1: Does it harm a SLA battery to fully depleate it's charge?
Yes. A normal 12V SLA battery should never drop below 11V.
2: Dose it harm a SLA battery to only recieve a partialy charge?
No.
3: If over charging a SLA battery occurs, how long would it take
before damage occurs?
Depends on how high the charging voltage is during the over-charge
period and the specific type of SLA battery. Old-style refillable
batteries start boiling off liquid and need to be refilled. Sealed SLAs
also start boiling off liquid, but can't be refilled. Newer glass-mat
(AGM) SLAs are less likely to be damaged.
Back in our warship days, quite a few SLAs died an early death because
of over-charging in the motel room when we forgot to turn down the
charging current before going to bed.
4: Is the life of a SLA battery measured in months/years or by charge
uses?
Both. SLAs only last a certain number of full re-charge cycles (roughly
250 for inexpensive batteries) and they only last about 5 years, whether
they are used or not. Basically, the plates used in the battery corrode
to the point where the chemical reaction isn't efficient enough to
provide current. Better chargers allow batteries to last longer (by
counter-acting the corrosive process with reverse voltage cycling) and
more expensive batteries last longer because of thicker/better plates,
but all SLA batteries are still limited by age.
Even a battery just sitting on the shelf will die over time because of
internal resistance which discharges the battery. If you leave the SLA
on a cold surface, like a cement floor, it will discharge even faster
because of thermal convection inside the battery.
5: How long is the life of a SLA battery?
4-5 years for normal SLAs.
6: Is it common for 1 SLA battery in a pair to go bad before the other?
Yes. Any difference in resistance between a pair of batteries will
cause one to be used slightly more than the other. Over time, the
difference widens because of the unbalanced use, so one battery dies
before the other. Large battery banks, such as those used in electric
vehicles, counter-act this problem by using simple electronic circuits
that continuously monitor the resistance of each cell/battery used in
the bank and electrically balance the resistance so that all
cells/batteries share the load equally. Those circuits are now fairly
inexpensive and can be used with a two cell/battery bank just as easily
as a 10 cell/battery bank. Typically, the same circuit is also used
during the charge cycle to ensure that all cells/batteries charge evenly
as well.
Frank P.
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