help needed wanting keep house batterys on boat charged while in port

[nath on the lath]

new to solar all help would be appreciated. right what i have as a system on board my boat is a house bank consisting of 8x 6 volt at (20hr ap 233) set up as 24 volt system which is generally charge by a 110 amp alternator on the main engine. can also be charged up using the invert-er if i can get shore power which is harder said then done. so in order of keeping the charge up while the boat is sitting idle at the dock i thought i'ld look into solar. i was thinking about 2 x 40 watt panels will this produce enough power to charge and maintain the system? what size charge controller would be required?
cheers Nathan

[Sunking]

Easy peazy. The size assuming you will be using a PWM charger is something larger than the panels Imp spec. A 12 volt 40 watt panel Imp is only 2.5 amps at most. If the controller is MPPT: Amps = Panel Wattage / Battery Voltage, or 80 watts / 24 volts = 3.4 amps.

Makes no effing difference, look for a 5 amp controller if you can find one that small for 24 volts. More likely 10 to 15 amps is the smallest you are going to find.

[nath on the lath]

Easy peazy. The size assuming you will be using a PWM charger is something larger than the panels Imp spec. A 12 volt 40 watt panel Imp is only 2.5 amps at most. If the controller is MPPT: Amps = Panel Wattage / Battery Voltage, or 80 watts / 24 volts = 3.4 amps.

Make no effing difference, look for a 5 amp controler if you can find a oty that small for 24 volts. More likely 10 to 15 amps is the smallest you are going to find.

excuse my novice knowledge at best what is ment by using a PWM charger? i was looking at getting a MPPT controller which seems easy to get in 10 amp. i will still need to run a fuse before power goes to the battery?
thanks for the reply

[Sunking]

PWM = Pulse Width Modulation = Cheap. Size fuse 50% larger than controller on the battery post, not controller.

[nath on the lath]

roger with you now so if it's a 10 amp controller 15 amp fuse?
also correct me if i'm wrong if i'm getting 3.4 amps from my panels that would mean i could draw say 3 amps from the batterys for say 4 hours a day and if i were to get atleast 4 hours of good sunlight a day batterys would stay good.

[nath on the lath]

Easy peazy. The size assuming you will be using a PWM charger is something larger than the panels Imp spec. A 12 volt 40 watt panel Imp is only 2.5 amps at most. If the controller is MPPT: Amps = Panel Wattage / Battery Voltage, or 80 watts / 24 volts = 3.4 amps.

Makes no effing difference, look for a 5 amp controller if you can find one that small for 24 volts. More likely 10 to 15 amps is the smallest you are going to find.

thank's Sunking for your time
i've been reading though other threads and in one of them you where saying about a electronic battery isolator using in a rv for the house batterys. is this so the alternator and solar pannels are not trying to charge at the same time? would i need one of these

[Sunking]

Thank's Sunking for your time.

You are welcome.

I've been reading though other threads and in one of them you where saying about a electronic battery isolator using in a rv for the house batterys. is this so the alternator and solar pannels are not trying to charge at the same time? would i need one of these.

No quite the opposite in fact. It isolates the House batteries from the SLI battery (starting lighting & igntion) battery. It prevents your SLI from being discharged by house loads when engine is off. I allows the alternator to charge both House and SLI batteries when the engine is running. So the house batteries can be charged by both the Alternator and solar at the same time. Keep in mind sola ris a soft poor charging source in cycle applications not really capable of fully recharging the batteries in ever day use. Alternator however can do it very quickly. Bonus is if the house batteries get low in the day while parked or docked, start the engine to recharge quickly.

Some Isolators even allow you to bridge the house and SLI battery to get you out of a jam when the SLI battery is dead so you can start the engine and get recharged. Basically the alternator when sized properly can do more work in an hour solar two, than a roof full of panels can generate in a week. Solar in the application is really just a fuel save and supplements power, but not the primary source.

[Sunking]

roger with you now so if it's a 10 amp controller 15 amp fuse?

