multiple battery banks

lazy_qlazy_q Registered Users Posts: 9
Background:
I have a fifth wheel. It originally was delivered with a single 12 volt battery. It has a typical 12 v converter (WFCO 55 amp 3 stage charger (not programmable as far as I know). I am in the process of equipping the fifth wheel with a solar charging system and an inverter to provide AC power when we boon dock.
My additions include: (2) kyocera 130 watt panels, a Tri-Star 60 amp charge controller, a Xantrex prowatt 3000 (MSW) inverter, a trimetric 2020, a progressive 50 amp EMS, and a battery bank of 4 Trojan T-105s series and parallel wired to provide 12volts.
I am thinking about leaving the battery bank 1 (original 12v battery) unchanged, and using battery bank 2 (4 Trojans) to provide power for the inverter.
Battery bank 1 would be the only battery connected to the stock converter and the wiring to the 7 way connector for towing. It would provide power for dc lights, furnace blower, landing leg motors, and the water pump and anything else wired into the stock distribution center. The converter has a dedicated breaker in the distribution center. At this time battery bank 1 is not tied into the solar system.
Battery bank 2 would be connected to the inverter and essentially only be responsible for providing ac power. It is wired to the solar charge controller.
The way that I see it is that when:
a) connected to shore power - battery bank 1 is charged by the converter. It is also charged when towing via the vehicle. AC power is provided via the shore connection.
Battery bank 2 is charged via the sun and not depleted as the inverter is not in use.
b) Boon docked - battery bank 2 is still charged via the solar charging system and it provides ac power via the inverter. The stock converter breaker is off and so Battery bank 1 is not charged and is depleted by all of the dc loads.

Questions:
1. Do you see any issue with the separation of battery bank 1 and 2.
2. Do I need an isolator between the battery banks?
3. Would I be better off merely tossing out the original battery and putting the dc load on battery bank 2 (4 Trojans)? Am I missing something?
4. If I toss out the original battery does the stock converter present any problem with regard to its charging profile “13.2 Vdc range “float” mode, 13.6 Vdc range “absorption” mode, and a 14.4 Vdc range “bulk” charge mode”. I realize it will not perform as well as the tri-star.
5. If I connect battery bank 1 to the solar charge controller (essentially putting it in parallel with battery bank 2) will that create any problem. I understand that it will increase the time required to fully recharge battery bank 2 assuming that both banks were 50% discharged.

Thanks for your advice.
ron q

Comments

  • GreenPowerManiacGreenPowerManiac Solar Expert Posts: 453 ✭✭✭
    Re: multiple battery banks

    One rule of thumb is NOT to mix different battery types: Gel, SLA, AGMs, etc. Different voltages are another story if the battery types are the same. Two 6v batts in series making 12v may be hooked up to a single 12v in parallel.

    If they are the same type: SLAs for example: the amp hours must be within 15% of each other in order to mix. One 100ah will mix with one 85ah batt, but the 100ah will keep drawing energy until it's full. This should not effect overcharging in the smaller battery, however with your charge controller should have some kind of diversion load to offset excess power, maybe to a D/C light bulb(s).

    If the converter can handle one big battery bank & the extra wattage, use it. Should never use two inverters or (two separate A/C sources) in unison. They are out of phase and will destroy one or the other inverter/converter. A very expensive mistake. If your converter will produce 240v in phase A/C, then I'm sure it can handle one big battery bank.

    Much, much safer to run one bank than two.
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  • KamalaKamala Solar Expert Posts: 452 ✭✭
    Re: multiple battery banks

    lazy_q,

    My situation is similar to yours. However, my inverter is also a 3 stage charger with a built in transfer switch. The breaker to my stock converter (Elixir EL-25) is switched off so it doesn't run from the inverter. I have pulled the stock battery (your bank 1) as well.

    Original plans (which may still be implemented) included wiring the converter to shore power ahead of the transfer switch (so that it can't run off the inverter) and installing a switch or relay so that the DC panel (and hence loads) are connected to the 400AH bank when boondocking or to the converter while on shore power or under tow. In this set up I would reinstall the single stock battery.

    See the My Progress link in my sig.

