Split Voltage

Allan_RAllan_R Registered Users Posts: 3
edited September 2021 in Solar Beginners Corner #1
I currently have a 20 foot Pontoon boat that I use on a small lake as transportation to and from my remote cottage.  The lake rules are, no gas motors and a less than 10 HP limit.  Currently I am using 2 transom mounted Minn Kota Endura 12 volt trolling motors. 2 @ 55 lbs thrust giving me a total thrust of 110 lbs.  These motors are connected to 2 pairs of 6 volt 232 amp hour deep cycle batteries, connected in series to get 12 volts on each bank.  The batteries are constantly under charge using a 55 watt 12 volt solar panel and Morningstar Duo charge controller which over a week is sufficient to keep the batteries fully charged.  This is necessary because the cottage is off grid and I do not have access to 120 Volt electricity at the lake.
I am finding that the motors are not powerful enough and want to upgrade to 2 80lbs thrust motors which run at 24 volts.  My question is, can I connect the 2 12 volt banks in series to get the 24 volts I need for the motors and still charge the 2 banks at 12 volts?
I do understand that it would be best to upgrade the entire system to 24 volts but that would increase the cost which is not feasible at the moment.


  • BB.BB. Super Moderators, Administrators Posts: 32,597 admin
    Welcome to the forum Alan.

    With ~70% more thrust--Do you have enough battery bank capacity to make your round trip (or possibly several trips over a weekend) between charges...

    Just as a guess... Looking at 56 amps per motor. Discharging to 50% capacity of bank:
    • 56 Amps * 2 = 112 Amp draw
    • 4x GC batteries in series = 24 volt @ 232 AH
    • 232 AH * 0.50 max planned discharge = 116  AH working capacity
    • 116 AH working capacity / 112 Amp draw (max power) = 1.04 hours per charge of operational time
    So--First question is does this work for you (~1 hour of operation between weekly charging)?

    Next, yes, you can wire up the 4 batteries in series--Run 24 volts, then wire them back to 12 volts to charge--That will get "old" over time. Do you have other 12 volt loads for the boat (splitting off 12 volts from a 24 volt bank is not great--It causes the low vs high batteries to discharge at different rates and unbalances them--May be less of an issue if you plan recharging at 12 volts during the week).

    I suggest that you may want to review your energy usage and solar charging... Guessing at Montreal Canada, summer usage, fixed array facing south:


    Average Solar Insolation figures

    Measured in kWh/m2/day onto a solar panel set at a 59° angle from vertical:
    (Optimal summer settings)

    Say May through August or over 5.0 hours (long term average) hours of sun per day:
    • 55 Watt solar panel / 17.5 volts Vmp = 3.14 Amps Imp (rated charging current) @ 12 volt bank
    • 3.14 Amps * 5.0 hours of sun per day * 5 days (weekday) charging = 78.5 AH per week average charging (ignoring 20% charging losses)
    Depending on your "real" weekend AH usage--This 55 Watt panel may not be enough for your needs...

    Typically, suggest 5% minimum charging current (10%-13% typical):
    • 24 volts * 232 AH * 1/0.77 panel+controller deratings * 0.05 rate of charge = 362 Watt array minimum
    • 24 volts * 232 AH * 1/0.77 panel+controller deratings * 0.10 rate of charge = 723 Watt array nominal
    Just from what is "optimal" for the battery bank (larger battery banks need larger solar array), a much larger array would be better (for longer battery life). As well as providing more energy for your weekend usage. Highly suggest a minimum of 5% rate of charge... And 10% is recommended by flooded cell lead acid battery mfg. for best battery life.

