String charging balance - 21.5A vs 23.2A

YehoshuaAgapaoYehoshuaAgapao Solar Expert Posts: 280 ✭✭
Is this too different? The left box is charging at 23.2A while the right box is charging at 21.5A. Was bulk charging after generator exercise (Xantrex inverter always charges batteries after AC source change). Voltage was currently 54.4V when I checked the current. Probably what makes it bad is that the current difference was formerly 0.2A.

Comments

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: String charging balance - 21.5A vs 23.2A

    You're talking about one array and charge controller putting 21.5 Amps to the batteries while the other array and controller is putting 23.2. This is not a problem in itself, nor is it unusual. The arrays may not be evenly illuminated, and even if they are slight differences in wiring and the controllers themselves can cause one to be "more powerful" than the other. On a different day they could work the other way 'round too.

    If you see a consistent, repeated lower output from one side disconnect the higher current array and see if the lower one comes up. If under those circumstances you have two identical array & controller set-ups producing significantly different amounts of power there may be a problem with one or more panels or connections.
  • BB.BB. Super Moderators, Administrators Posts: 29,482 admin
    Re: String charging balance - 21.5A vs 23.2A

    A 10% difference is usually not an issue. A 2:1 issue (30 amps:15 amps) when both are supposed to be near identical--That can indicate problems. In between, is a grey area (maybe yes, maybe no).

    If the charge controller are not coordinating (bulk, absorb, float--some controllers from Midnite, Outback, etc. can sync controllers)--Then there will be periods of times where they are not the same (one in absorb, the other in float)--and that is perfectly normal. The controller with the highest voltage set point, wins.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • YehoshuaAgapaoYehoshuaAgapao Solar Expert Posts: 280 ✭✭
    Re: String charging balance - 21.5A vs 23.2A
    You're talking about one array and charge controller putting 21.5 Amps to the batteries while the other array and controller is putting 23.2. This is not a problem in itself, nor is it unusual. The arrays may not be evenly illuminated, and even if they are slight differences in wiring and the controllers themselves can cause one to be "more powerful" than the other. On a different day they could work the other way 'round too.

    If you see a consistent, repeated lower output from one side disconnect the higher current array and see if the lower one comes up. If under those circumstances you have two identical array & controller set-ups producing significantly different amounts of power there may be a problem with one or more panels or connections.

    No talking about the two sets of 4/0 cables going from the PDP's DC busbar to each battery box. One string of batteries is accepting more current than the other string. They had the amp difference go up from 0.2 to 1.7 amps with a charging current of about 45 amps, measuring when charging off the inverter (because the generator was exercised, and I have AC2 set is preferred to have productive rather than idling exercise). It was 9PM and the charge controllers were asleep. The inverter was bulk charging (only because the inverter ignores ReCharge volts and charges automatically upon AC source change).

    I limited the inverter to 45 amps charging current so the generator is kept under 22A (Didn't like the eco-throttle cycling up and down as loads change as the inverter keeps it at around 22A; 22A appears to be right at a RPM change point in the generator's eco-throttle).

    I expect different currents to come from each of the charge controllers. 1st is 6 panels at 25 degrees. 2nd is 3 panels at 25 degrees. 3rd is 6 panels flat (0.5 degree). 4th is 12 panels at 20 degrees.

    I last EQ'ed in the first week of May. I will do an experiment this weekend to see if float @ 64V will work without rushing all kinds of current at that voltage into the batteries. I might like the bulk less than absorb idea better (bulk setting is maintained for 1 hour, then transitions to absorb setting), if it actually works. Maybe a blend of the two (bulk 58.8 absorb 60.0 float 64.0).
  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: String charging balance - 21.5A vs 23.2A

    This will probably sound like a dumb question, but it isn't really: why do you have two 'separate' battery banks charging off one (albeit compound) source? Normally sixteen 6 Volt batteries on a 48 Volt system would be two strings of eight, wired in parallel as in the familiar Smart Gauge method #2. All charge sources meeting at a common connection point, so there would be only one set of wires going to the batteries and the current would be as evenly shared through both strings as possible.

