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Factors Affecting Battery Life
State of Health
As batteries age, their overall health diminishes. The biggest sign that a
battery’s health has declined is seen as reduced capacity. Without the SG200,
the easiest way to see if this is happening is to monitor the time it takes
a battery to charge using the same charging source. If the time to charge
decreases over time, it is because the capacity has diminished. All lead-acid
batteries age due to sulfation as well as shedding, where lead falls off of the
plates and settles on the bottom of the battery.
There are several factors that affect the health and lifespan of a battery or
battery bank. For most lead-acid batteries, the two most important factors
affecting lifespan are depth of discharge/cycle count, and the concept of fully
charing the battery after each discharge. Each of these effect the mechanical
and chemical properties of batteries.
Sulfation and Shedding
At the mechanical/chemical level, there are two primary conditions that reduce
the life, or State of Health of a battery. They are Sulfation and Shedding.
Sulfation occurs when lead sulfate, formed naturally during battery use, is
converted to a more stable crystal form that covers the negative lead plates in
a battery. This happens when a lead acid battery is left in a partially charged
state. Different battery manufacturers have devised different methods and
chemistries to combat this effect, with varying degrees of success. Sulfation is
accelerated by Partial State of Charge (PSOC) cycling and deeply discharging
batteries.
Shedding in another naturally occurring phenomenon. During use, lead is
shed from the plates and falls to the bottom battery. When enough lead has
gathered, a partial-short starts to occur in the battery which will drain the
capacity on it own and hasten the battery’s demise.
In extreme instances, a battery can suffer a mechanical failure internally
and will quickly short a cell with no chance of recovery. When this happens
the battery must be replaced. If left in place the other cells can be
overcharged,risking excessive heat and fire.
Cycle Life
Batteries are rated to show how many charge/discharge cycles they are able to
endure before their health falls to a point where replacement is necessary. This
is typically expressed as XX # of cycles to XX% discharge. It means that if you
discharge to a lower value each time before re-charging, the total times this is
done is lower than if the battery is only discharged a smaller amount each time.
Of course, this is in a laboratory setting, as nobody discharges their batteries to
a set point each time before re-charging, and many other factors will contribute
to how long a battery lasts. For lead acid batteries, these also include:
1. Ambient temperature.
2. If the battery is fully charged after discharge and how much time passes
between the two steps.
3. If the battery is actually fully charged, instead of “Almost fully charged”
4. If the battery is over charged, at voltages higher than the manufacturers
recommendations
These factors together mean that in real-life applications, most batteries never
reach the cycle life proposed by manufacturers.
Partial State of Charge (PSOC) Cycling
All lead-acid batteries benefit from being fully charged back to 100% after
each use. PSOC means discharging a battery to some point, and then not
fully charging back to 100%, before discharging again. For some users, this
is not an issue as their batteries are not often left in a partial state of charge.
Boaters using battery powered trolling motors typically charge their batteries
fully right after use. For others, perhaps boondocking in an RV or cruising (and
anchoring) in a sailboat, this plays an important part in battery usage. Current
wisdom says when 24/7 charging is not available for lead acid batteries
(including AGM) the best balance between cycle life and maximizing capacity
is to discharge the batteries to 50%, and then charge back up to 80%. Then,
perhaps once week, charge up to 100%. As batteries close in on around 80%
charge, the current they accept diminishes. This means that the time it takes
to get the last 20% into a battery can become excessive. This becomes clear
when running an engine or generator to charge batteries.
