Parallel battery "Combiner"?

ljdavickljdavick Registered Users Posts: 3
I understand that arranging batteries in series makes sense. Arranging batteries in parallel is necessary sometimes, as we can't easily work with, say 192 volts of DC. I'm wondering if there is any kind of device that isolates each parallel bank, yet makes them available when the load requires them? Searching, for me, did not turn up a discussion about this - if one already exists then please point this nubee to it!


Thanks,

Comments

  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: Parallel battery "Combiner"?
    ljdavick wrote: »
    I understand that arranging batteries in series makes sense. Arranging batteries in parallel is necessary sometimes, as we can't easily work with, say 192 volts of DC. I'm wondering if there is any kind of device that isolates each parallel bank, yet makes them available when the load requires them?

    Why would you want to do that? Periodically disconnecting them would likely make matters even worse, unless you had a separate battery maintainer for each string.
  • ljdavickljdavick Registered Users Posts: 3
    Re: Parallel battery "Combiner"?

    My thought is, and please correct me if I am wrong, that batteries in Parallel can discharge each other as cells weaken. My Sister-in-law has an off-grid house with 16 6 volt L-16 batteries - 4 banks of 24 volts each. If one bank includes a weak cell it makes that bank weaker than the others, and they try to equalize, thereby discharging the stronger banks. If each 24 volt bank were isolated and combined by some intelligent controller this would not occur. Her solar array would produce electricity to be doled out to the banks and the draw would be consumed as needed. Is such a device available and if so is it practical?
  • NorthGuyNorthGuy Solar Expert Posts: 1,925 ✭✭
    Re: Parallel battery "Combiner"?
    ljdavick wrote: »
    My thought is, and please correct me if I am wrong, that batteries in Parallel can discharge each other as cells weaken. My Sister-in-law has an off-grid house with 16 6 volt L-16 batteries - 4 banks of 24 volts each. If one bank includes a weak cell it makes that bank weaker than the others, and they try to equalize, thereby discharging the stronger banks. If each 24 volt bank were isolated and combined by some intelligent controller this would not occur. Her solar array would produce electricity to be doled out to the banks and the draw would be consumed as needed. Is such a device available and if so is it practical?

    It would be better at 48 volts.

    You can measure voltage of each battery separately. You can do that manually. There should be ready-made monitoring devices which can do the monitoring and ring a bell if a cell fails.
  • vtmapsvtmaps Solar Expert Posts: 3,738 ✭✭✭✭
    Re: Parallel battery "Combiner"?
    ljdavick wrote: »
    My thought is, and please correct me if I am wrong, that batteries in Parallel can discharge each other as cells weaken. My Sister-in-law has an off-grid house with 16 6 volt L-16 batteries - 4 banks of 24 volts each. If one bank includes a weak cell it makes that bank weaker than the others, and they try to equalize, thereby discharging the stronger banks. If each 24 volt bank were isolated and combined by some intelligent controller this would not occur. Her solar array would produce electricity to be doled out to the banks and the draw would be consumed as needed. Is such a device available and if so is it practical?

    Welcome to the forum. Your thinking is quite clear. Parallel batteries are problematic in applications (such as solar energy) that require a lot of cycling. The device you are asking about is a Battery Management System (BMS).

    BMS are common in high energy density batteries such as lithium batteries. Those cells may burn and explode if overcharged. Boeing Airplanes has only recently learned how important a BMS is.

    Flooded lead acid batteries are much more tolerant of abuse. Therefore it is common to charge a FLA battery until ALL the cells are charged, even though this means most cells get overcharged. The lack of a BMS in most FLA battery banks is exacerbated by a parallel battery configuration.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: Parallel battery "Combiner"?
    ljdavick wrote: »
    My thought is, and please correct me if I am wrong, that batteries in Parallel can discharge each other as cells weaken.

    Not . . . really. (They can, in a way, but that's not the main problem.) A "perfect" setup of 100 battery strings, where every string receives the same voltage and sees the same impedance back to the charger, will not be any worse off from a life perspective than a single string - provided you're not doing anything extreme like trying to rejuvenate weak cells. The problems arise because:

    1) it is very hard to get exactly the same impedances back to the charger/load, so different strings will cycle to different depths
    2) you can't equalize just one string; you have to equalize all of them
    My Sister-in-law has an off-grid house with 16 6 volt L-16 batteries - 4 banks of 24 volts each. If one bank includes a weak cell it makes that bank weaker than the others, and they try to equalize, thereby discharging the stronger banks.

    I've seen the opposite problem. A weak cell has a higher resistance, and thus that string sees less charge and discharge current. It starts "loafing" requiring more DoD on the remaining strings. In bad cases that one cell can "reverse" and become seriously damaged. Note that this problem is the same in two cases:

    1) Several strings in parallel
    2) One string but not carefully monitored (charger has no way of knowing that the cell is bad unless you are looking at individual cell SOC's)
    If each 24 volt bank were isolated and combined by some intelligent controller this would not occur. Her solar array would produce electricity to be doled out to the banks and the draw would be consumed as needed. Is such a device available and if so is it practical?

