12V 7.2AH SLA battery (3): XL4015 buck converter charger

 I ordered step down modules from Amazon.  I bought 3 modules for $11.37 total.  The modules are rated for 5A with outputs from 0.8V to 30V. I haven't measured the dropout voltage but it appears to be slightly more than 1 volt. 

The unit has a voltage regulation adjustment and a current limit function.  I set the voltage at 13.8V and set the current limit to 0.72A, which is 10% of the battery's 7.2 Ah capacity.  After discharging the battery to about 50% state of charge (SOC) I recorded the current and voltage of the battery as it was charging.  See plot below: 

click to enlarge

The current is limited at 0.70A for the first hour or so. Then the current drops exponentially while the voltage approaches a constant voltage of 13.5 volts or so.   This is controlled 2 stage charging.  For a third stage the voltage could be dropped slightly for long term trickle charge.  This seems like a pretty good charger circuit and costs less and $4 for each module.