prevent battery banks from sharing amps

firerescue712
firerescue712 Solar Expert Posts: 95 ✭✭
in Off Grid Solar & Battery Systems #1
Is there a way to connect two or more battery banks, yet not allow these banks to back feed into each other? Example, can 5 battery banks, each charged by individual charge controllers and panels, be wired into on large (2000-watt or larger) inverter with one-way of current flow only towards the inverter?

Example:
405 ah 24 volt bank off one charge controller
225 ah 24 volt bank off another charge controller
Both banks connected to the same inverter, but some type of equipment that combines the current to the inverter, but prevents any back feed.

Or even several banks of the same voltage and the same ah, but not back feed between the battery banks.

Has anything like either of the examples been tried?
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Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #2
    Re: prevent battery banks from sharing amps

    Really, really big diodes.

    But seriously there's not any reason to do this; a battery bank is either whole or separate.
    If you need separate banks that can be used as either/both then they are connected via a switch like Blue Sea's.
    If it needs to be "on demand" in case the inverter needs to draw more than one bank can supply, the bank is really too small for the application.

    Can you clarify why you'd want to do this?
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #3
    Re: prevent battery banks from sharing amps
    firerescue712 wrote: »
    Is there a way to connect two or more battery banks, yet not allow these banks to back feed into each other? Example, can 5 battery banks, each charged by individual charge controllers and panels, be wired into on large (2000-watt or larger) inverter with one-way of current flow only towards the inverter?

    Example:
    405 ah 24 volt bank off one charge controller
    225 ah 24 volt bank off another charge controller
    Both banks connected to the same inverter, but some type of equipment that combines the current to the inverter, but prevents any back feed.

    Or even several banks of the same voltage and the same ah, but not back feed between the battery banks.

    Has anything like either of the examples been tried?

    It is simple to do, by putting a high current, low forward voltage diode (such as a power Schottky diode) in series with one of the wires from the each battery bank to the inverter input bus.
    What is not possible is to also charge those separate battery banks from a common inverter/charger.
    Since you specified separate chargers, you would be OK.

    The downside is that the inverter will see a voltage which is at least .2 volt (and maybe as much as 1 volt if you undersize the diode) lower than the battery voltage. For a 12 volt system this is painful. For a 48 volt system it may not be noticed.

    For most people this will not be an optimal configuration because it means that one set of panels will only be able to charge one of the battery banks, regardless of which one needs charging most. (You cannot in general connect two PWM or MPPT Charge Controllers to one array of panels and to different batteries. For MPPT CCs you cannot even connect two CCs between one array of panels and one battery bank.)

    So, as Cariboocoot said in effect, why bother?
    SMA SB 3000, old BP panels.
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  • firerescue712
    firerescue712 Solar Expert Posts: 95 ✭✭
    #4
    Re: prevent battery banks from sharing amps

    Since batteries are so expensive, and if this device was possible, one could add more batteries when money allowed, and still keep the old batteries from draining off the new batteries. When the current was pulled from the batteries, the newer batteries would provide the initial current until the old battery level was reached. Then, all batteries would allow current into the inverter. Just some redneck brainstorming. If this is possible, maybe someone is, or will, work on inventing such a device.
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  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    #5
    Re: prevent battery banks from sharing amps
    firerescue712 wrote: »
    Since batteries are so expensive, and if this device was possible, one could add more batteries when money allowed, and still keep the old batteries from draining off the new batteries. When the current was pulled from the batteries, the newer batteries would provide the initial current until the old battery level was reached. Then, all batteries would allow current into the inverter. Just some redneck brainstorming. If this is possible, maybe someone is, or will, work on inventing such a device.
    The argument is somewhat compelling, until you realize that you will only be able to use half of your panels some of the time.
    SMA SB 3000, old BP panels.
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  • firerescue712
    firerescue712 Solar Expert Posts: 95 ✭✭
    #6
    Re: prevent battery banks from sharing amps

    Inetdog, I understand about the charge controllers not sharing between two battery banks that are meant to be isolated. The connections between these banks would allow current flow both ways. I was just thinking for future expansion. I knew the gurus here would enlighten me to the validity of this set up. If not feasible, the only alternative is larger batteries as the system grows.
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  • firerescue712
    firerescue712 Solar Expert Posts: 95 ✭✭
    #7
    Re: prevent battery banks from sharing amps
    inetdog wrote: »
    The argument is somewhat compelling, until you realize that you will only be able to use half of your panels some of the time.

    I planned on individual panels into individual charge controllers in individual banks. This would allow a system to grow in increments instead of a large outlay at one time.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #8
    Re: prevent battery banks from sharing amps

    I see what you're aiming for:

    Old bank at 90% new capacity + new bank at 100% capacity separated by diodes. Inverter draws from both, but the old bank can not draw from the new bank. Yes, this can be done providing you can find diodes capable of handling the current potential.

    So 100 Amp hours gets drawn down to 90 Amp hours on Bank 1 and then Bank 2 (which starts out at 90 Amp hours) begins to contribute because the two are now evenly matched. Right?

