Solar Cell based 3.7V Li-Poly Charging

[sakibnaz]

Hi all.

I have a project where its power from 3.7V, 300mAH Li-Poly Battery. Currently its having USB based Charging by MCP73831T. I want to add a Solar Cell based Charging in adding with USB. My Board dimension 32mm Diameter which is very small hence I chooses a small Solar Cell. Below is the specification of Solar Cell:

Dimension : diameter 30.5mm, thickness 3mm
Open circuit Voltage: 2.5V
Short circuit current: 35mA
Operating Voltage: 2V
Operating current: 30mA
Rated Power: 0.06W

Currently to test Solar Charging I did the below:

Solar Cell -> DCDC Boost -> 5V -> MCP73831T -> Li-Poly Battery

But this is not working. DCDC Boost voltage dropped to less than 1v. I understand MCP73831T is tuned for 100mA charging currents due to that Boost Voltage dropped as Solar Cell not able to provide 100mA.

I like to have your advise:

The way of my design is Ok? or what is the best design idea to do a Li-Poly charger from such small Solar Cell.

Thanks in advance.

Awaiting for your reply.

 

[dick_freebird]

A deeply discharged cell might well drag the boost output
down that far. Why don't you concentrate on the converter
for now, see what kind of load-line it delivers with a solid
battery / power supply feed, and then what giving it a very
limited input current does.

There might be nothing wrong, besides needing time to
deliver coulombs. But you should characterize things to get
that understanding.

 

[sakibnaz]

Hi dick_freebird.

Thanks for your reply.

We also found MCP73831 consume around 500uA, even no battery. Is there any other Charging chip which will be better choice for Solar Based Charging than MCP73831?

Or how is, if connect the Solar Cell directly to Li-Poly through a Diode?

Regards.

 

[Audioguru]

A diode is not a complicated proper charging circuit. A diode does not limit the charging current to the charging ratings of the battery cell, and it does not limit the charging voltage to 4.20V.
The Li-Po might blow up or catch on fire if it is charged wrongly. Also it might be damaged.

 

[sakibnaz]

Hi.

To limit the charge current if I add a resistor in series with the Diode ... will it be helpful?

Or, can you advise me any other Schematic idea?

Regards.

 

[BradtheRad]

You might consider a 'joule thief' type boost converter. It will operate on a volt or two which your PV cell puts out.

A joule thief can be made from a few components. It will boost the 2V from your PV cell, so you can charge a 3.7V battery pack, at maybe 10 mA.

What would really help is if you were to add another solar cell or two.