battery specific gravities

I'm a newbie. Here's my vital statistics:

1750 watts pv Sharp 175's / Southwest Windpower H80 / Surrette C5/5000 48volts 1156 amp hours / Simpower 10Kw genset / Xantrex 4048 inverter

The system was commissioned in June of 04. there were a few hiccups at first...the first inverter would shut itself off...just off. It was replaced quickly by the installer. The replacement unit didn't charge well after the first winter and failed completely, lights out! The internal 35 amp ac breaker was replaced while a loaner was here from the installer (love that installer!). Just a few bugs to debug it seemed.

The failure of the first inverter I put down to having a high spike load on over the winter. The system powered a 240volt submersible well pump (through a 120-240volt transformer), whose initial load would read 40 amps ac on the Xantrex status screen (I suspect it was higher). Replaced with a Grundfos soft start 120 volt submersible and no problems since.

In order to utilize more of the genset capacity I purchased and installed a fork lift truck charger...48volt 100amps dc capacity. It's full capacity is never realized since the batteries are never discharged more than 30% (70%DOD). When charging with the genset, the truck charger runs using a "tapered" charge cycle...low dc volts=high dc amps. As the dc volts increase, the dc amps decrease. While the truck charger decreases it's output I gradually increase the charging output of the Xantrex while monitoring the total load on the genset (with a clamp meter). The total load on the genset is kept below the 7Kw mark (generators powering transformer/rectified loads should be de-rated by 30%). This has cut the charge time by about 30% overall.

Here's my first forum question: After 2+ years the specific gravities on my battery cells still vary considerably, up to .025 between cells. Is this an acceptable range of variation?

Any input appreciated.


  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: battery specific gravities
    After 2+ years the specific gravities on my battery cells still vary considerably, up to .025 between cells.  Is this an acceptable range of variation?

    Probably not. Trojan recommends equalizing batteries when the cell-to-cell SG variation exceeds +/- 0.015.


    Looks like your batteries are the Rolls / Surrette Series 5000 Model 6 CS 25 PS (1156 Ah, 100 hour rate). I recommend you charge and maintain your batteries in accordance with Rolls / Surrette specs and bulletins. Note that because of your cold location, your batteries’ charge algorithm will probably require temperature compensation unless you keep the batteries in an environmentally controlled room.

    the batteries are never discharged more than 30% (70%DOD).
    I'm confused by this statement. Limiting the battery bank's discharge to never more than 30% usually means that the state-of-charge (SOC) doesn't fall below 70%. A 70% DOD means that you've removed 70% of the rated capacity from the batteries. That's a severe discharge.

    Jim / crewzer
  • System2
    System2 Posts: 6,290 admin
    Re: battery specific gravities

    OOps! of course I meant that I discharged to 30% DOD. Math was never my long suit.

    the batteries are in a sealed vented enclosure in a heated basement...constant temp 20-23 deg C.

    The battery set has never been within .015 reading, even after equalization. I'll be in contact with Surrette to get the lowdown on inter-cell variations.

  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: battery specific gravities

    if they've never been right then they may be defective from the start like somebody failed to put enough acid in some of them upsetting the applecart so to speak. or is it they were ok at the start and went bad afterwards leading to the possibility of bad wires/connections causing unequal charging? if those aren't the case try to charge up some of the low specific gravity batteries on their own to see if the readings will come up. advice from the manufacturer is a good idea as they may know something like possible bad batches or an employee they may have let go because of not following proper procedures, etc., etc..