    In essence, then, you are experiencing an increased charging current imbalance between battery strings. The first thing to do there is check the individual batteries and cells for Voltage and SG (unless they were sealed of course) and inspect/tighten all connections. It may be time to run an EQ on the S530's.

    You may want to rewire those batteries too.
  • ChrisOlsonChrisOlson Banned Posts: 1,807 ✭✭
    Re: String charging balance - 21.5A vs 23.2A
    Is this too different? The left box is charging at 23.2A while the right box is charging at 21.5A.

    Assuming there is nothing wrong with a battery in either string, in reality that is so minimal of a difference between two parallel battery strings during what is probably bulk charging, that it's not even worth mentioning. At the end of absorb I would expect them to be closer to the same. But not during bulk stage.

    No two batteries are ever identical. So even if you take take two fully charged S530's standalone, and discharge the same amp-hours out of each one, then recharge them individually with two separate but identical chargers - the chances that both of them are going to pull the same amount of amps during bulk charging with identical chargers is nil to none.

    Edit:
    As a side note, we got 6 parallel strings of Surrette T12's and during the initial part of bulk stage at high amps the current to the individual strings can vary by up 7 amps, and the voltages of individual batteries in the strings can vary by up to 0.5-0.6 volts. That's because no two batteries discharge the same, nor do they charge the same due to slight differences in internal resistance.

    By the end of absorb all 24 of our batteries are almost identical on voltage (usually +/-0.1V) and less than .5 amps difference between the strings.
    --
    Chris
  • YehoshuaAgapaoYehoshuaAgapao Solar Expert Posts: 280 ✭✭
    Re: String charging balance - 21.5A vs 23.2A
    This will probably sound like a dumb question, but it isn't really: why do you have two 'separate' battery banks charging off one (albeit compound) source? Normally sixteen 6 Volt batteries on a 48 Volt system would be two strings of eight, wired in parallel as in the familiar Smart Gauge method #2. All charge sources meeting at a common connection point, so there would be only one set of wires going to the batteries and the current would be as evenly shared through both strings as possible.

    In essence, then, you are experiencing an increased charging current imbalance between battery strings. The first thing to do there is check the individual batteries and cells for Voltage and SG (unless they were sealed of course) and inspect/tighten all connections. It may be time to run an EQ on the S530's.

    You may want to rewire those batteries too.

    The use of bus bars (idea came from the Concorde SunExtender high-current-draw diagram) is most similar to Smart Gauge method #3. I'm using the PDP's DC busbars as the paralell string bus bar (it supports up to 3 battery cable lugs,and the PDP has 3 2.5" knockouts on the bottom for 3 sets battery cables). This shifts all the wiring distance in the SmartGuage Method #3 from pre-post (to zero) to post-post (whole length of inverter cable) and removes complexity of installing and insulating copper bus bars (or the post to be 2-4x tapped) in the boxes or running cables between the boxes.

    I haven't checked SGs while fully charged since early May. I'll do that too. I'll do my battery watering next week too (I checked 2 batteries last week and they were fine). Monthly watering was using less than 1/2 gallon of water so bi-monthly seems more efficient.

    I'm thinking it might be the acid spill (a couple ounces) from two cells from one of the batteries from the overwatering incident I had in April. The watering can I bought - the auto-stop appears to only work only if you hold it just right - I still use it but I don't rely on the auto-stop anymore.

    I voltmetered the wiring connections and the voltage between the 12" cable connections are 2.5-5.5 millivolts. This is for 21.5A on 4/0 cable.
  • YehoshuaAgapaoYehoshuaAgapao Solar Expert Posts: 280 ✭✭
    Re: String charging balance - 21.5A vs 23.2A
    ChrisOlson wrote: »
    Assuming there is nothing wrong with a battery in either string, in reality that is so minimal of a difference between two parallel battery strings during what is probably bulk charging, that it's not even worth mentioning. At the end of absorb I would expect them to be closer to the same. But not during bulk stage.

    No two batteries are ever identical. So even if you take take two fully charged S530's standalone, and discharge the same amp-hours out of each one, then recharge them individually with two separate but identical chargers - the chances that both of them are going to pull the same amount of amps during bulk charging with identical chargers is nil to none.

