A different slant on solar charging timing, crazy?

I have been thinking of maximizing the life of batteries and was pondering the recommended minimum charging current of 3-5%, depending on the battery type. When reviewing my charging rates of my system by looking at some graphs, I realized that the conventional solar system perhaps does not do the right thing. Most systems start charging the second there is enough energy received by the panels in the morning. The rates start at zero and rise as the sun goes up. In my case the rate ramps up until the batteries reach the programmed absorb voltage and the the charger goes in to absorb mode. The interesting point of this is that the batteries start recharging at a very slow rate and don't get a strong bulk rate current until just at the point that they switch to absorption mode. My solar panels are rarely used more than 1/3 of their capability and they are sized to meet this minimum rate before the batteries reach absorption voltage.

It occurred to me that perhaps it is better to wait until there is sufficient solar flux to meet the minimum 3-5% charge rate and then start charging. This would have two possible benefits.

The first is that the batteries would receive a charge current, right from the start, that was beneficial for optimum mixing of the electrolyte. Perhaps this would extend the life of the batteries as they would be better "equalized" all the time and the electrolyte would have better mixing.

The second benefit (although very small) might be that the specification of the solar panels could be relaxed as they would be start charging closer to their rated capacity with higher solar flux and thus would be able to provide the minimum current necessary for the battery's requirements.

There are some exceptional circumstances where I live so I need to account for them too. I live in arguably the best place in the world for solar production. The northern Atacama Desert has some of the highest solar irradiance in the world and the lowest number of cloudy days. This could skew my perception of the data I have. There are other case by case considerations such as battery type, battery size, and battery condition.

So the questions:

If a relay was added to the solar panel output to the solar charge controller so that he solar charge controller only could start charging when there was sufficient solar flux to provide the minimum 3-5% charge rate, would this possibly benefit the batteries over the long term? This would assume that the batteries had not reached a less than 50% charge and thus were not in danger of over discharging.

Given that my solar panels are over sized (by wattage) for the battery minimum charge current so that they get the minimum current before they reach absorption voltage, in other words I only see the first third of the solar panel charge curve being used, would it be reasonable to reduce the size of the solar panel capacity by around 30% so that when the relay connected the panels there would be sufficient capacity in the panels at the correct time?

OK, I know that at this point the solar panels are getting to be the least of the cost considerations in a total system, but the mounting, care and cleaning of panels have to be considered too. I also know that partial cloudiness will mean that the bigger capacity of over-specified panels will beneficial. They are also beneficial later in the day when the sun is going down to help support the load instead of the batteries having to take over earlier. In this pondering, the size reduction of the solar panels is a secondary consideration.

The main question is about the later start of charging to help extend the life of the batteries. They tend to be the running cost of a system. They are like fuel in essence. They can also be a real pain to the user when they start failing. Anything that can be done to extend the life of the batteries, within reason, is a good thing.

Please, comment, rant, call me stupid, whatever. I am looking for feedback.

Comments

  • BB.BB. Super Moderators, Administrators Posts: 29,980 admin
    Re: A different slant on solar charging timing, crazy?

    Sounds like your battery bank is on the "large" side--Perhaps discharge it to 75% or less state of charge every so often--Help the battery plates with some deeper cycling instead (i.e., turn off array for 1 day).

    Assuming true deep cycle batteries.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: A different slant on solar charging timing, crazy?
    Les Nagy wrote: »
    The first is that the batteries would receive a charge current, right from the start, that was beneficial for optimum mixing of the electrolyte. Perhaps this would extend the life of the batteries as they would be better "equalized" all the time and the electrolyte would have better mixing.

    No. mixing of the electrolyte does not occur until the voltage of the battery exceeds the gassing voltage.

    In general, lower charging currents are more efficient than higher currents. The charging currents must exceed the self discharge, of course.

    I think the main reason that some manufacturers specify a minimum charging current is so the battery can be charged in a reasonable amount of time.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • Les NagyLes Nagy Solar Expert Posts: 121 ✭✭
    Re: A different slant on solar charging timing, crazy?

    The batteries have been discharged to below 75% many times, so I am not sure they are over sized. You could be right and I will see because I had one battery fail and I had to cut out one string.

    The minimum charge, as has been said many times here on the forum, should be no less than 3-5% of the rated capacity so as to mix the electrolyte, or not just surface charge the plates. Maybe I have misunderstood this.
  • BB.BB. Super Moderators, Administrators Posts: 29,980 admin
    Re: A different slant on solar charging timing, crazy?

