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.
Good solar chargers will then go into what is often called "absorb" stage—where the charger holds the voltage just above the gassing point (voltage ideally temperature compensated). The batteries then absorb gradually less and less energy as they further charge.
Most manufactured solar chargers maintain the "absorb" voltage for a set amount of time—perhaps one to four hours before they go into the "float" voltage of about 13.2 volts. Often the better chargers allow you to set the exact absorption voltage, the holding time, and the exact float voltage. The float voltage is a maintenance voltage which is intended to be the ideal voltage to keep a battery at minimum wear for the longest time once it's fully charged.
Although just maintaining the "absorb" voltage for a fixed time is not a bad way to decide when to go into "float", many battery companies suggest that it is better to monitor the amount of current (amperes) going into the battery during this time and then go into float based on this. There are three variations on this method:
(1) Charge above the gassing voltage until the amperes drop to a sufficiently low value, say an "ampere" value that is 1%
or 0.5% of the amp hour "capacity" of the batteries.
(2) Charge until the value of amps into the batteries stops decreasing for a specified period of time—and stays at this constant value for perhaps a couple of hours.
(3) Charge until the charger has replaced a specified percentage of charge amp hours that was last removed from the batteries during its last discharge cycle.
These options are unusual with most solar chargers, but the first or third is possible with the SC-2030 solar charger when used with the TM-2030 monitor. The TM-2030 measures the previous amount of discharge (typically the night before), then when recharging requires returning 105-120% of that amount, adjustable by the user. The problem for many chargers is that they do not measure or know the exact value of amperes or amp hours going into the batteries. They may measure the amps from the charger going into the battery and loads together, but they don't know what percentage of this is going into the battery compared to the loads, so these methods of observing battery amps are not available.
By returning a constant additional percentage, excess charge that is returned depends on the amount that was previously removed. This has the effect that the "absorb" time is not always the same, but is adjusted to the previous day's usage to avoid overcharge or undercharge.
Undercharge is more common in many systems, but in applications where solar charging goes on for days when very little drain occurs on the batteries, such as for RV’s stored in the sun, or occasionally used cabins, measuring amp hours can avoid overcharge.
An additional method the SC-2030 uses to get in sufficient charge is that it has an (optional) finish charge stage to try to increase the intake of current into the battery by boosting the voltage when the current has declined to a safe enough value. This is explicitly recommended by some battery companies for liquid electrolyte batteries and recently even AGM types—but usually not gel batteries.). If the SC-2030 is programmed to do this, after the charging current decreases to a safe value while in "absorption" state the SC-2030 then increases voltage (while regulating current) to attempt to put more charge at the end when the battery is becoming extra resistant so as to attain the specified overcharge amount. The overcharge percentage, maximum voltage, and maximum permitted current are all values that can be programmed into the TM-2030.
The effect of temperature on charging: The ideal temperature for a lead acid battery is often considered to be about 25 degrees C (77 degrees F). When batteries are cold, the charging process is slower, so they take longer to charge.
The gassing voltage of the battery increases with lower temperature—and therefore the recommended "absorption" voltage should rise as temperature goes below the usual reference temperature of 25 degrees C (77 degrees F). If the battery temperature varies much, the charger should have the capability to adjust its voltage to temperature, especially for sealed AGM or gel types.
