Noob needs help sizing for 24v battery bank charging.

Dave.s
Dave.s Registered Users Posts: 2
Hello! I'm new to the forum and have a slightly odd situation I'm hoping you experts can point me in the right direction, or tell me I'm dreaming. I have a battery bank in my S10 Blazer consisting of 2 Trojan 31xhs batts wired in series. I use them on weekends to charge lipo batteries for my rc helicopters using roughly half their capacity. I'm trying to determined if a low cost solar charging system is an option for me, but have no idea where to start. Like I said I use the battery bank on weekends and my suv sits in the sun all week. I live in an upper apartment and outside power for an AC charger is not readily available and hauling the batts up stairs is a pain. Ideally I would like to keep my budget under $200.00 and keep the panels inside the truck. Am I dreaming, or is this doable, or should I wire them in parallel to my alternator when not in use?
Thanks in advance!

Comments

  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Noob needs help sizing for 24v battery bank charging.

    OK... It looks like these are 12 volt @ 130 AH batteries.

    You run them at 24 volts--Lets see what a "24 volt" solar power system would look like.

    First, normally I would suggest 5% to 13% as being a "reasonable" solar rate of charge--You can go lower, but for best battery life, you would need to monitor their state of charge and probably plug them into a higher capacity charger once a month or so...

    Any, solar charging:
    • 24 volts * 130 AH * 1/0.77 panel+controller derate * 0.05 rate of charge = 203 Watt array minimum
    • 24 volts * 130 AH * 1/0.77 panel+controller derate * 0.10 rate of charge = 405 Watt array nominal
    • 24 volts * 130 AH * 1/0.77 panel+controller derate * 0.05 rate of charge = 527 Watt array "cost effective" maximum
    Obviously, you are looking for a low cost solution, so a pair of "12 volt" ~100 Watt panels would be a good start at a "standalone" solar charging system (note: "12 volt" panels have Vmp~17.5-18.6 volt ratings each).

    Using PV Watts for Milwaukee Wisconsin, array mounted flat to roof (OK for summer, but for winter, may be issues with low sun angle):
    Month    Solar Radiation (kWh/m 2/day)
    1      1.89     
    2      2.61     
    3      3.48     
    4      4.79     
    5      5.86     
    6      6.55     
    7      6.10     
    8      5.44     
    9      4.28     
    10      2.94     
    11      1.80     
    12      1.44     
    Year      3.94      
    
    Tossing the bottom three months, we get 2.61 hours of sun per day (February). Lets say you are really "hard" on your batteries and discharge them to 20% State of Charge (near dead):
    • 24 volts * 130 AH * 0.80 charging capacity = 2,496 Watt*Hours energy to recharge
    • 2,496 WH energy / (203 W array * 0.77 panel+controller eff * 2.61 hours sun per day) = 6.1 days to recharge
    If you only discharge the batteries by 50% (really better for longer battery life):
    • 24 volts * 130 AH * 0.50 charging capacity = 1,560 Watt*Hours energy to recharge
    • 1,560 WH energy / (203 W array * 0.77 panel+controller eff * 2.61 hours sun per day) = 3.8 days to recharge
    So--Just a back of the envelope calculation, if you could fit two solar panels on the top of your truck--It appears that you have the makings of a stand-alone recharging system that would work for ~9 months of the year.

    You would have to shop around to find ~100 Watt panels at $1 per Watt--Typically they are closer to $2 per Watt (plus a lot of money for shipping and handling sometimes--Always check the price to your front door).

    And a question if you can fit two ~100 watt panels on the top of your truck.

    Another way to go would be to recharge from your truck--But it may be better to use a 12 VDC to 24 VDC battery charger or a 120 VAC inverter to 12 VDC battery charger.

    Normally, car/truck alternators are not designed to recharge at the higher voltages that deep cycle batteries like to see (~14.75+ volts for flooded cell batteries at room temperatures)--And vehicle alternators, even if they are rated at 150 Amps, they tend to fold back on their output current as the alternators get hot.

    Also, how long is the drive? You are looking at 4-6 hours (or more) to recharge your battery bank if 50% discharged. Of course, you can do heavy charging for a couple of hours when driving, and let the solar array finish charge the next 3-5 days.

    Anyway--That is how I would start. How much did I guess right? Does this look like anything you can use?

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave.s
    Dave.s Registered Users Posts: 2
    Re: Noob needs help sizing for 24v battery bank charging.
    BB. wrote: »
    OK... It looks like these are 12 volt @ 130 AH batteries.

    You run them at 24 volts--Lets see what a "24 volt" solar power system would look like.

