When using any Solar System with TM batteries, a
"Solar Charge Controller" is always required to lower the operating voltage (from one or more solar panels) downwards, to create a "battery charging voltage" which the batteries can actually accept.
"12 Volt" Solar Panels actually run at 17.9 Volts, or more. If a Solar Panel was connected directly to a water-filled Lead-Acid battery, it would pretty quickly overheat and ruin the battery. (The acidic "water" would start to boil away. A "sealed" battery would crack and leak as it began to overheat. AGM and Gel batteries would also overheat, being destroyed even a bit faster. Lithium batteries would suffer fatal damage to each of the internal cells.
"Solar Charge Controllers", like the 3-stage (and so-called 4-stage) "Power Converters" which charge from 120V shore power, must choose
how much voltage the batteries can safely accept. For "sealed" or wet cell batteries, the upper limit is set at about 14.4 volts, and usually limited by time (to also help prevent battery overheating). Charge Controllers have switches, or programming ability, to optimize settings for other battery types. (Lithium batteries can be charged somewhat higher, and kept at that charging voltage until nearly full. AGM and Gel Cell batteries require slightly LOWER maximum voltages.)
The lower limit, the "float voltage", is typically preset to about 13.6 Volts. While the Solar Controller is charging batteries, it will shut down charging, for very short periods of time, every few minutes to
measure the current Voltage of the battery connection - and it will adjust it's voltage accordingly, moving from the upper limit towards the "float voltage" in one or two steps.
Both types of Solar Controllers contain this "Voltage-Tuning" capability. (Although cheaper Solar Controllers may only have a switch for "Sealed or Wet Cell" versus "AGM or Gel Cell".) They should make approximately the same decision for choosing what voltage the batteries can accept. But, there is a big difference in the method they use to lower the Voltage from "Solar Panel Input" down to a Voltage which the batteries can safely use:
'PWM' SOLAR CHARGE CONTROLLERS use a very rapid switch on the Input "Solar Panel "+" connection, disconnecting and reconnecting the Solar Panel(s) many thousands of times per second. At any particular instant, the incoming Voltage is either above 17.90 Volts, and full panel power is being used (if not limited by the Controller's maximum current value). Or it is zero, and zero power is being accepted from the Solar Array. The effective charging voltage is the "weighted average" amount of time which the Solar Controller spends within the two alternating states. (
"PWM" means
"Pulse Width Modulation", the creation of an intermediate Voltage Value by varying the the width of each "ON" pulse, relative to the remaining "OFF" portion of each ON/OFF cycle.)
Gory details (you can skip this part): On the "Battery Charge +" wire going into the TM 12V system (either directly or indirectly reaching the batteries), the Source Voltage from the PWM Solar Controller is changing very rapidly. But "smoothing" capacitors, wire inductance, and the batteries themselves are all responding to the rapid shifts in received Voltage by sending power back into the wires during "zero voltage" periods, and offering greater resistance to incoming power at the end of each "full voltage" period. When viewed on a volt meter or measured by the Controller, the "zero voltage" and "full voltage" periods result in a "weighted average" - the "weight" corresponding to the proportion of time that the original Solar Panel in charged versus uncharged state.
SUMMARY: The important things to understand are: #1, During the very rapid and short "PWM" disconnect cycles,
power is being left behind in the disconnected solar panels, totally unavailable for charging batteries or running 12V electrical loads inside the TM trailer. But also, #2:
PWM Solar Charge Controllers are very simple, reliable, smaller in size, and much less expensive.