1. SOLAR BATTERY CHARGER CIRCUIT
Posted by Jim Keith in Solar | 15 comments
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This is the most simple and affordable solar battery charger that the
hobbyist can make. It has a few drawbacks over other similar controls, but
offers numerous advantages. It is intended for charging lead-acid batteries,
but may also be used for charging any battery at a constant voltage. Voltage
output 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 whistles
Solar 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)

2. Solar battery charger schematic
6V 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 Min
Minimum Head Voltage
This is also referred to ―drop-out voltage.‖ The input voltage must
exceed the output voltage by about 2.75V @ 1.5A. Fortunately, when

3. the battery discharged, the output voltage is lower so the solar
panel voltage will also be lower.
When fully charged, the battery voltage will be high, but the current
is very low—at this point, the drop-out voltage reduces to about 2V
and the open circuit solar panel voltage also comes into play. The
schottky rectifier was selected to reduce this head voltage
requirement—the voltage drop of the schottky is about 0.5V @ 1.5A
or about half that of a typical silicon rectifier.
More advanced controls have a much lower head voltage
requirement and will function better under marginal conditions.
Maximum Power Dissipation
The power is limited by the thermal resistances of both the LM317T
and the heat sink. To keep the junction temperature below the
125°C Max, the power must be limited to about 10W. If a smaller or
less effective heat sink is used, the maximum power dissipation
must be de-rated. Fortunately, the LM317 has internal temperature
limiting so that if it gets too hot, it shuts down thus protecting itself
from damage. Max power comes into effect when charging a 12V
battery @ 1.5A: e.g. battery voltage = 12V, solar panel = 18V. P =
(18V – 12V) * 1.5A = 9W. So thermally, it is carefully matched to the
current rating.
If a solar panel that is characterized for 12V is applied with a 6V
battery, 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 exceed
10W.
When charging, the heat sink normally runs warm. When beginning
to ―top off‖ or completing the charge at maximum voltage, the heat
sink runs hot. When fully charged, the heat sink runs cool. This heat
is not exactly wasted power—it is excess power that is unneeded in
the process of charging a battery.
Current Limiting

4. Current limiting is provided by the solar panel—it is not a commonly
understood fact that the solar panel tends to be a constant current
device. For this reason, a solar panel can withstand a short circuit.
Therefore, the control does not need current limiting.
Float Charge of Lead-Acid Batteries
This control charges the battery at a constant voltage and also
maintains a charged battery (float charge). The float charge voltage
specification is a little lower, so to accommodate both charge and
float charge voltage, a compromise is reached by simply reducing
the voltage slightly—that is how ALL automotive systems operate. To
obtain maximum charge in a 12V battery, set the control to 14.6V.
Automotive systems further reduce voltage to 13 to 13.5V in order
to accommodate high temperature operation as the battery is
usually located in the hot engine compartment—battery has a
negative thermal coefficient of voltage.

5. Application with other types of batteries
It is difficult to determine how to set the voltage in this case. The
easiest way to do this is to charge the battery fully using other
means and then transfer the charged battery to the control and
connect an ammeter in series. Increase the voltage setting until
there is significant current and then back off the potentiometer until
the current drops to perhaps 10mA or so. Some types of batteries
such as lithium ion types must be disconnected after charging to
prevent degradation.
Solar Battery Charger Protection
C1 provides substantial protection against static discharge.
There is no protection against reverse polarity or mis-wiring. It is
protected if solar panel is connected reverse without battery
connected, or if battery is connected reverse without solar panel
connected. However, if the battery or solar panel is misconnected
simultaneously, who knows? anyone care to experime