SparkFun Forums

[Nicola1337]

Hi,

I've just put my hands on the mighty Sunny Buddy V13 and I'm making some tests with a small 6V solar panel. This panel has got the following specifications:

Voc: 7,50V
Vmp: 6,00V
Isc: 1,07A

I'm experiencing this problem: when the panel is under good sun conditions, (e.g. 7,08 Voc) the charger doesn't start. (no current)
If, instead, I put temporary the solar panel in shadow (e.g. 6,33-6,50 Voc) and I disconnect-connect the input, the charger starts and has a small current running through it (and it's correct, it's in shadow). If then I move it to the sun, it gains the full amperage. But again, under the sun if I disconnect the input for a while and I reconnect it suddenly, the IC doesn't start anymore.

I have seen in LT3652 datasheet that:

if Vbat(flt) <= 4.2V
Operating Voltage is Vbat(flt) + 0,75V = 4.95V (and that's ok)
Start Voltage is Vbat(flt) + 3,3V = 7,5V (and that's might be a problem, here)

I have the sensation that there's something wrong in the startup voltage but I can't understand yet what's going on here. Do you kindly have any suggestion? This problem has put down all my enthusiasm because this chip is really really interesting and I would like to make it work.

Thank you in advance.
Best,
Nicola

[jremington]

What type and capacity battery have you connected?

[Nicola1337]

The battery is a Li-Ion 3,7V 6600mAh (24,41Wh). https://www.adafruit.com/products/353

[jremington]

And what is the battery open circuit voltage?

[Nicola1337]

At the moment 3,91V.

[jremington]

when the panel is under good sun conditions, (e.g. 7,08 Voc) the charger doesn't start.
...
Start Voltage is Vbat(flt) + 3,3V = 7,5V

That could be the problem. Try boosting the input voltage by adding a second panel, or a 1.5V D cell in series with the panel, to see if the charger starts up correctly in full sun.

Keep in mind that the open circuit voltage of a solar panel does not depend strongly on the illumination (the current does), so the unexpected startup behavior when the panel is in the shade might be explained by the chip entering a different, low current operational mode.

[Nicola1337]

Hi jremington,

Understood. To state that the IC operates from 4.95V is confusing, actually.
The problem is that the solar panel has been made custom from a Shenzhen company specifically for the project. Before using this chip I was using a linear charger <=6V (MCP73871). But it couldn't manage a current higher that 1A.
Do you think that if I make a custom solar panel with a little bit more voltage by adding two cells, reaching e.g. 8,52 Voc should I get it?

Thank you so much.

[Nicola1337]

Or is it even possible to use a 9,94 Voc solar panel to charge a 3,7V battery with this chip? I'm thinking about the design of a solar panel with cells cut smaller to put more of them, and to output a similar wattage by working with an increased voltage instead of current.

[jremington]

The chip is a switching regulator, and you can use up to 20V on the input. However, you lose the MPPT capabilities of the chip and maximum output of the solar cells that way.

For the smallest size and most efficient system, the solar cell voltage and current would be matched to the chip and battery combination.

[Nicola1337]

...you lose the MPPT capabilities of the chip and maximum output of the solar cells that way.

Could you please show me an example of that sentence? I can't understand how actually the system works.

If I have a solar panel with:

INPUT
Voc = 9,94V
Wp = 7,16W
Vmp = 7,95V
Isc = 0,90A

If I set the IC VIN_REG = 7,95V, when the chip tries to draw more current that 7,95V can give, it reduces the charge current speed in order to maintain the Vmp. So the result should be this one.

INPUT------------------OUTPUT
Wp = 7,16W-----------Wp =7,16W
Vmp = 7,95V----------Vmp = 7,95V
Isc = 0,90A------------Isc = 0,90A

But, given that the chip can't use 7,95V to feed the battery, it regulates down the voltage, even in order to satisfy the current limit for constant current mode. Is it right?

INPUT------------------OUTPUT
Wp = 7,16W-----------Wp =7,16W
Vmp = 7,95V----------Vmp = 3,58V
Isc = 0,90A------------Isc = 2.0A

I think that there's something missing in my argument. Help

[jremington]

A Maximum Power Point Tracker attempts to deliver the maximum obtainable power of the solar panel to the load.

However, the Sunny Buddy is current limited, which makes it impossible to accomplish the above unless the panel is carefully matched to the battery voltage and the current limit.

The more (or larger) solar panels you add, the more you waste of their capability.

[Nicola1337]

That could be the problem. Try boosting the input voltage by adding a second panel, or a 1.5V D cell in series with the panel, to see if the charger starts up correctly in full sun.

I tried to connect in series to the solar panel two 1,2V AA batteries in series. The resulting voltage is now 9,50V.
VIN_START = 7,50V
9,50V > VIN_START

But even this way, the chip doesn't start. Is my board born defective?

[jremington]

Could be. Contact SparkFun customer support.