eu2000i, iota DLS-54-13, Tesla 3, Leaf, Volt, 4 ton horizontal geothermal, grid tied - Green Bay, WI
  • System2
    System2 Posts: 6,290 admin
    Re: Grid tie with battery back up

    Thank you for all the information, pardon me for not RTFM but I didn't install it and I am not sure they want me to do anything other than install the 3rd party software that monitors everything the system does. So I didn't want to invest alot of time researching it. I sure do like this forum by the way, always great responses and not alot of patronizing.
  • System2
    System2 Posts: 6,290 admin
    Re: Grid tie with battery back up

    So it turns out it was in float mode and not in sell. They told me not to touch it, thanx again for all the help.