Classic LM317 used to charge 12V SLA battery
Close switch to start simulation.
LM317 adj pin is connected to POT wiper, Vin connected to switch, and Vout to 30 Ohm load.
21.2V is open circuit voltage from 36 solar cell string, which is also modeled with a 10 ohm internal resistor.
30 ohm load is used in place of battery, note: an Ideal-diode could prevent the 5mA discharge at night.
4.7K Ohm is NTC (B = 3500) Thermistor thermally coupled with battery.
http://www.murata.com/products/catalog/pdf/r44e.pdf
13.2V:2.69kOhm@40ºC
13.6V:4.7kOhm@25ºC
14.2V:13.4kOhm@0ºC
One good thing is LM317 is very low cost. Bad things include 3V dropout, huge bias current
There does not seem to be much point in worrying about maximum power point tracking with a 36 cell PV string when charging a 12V SLA battery, because the battery voltage is so close to the max power point. The float voltage needs to be accurately set to 13.56V. Another problem is that holding at the float voltage for long periods is sort of a bad idea, e.g. 3hr is good but 6hr is bad.