    The rules of thumbs addresses several issues:

    1. Recharge a battery quick enough that it sulfates less.
    2. Enough recharging energy to address self discharging losses (as a battery ages, flooded cell have higher self discharge losses than AGM, GEL, etc.).
    3. Allow you to to have enough power to charge the battery during the day even if you have some light loads.
    4. Enough "chemical activity" to keep pores open in the lead plates
    5. Enough energy available to "equalize" the battery bank (2.5% minimum rate of charge for equalization; AGM's are not really equalized, so not not an issue).
    6. Some battery Mfg. recommend 10% minimum rate of charge (Trojan). Some even recommend higher ("pure lead" type batteries).

    For a lightly cycled battery bank, 5% can work OK (AGM helps, weekend use, seasonal use + backup genset, etc.). For a heavily cycled battery bank and/or larger day time loads, a 10%-13% rate of charge helps.

    Also, having more solar panels usually reduces the amount of hand holding you need to do for your system... I.e., when somebody uses a bit more power than usual, leaves some lights on, guests come by for the weekend, etc... The "extra power" is nice to have (and solar panels are at a historical low for pricing).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Les NagyLes Nagy Solar Expert Posts: 121 ✭✭
    Re: A different slant on solar charging timing, crazy?

    Thanks for the clarification BB. But I see that some manufacturers do recommend that their AGMs should be equalized periodically.

    OK, so the recommendations for minimum charge rates do have relation to the life expectancy of the battery. The point is that the majority of the charging time for my system is that the current is not up to the minimums recommended. Only for the last 30 minutes of around 2 hours do my batteries receive the minimum recommend currents. Would it be better to wait until my panels can supply this minimum current for the whole time of the charging time? If not, why not?
  • BB.BB. Super Moderators, Administrators Posts: 29,980 admin
    Re: A different slant on solar charging timing, crazy?

    AGM absorb is typically just holding 14.2-14.4 volts for several hours extra--Not really a high voltage absorb like a flooded cell.

    Concorde does have a high voltage charging to restore a damaged/failing battery--But it is not something (I think) you would do several times a year "just for the heck of it".

    AGMs have a small catalyst in them to recombined hydrogen and oxygen gasses. This has a limited life (a certain amount of Amp*Hours of charging/recombining). And, it also gets hot as the gasses recombined (really a slow/low temperature combustion). Excessive gas generation will use up the catalyst and can overheat the catalyst.

    The other question about holding off charging until there is more sun/current available--I am not the person to answer that question. Hopefully somebody else here can.

    One poster here with lots of battery experience does recommend a tracker because it gives more hours per day for charging and a lower peak current required to recharge a battery (Dave Angelini/Dave Sparks). He has found that it gives him good battery life.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: A different slant on solar charging timing, crazy?

    No. This is backwards.

    As vtMaps said batteries actually prefer a low, slow charging. The reason why we target a peak current is because it is a short-cut to having enough power to recharge the bank in a reasonable amount of time with a moderate amount of concurrent loads. Solar applications do not use a constant current charging; they can't because of the nature of power production from PV. If you were to average the 10% peak with the minimum you'd see that over-all it's just above the 5% minimum recommended by manufacturers.

    No charging starts out at 0 Amps because by definition that is not charging. Some MPPT controllers, such as the venerable MX60, have a minimum current setting; if the panels can't supply above that charging does not occur. This is to keep the Bulk/Absorb timer from running up to high (not all chargers operate this way) resulting in longer than necessary Absorb time. I have mine set at 1 Amp and still use End Amps to make sure Absorb shuts off without excessive time.

    Many have reported that the bell curve charging of batteries by PV works better than the maximum current from start charging provided by AC based chargers. This has to do with charge acceptance. When batteries are discharged their resistance is low. If you put high current to them much of it goes to heat rather than actual charging. Think about it: at the start of the charge cycle you have a battery that has a real capacity of less than what it will be at the end. 100 Amp hours drawn down to 50 Amp hours: feed it 10 Amps (10% of total capacity) and that is a 20% rate! If it starts out lower and ramps up current as the battery charges you get greater acceptance and a more complete charge.

    There is no point in cutting off all of the early morning charging because it is not capable of the 10% current rate. That is a peak current, not a sustained one. All you will do is lose valuable charging time. If you wait until the 5% minimum is available you may not have enough hours of sunlight left to complete Absorb. Instead of better charging you will have incomplete charging.
  • Les NagyLes Nagy Solar Expert Posts: 121 ✭✭
    Re: A different slant on solar charging timing, crazy?

    I am not sure I understand your assertion that lower internal resistance causes more heat generation. I know that batteries are not resistors, but normally passing current through a lower resistance causes lower heat rather than more. I assume that the chemistry in the battery works differently in this case.

    OK, so my idea is probably not worth more consideration based on what has been said. As far a solar trackers go, IMHO they cost more and require much more care and maintenance than just adding the now cheaper panels to make up the needed output.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: A different slant on solar charging timing, crazy?
    Les Nagy wrote: »
    I am not sure I understand your assertion that lower internal resistance causes more heat generation. I know that batteries are not resistors, but normally passing current through a lower resistance causes lower heat rather than more. I assume that the chemistry in the battery works differently in this case.

