Expand a 12v solar system

[Sunking]

Ok, got it, but if Im using 600 wh a day thats only 12% of the batteries 4800 wh. I'd have to go 4 days with no sun for them to get to 50% discharge, and that's not happened in the past year.

Al that means is your batteries are over sized which is OK providing you have enough panel wattage to supply them with a C/10 charge current. That will cost you big bucks meaning now you are paying 20 times more for electricity.

Example if you use 600 wh per day with a 3 Sun Hour Winter days. All you need is a 300 watt panel, 250 AH battery, and 25 amp charge controller. But if you double the battery size to 500 AH means you must provide 50 amps of charge current to support the battery needs. Now you need 600 watt panels and a 60 amp Charge Controller. You just doubled the cost.

So be careful what you ask for. 600 wh is about 6-cents worth of electricity per day from the power company. Using standard practices of 5 day reserve will cost you 50 to 60 cents per day in battery cost. Go to 10 days and you are now paying over $1.20 per Kwh. You can buy for 6-cents.

[SunEagle]

Ok, got it, but if Im using 600 wh a day thats only 12% of the batteries 4800 wh. I'd have to go 4 days with no sun for them to get to 50% discharge, and that's not happened in the past year.

Ahh. I thought you were using the 2520 watt hours daily which is 50% of your battery system. I misunderstood when you stated you use 600W a day. You really meant 600 watt hours a day. So you do have a big enough battery system for your daily usage along with a fair amount of contingency in case you go a couple of days without sunshine.

I do see a potential issue by having 4 x 12volt batteries wired in parallel. Chances are you will start to get unequal charging and discharging due to different resistance values in each parallel "rung". That will more than likely reduce your battery life to much less then you have calculated.

[Solarson]

Yes, i meant 600 Watt Hours. I had a 'no sun' day a couple of days ago and noticed my MT40 meter showed 2.2kw of charge the next sunny day. Normally it shows 1.1kw charge per normal day - and 0.6kw discharge each day regardless of weather. Every day for a week or two of normally sunny days.

Now I'd assumed that it was the pv's that were creating the 1.1kw daily reading, but it looks like it's the batteries that are charging that amount, and taking double (2.2kw) after one cloudy day.

If that's the case, what's happening to the daily difference between the 1.1kw charge and 0.6kw discharge?

[inetdog]

The unneeded power is simply not being generated by the panels. Just like if you open the circuit no power is being generated at all even though the sun is still shining on the panels. This does not harm the panels and you are not wasting any energy; you are just not asking the panels to work to their maximum capability.

[Solarson]

Using standard practices of 5 day reserve will cost you 50 to 60 cents per day in battery cost. Go to 10 days and you are now paying over $1.20 per Kwh. You can buy for 6-cents.

The thing is I'm completely off-grid. A power line would have to come through national forest. So I have to balance having a low-key power lifestyle with a big investment to gear up to higher voltage.

It's a low key outdoor lifestyle so lends itself to a few inconveniences. To avoid the grey days of winter I go to Mexico, the cost partly offset against having to invest in double the days power storage otherwise needed.

The current objective (!) Is to maximise what I can get out if the existing set up and reinvent it when I need to replace the batteries (or I can't get into Mexico