Split Voltage

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.
Thanks,
Alan
Comments
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:
http://www.solarelectricityhandbook.com/solar-irradiance.html
Montreal
Measured in kWh/m2/day onto a solar panel set at a 59° angle from vertical:Average Solar Insolation figures
(Optimal summer settings)
- 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).
-Bill
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.
-Bill
Have a nice day. George
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