battery equalization strategies?

I have (2) strings of 12v-600AH (12V-100AH each) for a total of 12 batteries, 6 per string if that makes any sense.


I had been running on only 6 for a week, then made a 'Y' connector and have a single string of 12 now. What to watch for regarding battery performance?

I've just been testing each cell's voltage at the end of the daily charging cycle, and late at night after the drain has finished/inverter has been turned off. Basically every cell is within a couple hundredths of a volt after the discharge cycle, but after the charge cycle there are variations as high as 0.2 for one or two batteries.

although all my batteries are exactly the same (12V-100AH DCS-100L AGMs) some are only about 15 months old and the bulk of them are 24 months old, with 2 that are as old as 30 months.

I've noticed the newer battery is the one that charges up a hair higher, and the oldest ones are usually on the lower end.

I never cycle past 50%, and usually never past 75%.

Much ado about nothing? What are some guidelines? I've read elsewhere that anything north of 0.1 volt differential is a problem. Is that true?

Comments

  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: battery equalization strategies?
    JoeT wrote:
    I have (2) strings of 12v-600AH (12V-100AH each) for a total of 12 batteries, 6 per string if that makes any sense.
    I had been running on only 6 for a week, then made a 'Y' connector and have a single string of 12 now.


    I never cycle past 50%, and usually never past 75%.


    2 strings of 6, add a Y connector, and now one string of 12 ?
    Are these series strings or parallel strings? 12V or 144V ? (12eax12v)


    "I never cycle past 50%, and usually never past 75%." Is this a typo ?
    Does not make sense, are you talking "State of Charge" or "Discharged"
    25% charge = 75% discharged
    25% discharged = 75% charge

    I've assumed one should never mix batteries from different batches, as the weaker batch will drag down the rest, make 2 of anything, and one will be better.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
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  • System2
    System2 Posts: 6,290 admin
    Re: battery equalization strategies?

    1) parallel (each string is 24v) both strings together are 24v

    2) "cycle past 50%" means never past 50% DOD, but usually never past 25% DOD. aka battery pack is reading ~25.1 or so after nighttime drain. Yes I hit 'float' during the day.


    So I'm assuming it's inappropriate to call a charge/discharge event a "cycle" for a battery?


  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: battery equalization strategies?

    well joe, it seems you have done with batteries what you've done with pvs, but kind of mixed by age though. you may be seeing .1 or .2v difference and that difference would've been higher yet if the batteries weren't interconnected. the older or in worse shape batteries are dragging down the newer or in better shape batteries. i'd also bet those batteries at the end of the line are lower in voltage than those nearer the charge source. basically the rule of thumb is that the battery bank will be as good as the weakest in the bunch. you could throw another brand new battery in there and in no time it'll be about as good as the oldest battery in the bunch.
    do keep in mind that using poor battery interconnecting wires or methods can also lead to differences in identical batteries at the start. the result will be some batteries will age faster and thusly bring the rest of the bank down to its level in time.
    your mix and match pv system will keep giving even if not optimally, but mix and match batteries won't.
  • System2
    System2 Posts: 6,290 admin
    Re: battery equalization strategies?

    So this is an unsatisfactory battery bank?

    (2) DCS-100L that are 15 months old
    (eight) DCS-100L that are 24 months old
    (2) DCS-100L that are 30 months old

    They spent their entire life in 'float' in a freight elevator.

    My question is basically how do I determine when a battery is a problem? The 0.1 to 0.15 volt differences I observe after a float/charging cycle? After a 25 or even 50% discharge all batteries are within a couple hundredths.

    What else to watch for? Or is that it?
  • System2
    System2 Posts: 6,290 admin
    Re: battery equalization strategies?

    as a 'fer instance' I baselined the first bank of batteries against a state of charge curve I saw elsewhere for an AGM battery, used a C/20 load .
    I took half the bank down to 50% SOC per the graph for my temp/load and final voltage and it took 3.5 KW/hrs to get there. Theoretically this is a ~7.2 KW bank so it seems like it was pretty close? AKA I'm getting roughly ~90-95% of rated capacity based on a SOC discharge curve that I found on the internet. Maybe it isn't accurate?
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: battery equalization strategies?

    Joe,

    I have mixed feeling about the “batteries must be of the same age” rule. My research indicates this is more of a problem as batteries age. However, AGM batteries have demonstrated long-life capabilities, especially when used in float service, as yours apparently were.

    On the other hand, 3.5 kWh from a 7.2 kWh AGM battery bank discharged to 50% SOC isn’t as great as it might seem. AGM batteries often exceed specs during mid-life.

    My gut feel is that your batteries are probably in good enough shape for a starter set with a previous life. The .1 V to .15 V differences could be the result of poor connections between the batteries. I had a similar problem with my first set of batteries and it turned out to be a bad cable.

    So, here are some suggestions for getting the most from your batteries:

    1) Charge and maintain them in accordance with the manufacturer’s instructions and specifications.

    http://www.interstatebatteries.com/www_2001/content/products/specs/dcs100lmob.pdf

    2) Use a temperature compensated charge controller and remote battery temperature sensor.
    3) Check the connections between the batteries. Strings of interconnect cables should be bolted to opposite sides of the flag terminals using stainless steel hardware. Connections should be clean and tight.
    4) Charger/inverter cables to/from the battery bank should be connected at diagonally opposite corners of the bank. This configuration helps to evenly distribute charge/discharge current.
    5) Leave a little space between your batteries to allow for some air circulation. The inside batteries of a tightly packed bank often run a bit warmer, and that will affect their performance.
    6) AGM batteries usually require a long absorption cycle. Three to four hours is not uncommon.
    7) Measure the voltage at each battery near the end of a bulk cycle, when charging voltage and current are near their peak. Any problems will show up as a voltage differential. This is how I found my bad battery interconnect cable.
    8 ) State of charge measurements must be taken after the batteries have been disconnected from all charging sources and discharge loads for at least three hours.
    9) The term "equalization" is rarely used in association with AGM batteries. In fact, the direct response to your subject title: "battery equalization strategies?" is that, unlike flooded-cell batteries, AGM (and gel) batteries usually are not equalized.
    10) Other sources for good technical information for AGM batteries include Concorde, East Penn-Deka and MK Battery:

    http://www.concordebattery.com/
    http://www.eastpenn-deka.com/assets/base/0139.pdf (This document includes an SOC table for AGM batteries)
    http://www.mkbattery.com/images/AGMBatteryCharging.pdf

    HTH, and good luck!
    Jim / crewzer
  • System2
    System2 Posts: 6,290 admin
    Re: battery equalization strategies?

    Thanks, forgive my failure to use the solar-wind-etc. jargon term "equalization" for it's more general webster-usage.

    aka trying to keep the batteries on equal footing. Thanks for your informative post though.

    I am currently going to select the two weakest batteries and trim the herd down to (10) AGM batteries. My method will be to measure the voltage of each battery at the end of charge and discharge cycles every day. The two weakest one (Based on voltage) are getting re-assigned.

    Then I'll try to estimate capacity of the smaller battery string and compare. I have a feeling the two weakest batteries are beneficial overall but I'd like to be sure.
  • System2
    System2 Posts: 6,290 admin
    Battery Equalization Strategies?

    I've been looking for a good Solar Electric Forum for Years. A recent Google brought me here. This thread and several others convinced me their are some very bright solar folks on NAW&S. Hello!!!

    I've been living Off-the-Grid at 8500' on a Colorado mining claim since 1998. I have a Trace 4000 inverter, 2 - 8 panel solar panel grids, and 12 - 150# 6V batteries in 3 rows of 4 for a 24V system. Also a de-rated Onan 4KW generator for those consecutive cloudy days. It has 148 hours on its clock.

    I jumped into this thread because crewzer put a lot of effort into it and partially answered many of my questions.
    crewzer wrote: »
    So, here are some suggestions for getting the most from your batteries:

    1) Charge and maintain them in accordance with the manufacturer’s instructions and specifications.

    http://www.interstatebatteries.com/www_2001/content/products/specs/dcs100lmob.pdf
    I'll be reading this later.

    2) Use a temperature compensated charge controller and remote battery temperature sensor.
    I'm sure my Trace CC has this.
    3) Check the connections between the batteries. Strings of interconnect cables should be bolted to opposite sides of the flag terminals using stainless steel hardware. Connections should be clean and tight.
    While it all looks clean, I'll double check. There are a few weld marks. The carpenters who installed the batteries didn't have insulated wrenches. ;)
    4) Charger/inverter cables to/from the battery bank should be connected at diagonally opposite corners of the bank. This configuration helps to evenly distribute charge/discharge current.
    Not sure.
    5) Leave a little space between your batteries to allow for some air circulation. The inside batteries of a tightly packed bank often run a bit warmer, and that will affect their performance.
    Mine are packed tight. I'll have to see if the cables will allow movement, then figure out how to move the critters. A crowbar comes to mind, but oh what a welding rod!!!
    6) AGM batteries usually require a long absorption cycle. Three to four hours is not uncommon.
    I have standard flooded cell lead-acid batteries. I add 2-3 gallons of distilled water 3-times a year.
    7) Measure the voltage at each battery near the end of a bulk cycle, when charging voltage and current are near their peak. Any problems will show up as a voltage differential. This is how I found my bad battery interconnect cable.
    Can I make this measurement with the cables connected?
    8 ) State of charge measurements must be taken after the batteries have been disconnected from all charging sources and discharge loads for at least three hours.
    This is a tough one, everything runs off the batteries.
    9) The term "equalization" is rarely used in association with AGM batteries. In fact, the direct response to your subject title: "battery equalization strategies?" is that, unlike flooded-cell batteries, AGM (and gel) batteries usually are not equalized.
    So since I have flooded cell, please expand on the why and when of equalization?

    Thanks
    Roger
  • crewzer
    crewzer Registered Users, Solar Expert Posts: 1,832 ✭✭✭✭
    Re: battery equalization strategies?
    Quote:
    7) Measure the voltage at each battery near the end of a bulk cycle, when charging voltage and current are near their peak. Any problems will show up as a voltage differential. This is how I found my bad battery interconnect cable.

    Can I make this measurement with the cables connected?

    Quote:
    8 ) State of charge measurements must be taken after the batteries have been disconnected from all charging sources and discharge loads for at least three hours.

    This is a tough one, everything runs off the batteries.

    Quote:
    9) The term "equalization" is rarely used in association with AGM batteries. In fact, the direct response to your subject title: "battery equalization strategies?" is that, unlike flooded-cell batteries, AGM (and gel) batteries usually are not equalized.

    So since I have flooded cell, please expand on the why and when of equalization?
    Roger,

    Welcome to the forum! I’m happy you found my post to be useful. There are several very knowledgeable folks here on the board, and, although all of us except “Windsun” are volunteers, we do what we can to advocate this technolofy and help out with a broad range of technical issues.

    Re #7: Yes, you can take the measurements when the batteries and cables are all connected. This is how you’ll spot any significant voltage differences.

    Re#8: Yeah, that is a tough one. If our battery bank consists of multiple parallel strings, you can try isolating one string at a time to conduct this type of test. Or, perhaps you can run your loads from a generator while the batteries are disconnected and resting.

    Re #9: Regarding the “why”, let me refer you to “equalization” discussions and instructions from Trojan Battery and Rolls/Surrette:

    http://www.trojanbattery.com/Tech-Support/BatteryMaintenance/Equalizing.aspx
    http://www.rollsbattery.com/Bulletins/605.htm

    Regarding the “when”, there’s no set rule, although once-a-month is a common recommendation.

    HTH,
    Jim / crewzer