Low power solar Li-poly Charger suggestions?

Thanks..

Only Issue I am thinking about is....

If the sunlight is low and is producing about about 3V 50mA(0.15W), while the battery is 3.7V, this would mean that the battery would not get charged and some small amount of solar power is wasted.

But if there is an efficient step-up that that could produce about 4.2V 35mA(0.15W).

Ideally the step-up feature would be enabled if the solar voltage drops below battery voltage, so that even a small qty of charging can happen.

I am looking into bq25570, if it would do what i require.

bq25570 can only be used when it is boosting, i.e. only when output of the panel is below the battery voltage.

Thanks,

Just out of curiosity What would happen if the Vin(solar) voltage is higher then the output, as there is there is buck output?

I will keep that in mind and use a solar panel rated to about 3V 416mA instead of 5V 250mA, as this will insure that at all times there will all always be a charge for the battery.

But what sets the output current as I am aiming to have some flexibility in the output current due to sun? Ideally there will be a maximum but how do we insure that the battery will not cause the output voltage (4.2) to drop due to maybe the low charge current available and that even if the current is low as 5mA,the battery still gets this charge.

My application is simply a wireless TX and RX, 2000mAh battery last me about 4 days, so any little charge would be great to keep the system continuously running.

By the way do you know of any ready made dev kit i could use try my solar concept out?

Once again thanks

When VIN_DC > VBAT on the bq25570, a pull down resistor on VIN_DC turns on to sink the current from the solar panel. Keep in mind the bq25570 peak switch current is 200mA typical so if you use a 416mA rated panel, you will not extract all 416mA from the panel using the bq25570.

No problem, I will use a 3V solar panel to avoid current sinking and wasted.

In regards to peak switching current of about 200mA, is this in relation to Vin or Vout. As when Vout is 4.2V, max currrent will be 297mA


As if i can not use all of the 416mA or at least 90%, then it might be better to use a even small solar panel and reduce my costs.
Or is there any other IC solution i can use?

Thank You

DJ

The 200mA is the peak input current to the bq25570. The bq25570 will attempt to pull up to 200mA typical from the high output impedance solar panel until the voltage sensed at its VIN_DC pin droops to the MPP setpoint voltage (typically set to 80% of the panel's VOC) as stored on the VREFSAMP pin.

Hi Jeff

Thanks

I add that to the design book

What happens if i simply use a standard Step-up/down power regulator regulated to 4.1V, with a power switch at the output and then use a ADC to decide when to enable the charge.  Maybe i am wrong but this would insure the battery get the max charge output of a solar panel.  

Regards

DJ

The output of a solar panel is high impedance so the more current you pull the lower the output voltage of the solar panel becomes. Most dc-dc converters do not have way to manage input current so they will pull current up to their current limit and potentially collapse the panel. If you choose a dc-dc power supply with low enough current limit, you might be okay. TI's battery charger's have a VINDPM function that monitors the input voltage and reduces the input current draw to prevent collapsing the panel.

HI Jeff

Thank very much for your reply

That does make sense, in regards to the solar panel being high impedance and if the voltage does drop below the IC minimum input voltage than it could mean potentially the step up could be disabled and battery would not be charged at all. I think it would be best to use the bq25570.

Please do forgive my questions as i am new to solar design.

1) My solar panel was chosen as it was the max power i could get for the size i required (casing size). Now that you have mentioned that max switching current would be 200mA, but would that be at max input voltage of 3V?

If i am correct this would mean the max theoretical charge current i could achieve would be 142mA at 4.2V and given that this IC is 90% efficient this would mean 127.8mA charge current?

Currently my solar panels are 5V 250mA, but i am thinking about using 3V 416mA. Would this mean that about 200mA would be wasted OR should i just consider that that additional 200mA in solar panel will just insure that i am achieving my max switching current even the sun is low?

Ideally even with the bq25570 there will be a voltage drop at the input, but how do we adjust the charge current so that battery does not take to much current and forcing the IC to switch off. Is this automatic feature or something we must set?

Is there an online simulation i could try out?

Thank You