Low Amperage Battery Charging

tr0ytr0y Solar Expert Posts: 99 ✭✭✭
Can someone smarter than I please tell me, if and why, charging at like C/20 or C/25 as opposed to C/8 or less would be bad for AGM batteries ? I have seen most do not recommend but really the question is why?



  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Low Amperage Battery Charging

    short answer, if <~ 10% of amp hr rating of the battery, sulphation can occur. the higher the rate ( to ~13%) the less chance of a premature dead battery.

    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Low Amperage Battery Charging

    Actually the recommended minimum peak charge current is 5% (C/20) for just the reason westbranch mention: sulphation.

    Keep in mind that the charge rate is not constant. The current will decrease as the Voltage comes up. Too little current and the sulphor stays stuck to the plates. AGM's are less susceptible to this, but they are not immune. On average, AGM's are more "current tolerant" whereas FLA's are more "Voltage tolerant".
  • RCinFLARCinFLA Solar Expert Posts: 1,460 ✭✭✭✭
    Re: Low Amperage Battery Charging

    Just because you charge at a low rate does not mean you will get hard sulfated.

    There are three stages of sulfation, soft, medium, and hard. Soft is a a recently discharged battery and very easy to recharge. If you apply the low charge rate long enough you will convert all the sulfation due to discharge. It is like soft moss on the plates.

    Medium sulfate is a battery that has been left in a partially discharged state for one to four months. Like maple syrup, the sulfate will start to crystalize after a month or two. This can usually be recovered with a charge of greater then 10% to 20% AH capacity rate in amps. Some small hardened crystals may flake off and eventually drop to bottom of cell.

    Third stage is hard crystalization of sulfate. Hard crystal sulfate is for all practical purpose impossible to break down back into sulfuric acid in electrolyte and lead in plates. It builds up on the lead plates when battery is not recharge within two to six months. There is some promise of high current pulse charging to break down the crystal. Usually the best you can hope to do with high current recharge is cause the crystals to break loose from plate and eventually fall to bottom cell. This at least re-exposes plate lead that was insulated by the hard sulfate crystals and inactive area as far as plate is concerned.

    Too much, too long continuous float charging will wear on the positive plate. It will build up excessive lead dioxide and can oxidize the positve plates grid stucture increasing the battery effective internal resistance. Lead dioxide is like a fine white powered sugar donut coating. It is what makes a positive plate a positive plate but it has poor conductivity. Too much of it on the positive plate will degrade battery performance. It is a recoverable process but does eat away at positive plate base lead material. This depletion of positive plate material is a normal aging process and is why positive plates are made thicker on deep cycle batteries.

    AGM plates are nearly pure lead or a calcium-lead alloy plate structure. Deep cycle flooded batteries are usually lead-antimony alloy. This make AGM's have a low self discharge rate compared to a lead-antimony deep cycle battery. Because they have low self discharge they can sit for several months without enough overall capacity loss and associated sulfate build up. Lead-antimony is a more rugged plate, able to take more abuse, but have a fairly high self discharge rate.

    As to low charging rate, you must be able to replace what you have used with some overage for charging efficiency loss and any self discharge over the period of time the low charging rate is applied.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Low Amperage Battery Charging

    Further to what's been said already, if you charge at too low a rate you may not get the battery recharged before the sun goes down. Once is not a problem, but if this is repetitious you end up with a battery never getting fully recharged. This is bad.

    Also the net charge rate is the gross minus whatever is going out for loads. This is why I recommend shooting for 10% peak potential charge current. Coupled with limiting the DOD to 25% you should end up with enough current (between 5% and 10%) to keep the sulphation at bay, fully recharge the battery on a good day, and keep the batteries around for a long time.
  • DerikDerik Solar Expert Posts: 82 ✭✭✭✭
    Re: Low Amperage Battery Charging

    I have an Outback FM80 with just a little less than 2000 watts of panels and 12 L-16 RE trojans for a 24 volt system. I check the SOC with the hydrometer once a week or so and normally see numbers of 1.250-1.255 unless we had several days of clouds then it drops as low as 1.225 but never lower.

