On my Winnebago Journey, I have three Group 27 12 volt house batteries. I woud like to install some solar panels on the roof.
Using the recommended rule of thumb, I should use 50 watts of panel for each battery. So with what I have, I should only use 150 watts of panel.
Against the rule of thumb, I'd like to put 200 watts of panels (2 100 watt panels) on the roof now and use them with my current batteries, which are all new. Eventually, in a couple of years when we retire and hit the road full time, I will replace the 3 12 volt batteries with 4 6 volt batteries. I can fit 4 in the battery compartment.
My question is: can I use the 200 watt panels with a good charge controller and not damage the 12 volt batteries? I have thought about using smaller panels to cover the 150 watts vice 200 watts and add more panels later, but I'd like to limit the amount of small panels linked together and use fewer larger panels. I have an aversion to drilling more holes in the roof.
Best Regards!
bjoyce said
05:46 PM Jul 13, 2008
A good charge controller will handle most anything. Heliotrope, Blue Sky or Outback (the last is overkill for you) all all good brands with MPPT (gets you 5 to 30% boost depending on conditions), Xantrex is a good brand without MPPT. If you don't mind putting the controller by the batteries where you have to go out to read the numbers you can get a cheaper all-in-one unit like the Blue Sky 2000 or Heliotrope HPV-22 instead of a two piece unit with a separate display panel. Where did you get 50watts per battery from? Some home solar person? Five hours of effective power in winter at 50watts is 250watt-hours or 21 amp-hours of charging per panel so 200 watts will give you about 84 amp-hours of charging in the dead of winter in the Southwest in perfect conditions with tilted panels. Each group 27 should be 90-95 amp so a discharge of 50% is 45 amp-hours so three can be discharged to 135 amp-hours some of the time.
Old Snipe said
07:42 PM Jul 13, 2008
I knew I could count on BJoyce for some excellent information.
I found the rule of thumb of 50 watts per battery in an RV magazine article on solar panels. So a new question, is that wrong or correct?
Also, MPPT? Not familiar with that term.
So the bottom line, 200 watts of solar panels would charge the batteries in my MH in the dead of winter, facing south, with the panels tilted during perfect sunny conditions, or am I thinking wrong? Would that hurt my 12v batteries or should I wait until they are tired, replace them with 4 6v and then install the 200 watt panels?
Please forgive my obvious lack of understanding all this, it is a new area for me.
Thanks for the help!
bjoyce said
11:51 AM Jul 14, 2008
MPPT: Your solar panels put out about 16 to 17 volts DC and your batteries can only take 12.5 to 14.5 (unless it is cold) while charging. For example a 100 watt panel puts out 100watts at 16 volts or 6.25amps. Without MPPT if your panel is putting out 100 watts at 16 volts your batteries will get charged at (14/16) * 6.25 amps or 5.47 amps. With MPPT their is a transformer in the controller that gets you closer to the 6.25amps at 14 volts.
Fancy MPPT controllers, like the Outback MX-60, can handle input voltages of 60 volts and charge 12 volt batteries. People doing this wire their solar panels in series which has advantage of less power loss in the wiring and less problem with partially shaded panels.
A good controller does not overcharge your batteries, unless someone sets it wrong. You can have 1000 watts of solar trying to charge one group 27 battery and a good controller will discard the excess charge. This happens when you are on shore power in broad daylight, the solar panels electricity is being discarded. 200 watts of solar for three group 27s is not overkill in any way.
Solar can only charge the batteries if you don't run them down below what solar can make up. Notice that Howard and Linda use more than 200 amp-hours per day while dry camping. We also use more than 200 amp-hours per day while dry camping. Others seem to live much cheaper and only use 50 amp-hours per day. If you are using less than about 80 amp-hours a day of battery (which is less than a kilowatt of electricity) than you have a chance that 200 watts will charge your batteries given perfect conditions in winter.
Each RV seems to have its own power need. Some will us 3-5 amps per hour just sitting there with inverters off and others will only use 0.5 to 1 amps per hours. Turnng inverters on uses more power, for TVs, computers, microwaves, toasters, etc. You have to find out what works for you. Their are solar bigots out there that will insist you change what you eat, how you live, etc., to "do the right thing" and use as little electricity as you can. For them it is a religious issue. You have to find out what your usage is to determine your needs. A good thing is to add a battery monitor like a Trimetric or Xantrex, it will show you your usage.
Most RVs have limited roof space for solar panels and limited place to put batteries, plus those batteries and panels add weight and it all costs real money. You have to compromise somewhere.
Edit: I forgot to say anything about the 50 watts per battery. RV batteries run from about 75 (small) to about 240 amp-hours in size. 50 watts sounds like the person was refering to batteries at the small end only being discharged to 75 to 80% of capacity. If you boondock all the time you probably should size everything to only go down to 75 to 80%, but you have to know your usage so you can size things.
-- Edited by bjoyce at 12:58, 2008-07-14
-- Edited by bjoyce at 13:00, 2008-07-14
Old Snipe said
12:28 PM Jul 14, 2008
Wow, Bill! You really helped me learn some things. I appreciate you taking the time to answer my questions about solar.
So I'll get the best charge contoller I can get and not worry about too much power from the panels. I already have a factory installed 2000 watt inverter. I intend on only using the batteries to power the TV and other minor stuff at night while boondocking, so it looks like this is my solution to high diesel prices and not having to run the genny as much to re-charge the batteries.
