So what really counts as a "cycle"?

CALLD
CALLD Solar Expert Posts: 230 ✭✭
The most popular definition I've seen thrown around is:
"Going from fully charged to any depth of discharge and back to fully charged"

Ok, sure I can go with that but...

Here's another popular one:
"Going from 100%SOC to 20%SOC and back to 100%SOC.

There are a few others depending on battery manufacturer and they usually just change the lower number to suit their battery. Pretty much all of them require that the battery be returned to 100% SOC within 24hours.

But here's my big problem - "returned to 100% in 24hours"? Renewable energy systems can't do that everyday!
Sometimes it can take a week before you see 100% SOC after a particularly bad stretch of weather.

So in that week where SOC has been steadily declining, discharging at night and only being partially charged by the grey skies, how does that contribute to the cycle count on the battery odometer?

Supposing you just counted a week of really bad weather as "1 big long discharge" and the "hopefully" sunny week that follows as "1 big charge cycle"? Or do cycles have to be accomplished within 24hours? Is that the rule??

Comments

  • gww1
    gww1 Solar Expert Posts: 963 ✭✭
    Re: So what really counts as a "cycle"?

    I know member chris olsen takes the position that It is just one big disharge cycle as the battery is going down, no matter how long and then when it goes up to full that completes the cycle. Giant battery web site say opertunity charging is bad and that a thirty min charge counts as a cycle and if it takes more then one day to discharge, that is better then opertunity or short cycling. Others say it is just how much you take out and if you go down 20 percent that is a quarter cycle. With my knowlage of batteries, I apparently have no ideal or know who to listen to on this issue. I started using my batteries like chris promotes cause I was not really having good luck with any other way anyway.
    gww
  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    Re: So what really counts as a "cycle"?

    I see, depends on who you talk to then?

    I'm sure either point has it's merits. I doubt a hypothetical long discharge over several days would not be worse than a typical 18 hour winter time discharge though. At some point sulfation will begin to take hold - question is how long does this take?

    As for opportunity charging - I would always be inclined to believe that returning any amount of charge to a discharged battery is better than leaving it at a lower soc. I understand physical damage takes place during every charge cycle, but is this not mostly confined to the end of the charge cycle during the gassing stage? Please correct me where I am wrong...?
  • Photowhit
    Photowhit Solar Expert Posts: 6,002 ✭✭✭✭✭
    Re: So what really counts as a "cycle"?
    CALLD wrote: »
    ... Is that the rule??

    No rules...

    For me, it's a touchy feely sort of thing.

    I you read about the number of cycles they also include cycles at % of discharge, shallow discharges more cycles... you also want to, do your best, to keep the state of charge high, below 70-80% the battery will sulfate quicker. Along with this, the battery is aging, an unused battery will also died a premature death...
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
    - Assorted other systems, pieces and to many panels in the closet to not do more projects.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: So what really counts as a "cycle"?

    Difference between a "cycle" and a "micro-cycle":

    Cycle is to a given DOD then recharge to a given SOC. Specific levels are not necessary; only the operational action is. The levels do not have to be the same each time either.

    Micro-cycle is when a cycle is interrupted either on the charge or discharge with the opposite action for a short period of time. An example of this is when a heavy load comes on during charge cycle and PV is not able to supply it and continue charging (even at a reduce level) so for the time of the load the batteries are drain somewhat, then charging resumes after the load is off.

    For practical purposes there's no good way to figure micro-cycles much less predict them. We know they have an effect on battery life, but it's just one of the things you have to suck up. Otherwise you are dealing with averaged DOD cycles and recharging to near enough to 100% that it makes no difference and trying to avoid several days of partial recharging below 90%.

    For promoted cycle life purposes companies have specific parameters used in laboratory tests to determine this. It will involved controlled temperatures, fixed current discharge, and grid-based recharging ability to precisely meet the best performance characteristics for their batteries. It isn't exactly "real world" stuff.

    BTW "Going from 100%SOC to 20%SOC and back to 100%SOC" is called "killing the batteries as fast as possible"; you really shouldn't go to 20% SOC even with forklift batteries.
  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    Re: So what really counts as a "cycle"?