    If you go with a larger array and/or 24 volt charging--You need to make sure you always connect the battery power to the charge controller first, then the solar array. And disconnect solar first, then the disconnect the battery bank to charge controller. If you have solar power and no battery bank, the charge controller can get "confused" (not boot correctly, or make a 12 vs 24 volt battery bank configuration mistake) or even be damaged (lots of variables here--Just generic warning).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Allan_RAllan_R Registered Users Posts: 3
    edited September 2021 #3
    Hi Bill,
    Thanks for the info.  Will take me some time to digest for sure.
    With the current setup the average duration of a trip is 30 minutes and seem to be able to get between 2 and 3 one way trips.  On very windy days I can actually get 4 trips with the wind behind each time but only 1 with a strong head wind.  As you can see the wind on the lake is very strange as I can start a trip with a head wind and halfway through it can do a 180 deg. flip. That is the problem with being under powered.  I tried adding a 35 lbs thrust motor on the bow which improved the overall performance, especially into the wind.  That is why I think the 2 80s will give me a trade off between draw and run time.
    So to answer your first question, yes I think it will be enough.
    To answer your second question, currently only the motors draw power.  However, in the future I will be adding an electric tilt mechanism for the motors as I am getting too old to do it manually leaning over the back of the boat.
    I'm not sure if I was clear but the entire setup is mounted on the boat with the panel and charge controller mounted on the back over the motors.  When parked the panel always faces due south at around 35 degrees.  The boat is only in the water from May to end of September.
    Finally after a quick read through I get the sense that no it is not a good idea to do what I asked.  Additionally I also get the sense that the way I am running things now is not so good either.  I currently have one bank feeding power to both motors and then if needed or on the next trip switch to the second bank.  I also have the option, through a switch, to have both battery banks feed the motors.  I suspect now that the second option is the only way to go.
    Once again thanks for the detailed info.
  • BB.BB. Super Moderators, Administrators Posts: 32,597 admin
    edited September 2021 #4
    The good thing with putting batteries in parallel--It is easier on the batteries. 1/2 the current draw, and 1/2 the depth of discharge.

    Bad side, with A/B switch--If A "goes dead" and you need to travel--Switch to "B"... With A+B, if your bank goes dead, you are stranded until they get some charge back in them (it could take one day or two sunny days, at least, to get enough charge to make the trip from a "dead" bank).

    Good the panel is mounted to the boat--No charger/battery switching issues. Solar panels are getting pretty cheap these days... A 300 Watt panel can cost you something like $150-$300 (at least south of the boarder) if you shop around (get a Vmp~35-40 volt panel for charging a 24 volt battery bank--Not a Vmp~30 volt panel) if you look around (watch for packing & shipping costs--Can cost a lot to send a single panel).

    The smaller 150 Watt and less "12 volt" (Vmp~17.5 volt Vmp) panels usually cost a lot more $$/Watt.

    If you go 24 volts--You would need another charge controller.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • geosierranevadageosierranevada Registered Users Posts: 54 ✭✭✭
    Why not have a small gas generator on board with a 24 volt charger. The good ones are quiet and you could use it at your off grid cabin.
    Have a nice day.   George
    Off grid for 20 years. 5KW panels, Trace sw4024, Bergy XL1 wind generator, 3 EG4 Lifep04 200amphr batteries  3 SQF 2 well pump, 12kw back up gen.  Not easy living in the wilderness but it keeps you busy
  • Ralph DayRalph Day Solar Expert Posts: 975 ✭✭✭✭
    Does "no gas motors" include generators?  Sounds like it might.  Tough crowd.
  • Allan_RAllan_R Registered Users Posts: 3
    To George and Ralph, We are trying to reduce the noise as much as possible on the lake so electric motors only on the lake.  I have a 4500 watt generator at the cottage to augment my solar array for the cottage but try not to use it for any prolonged periods unless absolutely necessary.
    I will take all the info and suggestions provided and see what I can come up with as a layout and capacity.  If I add additional or larger panels I need to come up with a mount that will allow me to lower the panels to reduce wind resistance.  I'll have all winter to figure this out.

    Thanks to all for your input.
  • mike95490mike95490 Solar Expert Posts: 9,571 ✭✭✭✭✭
    My uncle had a hydrofoil boat with electric motor, it made lots of noise from the prop !!
    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 ,

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