Re: [VAC] Solar Power Again

[Don Horn <donhorn@xxxxxxxxxxxxxx>]

Jim,

To clarify, it's not that solar panels are *unable* to bring the battery to
full charge, it's just that they achieve maximum efficiency between the
floor of 50% charge and about 75-80%.  In that range (the bulk charge stage)
the battery can most likely accept the full amperage output of your panel0.
Above that level, the battery's acceptance rate drops considerably, so it
might only accept only 2 or 3 amps of a panel's 10-amp output per hour.
This is why it's recommended that you only plan to use the capacity between
50% and 80% of charge.

(Now you got me started again...)  ;>)
This also means that you need to balance the battery and solar panel size.
However, it's not as simple as choosing a panel having output that's equal
to or greater than your daily usage.   In fact, it's possible to have higher
daily panel output than daily usage and still not maintain the battery if
you don't have enough storage capacity, and if most of your usage occurs
after dark (however not likely for modest users).   You need to first
calculate the battery reserve you need based on your usage (amp hours used
between charge cycles), and from that, calculate the size of your battery
bank (i.e., 3 to 4 times your usage before recovery to 80% charge).   You
can then calculate the panel output you need to maintain that reserve,
whether for a few days at a time, a week, or indefinitely, and choose your
panel(s) accordingly.

Having said all this, for most small users 20 AH panel output per day will
probably be fine for 20 AH usage per day, even with a battery of only 80 AH
and with most of the usage taking place after dark.  However, it's
reassuring to be able to run the numbers and verify that this is true, and
when upgrading, it's the only way to determine whether you need to add
battery capacity, panel capacity, or both.  Both of the books I mentioned
earlier* explain all of this much more clearly and in more detail.

Don  

*(Disclaimer:  I have no interest in the sales of either of these two books,
although I have considerable interest in their contents).