    Edit:
    As a side note, we got 6 parallel strings of Surrette T12's and during the initial part of bulk stage at high amps the current to the individual strings can vary by up 7 amps, and the voltages of individual batteries in the strings can vary by up to 0.5-0.6 volts. That's because no two batteries discharge the same, nor do they charge the same due to slight differences in internal resistance.

    By the end of absorb all 24 of our batteries are almost identical on voltage (usually +/-0.1V) and less than .5 amps difference between the strings.
    --
    Chris

    I think I realize that when I clamp metered the battery cables the two prior times, it was about in the middle of the absorption stage. So differences during bulk isn't really an issue, but absorption would be.
  • NorthGuyNorthGuy Solar Expert Posts: 1,925 ✭✭
    Re: String charging balance - 21.5A vs 23.2A

    At the end of absorption, current is very small, so the difference is naturally small too. At the beginning of the bulk. current is higher, and the difference is higher too.

    Given all the same resistances, the difference between currents should be directly proportional to the current. Therefore, if you want to compare your measurements, you need to calculate a percentage difference. For eaxample, 1.7/44.7 = 3.8%.

    Also, your clamp (unless you have a $200 one) is probably only 2% accurate, so the difference you're getting is "hardly measurable" :D
  • ChrisOlsonChrisOlson Banned Posts: 1,807 ✭✭
    Re: String charging balance - 21.5A vs 23.2A
    I think I realize that when I clamp metered the battery cables the two prior times, it was about in the middle of the absorption stage. So differences during bulk isn't really an issue, but absorption would be.

    I would still expect to see some difference in the middle of absorb, and 1.7 amps between two big strings like that is really nothing at a 40-50 amp charge rate. I'd fully absorb them, where I expect them to be somewhere below 1 amp difference at the end of absorb. And if it's higher than that disconnect the load from them and check individual voltage with the surface charge still on to quickly "weed out" the ones that have more internal resistance difference (they'll have a lower voltage with the surface charge on, and those are the ones that are drawing more amps). Then rotate those (or some of them) to the other string.

    I do the above every time I service the batteries. They never live in the same spot on the same string from service to service.
    --
    Chris
  • VicVic Solar Expert Posts: 2,970 ✭✭✭✭
    Re: String charging balance - 21.5A vs 23.2A

    One of the disadvantages of multiple strings of batteries in parallel is that it can be difficult to maintain a fairly constant temperature for the batteries in each string.

    While the current differences noted in the OP are not great, it is possible that with two parallel strings, each in a separate battery box, could easily have a somewhat different average temperature for each string. This can cause different charge current in one string vs the/an other string.

    Vic
    Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH [email protected], 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.
  • YehoshuaAgapaoYehoshuaAgapao Solar Expert Posts: 280 ✭✭
    Re: String charging balance - 21.5A vs 23.2A

    The left box, being further away from the swamp cooler (the swamp cooler blows into the house, but some cool air does blow into the garage and the bottom foot of the garge is 10 F cooler than the ceiling, and the battery boxes inhale near the floor), runs 0.5-0.7C warmer on float and 0.7-1.5C warmer under load than the right box. The left box is accepting more current. Maybe this could be the reason?

    Xantrex said to remove one of the battery temperature sensors, but given I do have two battery boxes each with its own string, I would rather have one in each box. The inverter and charge controllers are documented to use the higher of the temperature readings for voltage compensation and it appears to behave that way. Charge controller #1 has the BTC for the left box and Charge controller #2 has the BTC for the right box. Charge controllers #1 and 2 power the right box's fans (Delta AFB1212M) and charge controllers #3 and 4 power the left box's fans (aux port).
  • NorthGuyNorthGuy Solar Expert Posts: 1,925 ✭✭
    Re: String charging balance - 21.5A vs 23.2A
    The left box, being further away from the swamp cooler (the swamp cooler blows into the house, but some cool air does blow into the garage and the bottom foot of the garge is 10 F cooler than the ceiling, and the battery boxes inhale near the floor), runs 0.5-0.7C warmer on float and 0.7-1.5C warmer under load than the right box. The left box is accepting more current. Maybe this could be the reason?