    Cariboocoot: thank you for answering most of my questions, just two more though:

    As for micro cycles how significant are they? I may have quite a few of them on cloudy days as the PV output goes up and down with every passing cloud. Let's say there is a steady load of 120watts on the battery, PV output can be over 700watts between clouds and as low as 50watts when a dark cumulus cloud passes. This can happen dozens of times a day!

    Then of course there is Port Elizabeth's oceanic climate - in a given month there will be 10 days of full sunshine, 12 days of interrupted sunshine where production is up to 70% and 8 to 9 days that are pretty much overcast for most of the day. In the latter scenario production can be lower than 30% of full sunshine days. How does a climate like this affect battery life if there is no way to quickly bring them up to 100% other than wait for sunshine? I have a small charger capable of adding a mere 6 amps to the batteries, but it's nothing compared to the 27-28 amps the PV can supply on a good day..
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: So what really counts as a "cycle"?
    CALLD wrote: »
    Cariboocoot: thank you for answering most of my questions, just two more though:

    As for micro cycles how significant are they? I may have quite a few of them on cloudy days as the PV output goes up and down with every passing cloud. Let's say there is a steady load of 120watts on the battery, PV output can be over 700watts between clouds and as low as 50watts when a dark cumulus cloud passes. This can happen dozens of times a day!

    If it is still charging the battery, even at a reduced rate, it doesn't count as a micro-cycle because the operation hasn't changed from 'charge' to 'discharge'. When it does you have the micro-cycle, and trying to determine its effect on battery life is like trying to pick the right lottery numbers; good luck. Even if you could determine the depth and length of the micro-cycle I'll bet no manufacturer has ever even tried to equate this to a full-cycle effect because it is something that really only occurs in RE applications so they simply would not consider it important.

    If you find you have a regular occurrence of noticeable micro-cycling you should consider adding more PV to try and prevent the loss of charging during the load time.
    Then of course there is Port Elizabeth's oceanic climate - in a given month there will be 10 days of full sunshine, 12 days of interrupted sunshine where production is up to 70% and 8 to 9 days that are pretty much overcast for most of the day. In the latter scenario production can be lower than 30% of full sunshine days. How does a climate like this affect battery life if there is no way to quickly bring them up to 100% other than wait for sunshine? I have a small charger capable of adding a mere 6 amps to the batteries, but it's nothing compared to the 27-28 amps the PV can supply on a good day..

    What you have to watch out for is how many days below what percentage of SOC you experience and how often. If you find you have regular bouts of <75% SOC for more than two days then you definitely have a condition which will affect the battery life. Again the first choice is upping the PV to improve charging on marginal days, but it is sometimes cheaper to just run the generator. Or the standard advice of "25% DOD one day one, 50% DOD on day two, start the gen on day three".

    This is also one of the situations where LiFePo's SOC tolerance can be advantageous. But you'll have to think long and hard about whether you want to put the money into the different (and as yet largely unused in RE) chemistry or merely up your PV and possibly battery capacity to give yourself more tolerance to the variable conditions.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: So what really counts as a "cycle"?

    Typical of company promotions it is somewhat exaggerated: most batteries that are used in RE do not require 100-150 cycles to reach full capacity. Large capacity ones (2V cells, forklift, and those produced by companies not interested in quality control or customer service) will (and it would be nice if they told you this up front) but the smaller ones will all be in-line within 30 days of normal use and you'll never even notice (unless you have a battery monitor which can be instantly wrong due to this initial increase in capacity).

    BTW I'm not impugning the company, just the standard "our are better because we say so" claim. Notice the other battery shown appears to be a Trojan, and could well be an industrial model rather than a small motive-force type (i.e. T105).
  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    Re: So what really counts as a "cycle"?

    Thanks cariboocoot, I think I get the gist.

    Having just purchased new batteries, I'm not in a position to purchase anymore PV or a generator/charger soon. Thankfully summer is on the way so that should (for now at least) have the same effect as additional PV.

