SOLAR BATTERY CHARGER CIRCUITPosted by Jim Keith in Solar | 15 commentsMore projects with: Solar charger Add to favoritesThis is the most simple and affordable solar battery charger that thehobbyist can make. It has a few drawbacks over other similar controls, butoffers numerous advantages. It is intended for charging lead-acid batteries,but may also be used for charging any battery at a constant voltage. Voltageoutput is adjustable.+ Uses commonly available commonly available components + Simple, small & inexpensive + Adjustable voltage + ZERO battery discharge when sun is not shining – High drop-out voltage—may be marginal for 6V application – Current limited to 1.5A – No LED indicators—no bells or whistlesSolar battery charger specifications Solar panel rating: 20W (12V) or 10W (6V) Output voltage range: 5 to 14V (adjustable) (may be reduced further by shorting R2) Max power dissipation: 10W (includes power dissipation of D1) Typical dropout voltage: 2 to 2.75V (depending upon load current) Maximum current: 1.5A (internally limits at about 2.2A) Voltage regulation: ±100mV (due to regulation of series rectifier) Battery discharge: 0mA (this control will not discharge the battery when the sun doesn’t shine)
Solar battery charger schematic6V Applicaton Output Voltage: Set for 7V Input voltage: o Battery discharged (6V): 8.75V Min @ 1.5A (this is a little high for panels that are characterized for 6V applications) o Battery charged (7V): 9V Min @ 10mA (e.g.)12V Application Output Voltage: Set for 14V Input voltage: o Battery discharged (12V): 14.75V Min @ 1.5A (Available from solar panel characterized for 12V operation) o Battery charged: (14V): 16V MinMinimum Head VoltageThis is also referred to ―drop-out voltage.‖ The input voltage mustexceed the output voltage by about 2.75V @ 1.5A. Fortunately, when
the battery discharged, the output voltage is lower so the solarpanel voltage will also be lower.When fully charged, the battery voltage will be high, but the currentis very low—at this point, the drop-out voltage reduces to about 2Vand the open circuit solar panel voltage also comes into play. Theschottky rectifier was selected to reduce this head voltagerequirement—the voltage drop of the schottky is about 0.5V @ 1.5Aor about half that of a typical silicon rectifier.More advanced controls have a much lower head voltagerequirement and will function better under marginal conditions.Maximum Power DissipationThe power is limited by the thermal resistances of both the LM317Tand the heat sink. To keep the junction temperature below the125°C Max, the power must be limited to about 10W. If a smaller orless effective heat sink is used, the maximum power dissipationmust be de-rated. Fortunately, the LM317 has internal temperaturelimiting so that if it gets too hot, it shuts down thus protecting itselffrom damage. Max power comes into effect when charging a 12Vbattery @ 1.5A: e.g. battery voltage = 12V, solar panel = 18V. P =(18V – 12V) * 1.5A = 9W. So thermally, it is carefully matched to thecurrent rating.If a solar panel that is characterized for 12V is applied with a 6Vbattery, the maximum current must be reduced to about 0.7A: e.g.battery voltage = 6V, solar panel voltage = 18V. P = (18V – 6V) *0.7A = 9.6W. In this case, the solar panel power may not exceed10W.When charging, the heat sink normally runs warm. When beginningto ―top off‖ or completing the charge at maximum voltage, the heatsink runs hot. When fully charged, the heat sink runs cool. This heatis not exactly wasted power—it is excess power that is unneeded inthe process of charging a battery.Current Limiting
Current limiting is provided by the solar panel—it is not a commonlyunderstood fact that the solar panel tends to be a constant currentdevice. For this reason, a solar panel can withstand a short circuit.Therefore, the control does not need current limiting.Float Charge of Lead-Acid BatteriesThis control charges the battery at a constant voltage and alsomaintains a charged battery (float charge). The float charge voltagespecification is a little lower, so to accommodate both charge andfloat charge voltage, a compromise is reached by simply reducingthe voltage slightly—that is how ALL automotive systems operate. Toobtain maximum charge in a 12V battery, set the control to 14.6V.Automotive systems further reduce voltage to 13 to 13.5V in orderto accommodate high temperature operation as the battery isusually located in the hot engine compartment—battery has anegative thermal coefficient of voltage.
Application with other types of batteriesIt is difficult to determine how to set the voltage in this case. Theeasiest way to do this is to charge the battery fully using othermeans and then transfer the charged battery to the control andconnect an ammeter in series. Increase the voltage setting untilthere is significant current and then back off the potentiometer untilthe current drops to perhaps 10mA or so. Some types of batteriessuch as lithium ion types must be disconnected after charging toprevent degradation.Solar Battery Charger ProtectionC1 provides substantial protection against static discharge.There is no protection against reverse polarity or mis-wiring. It isprotected if solar panel is connected reverse without batteryconnected, or if battery is connected reverse without solar panelconnected. However, if the battery or solar panel is misconnectedsimultaneously, who knows? anyone care to experime