Adding to an existing battery bank [RE-wrenches]

[Dan Rice]

Esteemed Wrenches,

Gubment job, to include doubling an existing, one year old battery bank of 8
Trojan L-16 G's (two strings of four -24 VDC nominal system) -final
configuration will be 4 strings of 4 (I recommended against doing this...).
The site is an existing Nat'l Park Service administrative site, complete
with 20 kW propane generator, stacked 4024's, and an 1800 Wp array. Mostly
summer time use; mothballed during snow season. 5-6 kW-h/day use when
occupied.

Question: what would be the least potentially problematic battery bank
reconfiguration? Should the existing strings be left intact, and the new
batteries be installed in two strings? Or, should the existing strings be
broken up, and made up into 4 new strings consisting of two old and two new
batteries in each string? I'm inclined to do the latter, to balance the
resistance of the parallel strings, but I'm not sure if that matters and if
that might introduce another type of problem that I'm not aware of. As an
aside, have others come across multiple string banks in which the cell
closest to either the positive or negative end has deteriorated faster than
the other cells? If the strings are broken up, should the new batteries be
on the ends of the strings, leaving the year olds as the center two in each
string? What is the failure mechanism that attends adding new batteries to
old? Is it primarily related to uneven string currents causing accelerated
and uneven wear?

The Park Service is already aware that this battery bank configuration is
not optimal, and is planning to upgrade to industrial batteries at the end
of this set's life, but for now they need additional capacity and the
existing battery set is only a year old. They're hoping to get 3-5 years out
of this set.

As always, your collective experience is much valued and appreciated.

Dan Rice
Abundant Sun, LLC.
435-826-4565


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