    First, normally I would suggest 5% to 13% as being a "reasonable" solar rate of charge--You can go lower, but for best battery life, you would need to monitor their state of charge and probably plug them into a higher capacity charger once a month or so...

    Any, solar charging:
    • 24 volts * 130 AH * 1/0.77 panel+controller derate * 0.05 rate of charge = 203 Watt array minimum
    • 24 volts * 130 AH * 1/0.77 panel+controller derate * 0.10 rate of charge = 405 Watt array nominal
    • 24 volts * 130 AH * 1/0.77 panel+controller derate * 0.05 rate of charge = 527 Watt array "cost effective" maximum
    Obviously, you are looking for a low cost solution, so a pair of "12 volt" ~100 Watt panels would be a good start at a "standalone" solar charging system (note: "12 volt" panels have Vmp~17.5-18.6 volt ratings each).

    Using PV Watts for Milwaukee Wisconsin, array mounted flat to roof (OK for summer, but for winter, may be issues with low sun angle):
    Month    Solar Radiation (kWh/m 2/day)
    1      1.89     
    2      2.61     
    3      3.48     
    4      4.79     
    5      5.86     
    6      6.55     
    7      6.10     
    8      5.44     
    9      4.28     
    10      2.94     
    11      1.80     
    12      1.44     
    Year      3.94      
    
    Tossing the bottom three months, we get 2.61 hours of sun per day (February). Lets say you are really "hard" on your batteries and discharge them to 20% State of Charge (near dead):
    • 24 volts * 130 AH * 0.80 charging capacity = 2,496 Watt*Hours energy to recharge
    • 2,496 WH energy / (203 W array * 0.77 panel+controller eff * 2.61 hours sun per day) = 6.1 days to recharge
    If you only discharge the batteries by 50% (really better for longer battery life):
    • 24 volts * 130 AH * 0.50 charging capacity = 1,560 Watt*Hours energy to recharge
    • 1,560 WH energy / (203 W array * 0.77 panel+controller eff * 2.61 hours sun per day) = 3.8 days to recharge
    So--Just a back of the envelope calculation, if you could fit two solar panels on the top of your truck--It appears that you have the makings of a stand-alone recharging system that would work for ~9 months of the year.

    You would have to shop around to find ~100 Watt panels at $1 per Watt--Typically they are closer to $2 per Watt (plus a lot of money for shipping and handling sometimes--Always check the price to your front door).

    And a question if you can fit two ~100 watt panels on the top of your truck.

    Another way to go would be to recharge from your truck--But it may be better to use a 12 VDC to 24 VDC battery charger or a 120 VAC inverter to 12 VDC battery charger.

    Normally, car/truck alternators are not designed to recharge at the higher voltages that deep cycle batteries like to see (~14.75+ volts for flooded cell batteries at room temperatures)--And vehicle alternators, even if they are rated at 150 Amps, they tend to fold back on their output current as the alternators get hot.

    Also, how long is the drive? You are looking at 4-6 hours (or more) to recharge your battery bank if 50% discharged. Of course, you can do heavy charging for a couple of hours when driving, and let the solar array finish charge the next 3-5 days.

    Anyway--That is how I would start. How much did I guess right? Does this look like anything you can use?

    -Bill

    Wow, thanks for such a detailed response! I'm west of Milwaukee so your sunlight calculations are on he spot. I only will be using these batteries spring through fall. Basically if the weather is over 50 F. Looking briefly at 100 watt panel dimensions and my small suv tells me stand alone solar probably will not be feasible, not to mention probable damage from hail or road debris. You did open my eyes to 12v to 24v chargers. I also was not aware of the limitations of alternators. Armed with this knowledge I'm going to investigate the 12 to 24 chargers some more and see if they put out enough during my 1 1/5 hour daily commute to get the job done. Or just keep hauling them up my stairs, glad I skipped the J185's!
    Thanks for taking the time to help educate me, I really appreciate it!
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: Noob needs help sizing for 24v battery bank charging.

    You are looking at around 5 hours a week of charging and 50% discharge:

    2*130 AH / 5 hours = 52+ from your vehicle alternator (26+ amps @ 24 volts)...

    That is a fair amount of current (plus you need to run the rest of the vehicle electrical).

    www.balmar.net makes (from what I understand) very good marine alternators with adjustable regulators. You might look at their offerings/solutions and see if this gives you any ideas.

    An alternative would be a Honda eu2000i genset + AC charger (run it during the weekend to keep your bank charged). Would only take a couple gallons of gasoline. They are not cheap (~$1,000), but will last you many years. And the AC charger can be used when parked overnight (on trips outside the hotel, etc.).

    Another option would be if you have a club house where you fly--Wire up your charging system there with solar (or park your batteries during the week).

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