    K
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: multiple battery banks
    lazy_q wrote: »
    Questions:
    1. Do you see any issue with the separation of battery bank 1 and 2.

    The only immediate issue I see is that when boondocking, RV furnaces are famous for draining down aux batteries in a hurry. I would rewire so that the furnace runs from the bigger bank.
    2. Do I need an isolator between the battery banks?

    Not unless they were being charged from the same source at the same time. I.e., engine alternator.

    While on that subject, there is a very good document here that argues strongly and convincingly *against* diode type isolators:

    http://www.smartgauge.co.uk/diodes.html
    3. Would I be better off merely tossing out the original battery and putting the dc load on battery bank 2 (4 Trojans)? Am I missing something?

    Probably not better off. If the original is capable of running lights and water pump and whatnot on it's own, then using it can't hurt. Again, I would probably move some loads (furnace for sure) to the bigger bank.

    The only real problem with keeping the original is that there will be significantly more wiring complexity. It would be much simpler all around to only have to deal with one aux bank.
    4. If I toss out the original battery does the stock converter present any problem with regard to its charging profile “13.2 Vdc range “float” mode, 13.6 Vdc range “absorption” mode, and a 14.4 Vdc range “bulk” charge mode”. I realize it will not perform as well as the tri-star.

    It should be fine. It's a multi-stage charger and voltages that you mentioned are right in the ballpark for a flooded lead-acid like the T-105s. It might not be the best if you were going with Gel batteries since they are sensitive to charging voltages and usually specify a bit lower than regular batteries.

    Also, at 55a it should be big enough for a bank of 4 T-105s.
    5. If I connect battery bank 1 to the solar charge controller (essentially putting it in parallel with battery bank 2) will that create any problem. I understand that it will increase the time required to fully recharge battery bank 2 assuming that both banks were 50% discharged.

    Yes, that could be a problem. As already mentioned, it's best not to mix battery types or voltages.

    It *can* be done, under certain circumstances, such as charging an engine start battery and a deep cycle aux battery from an engine alternator using a rig such as the SmartGuage split-charge relay.

    http://www.smartgauge.co.uk/

    (BTW, the entire SmartGuage web site technical info section is a must read for anyone mucking about with batteries and chargers and whatnot - MOST especially when mucking about with vehicular systems.)

    Doing it from a single MPPT controller is probably not a good idea at all.

    There is a solar charge controller designed to do two battery banks, the SunSaver Duo, but that's not an MPPT unit.

    It is certainly possible to connect two solar charge controllers to one set of PV panels - and it's been done often enough that it can't even really be called "uncommon". With PWM controllers it wouldn't be a problem - but I'm not an engineer and can't say how multiple MPPT controllers would behave.

    It maybe be possible to use the MPPT for the large bank, and also a small PWM for the single battery - again, I have to leave it to the engineers (plenty around here) to address that question.
  • lazy_qlazy_q Registered Users Posts: 9
    Re: multiple battery banks

    First of all: Thanks to you all for your insights.

    Power green – I was not aware of the 15% rule of thumb – thanks

    Kamala – Enjoyed looking at your project. Looks pretty compact. Interesting to note that I am not the only person thinking about this odd configuration. One thought – it looks like you are using wet cells, and I believe that you mentioned that they were under your bunk within the confines of the camper. I am sure that you are aware of this but just in case – when the flooded cells charge they may produce gases. You might consider enclosing the batteries and venting them to the outside just to be on the safe side.

    Dwh – thanks for the web sites and the detailed analysis.
    a. Great point regarding the RV furnace. I have a power distribution block wired into the second bank and so I can rewire the furnace to that point.
    b. Agree about diode based isolators. Had not considered that different banks would present different charging demand that would not be handled properly. I thought if the banks were in parallel that they would each draw what they needed. Having said that I realize that there is no way that the charge controller could figure out when to change stages i.e. bulk to absorption or float properly.
    c. Moving the furnace load is only a slight additional complexity. Charging both banks at the same time creates complexity.
    d. My concern about the converter arises from the Trojan website requirements for charging their batteries:

    Charger Voltage Setting 6V 12V 24V 36V 48V
    Daily Charge 7.4 14.8 29.6 44.4 59.2
    Float 6.6 13.2 26.4 39.6 52.8
    Equalize 7.8 15.5 31.0 46.5 62.0

    e. My charge controller (TriStar) is a PWM, but I do not see a way to individually sense each bank. The TriStar does not have the intelligence to control the charge voltage or the charge time for the different battery banks. I believe you are correct in that I could use the same panels to charge both banks but would need a separate charge controller for each bank. Its probably not worth the cost of the second controller and all that is implied with wiring it into the system for the slight advantage of having the lights and the water pump on a second battery bank.