    I can give you my production/usage characteristics, please let me know if they are matched suitably. I do try to alter them depending on weather:

    October data:
    Sunny trend:
    Day production: 250Ah @ 25 to 29.4v average
    Daytime consumption 65Ah
    Nighttime consumption 120Ah

    Partial cloud trend:
    Daily production 180Ah @ 24.5 to 28v average
    Daytime consumption:20Ah
    Nighttime consumption: 100Ah

    Overcast trend:
    Daytime production: 80Ah
    Daytime consumption: 20Ah
    Nighttime consumption: 100Ah

    There is a deficit on overcast days. I'm now on day two of overcast weather, battery voltage at sunset was 25.1 volts, high voltage of the day was 27.1v
  • Blackcherry04
    Blackcherry04 Solar Expert Posts: 2,490 ✭✭✭
    Re: So what really counts as a "cycle"?

    What about a Float and Re-Float scheme where the Float voltage ( 13.6 v ) is allowed to drop to say 12.7 v and then brought back up to the set Float voltage of 13.6 v ?? I see this in some chargers as a alternative to allowing a battery to set on a long term float current.
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: So what really counts as a "cycle"?

    Voltage is a pretty meaningless SOC indicator on an active system. The best we can say there is that at 25.1 'under load' Voltage the batteries are at least above 50% SOC.

    If you look at the deficit as 120 Amp hours used - 80 Amp hours produced = 40 Amp hours short and compare that to the 260 Amp hour battery capacity it indicates -15%, or an SOC around 85% which is still good. This isn't that accurate either, but at least it's encouraging.

    The immediate problem would be two such days in a row bringing the battery down to 70% SOC (15% deficit per day, using the basic math). You are now in the area where hard sulphation can take its toll if the battery is not recharged quickly. More such days and the SOC continues to drop despite whatever charging you get as well as spending more time below the hard sulphation level which is taking life off your batteries.

    So how long do these spells last, and can you fully recharge from such a low state in one day?
  • CALLD
    CALLD Solar Expert Posts: 230 ✭✭
    Re: So what really counts as a "cycle"?
    Voltage is a pretty meaningless SOC indicator on an active system. The best we can say there is that at 25.1 'under load' Voltage the batteries are at least above 50% SOC.

    If you look at the deficit as 120 Amp hours used - 80 Amp hours produced = 40 Amp hours short and compare that to the 260 Amp hour battery capacity it indicates -15%, or an SOC around 85% which is still good. This isn't that accurate either, but at least it's encouraging.

    The immediate problem would be two such days in a row bringing the battery down to 70% SOC (15% deficit per day, using the basic math). You are now in the area where hard sulphation can take its toll if the battery is not recharged quickly. More such days and the SOC continues to drop despite whatever charging you get as well as spending more time below the hard sulphation level which is taking life off your batteries.

    So how long do these spells last, and can you fully recharge from such a low state in one day?

    Thanks coot, this is the sort of valuable advice I am needing now. The time needed for hard sulphation to grip the batteries is what I wasn't sure of. These low production spells usually do not go on for more than 2 days but it is possible for them to return again after just one or two sunny days in a bad stretch. If it leads into day three I shut down the inverter and use the grid. I then switch on my 6 amp charger to help the PV. It then only partially charges the batteries on day one of shut-down if there is no sun. If the sun does break out before noon and holds for the rest of the day I will get two hours of absorb at least. A full day of sun always gets me to absorb by lunchtime regardless of how deeply I've discharged them.

    The big problem with Port Elizabeth is the weather forecasts are not reliable for PV. You can have sun when clouds have been predicted and vice versa. It's a case of "you take what you get one the day"
  • zoneblue
    zoneblue Solar Expert Posts: 1,220 ✭✭✭✭
    Re: So what really counts as a "cycle"?

    I appreciate the desire to understand your batterys, but at the end of the day they have minds of their own, and so long as you have a suitable charging regime, there isnt a whole lot to be gained in over thinking/ anxting about them. They were made to be used, and not cycling them is, in potentially as hard on them as cycling them.

    Get enough pv, and let them do their thing.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar