This HAS to be idiotic...but please explain why it is!

openplanetopenplanet Registered Users Posts: 33 ✭✭
OK, I'm still trying to figure out a way to upgrade my existing, healthy, 2 year old, series string of 4 Rolls S530 6 volt batteries from 24 to 48 volts by adding 4 new ones to the string...WITHOUT violating the apparently sacred rule that one Shalt Not Mix Old And New Batteries.

Here is the idea, which I presume is completely idiotic...but I'd really like someone who knows what they're talking about to try and explain to me precisely WHY!

*Keep my existing array, controller, and 4 battery, 24 volt  string. 
*Set up a second array with its own controller, charging a second  series string of 4 NEW batteries.
*Put those two strings in series...and there's your 48 volts, with old and new batteries being charged independently.

Yes, there will be times when one controller is in bulk while the other might be in absorb, or one in absorb while the other is in float.  Why does that matter?  End to end it's a 48 volt string, and a 48 volt inverter wouldn't know or care about the details.

I CAN see that there would be challenges getting each (Midnite Classic) controller to have its own WhizBangJr. for determining endamps.  But beyond that...???

So again, I know this must be lame-brained.  I'm just interested in hearing any clear explanations of WHY.

Thanks.
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Comments

  • Marc KurthMarc Kurth Solar Expert Posts: 409 ✭✭✭✭
    edited May 11 #2
    I would not be against your plan of putting four additional batteries in series to create a 48v battery bank - IF your existing battery string is in fact fully healthy.  I have in fact seen this done several times without any detectable problems. It beats the heck out of throwing away perfectly good batteries and starting over with all new.

    If it were my battery bank, I would perform a capacity test on the existing batteries to be sure that they really are fully healthy.

    Certainly, people will come along and say how bad the idea is based on something they read somewhere. I do not claim to be a world class expert on the subject, but I can offer opinions based on what I have seen or done.  I design and sell AGM battery systems for a living, so I am opinionated. There are some very talented battery engineers who agree with this.

    Just be aware that your battery bank will only be as strong as the weakest battery in the string.

    Marc

     


    I always have more questions than answers. That's the nature of life.
  • jonrjonr Solar Expert Posts: 845 ✭✭✭✭
    edited May 11 #3
    Your charging scheme solves the issues with getting old and new charged properly (even for the case of self discharge).   That leaves discharging, specifically measuring low voltage cutoff.  1/2 the string will have a different capacity and therefore fall to a lower voltage for a given amount of discharge amp hours.   But you could measure this and adjust your low voltage disconnect voltage upwards to account for this.  Which leaves no problems to explain.
  • LumisolLumisol Registered Users Posts: 374 ✭✭✭
    Just buy 8 batteries and sell the 4 older ones to help offset the cost, you will get an approximate pro-rated value of the old batteries that way. You already got some of your money out of the use of them so far.

    Are you going to keep the systems separate with diodes so you don't get flow between the two banks?
    As soon as they are connected in series won't there be a gateway for current to flow between banks?
  • BB.BB. Super Moderators Posts: 26,768 admin
    It is certainly an option... The biggest issue is the "shifted" ground between the charge controllers... Ground = zero volts for one controller, and the other has +24 volts as "ground". This means that typical communications and control circuits that use a "common signal ground" will not work (RS 232, RS 485, some contact open/ground outputs, one-wire bus for Wizbang Jr. comm, etc.). Others like standard cat3/cat5 Ethernet (transformer isolated), level shifting/isolating buffers (RS 232/485 buffers) can work around this issue.

    People do mix old and new batteries together--And get useful life out of them--It is just a recommendation (at least on my part) that you try to avoid it--If practical.

    If you need to mix old and new batteries--I would probably suggest paralleling into 2x24 volt strings rather than two 24 volt stings in series... But I am not sure that would never use an "old/good" lead acid string in series with a "new/good" lead acid string (same type of battery--model/mfg ideally).

    Older lead acid batteries tend to have high self discharge. So when you put old+new in series--You have to "over charge" the new battery to make up for the self discharge of the old battery. More equalization of the new cells will use more water, erode plates, cause more positive grid corrosion.

    How bad that will be--I am not sure I can tell you. Will the 2 year and new batteries "fail" (age/cycle out) in 6 years instead of 8? I cannot tell you.

    Would you be happy if your bank lasted 6 years (instead of 8-2 for old bank) and 8 years for new bank?

    When your bank gets ~6 years old--And one or two cells start to die every few months--When will you pull the plug on the whole bank and replace it?

    There is probably enough variability in any lead acid battery bank that, in the end, you may have troubles isolating failures in 4-6 years as being caused by mixed age of batteries--Or just the luck of the draw (battery mfg) and local factors (how you charge the bank, how you cycle the bank, any mistakes you may have made, etc.).

    If you need the extra capacity, install the extra charging (solar panels+charge controllers as needed) and cannot find anyone that needs your 24 volt bank for a reasonable price--Then build out your bank (24v or 48v) and just keep a good eye on specific gravity, battery voltages, etc.

    Your other option--Install the 2x solar array + controllers, and keep your current bank--And just cycle it more heavily until it wears out. The replace with new bank of your needs. You may need some more genset time during bad weather--But those costs are probably comparable or less than replacing your old bank now. You have good batteries and you can deeper cycle them--As long as they get recharged relatively quickly (i.e., don't take to 25% state of charge and let sit for a week until the sun returns--that will probably sulfate them--Get them recharging the next morning with solar and/or genset--Don't need to get to 100% but get 10-30% minimum state of charge back in the battery the next day). One of the alternative methods of running a battery bank is to cycle from 50-80% state of charge--The batteries do not sulfate or show "harm" from this cycling. And get to >90% State of charge every 7 days or so.

    For me, I suggest looking at the risk/rewards of decisions. Is the risk of old bank+new panels+genset run time better or worse than 1/2 old + 1/2 new bank, or all new bank (and more panels as needed).

    I do not think there is a good black and white answer here.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mvasmvas Registered Users Posts: 298 ✭✭✭
    edited May 11 #6
    Does NEC require a Ground on the 48 Volt Battery Bank?
    Are either of the Neg or Pos 24 Volt output wires of either charge controller internally bonded to ground?
    That could cause an issue.
  • BB.BB. Super Moderators Posts: 26,768 admin
    The "24 volt return line" (aka upper "ground" connection) would need a circuit breaker on both the floating ground and the +48 volt battery bus. The "high system" ground is now "hot" and needs an over current protective device.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mcgivormcgivor Registered Users Posts: 1,205 ✭✭✭✭
    Strange things can happen with mixing two sources, those electrons seem to find the path of least resistance, causing all kinds of weird unforseen problems. 
      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • mike95490mike95490 Solar Expert Posts: 7,249 ✭✭✭✭
    Lumisol said:.........
    Are you going to keep the systems separate with diodes so you don't get flow between the two banks?.........
    That's always a good trick, obtaining 300A diodes, heat sinking them properly and then adjusting everything to account for >0.5V loss across the diode. 
    Forget diodes.
    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 ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • LumisolLumisol Registered Users Posts: 374 ✭✭✭
    mike95490 said:
    Lumisol said:.........
    Are you going to keep the systems separate with diodes so you don't get flow between the two banks?.........
    That's always a good trick, obtaining 300A diodes, heat sinking them properly and then adjusting everything to account for >0.5V loss across the diode. 
    Forget diodes.
    That's the point I was making. Best to build a new bank with new cells.
  • mvasmvas Registered Users Posts: 298 ✭✭✭
    Please attach a diagram showing the location of these 300 Amp diodes (plural) and then explain WHY the diodes are needed.





  • littleharbor2littleharbor2 Solar Expert Posts: 644 ✭✭✭✭
    If your old bank is healthy, AND you can get a buyer for it at some prorated price you would probably better off by saving the cost of a new Midnite Classic controller and additional array if that was your intention. You can go from 24 to 48 volts with your existing controller with minimal fuss.You will want add more panels to your array though to get your charging rate back up.

    2.1 Kw Suntech 175 mono, Classic 200, Trace SW 4024 ( 15 years old  but brand new out of sealed factory box Jan. 2015), Bogart Tri-metric, 700 ah @24 volt AGM battery bank. Plenty of Baja Sea of Cortez sunshine.

  • jonrjonr Solar Expert Posts: 845 ✭✭✭✭
    edited May 11 #13
    > Forget diodes.

    Not to mention that there is no flow path here such that diodes are needed.
  • mvasmvas Registered Users Posts: 298 ✭✭✭
    I had considered purchasing a USED Rolls battery bank until I was told by the seller, "Sorry, the 7-year warranty is non-transferable".

  • Dave AngeliniDave Angelini Solar Expert Posts: 3,257 ✭✭✭✭
    edited May 11 #15
    Go ahead and do it if you have the grid and the time to do your plan.

    If  " I know this must be lame-brained." and you are off grid, and want to find the reason why, it is because a battery failure is always something we try and put off as long as possible. This is because we never want to lose power and when the bank starts acting funny it is our warning. You would be starting out with the warning. It also hurts my head to think about people making this more difficult because of money. If you can't afford it you really should not be doing this.

    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail offgridsolar@sti.net

  • mcgivormcgivor Registered Users Posts: 1,205 ✭✭✭✭
    Go ahead and do it if you have the grid and the time to do your plan.

    QIf  " I know this must be lame-brained." and you are off grid the want to find the reason why it is because a battery failure is always something we try and put off as long as possible. This is because we never want to lose power and when the bank starts acting funny it is our warning. You would be starting out with the warning. It also hurts my head to think about people making this more difficult because of money. If you can't afford it you really should not be doing this.

    Very eloquently said, no need for euphemisms, if you can't stand the heat, get out of the kitchen.

      1500W, 6× Schutten 250W Poly panels , Schneider 150 60 CC, Schneider SW 2524 inverter, 8×T105 GC 24V nominal 

  • LumisolLumisol Registered Users Posts: 374 ✭✭✭
    Yeah, bottom line is if you don't mind spending more money sooner than you might otherwise need to, have fun, that's what a hobby is for.
    Vocations make money and avocations are where it goes.
  • LumisolLumisol Registered Users Posts: 374 ✭✭✭
    mvas said:
    Please attach a diagram showing the location of these 300 Amp diodes (plural) and then explain WHY the diodes are needed.





    LOL you forgot the other half of the circuit where the positive and negative both connect to each other at the point of use, completing the circuit for the electrons to make the entire circuit back to the batteries.
    The diodes comment was to make that point, you evidently didn't understand it and took it literally

    Let me make it clearer where the electricity will flow between the two banks...
    .
  • mvasmvas Registered Users Posts: 298 ✭✭✭
    edited May 12 #19
    Lumisol said:

    Are you going to keep the systems separate with diodes so you don't get flow between the two banks?
    As soon as they are connected in series won't there be a gateway for current to flow between banks?

    My circuit clearly shows how to connect two Battery Chargers to a set of batteries that are connected in series - which is the very point of this thread.  And yes I did read what you wrote and you stated "DIODES".  And yet your diagram has no diodes - LOL.  No, I did not forget the "other half of the circuit" because it is obvious to everyone, where the 48 Volt Inverter would need to be connected (which is not part of this discussion).  Your diagram does not explain anything and it specifically does not explain why you claim "diodes" are needed.  Your circuit diagram is just one battery, a bulb and some wire. LOL
  • mike95490mike95490 Solar Expert Posts: 7,249 ✭✭✭✭
    The circuit you have, with 2 clip on battery chargers, will work fine, as long as both chargers are transformer isolated.  Or both solar arrays are ungrounded and the charge controllers not grounded.
       But the moment you install any required grounds, it shorts out one bank.
    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 ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • mvasmvas Registered Users Posts: 298 ✭✭✭
    edited May 12 #21
    mike95490 said:
    The circuit you have, with 2 clip on battery chargers, will work fine, as long as both chargers are transformer isolated.  Or both solar arrays are ungrounded and the charge controllers not grounded.
       But the moment you install any required grounds, it shorts out one bank.
    Yep, I already mentioned this potential issue on May 10th in Message #6.
    And BB mentioned in message #7 that the "upper" charging circuit needs floating outputs - both (+) and (-) need to be fused
  • mvasmvas Registered Users Posts: 298 ✭✭✭
    edited May 12 #22
    Lumisol said:

    Let me make it clearer where the electricity will flow between the two banks...
    .
    LOL, That appears to be the circuit diagram of a flashlight.

  • LumisolLumisol Registered Users Posts: 374 ✭✭✭
    mvas said:
    Lumisol said:

    Let me make it clearer where the electricity will flow between the two banks...
    .
    LOL, That appears to be the circuit diagram of a flashlight.

    It is. But don't let that fool you into thinking it isn't related.
    Remember:


    It's chapter 1 volume 1 of basic electronics concepts. Adding more to the circuit does not alter the flow.
  • mike95490mike95490 Solar Expert Posts: 7,249 ✭✭✭✭
    Well, if you have it all figured out, go for it,   But don't be surprised when phantom ground loops muck it all up.. 
    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 ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • bill von novakbill von novak Solar Expert Posts: 694 ✭✭✭✭
    Lumisol said:


    Let me make it clearer where the electricity will flow between the two banks...
    .
    I don't see any ground return paths in there.  Will your system be completely isolated?  Such systems are extremely unsafe.
  • BB.BB. Super Moderators Posts: 26,768 admin
    Before we go down the rabbit hole--Lets get back to what OpenPlanet wants to do. In general I would suggest that there is no "good way" of adding old and new batteries together--Other than to avoid a single series string of mixed age batteries. (i.e., do not take an old 24 volt string and add a new 24 volt string in series for a 48 volt battery bank (mixing old/new in series and/or different brands/model/capacity/type of batteries in series).

    If I had to make the best of what I have--I would suggest adding the second new string of 24 volts in parallel. No diodes. Upgrade amoutn of solar panels+charge controllers+backup genset+battery chargers to support the larger battery bank and loads.

    Get/use a good hydrometer to monitor the state of charge for each cell (if flooded cell lead acid battery). Get a DC Current Clamp meter to measure the charging/discharging current between old and new string (you want each bank to handle its portion of charging and discharging current--They will not be identical, but they should be reasonably close). Watch battery/cell temperature. Monitor voltage across each battery. Log specific gravity and battery voltages and watch for things that change--Investigate why changed (i.e., dirty/corroded wiring or connections, failing cell, etc.).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Solar Expert Posts: 3,257 ✭✭✭✭
    The OP never came back and even he knew this was "idiotic and lame brained" What a waste of time....
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
     http://members.sti.net/offgridsolar/
    E-mail offgridsolar@sti.net

  • LumisolLumisol Registered Users Posts: 374 ✭✭✭
    Yep
  • LumisolLumisol Registered Users Posts: 374 ✭✭✭
    Lumisol said:


    Let me make it clearer where the electricity will flow between the two banks...
    .
    I don't see any ground return paths in there.  Will your system be completely isolated?  Such systems are extremely unsafe.
    LOL grounded flashlights are extremely rare even on today's market. The need to keep driving in a rod before switching it on is a real detriment to the portability factor that is a major selling point. It could catch on with law enforcement officers however as the rod could also be used as a baton, and they are generally not Phd.s
  • bill von novakbill von novak Solar Expert Posts: 694 ✭✭✭✭
    Lumisol said:
    LOL grounded flashlights are extremely rare even on today's market.
    And flashlights wired to solar arrays and home wiring are even rarer.  Which is why they don't need grounding.  (That and 3V at 100ma isn't going to harm anything.)
  • LumisolLumisol Registered Users Posts: 374 ✭✭✭
    Lumisol said:
    LOL grounded flashlights are extremely rare even on today's market.
    And flashlights wired to solar arrays and home wiring are even rarer.  Which is why they don't need grounding.  (That and 3V at 100ma isn't going to harm anything.)
    Your comment was that you didn't see any ground paths, I was surprised you would expect them in a flashlight.
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