    Once again thanks for you time and advice.

    ron q
  • KamalaKamala Solar Expert Posts: 452 ✭✭
    Re: multiple battery banks
    lazy_q wrote: »
    ... it looks like you are using wet cells ...

    You can't judge a battery by looking at its picture.;) They're AGMs.
  • dwhdwh Solar Expert Posts: 1,341 ✭✭✭
    Re: multiple battery banks
    lazy_q wrote: »
    d. My concern about the converter arises from the Trojan website requirements for charging their batteries:

    Charger Voltage Setting 6V 12V 24V 36V 48V
    Daily Charge 7.4 14.8 29.6 44.4 59.2
    Float 6.6 13.2 26.4 39.6 52.8
    Equalize 7.8 15.5 31.0 46.5 62.0

    According to your OP your converter/charger does “13.2 Vdc range “float” mode, 13.6 Vdc range “absorption” mode, and a 14.4 Vdc range “bulk” charge mode”

    The difference between 14.4 (your charger's bulk) and 14.8 (Trojan's specs) isn't very significant - either one will eventually push your battery up to the float point of 13.2v. I would probably interpret the Trojan specs as "Max Bulk Voltage" rather than Min - i.e., don't go over 14.8 on the bulk stage.

    [EDIT: Having slept on it, I came up with a clearer way to say the same thing; The battery's specs allow you to use UP TO 14.8v on the bulk stage - so 14.4v is enough to charge the battery, but doesn't go over the 14.8v limit.]

    The Trojan specs you quoted don't specify an absorption stage, but still, you won't hurt the battery with 13.6v for a while.

    And the final Float stage is right on the money, 13.2 specced by Trojan, and 13.2 from the charger.

    Good to go...almost.

    You also have to consider the C/* rate in making sure that you have enough amperage available for whatever size your battery bank is. I think 55a is probably enough, but I haven't bothered to figure out exactly what your bank's ah rating is or what those particular batteries like for a C/* rate.
  • brulazbrulaz Solar Expert Posts: 31 ✭✭
    Re: multiple battery banks
    dwh wrote: »
    According to your OP your converter/charger does “13.2 Vdc range “float” mode, 13.6 Vdc range “absorption” mode, and a 14.4 Vdc range “bulk” charge mode”

    The difference between 14.4 (your charger's bulk) and 14.8 (Trojan's specs) isn't very significant - either one will eventually push your battery up to the float point of 13.2v. I would probably interpret the Trojan specs as "Max Bulk Voltage" rather than Min - i.e., don't go over 14.8 on the bulk stage.

    [EDIT: Having slept on it, I came up with a clearer way to say the same thing; The battery's specs allow you to use UP TO 14.8v on the bulk stage - so 14.4v is enough to charge the battery, but doesn't go over the 14.8v limit.]

    The Trojan specs you quoted don't specify an absorption stage, but still, you won't hurt the battery with 13.6v for a while.

    And the final Float stage is right on the money, 13.2 specced by Trojan, and 13.2 from the charger.

    Old thread, but I'm in almost exactly the same situation. Same WFCO charger/converter, US Battery golf carts (US-2200xc) with similar charging specs to the Trojans.

    About this: "you won't hurt the battery with 13.6v for a while". What if we live in our trailer for a week or more with 120V. The WFCO converter will sit at 13.6V ... any potential battery problems? Or should we disconnect the batteries when on 120V for an extended time ... more than 2-3 days or ... ?
  • brulazbrulaz Solar Expert Posts: 31 ✭✭
    Re: multiple battery banks
    brulaz wrote: »
    Or should we disconnect the batteries when on 120V for an extended time ... more than 2-3 days or ... ?

    Well, guess we'll do this when on shore power for more than a couple of days... just for peace of mind.
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