MPPT (LT3652) + Solar Panel Voltage

Question

We are trying to charge a 3.7V LiPo 150mAh battery with a 30W 12V Solar Panel.

The circuit we're using is a simple derivative the of the MPPT Solar Charger for Sparkfun's Sunny Buddy:

Our circuit is almost the same, except for on our prototype board, we use different resistances for \$R_3\$, \$R_6\$, and \$R_2\$. 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.

When powering the circuit with a DC supply, everything works OK. 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). With the solar panel disconnected, we measured ~15 V with a voltmeter.

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). Even when we connect another 6V @ 9W solar panel, we still measure 4.485V. We have tried using the two separately and in series, all of which maintain the 4.485V.

We considered exposing the solar panel to more light. However, since we measured the same voltage from different panels, we think the circuit itself is the problem.

How do we "turn on" this circuit with the solar panel(s) connected (how do I approach this / what do I try next)?

Note: I've read the datasheet & know a good amount about the LT-3652's capabilities.

Answer 1

Make sure you are resetting your potentiometer for the different panel configurations as this sets the holding voltage for MPPT. 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. Set voltage is measured from the SET pin to the GND pin. 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.

'The weirdest thing to us is that with a DC supply, everything works OK. After 6.5V, the circuit "Turns on" ' ... this is interesting since the start-up voltage listed in the datasheet is 7.5 V. Actually reading a little further there is a "Note 4" on page 4 of the datasheet referring to the startup voltage. The note reads as follows...

Note 4: This parameter [startup Voltage] is valid for programmed output battery float voltages ≤ 4.2V. VIN operating range minimum is 0.75V above the programmed output battery float voltage (VBAT(FLT) + 0.75V). VIN Start Voltage is 3.3V above the programmed output battery float voltage (VBAT(FLT) + 3.3V).

This note tells me that your VIN minimum is 4.65 V (4.1 V + 0.75 V) and your VIN Start Voltage is 7.4 V (4.1 V + 3.3 V). If you are currently at 4.485 V that may not be enough to startup or maintain.

Backing up a little Note 3 may also be of interest...

Note 3: VIN minimum voltages below the start threshold are only supported if (VBOOST-VSW) > 2V.

In the schematic VBOOST and VSW are on connected on the battery management side so my guess is that VIN minimums below the start threshold might need help from the battery. Page 12 and 13 of the datasheet elude to this.

I suggest setting your MPPT voltage per the Sunny Buddy Hookup instructions but use 2.8 V as your set value. Check the voltage across your panel and if it isn't above 7.4 V try to tweak the potentiometer above and below the recommended set point and see if you can measure a VIN value above 7.4 V. Make sure you are measuring good VOC levels to start, such as the 15 V you have achieved in your lab. MPPT will be around 80% of VOC as an approximation...from what I've read.