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battery life on "float"?

Discussion in 'Photovoltaics' started by Robert Morein, Jun 5, 2004.

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  1. Figures for golf cart battery life seem to be based on "typical"
    discharge/usage patterns.

    Are there any figures for life of these batteries if maintained constantly
    at "float" with zero discharge cycles?
     
  2. Well, it's probably measured in decades if not centuries, but it's not
    a real-world situation, so it's not very interesting. It also depends
    on how you define "lifetime" if you aren't using them for anything.

    Initial battery quality is probably important. Satellites get one
    cycle per (90-minute or more) orbit, and those NiCads last decades.
     
  3. Guest

    I'm not sure that's really true. I can't be the only on-grid solar
    installation with batteries. I just felt that it was absurd to have a
    grid-connected photovoltaic system and still go dark when the grid went
    down at noon on a sunny day! So I spent the extra 20% to have a
    battery-based system.

    And, of course, it spends 99% of its time at float. The only time it's
    not at float is during a scheduled or unscheduled power outage, or when
    I run an equalization cycle twice a year.
    Interesting. NiCads are well known for relatively rapid dropoff in
    capacity. I wonder if they launch with huge overcapacity so that
    they'll still have enough to get through the 45 minute blackouts at the
    end of their scheduled lifetimes.
     
  4. Well, OK, is it zero cycles or is it a few cycles a year?How deep are
    those cycles? How old is the battery bank? How do you know the
    batteries are still any good?
     
  5. Guest

    Typical outages are 2 hours, and they tend to happen no more than 4-6
    times per year. Perhaps half of those outages occur in the absence of
    significant solar power.

    I sized our battery bank so that we can go a full day with no sun or
    grid power, and draw it down to 50%. So 2-3 outages that take the
    battery banks down by perhaps 5-10%.

    Bottom line: my batteries are rarely taxed significantly. They spend
    nearly all their time at float.

    I'm sure there are stats for this sort of thing. My batteries have a
    similar duty cycle to, say, UPS batteries. UPSs have been around for
    decades.

    I'm guessing that my batteries will last for something like 10 years,
    which means I've got 8-ish years to go...
     
  6. :
    [System details, sounds nice!]
    And my point is it's hard to tell by guessing. If someone were to
    tell you that your batteries were now at only 25% of their initial
    capacity, how would you "know" they were wrong? Sure, they work fine
    for the 10% you are using them...

    http://compusmiths.com/~w_smith/DailyBatteryVoltage.htm shows the
    daily high and low battery voltage for a pair of "golf cart batteries"
    that had what I would have thought was a pretty easy life but failed
    with a shorted cell after less than 3 years.

    The OP was asking "If they are on float 100% of the time and have zero
    discharges, how long will they last", and my first thought was "Use a
    1uF capacitor."
     
  7. daestrom

    daestrom Guest

    Then you may have the wrong type of battery. This sort of battery
    application is used in a lot of power plants and substations. The batteries
    are on 'float' charge almost 100% and only used as backup power when the AC
    goes out. For these applications, a lead-calcium grid type of 'station
    battery' is used. This battery will not suffer from self-discharge as much
    as deep-cycle lead-antimony grid, and they last longer when kept on 'float'
    most of the time.

    Of course, you've already got your batteries and are not about to replace
    them until they fail. But for those considering this type of grid-tied,
    battery backup system, they may wish to look closer at the type of battery
    they choose.
    Look into large UPS systems or 'station batteries' and you will find they
    are usually not deep-cycle units.

    daestrom
     
  8. Not to put too fine a point on it, but I believe the term is
    "stationary" and is used to distinguish this type of application from
    "motive" batteries as used in golf carts, forklifts... and (a-hem)
    stationary applications like home power.

    Or I could be blowing bubbles out the wazoo.

    -=s
     
  9. daestrom

    daestrom Guest

    No, you're right. I've heard them referred to as 'station' (as in
    substation/powerstation) and 'stationary' (as you say, not mobile). Either
    way, it may be the more appropriate choice for this particular type of PV
    setup (not discharging except from blackouts).

    daestrom
     
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