isolator question

[idiggplants]

Your mistake is off grid systems are designed for worse case. You cannot use summer or annual usage because in winter you will go DARK. Sound familiar?

You design for winter by knowing your daily watt hour usage. From that you then determine battery size and panel wattage. For you that means both more panel wattage and larger battery.

So if you use 40 AH a day means you need a 200 AH battery. As for panel wattage depends on location, shade issues, and what controller type you use. Let' say you live south of Interstate 40 and use a MPPT controller would need a inexpensive GT 200 watt panel with a 15 amp MPPT controler. If you have a 15 amp PWM controller you must use expensive battery panels, of 300 watts using 100 or 150 watt panels in parallel.

Now say you are in Gloomy Doomy Pacific NW winter you would need a 500 watt GT panel with 40 amp MPPT controller, or 750 watts of expensive battery panels with a 40 amp PWM controller. In addition to a lot more power wil also require a much more expensive AGM battery to handle the extremely high charge current.

FWIW 12 volt is more than nervous. That means you have used just over 50% of the capacity which is a NO-NO. Limit daily use to no more than 25%. Just one cloudy day and you are toast.

you misread. 40ah in a 4 day weekend.

according to our meter we are hitting 12v after much less than 50ah of draw. the battery is on its way out... but regardless. we arent using nearly half the battery's capacity in 2 days, let alone 1.

idk. my numbers arent exactly accurate. id just like my actual question answered... how can i add that second battery to the charge controller while keeping the batteries separated, but the controller charging both.

[Sunking]

Your mistake is off grid systems are designed for worse case. You cannot use summer or annual usage because in winter you will go DARK. Sound familiar?

You design for winter by knowing your daily watt hour usage. From that you then determine battery size and panel wattage. For you that means both more panel wattage and larger battery.

So if you use 40 AH a day means you need a 200 AH battery. As for panel wattage depends on location, shade issues, and what controller type you use. Let' say you live south of Interstate 40 and use a MPPT controller would need a inexpensive GT 200 watt panel with a 15 amp MPPT controler. If you have a 15 amp PWM controller you must use expensive battery panels, of 300 watts using 100 or 150 watt panels in parallel.

Now say you are in Gloomy Doomy Pacific NW winter you would need a 500 watt GT panel with 40 amp MPPT controller, or 750 watts of expensive battery panels with a 40 amp PWM controller. In addition to a lot more power wil also require a much more expensive AGM battery to handle the extremely high charge current.

FWIW 12 volt is more than nervous. That means you have used just over 50% of the capacity which is a NO-NO. Limit daily use to no more than 25%. Just one cloudy day and you are toast.

[idiggplants]

ok so a quick background...

we have an off grid cabin... we use very very little electricity there. we currently have 200w of panels through a pwm controller charging a traditional 12v marine deep cycle battery. it meets our needs 9 months out of the year. during the summer that sucker is topped off by 9am. we typically use 40ah in a 4 day weekend between our led lights, cell phone chargers, radio, and our 12v pump. about 100 yards away i have a pavilion with 100w of panels and another deep cycle battery that runs led lights and car radio. again... plenty of power to get that thing topped off within an hour of sunrise.

now, i mentioned the 9 months.... well the other 3 months, we have a trifecta of things that stress the cabin's system... 1, shorter daylight, so less charging, 2 its cold out so we arent hanging at the pavilion, so we are in the cabin the whole time, and 3. we are using a lot more lights in the cabin...

so in the winter during hunting season we end up dropping the voltage down to 12v and we get nervous and start using propane lanterns. not ideal.

but... we dont use the pavilion at all in the winter. so i was thinking i could move the battery up from there to cover our extra needs during those 3 winter weekends we go up there. we have figured out that a water pump and one other circuit is almost always close to half of our load. so we could easily move a pair of leads to the other battery on the "load" end of things... but for charging... how could we wire it so it kept the batteries isolated so they dont kill each other, but still both get the same amount of charge from the panels?

we also plan on adding a 3rd 100w panel at the cabin. our controller can handle 4.

thanks a lot for your advice!