    A case of the amount of resistance vs. the current put to it, or as I mentioned what the charge rate percentage is for a given amount of current against the SOC (real capacity) of the battery at the time.
    OK, so my idea is probably not worth more consideration based on what has been said. As far a solar trackers go, IMHO they cost more and require much more care and maintenance than just adding the now cheaper panels to make up the needed output.

    Yep. Hard to be the per Watt panel pricing these days for cheapest method of getting charging earlier/later in the day.
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: A different slant on solar charging timing, crazy?

    les nagy,
    you are fine charging as you are doing, but you may want to have an occasional deeper discharge to cycle the batteries. do not go beyond 50% dod of course. if this means shutting down the solar for x hours or even the whole day without going below the 50% dod point then so be it. this would facilitate a deeper charge to the batteries and don't worry about the high or low charge rates too much as agm batteries are more efficient so they start charging earlier on and they tend to be able to take higher currents fairly well compared to some fla batteries. yours is an off brand so do not go beyond 20% charge rate as a precaution. i would watch for the outgassing condition regardless as the elevation may be a factor in the max voltages/currents these batteries accept. something i admit i hadn't thought of, but it makes sense that it could cause earlier outgassing with lower humidity and air pressures no matter if your battery is agm or fla.

    i also read of the failure you had with a battery and stuff like that can happen to any battery unfortunately as occasionally they fail through no fault of yours. it does pose its problems being the battery bank is now much smaller and leaves a few other batteries out of the loop unless you were to buy new to replace. at almost 5yrs of age i wouldn't put any new ones into the mix and i hope your present 2 banks of 4 in series are filling the needs you have.

    btw the initial hiss you heard was venting for sure and may have hurt the bank a bit, maybe in a slight loss of battery capacity and life. it depends on the batteries involved and how long and deep the venting was. in other words, it's hard to say exactly how far negative that event was. i might ask you to disconnect all of your batteries so that you can measure and write down all individual battery voltages after a rest of say 8-10hrs. less time if need be. this is to see variations of voltages in the batteries. when you do this you can physically put the higher voltage batteries into the places of the lower voltage batteries. it is a kind of eqing, but a pain to do. much better than higher voltage eqing overcharging which is more or less a last ditch effort to bring them all into line. the electrical eqing (referencing with higher voltage and not to be confused with extended absorb charges) can outgas the good cells to bring them closer in line to the weaker cells, but physically moving the batteries can make the batteries eq somewhat over time due to minor variations the wires and connections may have imposed. the voltage variations should be somewhat small, but if you see variations that are larger like .05v or more then do both the physical rearranging and an extended absorb charge. that voltage variation isn't written in stone either as it is a good guesstimate by me. ideally, they all should be identical in their voltage, but this is rare for all to be exactly the same.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: A different slant on solar charging timing, crazy?

    An unfortunate fact of 8,000 feet of elevation is that it is much lower pressure than sea level. Water boils at a lower temperature, sealed batteries will gas earlier (pressure differential), and hydrometers do not read correctly. These effects are noticeable at my elevation of 3,200 feet; at 8,000 they must be painfully evident.
  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    Re: A different slant on solar charging timing, crazy?

    Our setup behaves similarly. Todays a bright and sunny day, (need to get the hot water hooked up). The green line is more or less what i could be producing.

    Attachment not found.

    Last summer i used midnites days between bulk setting to only absord every fifth day. Not much else you can do. Thats the price of genset independence.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • ramlouiramloui Solar Expert Posts: 103 ✭✭
    Re: A different slant on solar charging timing, crazy?

    Zoneblue! How do you get this graph/display? This is amazing!!! I'd love to get the same for my small system.

    Wow!
    Off-grid cabin in northern Quebec: 6 x 250 W Conergy panels, FM80, 4 x 6V CR430 in series (24V nominal), Magnum MS4024-PAE
  • zonebluezoneblue Solar Expert Posts: 1,218 ✭✭✭✭
    Re: A different slant on solar charging timing, crazy?

    http://code.google.com/p/theblackboxproject/

    Note that rest voltage and min/max daily SOC were added recently.

    "Rest voltage" is calculated by taking a specified half hour period in the morning before you generally get out of bed, and scanning all the readings to find the highest.
    1.8kWp CSUN, 10kWh AGM, Midnite Classic 150, Outback VFX3024E,
    http://zoneblue.org/cms/page.php?view=off-grid-solar


  • ramlouiramloui Solar Expert Posts: 103 ✭✭
    Re: A different slant on solar charging timing, crazy?

    So... this is not yet available for purchase?
    Off-grid cabin in northern Quebec: 6 x 250 W Conergy panels, FM80, 4 x 6V CR430 in series (24V nominal), Magnum MS4024-PAE
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