    Problem: Peukart.
    The bank that is at 90% to begin with will not discharge along the same power curve as the one that started out at 100%. The same goes if they actually are different capacity batteries (but at their same % level) to begin with. The power would shift back and forth between the two a bit, but it wouldn't be noticeable.

    Now, how practical is this to do? Good question.
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  • firerescue712
    firerescue712 Solar Expert Posts: 95 ✭✭
    #9
    Re: prevent battery banks from sharing amps
    Cariboocoot wrote: »
    I see what you're aiming for:

    Old bank at 90% new capacity + new bank at 100% capacity separated by diodes. Inverter draws from both, but the old bank can not draw from the new bank. Yes, this can be done providing you can find diodes capable of handling the current potential.

    So 100 Amp hours gets drawn down to 90 Amp hours on Bank 1 and then Bank 2 (which starts out at 90 Amp hours) begins to contribute because the two are now evenly matched. Right?

    Problem: Peukart.
    The bank that is at 90% to begin with will not discharge along the same power curve as the one that started out at 100%. The same goes if they actually are different capacity batteries (but at their same % level) to begin with. The power would shift back and forth between the two a bit, but it wouldn't be noticeable.

    Now, how practical is this to do? Good question.

    BINGO! I know it will not share the current drain equally. I get these wild ideas and have to ask why. Thank you Cariboocoot for enlightening me. It seems if someone could come up with a device that would easily connect and perform, they could change the way battery banks are looked at. Thank you for sharing this information.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #10
    Re: prevent battery banks from sharing amps

    Well two problems with diodes: the current and the Voltage drop. Most inverter systems would need a diode capable of 200+ Amps. Some of those used for DC welders would work there. But they will also remove the better part of 1 Volt from what is available. Schottky diodes minimize Voltage loss, but I don't think you'll find one capable of the high current. Paralleling diodes is not a good idea either, because if one blows the current falls on the other(s) (and so on) and if one shorts it will try to carry all the current - right before it goes "boom".

    Anyone who wants to test this design needs a couple of new batteries of different capacity but same Voltage, fully charged (with separate charge sources so the experiment can be repeated), a couple of battery monitors properly programmed for each, the isolation diodes (smaller than full capacity can be used for testing purposes as the current would be held to less than max - probably about 5 Amps constant), and a resistive load to provide a constant drain. Hook it up, watch the monitors, see what happens.
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #11
    Re: prevent battery banks from sharing amps

    Here's a chap from Italy who built such a thing using different sized car batteries and schotky diodes: http://offgrid2.altervista.org/viewtopic.php?f=7&t=6&start=140#p4245

    file.php?id=297
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #12
    Re: prevent battery banks from sharing amps

    stephendv,
    i know you are trying to help here, but without input in english as to what he is doing that it's not very helpful. for instance he has batteries near each diode array, as expected, but also another battery bank below (agm) and yet another whole section to the right that seems to be wired from the controller's load to the agm bank below rather than the diode output array leading me to think it's possibly miswired. i can't say for sure as his link is in italian making his train of thought on it difficult.

    please try to refrain from using non-english links for this reason and that we moderators can't make proper determinations on content when it's in a language we don't know.
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  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    #13
    Re: prevent battery banks from sharing amps
    niel wrote: »
    please try to refrain from using non-english links for this reason and that we moderators can't make proper determinations on content when it's in a language we don't know.

    Hi niel, I thought the diagrams and google translate would be helpful to those interested in this discussion. Here's the google translated version of the link: http://translate.google.com/translate?sl=auto&tl=en&js=n&prev=_t&hl=en&ie=UTF-8&eotf=1&u=http%3A%2F%2Foffgrid2.altervista.org%2Fviewtopic.php%3Ff%3D7%26t%3D6%26start%3D140%23p4245&act=url

    I was under the impression that the purpose of moderation was to remove spam links (which this isn't, since it's to another off-grid forum) and to remove language that would make a 5-year old blush (which I think google takes care of). Surely more information is always better than none, and the readers can then choose what to read and what to ignore?
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #14
    Re: prevent battery banks from sharing amps

    i am a bit confused here as to what the op really wants here as the title indicates preventing a sharing of output currents, but then why parallel them then if the outputs aren't to be combined? a diode will not prevent the sharing of output current to the loads. the diodes would isolate one battery from another, but will not isolate to the loads. this means 1 battery could pump out 20a while the other 14a and the diodes will allow this imbalance to pass to the loads.

    stephen,
    how can we determine the content if we can't read it? i can read a schematic well, but explanation is warranted as it doesn't make sense to me as it is. as far as moderating goes we do more than just remove spam liinks, thank you. how are we to know if there's swearing or whatever in it? we have asked for english links to be used here as english is key to this site's main body of members.
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  • firerescue712
    firerescue712 Solar Expert Posts: 95 ✭✭
    #15
    Re: prevent battery banks from sharing amps

    Niel, sorry for the poorly worded title. I was questioning the configuration of have two or more battery banks, same voltage, charged with separate charge controllers and panels, but isolated from passing current from one bank to the other. These banks would power one large inverter. This would allow the currents to flow out into the inverter. I hope this makes sense.