Basically yes. Catch is fuses are based on wire size as a fuse is to protect wiring. So as long as the wire can safely handle the fault current you got the idea. Use 12 AWG or larger and you are good to go with a 15 amp fuse.

also correct me if i'm wrong if i'm getting 3.4 amps from my panels that would mean i could draw say 3 amps from the batterys for say 4 hours a day and if i were to get atleast 4 hours of good sunlight a day batteries would stay good.

No does not work that way. The panels can only generate maximum power for a few brief minutes at solar noon, and no panel produces it rated maximum current.

What we work with is Sun Hours in a day. Do not confuse Sun Hours with how many hours of sun light you get. As a rule of thumb to wrap your brain around it is for every 3 hours of sun light = 1 Sun Hour. On average across the USA Summer time Sun Hours are around 4 to 6 Sun Hours, and winter around 2 to 4 Sun Hours. Place like Seatle in Winter recieve 1.2 Sun Hours.

So if you have a panel that specs 2.5 amps at maximum power with a 4 Sun Hour day generates 2.5 amps x 4 Hours = 10 AH. But there is another catch, efficiency. For a battery about 80% which turns the 10 AH out of the panels = 8 AH usable in a 24 hour period.

[nath on the lath]

You are welcome.



No quite the opposite in fact. It isolates the House batteries from the SLI battery (starting lighting & igntion) battery. It prevents your SLI from being discharged by house loads when engine is off. I allows the alternator to charge both House and SLI batteries when the engine is running. So the house batteries can be charged by both the Alternator and solar at the same time. Keep in mind sola ris a soft poor charging source in cycle applications not really capable of fully recharging the batteries in ever day use. Alternator however can do it very quickly. Bonus is if the house batteries get low in the day while parked or docked, start the engine to recharge quickly.

Some Isolators even allow you to bridge the house and SLI battery to get you out of a jam when the SLI battery is dead so you can start the engine and get recharged. Basically the alternator when sized properly can do more work in an hour solar two, than a roof full of panels can generate in a week. Solar in the application is really just a fuel save and supplements power, but not the primary source.

thanks for all the info it's helping me learn the system that i have got as my alt charges both set of batterys so somewhere in my engine room must be a isolater. there is also abilty to bridge the banks in order to start the boat if starting bank is down. i'm really looking ford to this project after all the reading i've done today i think i've got my head around it all. the only thing that runs off the battery bank while in port is the automatic bilge pump with draws 6amps and goes all of 10 seconds every half hour to hour so hopefully i can make up for some of that and the other factors. just trying to get the most out of my batterys
once again thank you for your time and knowledge it has been helpful

[littleharbor]

If I might add... Be careful when buying a 10 amp MPPT controller. There are a lot of SO CALLED MPPT controllers out there that aren't true MPPT. One red flag would be price. Any real MPPT controller isn't going to be selling for 20 - 50 USD. A 10 amp MPPT controller is actually kind of odd although not unheard of. Make sure it has a VOC limit in the 100 to 150 volt range.

[Amy@altE]

Outback Power has a 10A 12/24V MPPT charge controller called Smart Harvest. SCCM10-100

[nath on the lath]

If I might add... Be careful when buying a 10 amp MPPT controller. There are a lot of SO CALLED MPPT controllers out there that aren't true MPPT. One red flag would be price. Any real MPPT controller isn't going to be selling for 20 - 50 USD. A 10 amp MPPT controller is actually kind of odd although not unheard of. Make sure it has a VOC limit in the 100 to 150 volt range.

inertesting you say that i noticed when looking at MPPT controllers on ebay all the cheaper ones in the spec's said they charging mode is PWM i take it this is the type you are talking about that are not true MPPT controllers
thanks for your input

[littleharbor]

Exactly.

[Sunking]

thanks for all the info it's helping me learn the system that i have got as my alt charges both set of batterys so somewhere in my engine room must be a isolater. there is also abilty to bridge the banks in order to start the boat if starting bank is down.

Most likely it is mechanical and not electronic.