Battery Cycle Life Ratings [RE-wrenches]

[Allan Sindelar]

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Wrenches,
As I follow this battery life thread, and we learn that some of our
long-held assumptions may be myths (such as that L16s inherently last longer
than golf carts) a couple of other questions come to mind:

1. It has been commonly understood that new flooded batteries are never to
be mixed with old batteries; the limit is 6 months to about a year,
depending on use. Why? Is it, as I have believed, that the new batteries
would only perform as well as the old--that is, remaining cycle life,
charging characteristics, etc. would match the older cells. Is it worse than
this? Will the mixed bank perform worse than the old cells, or cause the
entire bank to fail prematurely? What if eight 6V batteries are doubled to
16, and the new and old units are evenly split among each parallel string
(in either 24V or 48V configuration)?

2. I have also designed systems with smaller banks relative to charging
capacities, based on manufacturers' recommendations for best performance.
But a C/15 to C/20 rate is still hard to achieve, and can lead to excessive
discharges if the owner isn't attentive. In the early days of offgrid
system, battery banks were way oversized relative to arrays (I still get old
timers thinking in terms of one "battery" (a golf cart) to one module (which
back then was a 35 watt Arco 16-2000 or the like). That's about a C/100
charge rate.

What I have noticed is that the grossly oversized battery banks batteries in
these old systems still tended to last about 5-8 years; that is, about as
long as well-sized banks. So I could use some real-world discussion of how
others size batteries to arrays and loads, outside of all the textbook
numbers.

Allan at Positive Energy

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