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RV solar






#1
So, trying to size a small starter system. The one thing I don’t understand is the charge in amps. Harbor freight has one that says something like 6 amp output. Now is that per hour, or what?
 

MAC702

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#2
Current is an instantaneous measurement. It can give you 6A of current flow whenever you ask for it, depending on if it is receiving sufficient input solar energy at the time.

Battery delivery is often in amp-hours because of their storage size. Battery lasts twice as long when halving the amperage draw.
 
#3
Got that. I’ve been in the trade nearly 40 years, but this new fangled solar is new to me. Does the 6 amp claim mean I can use 6 amps per hour and the panels will replace that hourly ( of course not including cloudy day ), or is it something different. I only have (2) 81 AH batteries, so I’m trying to figure it out for now
 

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#4
The 6 amps means that in its best conditions, it can provide 6 amps. Meaning at that moment in time, there is 6 amps of current flowing.

Amps per hour is not a thing. Amp-hours per hour is.

You'll need to know the charge rate your batteries can take or what you wish to provide them with. You may need a regulator for trickle charging, which is usually less than 2 amps.
 

Bumper

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#5
The more definitive parameter, normally used, is watts (which is amps X voltage). 6 amp output means that's the maximum current output with the panel in full sun and the leads shorted together. The panel will not provide that much charging current when connected to a 12 volt battery. However, with all but the smallest solar panels, you would still normally want to use a "solar charge regulator" to avoid overcharging the battery.

The panels HF sells are typically amorphous silicon, these will be larger and less efficient, for the same output, than either polycrystalline or better still, monocrystalline panels. The only real advantage of the amorphous silicone panels is that partial shade doesn't shut them down completely as it can with the others.

Bottom line, you can get better deals on-line than at HF.

If you just want a small panel to keep a battery topped up, you can buy a small 5 watt panel to use as a trickle charger (no charge regulator needed). If you are looking for a medium size system, the rule of thumb is one watt of solar for each amp hour of battery capacity - this would be for an actively used system like that in my RV that has 400 watts of solar panels.
 

Mikeee54

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#7
What are you trying to accomplish with the solar on your rv? That would help on recommending sizing. Harbor frieght new kit is 100watt I believe. You could probably buy a 100w panel and charge controller for the same and have better quality and room for expansion. But 100w isn't much...
 
#8
Trying to figure this. If I use “X” amount of watts per day, how many watts of panel do I need to completely recharge. I guess that makes more sense than the way I previously stated 🧐
What are you trying to accomplish with the solar on your rv? That would help on recommending sizing. Harbor frieght new kit is 100watt I believe. You could probably buy a 100w panel and charge controller for the same and have better quality and room for expansion. But 100w isn't muc
 

Mikeee54

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#9
If you get 5 hours of GOOD sun in a day a 100 watt panel produces 500 watts. So if you are pulling x watts out of batteries, hours of sun at your location (no clouds or other obstructions) x watts per panel needed to replace consumption....
 

Fogie

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#11
A big 325-335 watt, 24 volt panel is usually on Craiglist for around $125...quality residential panels. An overkill 40 amp rated MPPT Epever Tracer charge controller on Amazon for around $135...enough for most RVs, would be considered a "small cabin system." Will produce a lotta amperage, enough to run forced air heaters, 12v fridges, water pumps and lights within reason, while keeping batts charged, almost all the time/weather.
An MPPT controller will reduce the voltage, while increasing amperage..keeping wattage nearly (98% or so) the same.
Some PWM controllers can decrease the voltage, but will consume/lose up to half the wattage doing so...not all PWM controllers can reduce the 24v to 12v, so MPPT is the way to go.
 
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Mikeee54

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#12
A big 325-335 watt, 24 volt panel is usually on Craiglist for around $125...quality residential panels. An overkill 40 amp rated MPPT Epever Tracer charge controller on Amazon for around $135...enough for most RVs, would be considered a "small cabin system."
An MPPT controller will reduce the voltage, while increasing amperage..keeping wattage nearly (98% or so) the same.
Some PWM controllers can decrease the voltage, but will consume/lose up to half the wattage doing so...not all PWM controllers can reduce the 24v to 12v, so MPPT is the way to go.
Thats what I would do....
 