Again, thank you for your help. You're a wealth of information!!!
Best Regards!
bjoyce said
01:59 PM Jul 14, 2008
It does take a while to wrap your head around how your 12 volt system and 110 volt system interact. Add the wierd way batteries work and all the numbers being slightly different with 12 volt DC and 110 volt AC and life gets even more interesting.
Solar works well for topping off batteries. Since batteries need to taper charge to fully charge you have to run a generator for a long time to finish the charge and the last bit the generator is barely working but still using fuel and making noise. Solar is quiet and doesn't care if it takes three hours to finish the charge.
dgorila1 said
07:50 PM Dec 9, 2008
Bill,
Can you recommend any good books on the subject that are easy to understand? Thanks.
Using the recommended rule of thumb, I should use 50 watts of panel for each battery. So with what I have, I should only use 150 watts of panel.
Against the rule of thumb, I'd like to put 200 watts of panels (2 100 watt panels) on the roof now and use them with my current batteries, which are all new. Eventually, in a couple of years when we retire and hit the road full time, I will replace the 3 12 volt batteries with 4 6 volt batteries. I can fit 4 in the battery compartment.
My question is: can I use the 200 watt panels with a good charge controller and not damage the 12 volt batteries? I have thought about using smaller panels to cover the 150 watts vice 200 watts and add more panels later, but I'd like to limit the amount of small panels linked together and use fewer larger panels. I have an aversion to drilling more holes in the roof.
Best Regards!
Where did you get 50watts per battery from? Some home solar person?
Five hours of effective power in winter at 50watts is 250watt-hours or 21 amp-hours of charging per panel so 200 watts will give you about 84 amp-hours of charging in the dead of winter in the Southwest in perfect conditions with tilted panels. Each group 27 should be 90-95 amp so a discharge of 50% is 45 amp-hours so three can be discharged to 135 amp-hours some of the time.
I found the rule of thumb of 50 watts per battery in an RV magazine article on solar panels. So a new question, is that wrong or correct?
Also, MPPT? Not familiar with that term.
So the bottom line, 200 watts of solar panels would charge the batteries in my MH in the dead of winter, facing south, with the panels tilted during perfect sunny conditions, or am I thinking wrong? Would that hurt my 12v batteries or should I wait until they are tired, replace them with 4 6v and then install the 200 watt panels?
Please forgive my obvious lack of understanding all this, it is a new area for me.
Thanks for the help!
Fancy MPPT controllers, like the Outback MX-60, can handle input voltages of 60 volts and charge 12 volt batteries. People doing this wire their solar panels in series which has advantage of less power loss in the wiring and less problem with partially shaded panels.
A good controller does not overcharge your batteries, unless someone sets it wrong. You can have 1000 watts of solar trying to charge one group 27 battery and a good controller will discard the excess charge. This happens when you are on shore power in broad daylight, the solar panels electricity is being discarded. 200 watts of solar for three group 27s is not overkill in any way.
Solar can only charge the batteries if you don't run them down below what solar can make up. Notice that Howard and Linda use more than 200 amp-hours per day while dry camping. We also use more than 200 amp-hours per day while dry camping. Others seem to live much cheaper and only use 50 amp-hours per day. If you are using less than about 80 amp-hours a day of battery (which is less than a kilowatt of electricity) than you have a chance that 200 watts will charge your batteries given perfect conditions in winter.
Each RV seems to have its own power need. Some will us 3-5 amps per hour just sitting there with inverters off and others will only use 0.5 to 1 amps per hours. Turnng inverters on uses more power, for TVs, computers, microwaves, toasters, etc. You have to find out what works for you. Their are solar bigots out there that will insist you change what you eat, how you live, etc., to "do the right thing" and use as little electricity as you can. For them it is a religious issue. You have to find out what your usage is to determine your needs. A good thing is to add a battery monitor like a Trimetric or Xantrex, it will show you your usage.
Most RVs have limited roof space for solar panels and limited place to put batteries, plus those batteries and panels add weight and it all costs real money. You have to compromise somewhere.
Edit: I forgot to say anything about the 50 watts per battery. RV batteries run from about 75 (small) to about 240 amp-hours in size. 50 watts sounds like the person was refering to batteries at the small end only being discharged to 75 to 80% of capacity. If you boondock all the time you probably should size everything to only go down to 75 to 80%, but you have to know your usage so you can size things.
-- Edited by bjoyce at 12:58, 2008-07-14
-- Edited by bjoyce at 13:00, 2008-07-14
Wow, Bill! You really helped me learn some things. I appreciate you taking the time to answer my questions about solar.
So I'll get the best charge contoller I can get and not worry about too much power from the panels. I already have a factory installed 2000 watt inverter. I intend on only using the batteries to power the TV and other minor stuff at night while boondocking, so it looks like this is my solution to high diesel prices and not having to run the genny as much to re-charge the batteries.
Again, thank you for your help. You're a wealth of information!!!
Best Regards!
Solar works well for topping off batteries. Since batteries need to taper charge to fully charge you have to run a generator for a long time to finish the charge and the last bit the generator is barely working but still using fuel and making noise. Solar is quiet and doesn't care if it takes three hours to finish the charge.