    My system seem to only charge at the high amps for an hour or just a little more and then quickly drops off as the absorb voltage is reached and maintained at 29.6. I set the absorb for 5 hours in the winter and 2-3 in the summer.

    Is there a number of hours/min the amps need to be held at the 5-10% ?

    Is it better to use more power so your system will charge longer at the higher amps?
  • SevenSeven Solar Expert Posts: 292 ✭✭
    Re: Low Amperage Battery Charging

    12 l16 for a 24v system would mean that you have three parallel banks. Presuming 320ah l16s would mean you have 960ah of bank. My quick math says your system will only produce about 51a to 64a of current. That seems to be on the very low end, especially for such a tall battery. What are your loads like?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Low Amperage Battery Charging

    Twelve L16's? That is a lot of battery. Four to make 24 Volts means you have three strings, or at least 960 Amp hours.
    That wants a peak potential of 96 Amps, meaning two charge controllers and 3 kW of panel.

    Unless the SG read 1.255 on your hydrometer when these were brand new they have been undercharged consistently. What sort of current numbers do you see on the FM80? I would hazard around 65 Amps? That's only about 6-7% not including load deduction. L16's seriously want the high current for proper charging.
  • DerikDerik Solar Expert Posts: 82 ✭✭✭✭
    Re: Low Amperage Battery Charging

    My use is low I am only there on the weekends and once a week during the week. I have a Outback Mate which tells me what I make daily, I realize this number is not directly related to what is used but it's close. 4.5KWH a day seems to be the norm maybe a little higher in the summer and when I am there but never above 7KWH, unless I intentionally turn on a lot of stuff I saw 14,000 watts made once in one day when I ran a small ac unit.

    I have seen amps as high as 80 but normally they are 60-65. This is on the FM80 log and on the mate read out.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Low Amperage Battery Charging

    4.5 kW hours per day is quite reasonable for a 2kW array. For an off-grid system, your FM80 should reflect not only your daily production but also your daily usage. For instance you say you sometimes use 7 kW hours per day. That can not be replaced by a 2 kW array in one day The result will be deficit charging, and that is bad for batteries.

    I don't think you saw 14 kW hours made in one day. That would require the 2 kW array to operate at 100% efficiency for 7 hours. This does not happen. 14 kW hours used does. That would be about 584 Amp hours @ 24 Volts, or about 60% of your battery capacity. The one saving grace here is that since it was A/C running the bulk of the drain may have occurred during daylight hours when the batteries were charged and there was "surplus" power available from the panels.

    At any rate I see three things wrong with your system:
    1). Your peak usage is too high for the rest of it.
    2). Your battery capacity is larger than your panels can comfortably recharge.
    3). As a result, your SG readings are too low - and indication of chronic undercharging.

    That you are only there occasionally helps, but when you have to recharge batteries over more than one day (instead of quickly and immediately after discharging) their lifespan is shortened. You say you have the Absorb time set to 5 hours. It would be a miracle if this could happen in one day, as they have to go through Bulk first then Absorb all in the course of having enough "equivalent good sun" time to do the job. With the OB charge controllers, the Absorb time is a maximum limit. The actual time will be equivalent to how long the Bulk cycle took to raise the batteries to Absorb Voltage. If you only have 4 hours of sun a day, you will never see 5 hours of Absorb time in one day. Fortunately on Day 2 (without additional use) the Bulk cycle will be shorter, and thus the Absorb time.

    Have you ever reached Float? That is key to a balanced system. If you can use the power one day and replace it the next good sunny day and achieve Float then it should work.

    Just my opinion, but it sounds like you are "under paneled" by about 50%. You might want to disconnect one or two of those strings of L16's and try to charge them up enough to reach Float, and perhaps run an equalization charge (especially if there is any discrepancy between cell SG readings). The connect another string and do the same. Otherwise (and again, my opinion - no one has to agree) you're looking at replacing that expensive battery bank sooner than you should have to.
  • DerikDerik Solar Expert Posts: 82 ✭✭✭✭
    Re: Low Amperage Battery Charging

    I reach float almost every day with a 5 hour absorb time in the winter. In the summer I have to adjust the absorb to 2-3 hours because the batteries bubble and off gas so much that I am adding water more than once a month. Keep in mind that batteries are going into absorb by 10:30 -11:00 a.m. when I am not there and by noon when I am there.

