12V 7.2AH SLA battery (4): LM317 battery charger test

 When I originally tried a LM317 circuit, I used the circuit I found at this link.  The LM317 charger circuit is quite popular and can be found all over the web. The circuit I'm using differs from that at the above link in that I added a 100 ohm resistor in the base of the 2N3904 current limiting transistor. I did this to protect the transistor. The circuit is design for a charge voltage of 13.8V and a charge current limit of 0.75A. 

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The charging circuit works as follows: 

• The LM317 regulator controls the voltage drop between the power source on the left and the battery on the right to maintain 1.25V from the VO pin to its ADJ pin. 
• The regulator voltage is set by VO = (1+(RV1+R5)/(RV1+R5+R4)*1.25V)
• The battery charge voltage is set to 13.8 volt. Given the values in the schematic, this corresponds to RV1 = 2.52Kohm
• R6 and Q3 form a foldback current limiter circuit. The current limit cuts in when the voltage at R6 reaches about 0.6V.  If R6 = 0.82 ohms (four 3.3 ohm resistors in parallel), then the current limit will be 0.6V/0.82ohm = 0.73A. 
• In actual operation with a discharged SLA battery, the circuit will initially attempt to regulate the battery voltage at 13.8V but current limiting would cut in, resulting in constant charge current as the battery charged.  As the battery charge voltage approached 3.8V the current limiting would cut out and battery charge current would fall while the charge voltage remained constant. 

I built the charger circuit 

After building the circuit, the LM317 charger was tested as shown in the photo below.  In the upper left is a 23V source taken from the unregulated side of a 13.8V power supply. The charger circuit is in the lower left while a voltage and current monitor for the battery is in the lower right while the battery itself is in the center right. 

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The initial test of the LM317 charger is shown below.  A problem with is test is that the voltage was not set to 13.8V as required.  The voltage was set to 13.4 volts. The battery fails to charge fully and final battery charge voltage settles at 13.3 volts.  

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The test will be conducted after adjustment of the set voltage to 13.8V.