Last year I put together a small starter solar package for my summer
cottage. I originally started with four T-105 batteries. I am considering
purchasing two more T-105's to add to the system, but was told that adding
new batteries to an existing bank is not good. Is this true and is there
anyway around it?
Adding more capacity with out more panels will eventually harm the
efficiency of the system. As you add more batteries you need to add more
X number of panels can only charge X number of batteries. Unless you
undersized the batteries on the original installation you will need more
panels to charge the additional batteries. Sorta like you change your
battery from a 100 amp to a 1000 amp. The charger stays the same at 10 amp.
You will never be able to charge the new battery completely because the
charger is not big enough. example exaggerated
I'd think that you would be reasonably safe to add a couple batteries to
your bank. One years use at a summer cabin probably has not worn down your
existing batteries too much, unless you have been really discharging them to
the max r otherwise absing them.
The issue is that if your existing batteries are aged (many deep cycles,
abuse, time) then they have used up some of their useful life. When you add
new batteries, they will be limited by the useful life left in the 'oldest'
Think of it like adding a single new 60k mile tire to a set of tires that
already have 50k miles on them. You are going to have to get a new set of
tires in another 10k miles. The new tire does not add to the life of the
whole set. It is even a bit worse with batteries, because they are
interconnected and a bad battery will place additional strain on a good
As another reply stated, you do want to make sure that you are adequately
charging the bank. Adding additional batteries will not compensate for a
situation where you have an indaequate charging setup.
Wow Mr Bughunter, you gave me something new to worry about. While I
only have 140 Watts of panels, I also only have 1, 75ah deep cycle
(Marine starting) battery, with a 400 watt inverter. As soon as I am
working again I do plan to get proper (Real) deep cycle batterys. But,
how many ah's should I concider (I am still in the experimental
stage)? I would like to boost my wattage to 500 before next winter
starts. I would like to balance my batterys to my (Panel) Wattage. I
can always plug in more appliances as I produce more power (and learn
Should I get more panels, or should one get batterys sized to ones
"estemated" eventual panel Wattage size?
Please remember that there are no stupid questions (I hope)
You don't really have much to worry about. About all that you are risking
with your current setup is the value of a 75ah marine battery. My guess is
that it is way undresized, and that you are probably well on your way to
sending it to a recycling center. Not a big deal. I have burned up countless
marine batteries in my lifetime. A polititian would claim that it is a
stimulis to the economy.
Your concern seems to be to design a well balanced system. While it's not as
complcated as rocket science, you do need to have a good undrestanding of
your usage patterns and needs, as well as being able to translate those into
numeric values. There are a number of spreadsheets around that you can use
to calculate your esitmated loads, battery sizing, inverter size, and
charging requirements. Try www.homepower.com.
The first place to start is to determine what your loads will be. For
example, how many lights do you use, at what wattage, and for how long each
day. Repeat for other types of loads. One advantage of an experimental setup
like yours is to keep records of the loads that you actually use. Keep a
log. Your particular usage patterns are a critical factor in the design that
only you can estimate with any accuracy. Time of usage is an important
metric. You post never mentioned it at all, so it is something that you do
need to consider in a dynamic system.
Loads are expressed in watt-hours, battery capacity is expressed in
amp-hours. Time must be taken into consideration in charging, and
discharging of batteries, as well as in estimating loads.
You loads can help you estimate how large an inverter you need. You want to
buy a large enough inverter to meet future needs, but not so large that you
are only utilizing a small fraction of the capacity. Keep in mind that all
of your loads will not be on at the same time. You can estimate how many you
do desire to use simultaneously, and then size to that. Leave youself some
headroom for future expansion in loads. Upgrading inverters multiple times
might be more expensive in the long run that getting one large enough in the
first place. A few simple estimates and calculations can help you choose.
The battery would be my next consideration. How big it needs to be depends
on your loads, as well as how long you hope to run from it without needing
an input in the form of a charge. Maybe you are happy to have enough
charging in a single sunny day to keep your lights and TV running for the
evening hours of the same day. Or maybe, you want to have 3 or 4 days of
capacity to get you through a few cloudy days. Most systems make provisions
for at least a few days of battery autonomy. Incremental upgrades to a
batery bank is not a good idea, so take care in sizing it on the first shot.
Think of your battery bank as a water resevoir. You don't want to go thirsty
just because it doesn't rain for a few days. Think of your charging system
as analagous to water collector (roof?). If it is too small, the resevoir
will never fill. Too big, and you just waste the excess production. It's a
Finally, there is the charging system. This is often the most costly part of
the system, unless you are grid connected. For alternative sources like PV
and wind, you need to consider the environment in which they will run. Not
every site has ideal wind conditions, and the same is true of the sun. An
good system will keep just ahead of your ability to run the batteries below
50% DOD. Some systems, will augument alternative sources, which are capitol
intensive with genertors (not quite as capitol intensive), but more
expensive to run.
Now, the calculations are not complex, but there are so many variables that
are tricky to estimate that the output will never be "perfect". Shoot for
So, my bottom line suggestion woud be to add a spreadsheet to your
You might also look at the specs on some "packaged systems" to get a feel
for what other designers thought were reasonably balanced systems.