Posted by N9WOS on August 15, 2008, 4:22 am
And if anyone has access to them, the two papers it references.
Operational characteristics of VRLA batteries configured in parallel
strings, by cole et al.
The operation of VRLA lead acid batteries in parallel strings of dissimilar
capacities or can we now sin, by giess.
I can just see the smoke coming out of peoples ears now. :-)
Posted by bealiba on August 15, 2008, 12:55 pm
Thank you for this. Although I have yet to read it right through it
appears to be related to industrial back up systems and not home power
systems. Two different games. Still looks an interesting read.
Posted by N9WOS on August 15, 2008, 7:03 pm
Then the question becomes, how do we do are own test to see how applicable
it is to RE applications?
One way that I can think of for full scale test is to have two battery banks
with a trimetric hooked up to each. And have them paralleled after the
trimetric meters. That way you can keep a track of how much each one is
charging and discharging.
Another thing I have been thinking about is a small scale test. Like a 33Ah
18Ah 10Ah and 2.5Ah batteries in parallel. 63.5Ah total. Or possibly an old
battery and a new one. Hook up current shunts between the buss bar and the
batteries. Set them in a small scale RE system and see how they do.
Keep track of them once in a while and see if the charge and discharge
currents are equally distributed among the strings based on AH capacity in
each string. And to see if, and how much new batteries would hog the current
in a system they share with older batteries.
There is many interesting things people could do.
Posted by bealiba on August 16, 2008, 1:06 pm
Read the whole thing from beginning to end, This report refers to
float systems as use for backup in industrial applications and "NOT"
home power systems and in fact says that
"This is not a problem for occasional discharges, or for more frequent
discharges that are taken to completion. It can become an issue,
however for a system that is designed for long discharges, but is
subjected to frequent shallow discharges. In this case, the high rate
battery will receive the brunt of the cycling duty, and may age
prematurely as a result."
Given that in parallel strings one string will always have a higher
rate than another string, batteries /cells never being perfectly
matched, failure is always an option. The paragraph above gives a
pretty good description of a home power system.
Whether you like it or not, no matter how you try to rationalize it,
no matter how you twist the words of the author to meet your needs, a
home power system with parallel batteries is not a good idea.
The real problem is the Tweedledee's and Tweedledum's of the world
spot such a report as this on the net, read the title and jump to the
conclusion that it says that parallel strings are ok for a home power
system, never bothering to read the article to find out that it
describes industrial float systems which may or may not ever be called
upon to fulfill its intended use.
Sorry, the report really has nothing to do with home power systems.
Posted by daestrom on August 15, 2008, 11:52 pm
From the report, "...for a system that is designed for long discharges, but
is subjected to frequent shallow discharges...the high-rate battery will
receive the brunt of the cycling duty, and may age prematurely..."
I read that as saying mixing two different capacity strings for a system
such as solar that is going to be charged/discharged on an almost daily
basis, the higher-capacity string is going to take the brunt of the cycling
and fail early. So mixing different capacity batteries in a system that
sees frequent shallow discharges (such as an off-grid solar installation)
would be less than the best solution from a battery life standpoint.
Batteries installed for frequent cycling such as an off-grid solar
application are going to have issues as soon as one string starts to degrade
more than the other(s).
For installations such as telecom, where the discharging is relatively rare
and short term (don't many use backup generators and the batteries only
carry through for less than a minute while the generator starts??), the
parallel strings stay on a constant float charge almost all the time.
The paper also points out that one of the reasons for parallel strings is
redundancy. The author explains this is done for large UPS systems this is,
"...prudent for VRLA batteries, in which cells sometimes fail open or
near-open..." But again, the type of service these batteries are intended
for is *not* the type of service that would be seen by most off-grid solar
Regarding Panasonic's VLRA batteries, the recommendation number 5 on page
seven of this document, "Avoid parallel charging of batteries in cycle use,
as this may shorten the service life of the batteries by causing an
imbalanace in charge/discharge state among the batteries connected in
Paralleling two strings for *redundancy* in life-critical UPS installations
where the batteries are on float > 90% of the time is one thing (a typical
trade off between battery life and the need for redundancy). But
paralleling a large number of low voltage batteries for additional
*capacity* in an application that involves frequent charge/discharge is
From the conclusions of the paper, "Parallel strings can improve system
reliability under the right circumstancs, but it is important to know just
what paralleling can do under the specific conditions of a given