Fogie

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#13
Thats what I would do....
yep, what we have been doing. My system was done before the big panels became a dime a dozen, so is 100 watt, 12v panels wired in parallel...way more expensive (5 or so years ago) and much more conglomerated wiring, fuses and such.
Also went 6v batts in series, a huge improvement and truly deep cycle vs the two 12s in parallel that preceded them...so much deeper, cool running and trouble free.
If anybody wants to run microwaves, coffee makers and other high draw appliances, four 6v, 110 amp hour batts are minimum. I don't, so two run the ARB fridge/freezer, lights and pump just fine.
 

Mikeee54

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#14
I am building a system for our property. Some great deals on leftovers from home systems. Ive done several systems but this is my first mppt wow what a difference much better. batteries are pricey especially when you don't have cores ugh.
 

Fogie

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#15
I am building a system for our property. Some great deals on leftovers from home systems. Ive done several systems but this is my first mppt wow what a difference much better. batteries are pricey especially when you don't have cores ugh.
still have a simple PWM on mine and it works ok since mine is 12v to 12v, have been thinking about MPPT for it's other advantages though. The system def keeps up with my usage, can run the freezer all night after putting in a warm antelope and by 10am..antelope nearly frozen and batts full charge.
 

Bumper

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#17
I know folks here have good intentions, but there's some misinformation in some of the preceding posts.
Not trying to be a wisea$$, but I've been in electronics since the Navy trained me as an ET in 64, I'm a licensed electrician (though I sure wouldn't want to live in a house I wired. :c(

Okay. First of all batteries. If you are using Pb (lead acid), for longevity you don't want to discharge below 50%, best life is to stay above 80%, but that's unrealistic, so we'll use 50%. If you have a fairly new 12 volt 100 amp hour battery, discharging by 1/2 gives you 600 watts of power (the formula is P=IE, where P is power in watts, I is current in amps, and E is voltage) - in this example you can draw 5 amps for 10 hours or 2.5 amps for 20 hours.

The above is simplistic and ignores lots of variables such as temperature, and the fact that a given battery will deliver power more efficiently at lower discharge rates, etc.

MPPT chargers are better in many applications, but not for the reasons stated. Consider that a PV panel is more a constant current device. EOC (voltage open circuit) means little when you connect a battery to it, as the battery will clamp the PV panel voltage to the battery voltage. Say the panel open circuit voltage is 20 volts with 10 amps (200 watt panel). If you use a PWM (pulse width modulating) or other simple switching panel, to a battery that has discharged to 10 volts (to keep the math simple), the panel voltage out will drop to 10 volts and the current will stay roughly at 10 amps, so now we have lost half the panel's output charging potential. The beauty of MPPT (maximum power point tracking) charge controllers is that they use a DC to DC switching circuit to "trade" the panel's higher voltage for more output current than the panel alone can provide. Typical boost will be as much as 30% more power (amps) available to charge the battery.

And again, a rule of thumb is 1 watt of solar panel for each amp hour of battery capacity. In my RV I have 400 watts of PV panels and 450 amp hours of battery capacity in two banks. I might use 80 amp hours evening and overnight, and with a reasonably sunny day that will be replaced by 10:30 to 12:00. I'll see 25 amps up to 30 max (the limit of my MPPT charge controller).

Note that my panels are flat on the roof (with air space) and I don't go up there to tilt them directly towards the sun - that costs me something like 17% efficiency, but adding a panel or two is easier than worrying about which way the panels face. For a home, of course, the panels want to be tilted according to your latitude. Some even have their panels track the sun's arc - - but again, often cheaper just to add panels and keep it simple.