    Example:
    an older 405 ah 24 bank in use
    place a new 405 ah 24v bank online.
    Each bank is using a different cc and panels.
    Both bank are wired into one inverter to provide more power for larger AC loads.
    The new batteries cannot pass current back into the older batteries.

    Cariboocoot hit the nail on the head in his response above. It would take big diodes. I was looking to see if there was a device that could be placed inline with the battery wiring between the battery banks and the inverter that would prevent the back feeding between banks. This would allow adding new battery banks to existing systems as demands grow.
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  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    #16
    Re: prevent battery banks from sharing amps

    other than the diodes the only other thing may be a switch to pick one or the other battery bank.
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #17
    Re: prevent battery banks from sharing amps

    Looks like the gist of the diagram provided by stephendv is to drop the Voltag between flooded cells and the AGM battery while preventing the latter from back-feeding to the former. Note the V-sense leads of the charge controllers connected directly to the batteries for accuracy in measuring charge Voltage.

    Frankly, it's an illogical mess. For one thing according to the legend the FLA's are 12 Volt and it shows two in series for each bank, then paralleled to a single 12 Volt AGM. That sure won't work!The second FLA bank is connected to the (+) of the AGM via the wrong side of the diodes, so the AGM can feed them but not vice-versa calling in to question the whole of what he was trying to accomplish. It shows an optional Schottky diode reversing around one of the isolation diodes for the CC & desulphator, thus negating any advantage of those (of which there would be none). The second desulphator is connected directly to the AGM but to the second FLA bank via isolation diodes.

    Okay, so I can read a little Italian. But I can read a simple schematic and this one doesn't makes sense for any function I can think of. It also does not address the original issue of being able to share output from two dissimilar battery banks without one back-feeding the other.

    But don't hire the guy who drew it. :p
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  • firerescue712
    firerescue712 Solar Expert Posts: 95 ✭✭
    #18
    Re: prevent battery banks from sharing amps

    I am glad ya'll can read the schematic. I tried, but got lost. Just a simple mind here trying to safely do complex things. Scary, ain't it?
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #19
    Re: prevent battery banks from sharing amps
    firerescue712 wrote: »
    I am glad ya'll can read the schematic. I tried, but got lost. Just a simple mind here trying to safely do complex things. Scary, ain't it?

    That describes the whole human race.
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  • H2SO4_guy
    H2SO4_guy Solar Expert Posts: 213 ✭✭✭
    #20
    Re: prevent battery banks from sharing amps

    Hi Fire,

    I am trying to do something similar as to what you are doing with the same reasoning that batteries are expensive. That's why I try to get them for slightly over scrap value and work out switching issues. I am building inside of a barn and in the electrical room are 5000 pounds of batteries, all purchased near scrap prices, so if I don't make it work, I'll scrap some and get more. The charging seems to pose the most problems with switching chargers around so I, like you, have decided to use dedicated chargers for each battery pack. That way you can have dedicated BTS's and 2 of the controllers are Morningstar TS-60's so they have sense lines. There are 20 Evergreen 210 watt panels on the new south facing part of the barn. 4 series strings of 4 coming in about 88 VOC and 11.48 amps in STC. I can switch these to different charge controllers to charge the various battery packs. The last 4 panels come in individually for the 12 volt batteries and I can switch them around too. I am still working out the details and figuring out what I want it to be able to do, wishing I could automate it too!

    2 of the 48 volt battery packs are on a Blue Sea battery switch (Up to 350 amps switch capacity, they are good http://www.solar-electric.com/basw1300amp.html) for the Outback VFX-3648 inverter and the rest are on Exeltech XP-1100 inverters at 12 or 48 volts.

    My idea was to have a distributed system where the frig and freezer would be on one set of batteries, general use and stuff would be on another, and a large pack would be for air conditioning in the summer. The goal would also be to utilize unused wattage to preheat water before the on demand water heater. I don't live there yet, planning to move in late next month. Still have to get water issues resolved.

    I would be happy to share more with you if you would like and we could chat on the phone too. Perhaps we could help both of us. It does seem like I am trying to do stuff out of the norm and I can't find parts to accomplish the goal. The forum has been wonderful help, but I often am left with more questions than answers, but it sure is fun trying to figure this stuff out!

    Skip
    12K asst panels charging through Midnite Classic 150's, powering Exeltechs and Outback VFX-3648 inverter at 12 and 48 volts.  2080 AH @ 48 VDC of Panasonic Stationary batteries (2 strings of 1040 AH each) purchased for slightly over scrap, installed August 2013.  Outback PSX-240X for 220 volt duties.  No genny usage since 2014. 
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  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    #21
    Re: prevent battery banks from sharing amps

    Not too far out of the norm, Skip. :D
    Really big or expanded systems are often wholly separate with dedicated divisions among loads. The only odd bit in any of this is trying to utilize dissimilar batteries on the same load. Which is where the trouble comes in.
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  • _OS_
    _OS_ Solar Expert Posts: 207 ✭✭✭
    #22
    Re: prevent battery banks from sharing amps

    Maybe a battery combiner will do the trick: http://www.victronenergy.com/battery-isolators-and-combiners/argo-diode-battery-combiners/
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