    The temperature compensation (about 0.1V/degree) is normally used. It decreases the voltage on warmer batteries to keep the same current. This means that the warmer batteries should get more current at the same voltage - exactly what you observed.
  • ChrisOlsonChrisOlson Banned Posts: 1,807 ✭✭
    Re: String charging balance - 21.5A vs 23.2A
    NorthGuy wrote: »
    The temperature compensation (about 0.1V/degree) is normally used. It decreases the voltage on warmer batteries to keep the same current. This means that the warmer batteries should get more current at the same voltage - exactly what you observed.

    Except we're only talking about 1.7 amps between 400 amp-hour strings! That's not even 0.5% of the ah capacity of the string. You need a shunt to measure amps that accurate. And even then you're limited by the +/- accuracy of the meter reading the voltage drop across the typical 75 mV high-amperage shunt. The same string can vary by that many amps at the same SOC on different days, and reading it with a clamp-on hall-effect type meter can vary by more than that just by you hold the meter, or adjacent magnetic fields in other wires.

    There's mountains and there's mole hills, and you know what they say about trying to turn one into the other.
    --
    Chris
  • NorthGuyNorthGuy Solar Expert Posts: 1,925 ✭✭
    Re: String charging balance - 21.5A vs 23.2A
    ChrisOlson wrote: »
    There's mountains and there's mole hills, and you know what they say about trying to turn one into the other.

    Just to clarify, I do not suggest to install a fine climate control to fix the temperature difference. I just said that the small difference in current might be explained by small difference in temperatures.
  • ChrisOlsonChrisOlson Banned Posts: 1,807 ✭✭
    Re: String charging balance - 21.5A vs 23.2A

    Of course. All I'm saying is that, assuming the meter is even accurate, 1.7 amps is nothing to worry about. That can be caused by cable routing, slight differences in internal resistance in the batteries, the fact that one string may not have discharged at the same rate as the other due to temp difference, etc., etc., etc.. It's kind of like going out to mow your lawn and trying to set the cutting height of the lawnmower with a micrometer.

    Batteries on a bank should be rotated to a different spot in the strings from time to time - few people do it. But it's like rotating the tires on your car - you'll get more even wear and life on the tires if you do. And that's because I don't care how you wire 'em up there's differences in batteries.
    --
    Chris
  • NorthGuyNorthGuy Solar Expert Posts: 1,925 ✭✭
    Re: String charging balance - 21.5A vs 23.2A
    ChrisOlson wrote: »
    Batteries on a bank should be rotated to a different spot in the strings from time to time

    I guess your batteries do not weigh 315 lb each :D
  • YehoshuaAgapaoYehoshuaAgapao Solar Expert Posts: 280 ✭✭
    Re: String charging balance - 21.5A vs 23.2A
    ChrisOlson wrote: »
    Of course. All I'm saying is that, assuming the meter is even accurate, 1.7 amps is nothing to worry about. That can be caused by cable routing, slight differences in internal resistance in the batteries, the fact that one string may not have discharged at the same rate as the other due to temp difference, etc., etc., etc.. It's kind of like going out to mow your lawn and trying to set the cutting height of the lawnmower with a micrometer.

    Batteries on a bank should be rotated to a different spot in the strings from time to time - few people do it. But it's like rotating the tires on your car - you'll get more even wear and life on the tires if you do. And that's because I don't care how you wire 'em up there's differences in batteries.
    --
    Chris

    Actually for me, its easier to move the inverter cables since they are too long (they can reach the batteries on the far side of the box and then a little bit more - should have bought 5' cables) than it is to move the batteries themselves. My batteries are about 130lb each. I cannot lift them out of the box. I can only lift them about a half a foot off the ground (I have strong legs but not strong arms or chest). The inverter is worse because not even the installer or his helpers could lift that to its installed height, though my 6'8" 275lb nephew lifts it up by himself.

    The meter may not be accurate but it should be wrong by an almost equal amount on each battery string.

    What is a charge balance difference that would be reason to worry?

    I am EQing now. Raising absorb voltage in steps (Normal 58.8, then 60, then 62, soon 64) to keep battery current under 15 amps or so. Inverter is keeping battery voltage (in enhanced interactive mode) at the temperature-compensated set voltage minus 1V so the batteries are actually running 60.8V right now.
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