I didn't speak to PWM or (pulse width modulating) charge controllers. They control the charging current by electronically switching the output on and off rapidly - the on time will determine the rate of charge so that the apparent charge rate can be tapered off to be kinder to the batteries (for lack of a better description) instead of the simple on and off of a simple switching controller.

best,

bumper
 

Fogie

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#18
I know folks here have good intentions, but there's some misinformation in some of the preceding posts.
Not trying to be a wisea$$, but I've been in electronics since the Navy trained me as an ET in 64, I'm a licensed electrician (though I sure wouldn't want to live in a house I wired. :c(

Okay. First of all batteries. If you are using Pb (lead acid), for longevity you don't want to discharge below 50%, best life is to stay above 80%, but that's unrealistic, so we'll use 50%. If you have a fairly new 12 volt 100 amp hour battery, discharging by 1/2 gives you 600 watts of power (the formula is P=IE, where P is power in watts, I is current in amps, and E is voltage) - in this example you can draw 5 amps for 10 hours or 2.5 amps for 20 hours.

The above is simplistic and ignores lots of variables such as temperature, and the fact that a given battery will deliver power more efficiently at lower discharge rates, etc.

MPPT chargers are better in many applications, but not for the reasons stated. Consider that a PV panel is more a constant current device. EOC (voltage open circuit) means little when you connect a battery to it, as the battery will clamp the PV panel voltage to the battery voltage. Say the panel open circuit voltage is 20 volts with 10 amps (200 watt panel). If you use a PWM (pulse width modulating) or other simple switching panel, to a battery that has discharged to 10 volts (to keep the math simple), the panel voltage out will drop to 10 volts and the current will stay roughly at 10 amps, so now we have lost half the panel's output charging potential. The beauty of MPPT (maximum power point tracking) charge controllers is that they use a DC to DC switching circuit to "trade" the panel's higher voltage for more output current than the panel alone can provide. Typical boost will be as much as 30% more power (amps) available to charge the battery.

And again, a rule of thumb is 1 watt of solar panel for each amp hour of battery capacity. In my RV I have 400 watts of PV panels and 450 amp hours of battery capacity in two banks. I might use 80 amp hours evening and overnight, and with a reasonably sunny day that will be replaced by 10:30 to 12:00. I'll see 25 amps up to 30 max (the limit of my MPPT charge controller).

Note that my panels are flat on the roof (with air space) and I don't go up there to tilt them directly towards the sun - that costs me something like 17% efficiency, but adding a panel or two is easier than worrying about which way the panels face. For a home, of course, the panels want to be tilted according to your latitude. Some even have their panels track the sun's arc - - but again, often cheaper just to add panels and keep it simple.

I didn't speak to PWM or (pulse width modulating) charge controllers. They control the charging current by electronically switching the output on and off rapidly - the on time will determine the rate of charge so that the apparent charge rate can be tapered off to be kinder to the batteries (for lack of a better description) instead of the simple on and off of a simple switching controller.

best,

bumper
most PWM controllers will not accept higher voltages, will not charge 12v batts from a 24v panel..certain models will but at a cost concerning amperage. As of late, controllers of both types will not attempt to charge a battery that has been discharged below a certain point, also.
All recommend that batteries are fully charged before attaching line in from the controller, the more advanced read the bank capacity and voltage at the time of connection, measure panel output regularly and choose the parameters for charging automatically.
MPPT's offer extras such as de-sulphating schedules, the option of using 24v or higher panels (which are much cheaper per watt than 12v panels) without a substantial penalty concerning amperage/voltage conversion, bluetooth pairing and device enabled monitoring and precise over-charge protection. So, I am thinking of upgrading.
No controller can up the amperage available from a given panel, some are more efficient at converting the over-voltage to output of amperage. None that I know of can deliver 100% of a panels wattage after reducing input from 24 or higher to 12, of course. The best advertise 98% efficiency. Ideally, matching panel voltage to battery voltage is most efficient..but the cost savings realized by purchasing higher voltage panels is worth the 2% loss in amperage during conversion. 24 to MPPT to 12 has become the standard because of those savings.
 
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