Lipo charging from solar cell

[vinodstanur]

Hi,
I am charging a lipo from solar cell. Lipo capacity is 2000mAh 3.7v.
My solar cell is 9v 3W.

Currently what I am doing is, I am using a buck regulator to convert 9v to precise 4.2v. I connected the 4.2 to 2000mAh lipo and here I am taking advantage of the maximum current limitation of solar cell, I am getting around 330mA when I am keeping it on heavy sunlight, pointing directly to sun, so I am not worried of constant current charging as the current can never be more than 1/2C (1000mA).

I have few questions regarding this approach:
1> I will not be terminating the charging when the cell voltage matches 4.2v. Generally I observed in normal lipo chargers the charger will terminate giving potential when charging current is less than a threshold at CV mode and then it starts charging only when cell voltage comes down below a certain voltage (say 4v). Does this feature really required for a 2000mAh cell ? What if I keep the above buck regulated 4.2v continuously on the cell ?
If it is a problem, what if I keep 4.12v (little less) to be in the safe side? If so, can I keep it for infinite time ?

 

[BradtheRad]

What if I keep the above buck regulated 4.2v continuously on the cell ?
If it is a problem, what if I keep 4.12v (little less) to be in the safe side? If so, can I keep it for infinite time ?

Seems to me you can adjust the voltage until you find one which maintains a tiny trickle charge indefinitely. Say between 1uA and 1mA. (Expect it to wander up and down.) That should keep the battery fully charged, without overcharging it. The exact voltage might turn out to be 4.12 or 4.15 or 4.20.
Temperature may play a role.

 

[vinodstanur]

Also I am not able to extract the maximum power.

Is there any simple MPPT method for this approach ?

What if I can keep an LDR as a pilot circuit to detect light and can map it to OCV of the solar cell and accordingly if I adjust the buck PWM so that the CCV of solar cell will never go below around 80% of instantaneous LDR-MAPPED OCV of solar cell ?

Can this be a good approach or is there any even simpler method possible without much expensive circuitry ?

 

[KlausST]

Hi,

Is there any simple MPPT method for this approach ?

Keep the PV voltage constant.

Klaus

 

[Audioguru]

A Lithium rechargeable battery kept at its maximum voltage of 4.20V does not last long. It should be stored at 3.7V then charged before you need it.
When charging a Lithium battery cell it is not fully charged when its voltage reaches 4.20V, it is about 75% to 80% fully charged. It is fully charged when its charging current reduces to a fairly low amount which takes time.
ALL half-decent charger ICs detect the low charging current then they shut off.