    This doesn't get you any benefit if you aren't monitoring the cells yourself. However, a system where you could equalize strings independently might make sense and let you "repair" a bad string. The manual way to do this would be to just disconnect and equalize.
  • jcheiljcheil Solar Expert Posts: 722 ✭✭✭
    Re: Parallel battery "Combiner"?

    Question: Can you accurately measure the voltage of an individual battery in a series by simply measuring the voltage at the battery posts while still connected to other batteries or will the fact that they are connected upstream or downstream to other batteries effect that voltage?
    Off-Grid in Central Florida since 2005, Full-Time since June 2014 | 12 X Sovello 205w panels, 9 X ToPoint 220w panels, 36x ToPoint 225w panels (12,525 watts total) | Custom built single-axis ground mounts | Complete FP2 Outback System: 3 x FM80, 2 x VFX3648, X240 Transformer, FLEXnet-DC, Mate-3, Hub-10, FW500 AC/DC | 24 x Trojan L16RE-B Batteries 1110ah @ 48v | Honda EU7000is Generator and a pile of "other" Generators | Home-Made PVC solar hot water collector | Custom data logging software http://www.somewhatcrookedcamp.com/monitormate.html
  • BB.BB. Super Moderators, Administrators Posts: 30,625 admin
    Re: Parallel battery "Combiner"?

    Ideally, you would measure the temperature corrected specific gravity in each cell (and log once a month) and measure the cell voltage (resting/discharging/charging) and log those.

    For sealed batteries, you cannot measure specific gravity. For many multi cell batteries, you cannot measure the per cell voltage (you cannot get to the bus bars).

    With paralleled battery strings--If all is working well (good connections, good cells, no hot/cold spots in the bank, etc.)--There is no advantage to disconnecting a single string. If you have problems, sometimes there is. One way a few people have done connected two parallel strings (they like the redundancy)--Is to use an A/B battery switch:

    wind-sun_2269_21868681

    The "worst" battery to figure out if everything is OK is a sealed 12 volt battery bank with multiple parallel strings. Cannot measure per cell/battery voltages, cannot measure specific gravity.

    Anyway--I like to recommend a DC Current Clamp meter. You can put the bank under heavy load or heavy charging current and measure the current in each parallel string by just "clamping" a single cable. If you find a string that is taking too much or too little off the shared current, you have issues. Do that testing at least once a month and you should be OK.

    This meter from Sears is cheap and "good enough" for the job (AC/DC current clamp DMM--currently $51 in US). It does do the "DC current zeroing" differently than any other meter I have ever used--But once you understand how it works--it is easy to do.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • ljdavickljdavick Registered Users Posts: 3
    Re: Parallel battery "Combiner"?

    Thank you all. Lots to consider.
    My thinking came around to something like DROBO, the computer disk array box that takes any collection of disks and makes them one. As you run out of space you replace the smallest disk. With batteries it would be nice to be able to keep older batteries, as they have some capacity, but not impair the whole magilla.
  • BB.BB. Super Moderators, Administrators Posts: 30,625 admin
    Re: Parallel battery "Combiner"?

    I am not a big fan of "linking together" different batteries or disk drives...

    Just two batteries (or two disk drives)--Doubles capacity, but also (roughly) doubles the failure rate... And if one of the devices is already "old"--Your battery bank (or disk array) is just as reliable as the weakest member.

    Also, if you have small and large batteries (or disks)--The older (and smaller) devices simply do not have that much capacity... For a battery bank, the same batteries can differ by upwards of 20% (new/middle age/old)--Saving an older battery to add 20% capacity to your battery bank--The natural varibility in the many battery bank can swamp the preservation of the older battery...

    Remember that batteries can fail different ways... A battery can fail "open" and take parallel strings out of service. Or the battery can fail shorted and discharge the rest of the battery bank and even cause the failing cell to overheat (or worse).

    Anyway, my two cents worth.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • bill von novakbill von novak Solar Expert Posts: 891 ✭✭✭✭
    Re: Parallel battery "Combiner"?
    jcheil wrote: »
    Question: Can you accurately measure the voltage of an individual battery in a series by simply measuring the voltage at the battery posts while still connected to other batteries or will the fact that they are connected upstream or downstream to other batteries effect that voltage?

    They will affect the voltage. However you can easily get a rough sense of where bad cells are by measuring terminal voltages. For example, in a large array of batteries, checking any one battery should give you (Vstring/stringsize) voltage. If a battery is much higher during charge and much lower during discharge chances are there's a bad cell in the battery.

    Clamp on ammeters are another good way to see what's going on. Whichever string is seeing the lowest currents during charge or discharge likely has problems.
  • BB.BB. Super Moderators, Administrators Posts: 30,625 admin
    Re: Parallel battery "Combiner"?

    In the end, you have a bunch of "identical" cells/batteries and electrical connections... Anything that is "different" from the group is probably indicating problems (either failing directly, or showing issues elsewhere).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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