Actually reading a little further there is a "Note 4" on page 4 of the datasheet referring to the startup voltage. this is interesting since the start-up voltage listed in the datasheet is 7.5 V. ' The weirdest thing to us is that with a DC supply, everything works OK. I could see the voltage you are measuring being the same for any number of panel configurations if the MPPT holding voltage is still set at the same voltage. Set voltage is measured from the SET pin to the GND pin. The Sunny Buddy hookup guide refers to a set Voltage of 3.0 V while the schematic refers to a set voltage of 2.8 V. Make sure you are resetting your potentiometer for the different panel configurations as this sets the holding voltage for MPPT. Note: I've read the datasheet & know a good amount about the LT-3652's capabilities. How do we "turn on" this circuit with the solar panel(s) connected (how do I approach this / what do I try next)? However, since we measured the same voltage from different panels, we think the circuit itself is the problem.
We considered exposing the solar panel to more light. We have tried using the two separately and in series, all of which maintain the 4.485V. Even when we connect another 6V 9W solar panel, we still measure 4.485V. However, when the solar panel is connected to the circuit, the voltage is a consistent 4.485V and current is drawn from the battery since the charger is "off" (circuit will only "turn on" with >=6.5 V power supply). With the solar panel disconnected, we measured ~15 V with a voltmeter. When supplied with at least 6.5 V, the circuit "turns on" and supplies a small but significant charge current to the battery at around 60-100 mA (to measure this, we cut the trace at JP5). When powering the circuit with a DC supply, everything works OK. This is done to set the float voltage to 4.1 V as described in the datasheet for the LT-3652 2A Solar-Powered Battery Charger. Our circuit is almost the same, except for on our prototype board, we use different resistances for \$R_3\$, \$R_6\$, and \$R_2\$. The circuit we're using is a simple derivative the of the MPPT Solar Charger for Sparkfun's Sunny Buddy:
We are trying to charge a 3.7V LiPo 150mAh battery with a 30W 12V Solar Panel.
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