    I have been told on the Outback site that you can't see what your use is from the FM80 & the mate but only what you made for the day. Please let me know how to do this. I watched the mate the day I made the 14,000 watts and yes we had over 10 hours of sun, I did go out and turn the panels towards the end of the day to get my system into float. This was summer and absorb time was only 2-3 hours. Panels were new and seemed to make a little more power than what they make now not sure why but they were more in the 2,200 watt range.

    All of my power use, dishwasher, garbage disposal, micro wave, toaster, washer dryer are used during the day under full sun.
    At night I watch a 100 watt tv and run a few low watt florescent lights. When I am not there 4-5 days a week only the lights are used but the fridge is on, not running much I would guess with nobody opening the door.

    SG is has always been 1.255 - 1.260 at a high. I have followed Trojans recommendations for running EQ and have only done it maybe 4 times or so. All cells are very close in readings and very seldom have been below 1.250.

    So assuming that I may have too much battery but batteries seem be be getting 5 hours of 29.6 volts in the winter and 2-3 hours in the summer and SG is stable how much time will this shorten the life of my batteries & what kind of sulfation if any is or has occured?
    In summary I think I have an example of low amps being used daily to charge my system but reaching all set parameters almost daily how much time will this shorten the life of my batteries?
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Low Amperage Battery Charging

    Lots of bubbling and excess water use is a sign that the Absorb Voltage may be set too high. Do you have a remote temperature sensor on the FM80? $30 and worth it.

    The FM80 will not tell you exactly how much power you have used as it is not a battery monitor. However, the Watt hours it records is what comes out of the panels which is what was used from the batteries during non-charging hours plus loads during charging hours plus the bit more needed to full recharge the batteries (charging and system efficiency losses). In that sense it shows what your total system usage is, providing you get through a full recharge in one day. If all the "used power" is not put back (no Float) it will only be panel production, and additional power may have been used from the batteries to provide loads (deficit charging). It's all in how you look at it. A battery monitor is much better for full analysis (once programmed), but with a little interpretation you can get an idea of total use from the FM80.

    It sounds like your doing good on the load-shifting power management. You can really squeeze a lot more Watt hours out of a system this way! A refrigerator on its own will use around 1 kW hour per day, even with no one there. Some a little better, most of them worse. But the usage when you're away does not appear to be a problem at all.

    Don't think that 10 hours of daylight is 10 hours of equivalent good sun. Even with a tracker. The early and late light travels through more atmosphere and therefor is not as intense as midday +/- a couple hours. We have 16 hour days up her in Summer, but my array is fixed so it doesn't amount to even that much in exposure never mind "good sun".

    I'm a bit confused because in your first post you said you had SG of 1.250. I assumed that was fully charged, but maybe you meant that was the lowest?

    Regrettably it is impossible to accurately predict battery life, even if "perfectly" maintained. The darn things can just drop dead from plate failure if they "want" to. It's just that with as much battery bank as you have I'd normally expect 50% more panel on them.

    You aren't by any chance at a high elevation (say over 3,000 feet)? That can help a lot with panel efficiency.

    You don't have to take my advice (no one has to), but I'd re-evaluate the loads and then either take one string of L16's out of the mix or add another 1kW of panels somehow, preferably the former as that is cheaper. You could use a battery switch to have them available if needed, and use the FM80's AUX function to control a relay and switch over to charging the back-up bank.

    If you're happy with the way things are working now, don't change anything.
  • DerikDerik Solar Expert Posts: 82 ✭✭✭✭
    Re: Low Amperage Battery Charging

    Yes I am just a little below 3,300 feet so I do see some big numbers out of my panels in comparison to the ratings. Amps are big at times as well and I have seen it put out 80 amps before so I would have to change the FM80 if I added panels. With the Outback Mate I can watch the watts being produced by the panels I see the difference from the sun at say 8:00 a.m. and the sun at noon.

    Since my system is under used, I have tried to adjust the panels to capture more sun early in the day in order to get the amps up prior to the batteries reaching my absorb setting. Seems the amps are highest trying to get to absorb and then taper off quickly maintaining the 29.6 setting.

