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Hi, i have buy a SparkFun Sunny Buddy, connected a 3.7v 1800mah lipo, and some solar panels (or dc, same things).
The battery never charge more than 4v. With an external lipo charger, same battery, no problem to go to 4.2v - full charge.

I have increase current with R_SENSE = 0.05ohm but no change, only 4v in battery.

Any idea?

Many thanks in advance.
Solar panels: 6v in series, 18v in parallel
That cannot be correct.

If you want help, please provide a meaningful answer to this question: what is the voltage and the current provided by the power source used as input to the Sunny Buddy?
It seems i have the same problem. My LiPo batery charges only up to 4.0V
It is a 1s 4000Mah LiPo.
My solar panel is:

Material polycrystalline silicon
Body dimensions 290 x 330 x 25mm
Voltage at max power 17.49V
Current at max. power 0.58A
Short circuit current 610mA
Maximum power 10W

The load on the buddy is a 5v step up converter --> Arduino Nano, a ultrasonic sensor hc sr04 and a HC-12 Radio.
I live in Greece, and believe me, its bright outside...

I added a 0.1 Ohm resistor in parallel on R_SEN, so 0.1/0.07=1,428 A
I turned the poti on the buddy to 3v with the panel pointing to the sun and no load and battery connected.

If you need any other information, please ask.
4 volt battery voltage is as it is designed for. In the LT3652 datasheet on page 9 it is explained that charge function operates to achieve a final float voltage of 3.3 volt on the Vfb pin. If you look in the schematic you'll see that the voltage Vfb is made from the output (battery) voltage with a 220k/47k resistor voltage divider (R2 and R3). If you feed 4 volts into such a resistor pair then 3.3 volt comes out between them. I'm sure Sparkfun chose those values to cover most lithium-based battery chemistry.