BQ24650: Maximum Charge Current Problem

Solar Array (Vin) open circuit is around 20v

When connected and charging, MPPSET = 1.2v @ Vin = 18v.

Solar Array Current = 2.79A, Battery Current = 3.31A

I am not 100% sure of the MPP voltage of this panel as it is not published however, I have some experience in using this panel with other MPPT chargers and can estimate it pretty well to be about 18v.

The solar panel are in full sunlight and never shaded.

>>If your MPPSET voltage is regulating at 1.2 V, this means the charger is drawing the most current it can without crashing the input below 18 V.<<

I am a bit confused about how to set the voltage at the MPPSET pin. From what I understand, the goal is to choose the resistors in the voltage divider that will set the voltage on that pin to 1.2v to have the part regulate Vin at the desired MPP voltage. (18v in my case) OTOH, having this voltage set to 1.2v puts the device into its current limiting mode so that it doesnt tank the input voltage. It seems that this is where my problem is. 

Also, is there a way to disable precharge mode or at least lower Vlowv to below, say, 8v? Would like to disable if possible. I found an appnote, SLVA473, that describes a method to do this but heard previously that TI no longer recommends doing this.

Hello,

If your VIN is 18 V and the input current is 2.79 A, your input power is only approximately 50 W. With a 10 V battery you will get maximum 5 A charge current. This is why you cannot charge at 10 A, there is not enough input power. Careful selection of the solar panel and testing of its Maximum Power Point will help to yield full input power.

Your description of the MPPSET pin is correct. Set the voltage divider so that the MPPSET pin is 1.2 V at the desired input MPP voltage.

I have not verified the hardware of the SLVA473 Application Note, but I simulated it and it works in theory. Please make sure that the FB pin is above VLOWV for the desired battery voltage range you want to have fast charge only. A full charge cycle would verify the behavior once designed.

Thanks,

Mike Emanuel

Please click "Resolved" if this answered your question.

>>If your VIN is 18 V and the input current is 2.79 A, your input power is only approximately 50 W<<

I must be missing something. (Not the 1st time :) ) The solar panel I am using outputs up to 5.8A at about 19v (110W) I am still a bit unclear as to why the part thinks that my input source is underpowered. 

Hello,

An example PV curve of the solar panel is shown in the link below. If you set your Maximum Power Point (MPP) below the optimal voltage, you may not get as much power out of the panel. It is crucial to set the MPP voltage as close as possible to the solar panel's MPP.

https://www.pveducation.org/pvcdrom/solar-cell-operation/iv-curve

Thanks,

Mike Emanuel

Please click "Resolved" if this answered your question.

I see, so I guess that my only choice (since I don't know the exact MPP voltage of this panel) would be to experiment with changing the divider until I get the part out of it's current reducing mode. Does this sound right?

And, just to ask a stupid question, if I need to know the MPP of the panel and set the voltage on MPPSET to have the part regulate that voltage, does the part actually "track" the MPP i.e. calculate and find  it using an algorithm like P&O or does it just regulate around the set point. I noticed when scoping the gates of the MOSFETS that the duty cycle doesn't appear to change much so a assuming that the parts acts more like a straightforward adjustable voltage regulator.

Thanks for the info and will close this case on your answer.