    I am happy with the way things are working but just wondered how long I can expect these batteries to last given that I don't have the high amps most recommend. My batteries are currently just 2 years old and I hope to get another 7 -8 years out of them. If taking some out of the string or putting in an Aux, switch will get me a few more years then it's worth it. If my slow but steady trickle charge of 60 amps is sufficient then I'll just not worry about it.

    SOC of 1.250-1.260 are fully charged (end of day) and vary from summer to winter but I seldom see lower than 1.250 (at the end of the day) and if so, I get them up with an EQ which I have done just about 4 times or so in the time they have been in service.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Low Amperage Battery Charging

    Good planning on your part. :D

    The thing about the "10% rule-of-thumb" is that it is a guideline for bring the whole system into balance, not an absolute maxim. Even if followed it does not mean you will see that 10% current; it is a potential peak current. If it is "available" (possible with the system design) then daily recharging from 25% DOD is possible within the usual 4 to 5 hours of equivalent good sun with a panel + controller efficiency of 77% (typical). Lead-acid batteries do not get recharged at a constant current rate; as the Voltage comes up the current drops off*. This is normal.

    You have some advantages working for you. The higher elevation will increase panel efficiency (probably 80% instead of 77%). The infrequent use and usually low DOD reduces battery stress and time needed to recharge. The long hours of sunlight help with over-all charge time. Your mindful EQ cycles will help keep the electrolyte from stratifying.

    The major concern I see now is that you occasionally pull 14 kW hours from the system, and if too much of this occurs in "no sun" times it can be dragging the batteries a bit too low. If this were to happen daily it would spell doom for the batteries. As it is only one day or two out of a week every now and then it probably will only shave a year off the over-all life of the batteries. You know your usage pattern and how much/how often the batteries are pulled below 50% and how quickly they are recharged after. Probably the only thing your system could really benefit from would be a battery monitor, so you'd know exactly if you were encountering deficit charging.

    *The Voltage vs. current graph for charging from solar is interesting because charging starts before the panels are capable of full power. As such, the initial charge rate will be much lower than the peak potential. Nonetheless charging occurs and the Voltage and current will increase as the batteries charge and panel output increases. Then the current will decrease even though panel output increases because the batteries continue to rise in Voltage. This is different from charging from battery chargers or inverter-chargers which are able to start out with full current.
  • jtdiesel65jtdiesel65 Solar Expert Posts: 241 ✭✭✭
    Re: Low Amperage Battery Charging

    Was the battery really that low or were there other loads on the system?

    A quick look at records, and I've seen 15kwh out of my two 1360w arrays and 8.3 out one of them. This represents charging the batteries and using two air conditioners and a dehumidifier. In the same month ( July 2010), the same arrays made 331 kwh with 43 hrs float.

    If I don't run something that uses excess juice what happens is my daily kwh will look small, but my float hrs will be huge. For example, May 2010 was 151kwh and float was 212 hrs (combined hrs from two controllers: one at 140 hrs, the other at 72).
  • DerikDerik Solar Expert Posts: 82 ✭✭✭✭
    Re: Low Amperage Battery Charging

    Thanks Coot for the reassurance. I only did the big draw once to see how much power I could make. I don't use the A.C even in the summer and rely on ceiling fans and cool breezes. I don't have the exact amount but I would guess 5-7 KWH would be the norm or less.
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Low Amperage Battery Charging

    I'm going to add this even though I think it's supposed to go in another thread. :blush: It applies here as well.

    One reason why quick Bulk/Absorb is favoured is because the longer time spent at higher Voltage the more heat stress is placed on the battery plates. This is why Trojan doesn't recommend automatic EQ too; stresses the plates. But it is a judgment call as to just how much Voltage/time/heat is too much. Obviously some must take place. The L16's with their large plate surface area run a greater risk of failure from this. AGM's have almost no risk of this due to their "sandwich" construction.

    If you get DC clamp-on Ammeter you can compare the current going out to loads to what is coming in from the FM80 and get an idea what your maximum charge rate really is (do it several times on different days when you see high current at the controller). If it's really in the 5%-13% range the batteries should last a reasonable lifespan.
Sign In or Register to comment.