Bogart Engineering SC-2030 Technical Manual

Bogart Engineering
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None of these analogies apply to batteries. Lead-acid batteries can accept higher charge currents when they are
nearly empty, but must be charged more slowly when they are nearly full because then they will not readily accept the
charge. Batteries need to be frequently fully charged. If a battery is allowed to operate consistently at a low charge level,
its capacity to hold charge decreases over time. This means that a battery needs to be fully charged every now and then
for the proper maintenance of its capacity. In solar applications, this sometimes cannot be achieved in the course of a
single day, depending on sunshine and energy usage.
Sometimes a generator is used instead of sunlight to charge the battery at a high current for one or two hours each
day. This can easily shorten the life of lead-acid batteries, which cannot get fully charged in two hours even with a very
powerful generator. If only charged with very high currents over a short time, a battery’s charge capacity will decline
over time. A “hybrid” generator-solar system would be preferable with a strategy of using the generator to provide a fast
charge early in the morning, when the battery is at its lowest charge level, and stop when the battery can no longer
accept high charging currents. By the time the sun is high enough, the solar panel can take over the charging at a lower
current for the following six to nine hours. Alternatively, a surplus of solar panels can accomplish something similar, but
that means the peak solar power in the afternoon, when batteries are accepting less current, may not be used. To
overcome this, plan to use heavier loads, such as laundry, dish washing or well pumping, during the mid afternoon,
when batteries accept less solar energy.
6.2 Specifically how solar chargers, including the SC-2030 charge batteries
The following charging description applies when the SC-2030 Solar charger is connected with the TM-2030. If for
some reason the TM-2030 is not connected, it uses a basic charging procedure, to be described in section 6.3.
This discussion refers to 12V systems—for 24V systems multiply voltages by two.
To charge batteries, a charger supplies electrical energy to the battery with a certain "voltage." "Volts" is a measure
of how hard the charger is attempting to push the energy (electrons) into the battery. The battery always tends to resist the
tendency to push the electrons in—the voltage of the charger must be high enough to overcome the resisting force of the
battery. This is a little like pushing water into a pipe which is under pressure—enough force must be provided to push it
in or it will not go. The "current" or "amperes" is a measure of how much (charge) energy is actually flowing in. The
actual flow (“amps” or amperes) depends on two factors: how hard the charger is pushing (voltage) and how much the
battery is resisting.
When batteries are at a lower state of charge they do not push back very hard, and the battery will easily absorb all
the charge (amperes) that the charger can supply. This is called the "bulk" stage of charging, and the "voltage" from the
charger during charging will be below 14 volts or so. This is when most of the charge can go into the battery, and is the
simplest part of the charging process; usually the batteries will be able to absorb all the energy the charger is capable of
delivering.
When the batteries reach about 85% full, the job of the charger gets more difficult. The batteries begin to resist more,
and absorb amps at a lower rate, meaning that it takes a longer time to do the rest of the charging. One might say, "why
bother, then to go beyond 85% full? Wouldn't this make the job easy on the charger? Just always operate the batteries
from 55%-85% charged." Well, yes it would, but the reason this is not a satisfactory strategy for lead acid batteries is that
if you don't fully charge them regularly, it makes it harder in the future to charge them as much. It is remarkable how
often even authoritative sources on lead acid battery charging repeat the phrase that "lead acid batteries do not have
memory." Lead acid batteries DO have a memory—if you do not fully charge them, they will remember that, and if this
is repeated often their capacity will gradually "walk down" as is correctly described in charging information from the
Concorde battery company.
This presents a challenge to solar charging— because the solar day starts to end as the batteries become more
resistant. This can result in a battery that is not fully charged when the day ends. It is frequently observed that batteries
being charged only by solar tend to lose capacity to hold energy— described as batteries becoming “sulfated”. This
conveys the fact that the lead sulfate, which is the byproduct of discharging gets more difficult to convert back to fully
charged lead and sulfuric acid if it sits around too long before recharging.
To continue the charging story, once the batteries become more resistant to charging—when the charger rises to 14.4
volts, (at 77 degrees F or 25 degrees C) liquid electrolyte batteries will begin to "gas" which means that although part of
the energy is still doing some slower charging, part of the charger energy is breaking down the electrolyte in the battery
into oxygen and hydrogen gases—and in addition a higher amount of the energy begins to go into heating the battery
instead of the desirable conversion of the chemical charging. Although the gassing does waste some energy, this turns out
to be desirable in liquid electrolyte batteries because the gas bubbles stir up the electrolyte which otherwise can stratify—
because without the stirring the heavier acid can sink to the bottom while weaker acid goes to the top causing unequal
charging at the top and bottom. In AGM batteries, the design is different, so gassing typically doesn't occur, which makes
them a little more efficient.