Battery Life in Grid-Tie Systems [RE-wrenches]

[Eric Smiley]

>can you expect a greater life expectancy from a battery (sealed or
flooded) >when it is in grid-tied mode verses stand alone mode?

It depends...

Different battery chemistries are optimized for float service or cycle
service (i.e. grid-tied or stand-alone) The difference is in the plate
alloys used to strengthen the lead -- which are usually antimony or
calcium, but tin, cadmium, selenium, aluminum, arsenic, tellurium and
silver also show up.

In float duty battery life time is dictated by internal corrosion which
depends on battery temperature. The corrosion rate approximately doubles
for every 10 degree C rise above the rating temperature which is usually
20 degrees C. Low-antimony or calcium alloys give longer float life.
Generally, sealed batteries don't have any antimony since gassing and
water loss are one of the disadvantages of this type of alloy.
Corrosion of the positive plate is the main life time limitation in
float service. Tubular plates are also used to extend battery life. Only
the postive plate is tubular and corrosion of the positive plate is
reduced in this format.

In cycle duty, battery life is determined by number and depth of each
cycle. The change from Lead or Lead Oxide to Lead Sulphate during each
cycle results in a volume change (lead sulphate is less dense) and this
will lead to eventual shedding of active material from the plates. High
antimony batteries have a better cycle life, especially for deep
discharges, but antimony dissolves into the electrolyte as the battery
ages (corrodes) and this leads to gassing, high water loss and
self-discharge.

Since stand-alone PV systems often have 5 or more days of autonomy they
don't usually cycle very deep and high antimony content is not very
useful. So most "Solar batteries" are low-antimony.

Thus the main choice is between low-antimony or no-antimony (i.e.
lead-calcium, lead-calcium-tin, ...) design with low-antimony generally
giving better cycle life and lead-calcium generally giving better float
life.

Stratification, plate thickness and acid concentration are other factors
to consider.

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